SPE C J M Putot

SPE, lnstitut Frangais du Petrole P J Perreau

ASME A Constantinescu

Field Data vs. Theoretical Model to Quantify Drilling Efficiency and Disruption

Mechanical and kinematics data at the bit have been collected during drilling operations and organized to identify optimal drilling efficiency. Transitional conditions towards abnormal behavior have been distinguished. Normal drilling is usually characterized by regular linear relationships between global control variables at the bit, which approximately correspond to an up scaling of a local Mohr Coulomb type failure mechanism of the rock. Most of our experimental data confirm this classical disposition. However, considerable departure from these relationships is noted, especially when there is BHA resonance or when there is partial balling at the bit. The field tests presented here allows us to verify classical theory and to draw up new guidel-ines for behavior in non optimal or dangerous, evolving operating conditions, such as those that occur under poor cleaning conditions. The main features of a mechanical structuring model involving transition towards low efficiency conditions are presented, and it is shown how this model confirms our interpretation of field data. The model couples the drillability of rock and the general dynamics of the bit in an original and synthetic way. It makes it possible to understand and test the sensitivity of bit response to variables such as "weight on hook", torque on the drillstring and flow rate of the fluid.

Introduction

Common perception of drilling efficiency. The idea of combining the specific energy associated with rock crushing and the energy from hydraulics in order to achieve a synthetic view of bit efficiency in soft rock is not new. Refs. 1 and 2 have defined semi-empirical relationships which take into account the tendency for the rate of penetration to level off when weight on the bit increases beyond normal values. These schemes have been verified through numerous tests but, as they cover ordinary conditions, cannot describe conditions like transients to balling, for example.

Meanwhile, some experts think that it is time for a new perception of hydraulics. According to Warren (Ref. 3), "Generally, when people talk about bit hydraulics, they are referring to some measure of flow rate and pressure drop, but those factors cannot be considered independent of the particular bit design. The major issue is that the bit should be designed and operated in such a way that drilled cuttings do not impede drilling efficiency. When drilling in soft rocks, preventing bit and cutter balling is a primary concern." Drilling conditions. It appears from these considerations that two kinds of drilling conditions must be distinguished: I. Those for which a slight perturbation of operating parameters will not significantly modify the drilling conditions. These drilling conditions will be considered as stable, and they do not significantly evolve in time. They correspond to rather efficient conditions for which vibration is low. An optimum may be distinguished. 2. On the other hand, transitions towards abnormal conditions may be observed when control variables are poorly chosen. Among these are:

2.1. High levels of vibration and correspondingly low rates of penetration, such as those that are observed with poorly designed bits when crossing hard and abrasive formations (Fig. 1, bottom part, for axial vibrations). The art of the designer is to introduce elements that reduce both the severity and sharpness of vibration peaks and the range of operating conditions where these detrimental conditions appear. The strategy, when conducting drilling operations, is to find ways to escape from these conditions either by reducing rotary speed or decreasing weight on the bit.

2.2. Low efficiency associated with balling mechanisms, principally when drilling soft rocks, also characterized by very low penetration rates (Refs. 4 through 6). This phenomenon generally appears after a temporary increase in performance with excessive generation of crushed material and, consequently, sudden saturation of the cutting head (see for example Fig. 14 of Ref. [START_REF] Hemphill | Effects of PDC-Bit Selection and Mud Chemistry on Drilling Rates in Shale[END_REF]). Excessive weight on the bit or rotation speed are generally at the origin of this disruption. Certain forms of balling may be irreversible. We will concentrate on this second type of disruption. Numerical Analysis and Validation. Considering that drillability and dynamics are correlated, an assertion that has often been made, but that has yet to be practically applied, it is useful to demonstrate the complementary roles of bit dynamics and drillability with a model that covers unusual conditions. This model is intended to show the effects of mixed cutting and cleaning conditions on bit dynamics. Among these are some special forms of bit bouncing (Fig. 1) that occur within soft rocks when the cleaning flow of crushed rock is plugged. Brett (Ref. 7), among possible causes of angular excitations, cites bad cleaning conditions as possible candidates for stick-slip. Our model, called CUTCLEAN, dynamically couples axial and rotational degrees of freedom at the bit with a concentration variable at the bit in a very simple and synthetic way. The forces and moments of various origins encountered at the bit explicitly depend on these three state variables. The solution expresses the evolution in time of these three state variables, for a convenient choice of the control variables W, N, Q.

The second step of our approach is to verify the expressions for forces and moments included in the model using examples taken from field cases. The main difficulty with field data is the assumption, which is rarely fully justified, that rock is homogeneous. The way to overcome this difficulty is to use stabilized sequences of drilling behavior that are, when possible, sufficiently long to ensure stabilization of the process, but not long enough to risk crossing a geological transition. Nevertheless, it is impossible to eliminate variability on a centimeter scale.

The validation examples are of two types. The usual way is simply to describe systematically the global parameter and response of the drilling system and to observe the effects as a function of time, under the control of the operator. Stable behavior can be reversible, whereas some types of balling are not. A more systematic and rational way to observe response behavior is to carry out specially designed experiments with programmed sequences of weight on bit, rate of rotation and hydraulics. The two types of experimental data have been carefully collected and combined to create a "representative" state.

Experimental Evidence of Three Typical Regimes

Three kinds of typical behavior have been observed and classified (Fig. 2) (An excessive level of vibration associated with hard rock interaction is disregarded).

Type I: Normal case. Interpretation of control parameters Rv and Tv according to a classical representation derived from Warren (Ref. 8) and proposed by Falconer (Ref. 9) for a Tricone bit (extended to PDC). The dimensionless torque, To. plotted as a function of reduced rate of penetration, ...J R0. leads to a linear relationship (Fig. 3). The distribution of points reflects the intrinsic variability of conditions imposed by the natural variation of rock properties, even when the driller does not modify control parameters. Small-scale properties induce slight perturbations of parameters. The ordinate T0 represents (Pessier and Fear, Ref. IO) the "friction coefficient" of the bit, insofar as the failure mechanism might be comparable to friction. A rapid calculation suggested by the authors shows that the actual friction coefficient m is three times that value. Some fundamental experimental research (Scholz, Ref. II) relates this coefficient to rock properties and normal stress. This evaluation corresponds to normal behavior and has a secondary dependence on surface roughness and relative velocity .

. Orders of magnitude. The effects described above are quite difficult to quantify. Nevertheless, the order of magnitude (m=0.6, T0=0.2) is in accordance with the observed values. These are lower for Tricones ( about O.IS) than for PDC's ( about 0.3), and this observation is related to the fact that the kinematics of the bit types are rather different. The rolling movement with relatively low shearing and "friction" for Tricones is in contrast with the high shearing interaction for PDC's. Some possible justification of variations. Whether the straight line that represents dimensionless torque T 0 as a function of the dimensionless rate of penetration...J R0, which varies monotonically with weight on bit in this case, increases or decreases could depend on hydraulics. Detournay's (Ref. I2) analytical expression seems to confirm this assumption for PDC's. With this interpretation in mind, the direction of the slope is obviously related to the friction parameter, which could logically increase with decreasing hydraulic efficiency. This could be the reason why we observe negative slope when hydraulics are unfavorable. The common feature of this class of behavior is the moderate value of T�. From the "rock mechanics" point of view, this behavior calls to mind a possible saturation of admissible shear as confinement increases (hydrostatic stress state), which has been demonstrated by different authors. Positive and negative slopes could be two facets of the same failure characteristic (increasing admissible shearing stress for moderate mean stress and decreasing for higher values).

Type II: Slow balling with low weight on bit. We refer to this type of behavior when excess T 0 is observed, with relatively low weight on bit. This behavior is referred to by Falconer to justify significant departure from the normal failure rule and it may be due to failure of a seal or a roller bearing (cone locking). In that case, the teeth of the locked cone bite into the formation very aggressively, resulting in a significant jump in downhole torque. Our own further interpretation is that this kind of behavior is not always irreversible and could also be linked to polluting the cutting front with very fine particles. This might result from a special kind of packing mechanism at the front caused by too little weight on bit and involving very small rock cuttings and generating high relative torque, probably resulting in an inefficient abrasive mechanism (below the threshold of normal cutting). See Ref. 10, Fig. 6 "bottom balling with buffer of fines; clean and contaminated bearing". It is quite difficult to distinguish between mechanical inefficiency and pollution by very small particles.

Type Ill: Rapid balling with excess weight on bit. This behavior, with simultaneous low R0 and low T0 is, according to Falconer, symptomatic of balling mechanisms or drilling very compact, low porosity rocks. These two elements are mutually exclusive and therefore easy to separate. Concentrating on the balling mechanism, it is important to stress that, as with type II and anticipating subsequent interpretation, such• piling up of cuttings does happen with excessive weight on bit. We will see below the mathematical significance of types II and III. Table 1 sums up the main characteristics of the three mechanisms.

Major Features of CUTCLEAN Models General presentation of specific models coupling dynamics and drillability

First preliminary version, quasi-static (CUTCLEAN) Ref. 13 (Fig. 4 and Fig. 5): It defines, using hydrodynamics, equations that show the time evolution of general balling conditions at the cutting front. It shows critical regimes (Fig. 5) as a function of the controlling parameters weight on bit, rotary speed and flow intensity. Excessive weights (regime 5) lead rapidly to balling with accumulation of coarse material whereas insufficient weights (regime I) induce slow, progressive pollution with small particles and contribute as well to an inefficient regime. This, too, could be defined as balling, making the description of the mechanical threshold, which is usually associated with abrasion and wearing mechanisms, more comprehensive by adding a hydraulic dimension .

Bit dynamics when considering the concentration of cuttings at the cutting front (CUTCLEAN 2) Ref 14: In a simplified version, for which a numerical simulation exists, two dynamic state variables have been defined with axial and circumferential degrees of freedom and one to describe the concentration of solids. The solution of the system of differential equations coupling these state variables is as much a problem in dynamics as it is in describing this concentration. As will be seen, the influence of initial conditions, the gradient of independent loading control parameters weight and torque, have been studied. The advantages of this approach over the quasi-static model are significant, as it explains the influence of bit dynamics on the evolution of concentration and is a contribution to the understanding of drilling system dynamics in balling problems (See, for example,

dt = B( t , l, r, v) = B+(r, v) -B-(l, r, v) dr dt = W ( t , l, r, v) (2a) = w+( t ) -w-u, r, v) (2b) dv dt = T ( t, l, r , v) = r+( t ) -r-(l, r , v) (2c)
Here, B+, s•, W'", W, r. T are respectively the positive and negative parts of the balance of mass, weight and torque functions. The structure of the functions has been chosen so that conformity to general observations is qualitatively ensured. Experimental results for balling phenomena are scarce, so systematic rules for determining WH must be coherent with general intuition as much as with data borrowed from diamond bit studies, for which pump-off effects are important, even in the absence of balling (see, for example, Refs. • For lower and higher values of W'", the drilling system plugs up (r-70) with respectively small and large values of cutting bed parameter l.

• For intermediate values of W'", the dynamic system reaches an equilibrium point or a limit cycle. Transition between these regimes is sudden and occurs when the trajectory reaches a bifurcation point B(r,l) = W(r,l)=O.

Important remarks: The rate at which the weight on bit increases to a given final value may influence whether a possible equilibrium state occurs, as may initial conditions. For a stabilized level control variable, several types of accumulation points can be distinguished (see Figs. A-3 and A-4, for example):

• On the r = 0 axis, reaching balling state.

•

On the l = 0 axis, when B(r,l)<O , conditions for which excess cleaning possibilities exist.

• On B(r,l) = 0, which corresponds to singular points of the dynamic system.

Interpretation of field data

Specific tests while drilling have been conducted on two Gaz de France well sites. One goal was to verify theoretical models that describe some of the typical vibration modes that occur during drilling operations. A specific data acquisition system, Trafor (Ref. [START_REF] Pavone | Application of Sampling Rate Downhole Measurements for Analysis and Cure of Stick-Slip in Drilling[END_REF], was used for this purpose. The operation was conducted with a 12 J.A'' Tricone bit, in rather soft formations, prone to balling, for one site, and hard rock for the other.

General remarks. Field data show: Extrapolation of field data with the help of a systematic program. A grid of independent pairs of the input parameters rotary speed, N, and weight on bit, W, is formed for testing on the well site and the rate of penetration is observed for each (Fig. 6). Hydraulics has been modified in a limited way with no significant effect. Sequences have been carried out for a sufficient duration to ensure stabilization of the process and representative values of the control and response parameters.

Considering R (rate of propagation) as a function of N (rotary speed) for 4 levels of W (weight on bit) (30, 50, 70, 90 kN), a very clear trend to performance saturation appears for low weights, which agrees very well with results previously described for CUTCLEAN calculations (weight threshold with a hydraulic correction). More intuitive and well known is balling that occurs with high weights, which seems, in our case, to appear in the vicinity of W=100 kN and N=100 rpm (Fig. 9).

Since weight, W, and rotary speed, N, are the two engines for producing rock cuttings, it is normal that, for given hydraulics and extension of the cutting head (corresponding to a potential mass flux), a saturation point is reached when upper bounds of couples (W*, N*), characteristics of collapsing performances, are observed. This saturation is observed with R(N) curves for constant W as well as with R(W) curves for constant N.

Investigations for non vanishing values of R are also of undeniable interest and are now considered.

Extrapolation induced by CUTCLEAN analysis: The synthesis presented here is based upon compatibility, for given hydraulics, of two families (sets) of parametrically indexed curves (Fig. 7):

( 1) R(N), parameter W (2) R(W), parameter N • Set of curves R(N), parameter W: A careful study of gradual modification of set (1) with parameter W can be summarized as follows:

(i) For low weights on bit, typically 20 to 40 kN, we generally observe low efficiency, but with a maximum of performance for a relatively low rotary speed (50 to 75 rpm). Performance collapses at about 100-150 rpm, perhaps due to a balling mechanism with fine particles, possibly connected with seal bearing deterioration, as Pessier and Fear seem to propose. The prototype for this regime is sequence A, case 870 (Fig. 8), for which an unusual increase of T 0 is observed when weight on bit decreases and there is simultaneously evidence for reduced efficiency, R.

(ii) For high weights on bit, typically 90 to 100 kN, we observe good efficiency, but for a range of N values decreasing progressively with increasing weight on bit, W. It appears to be symmetric with behavior (i). In this case, a balling mechanism provides an upper limit to rotary speeds. Case 867, Sequence A (Fig. 9) is representative, showing a balling tendency for a rotary speed around 100 rpm.

(iii ) For intermediate weights on bit, typically 50 to 80 kN, the acceptable range of rotary speeds is wider than in the preceding limit cases. Efficiency continues to be significant. This average value of weight will offer a maximum margin of safety without penalizing performance.

• Set of curves R(W), parameter N: The gradual modification of family (2) with parameter N can be summarized as follows: (i) Except for very low values of N, the acceptable domain of weight on bit increases considerably with decreasing rotary speed.

(ii) The higher rotary speed, N, the more sudden and earlier the onset of balling with excessive weight on bit. (iii) The hydraulic threshold described above is hardly noticeable for reduced rotary speeds, but becomes apparent at about N=lOO rpm and is 30 kN for that speed. (iv) Maximal estimated efficiency is obtained for these hydraulics over a large range of control parameters W and N. Special care must be taken for those values close to the "cliff' at the edge of the large "shelf' of maxima, which indicate a tendency towards balling. (v) The choice of N values seems to be relatively unimportant in the upper range. On the other hand, once this choice is made, it is imperative to control W, in order to avoid balling.

Remark concerning the CUTCLEAN model:

CUTCLEAN is a general simplified model for which only full balling is assumed. Consequently, the phenomena described by this code are usually more extreme than in real cases, where partial plugging of cutters is a more realistic hypothesis.

Summary of field testing:

Interpretation and representative types of behavior

These tests:

1. Confirm the classical Falconer representation, which is linear in both the dimensionless rate of progression and the specific energy (behavior type I).

2. Show the existence of numerous sequences where this linear rule is not valid, either because the nature of the rock has radically changed and induced a different failure mode (adjusting t/s defines other Falconer-type lines, higher for porous, lower for compact rocks), or because some type of disruption has occurred, either due to drilling hard rock, where there is bit bouncing or whirling, with a large amount of dissipated energy that is not involved in drilling, or to mechanisms for clearing rock cuttings, which phenomena also have a dynamic component. At this point, two types of situations may occur:

• Type II behavior: When hydraulics is limited but rate of penetration is not too low, (Warren leveling off curves) the behavior, in Falconer's variable space, seems to translate into high negative slope lines. In the closely related Detournay coordinate system, more specific to PDC's, this type of behavior is related to increased friction. It is physically compatible, in the spirit of the model, with the idea of increased dissipation, which here finds its origin in viscous flow pressure losses.

• Type III behavior: When behavior tends towards balling by excess weight, the evolution can no longer be represented by type II straight lines. The operating point reaches very low values of R0 and T0, as has been observed by other authors, and which we effectively confirm with our tests. Balling with excess weight (type III ) is a rather well known phenomenon for which a sound interpretation exists. Its sudden onset has been confirmed by field and laboratory experience (under �ur test conditions, W=100 kN and N= 100 rpm)

Balling with insu f ficient weight (type II) is less intuitive and tends to appear in combination with threshold effects associated with inefficient cutting (see, for example, Glowka,Ref. 19). Its existence has been mathematically proven, using CUTCLEAN (Ref. [START_REF] Putot | Modele de foration fonde sur le couplage de Ia destruction de Ia roche et d'evacuation des debris[END_REF]), for conventional hydrodynamic hypotheses concerning particles of assumed granulometry. However, these physical relationships must be confirmed in order to validate the result. This effect has been identified during tests which show performance saturation for low weights (30 and 40 kN) and relatively high velocities (100 rpm).

After extrapolating and fitting the data, a representative case, which can act as a reference for a semi-quantitative but instructive discussion of drilling efficiency, has been formulated. The data analysis is an indirect step in validating the mathematical functions, in terms of We, Wp, WH, introduced by CUTCLEAN.

Consequences for the driller's response to ba lling. Distributing sets of characteristics R(N), for parameter W or R(W), for parameter N, could be useful to drillers. It could give them quantitative indications of the response of the drilling system in the neighborhood of a selected operating point. Conversely, this map could allow them, since it provides all the useful information, to modify this point, if necessary. where Llp and ilv are two given functions which characterize pressure and velocity losses at the bit and L is a characteristic length.

Nomenclature

• Axial force balance (dynamic equilibrium)

d 2 � M dt 2 = Mg -WM + K�� -WH -We -WF (A-5)
where Mg is the drillstring weight, WH, We , WF represent respectively the hydraulic lift (pump off force), the cutting and the frictional components of true WOB, W M is the weight on hook, the part of the weight controlled by the driller with the brake and which will be considered as a given function of time.

• Torsional balance (dynamic equilibrium of moments) d 2 e de J dt 2 + Ve dt + Ke e -Ke e o -1'c -TF (A-6) where J, V8 and K8 are respectively rotational inertia, damping and stiffness characteristics, Te and TF represent respectively the torque components attributed to the cutting mechanism and that are involved in wear flat friction. K900 is the driving torque and represents rotation control at the bit. In summary, the bit state is characterized by the six variables � 0, l, h, x, and vR and is described by six ordinary differential equations. A discussion of the particular expressions for forces and moments is given in Ref. 14 and is the translation into our own variables of the expressions found in the literature (see, for example, Refs. 15 and 16). We will not debate that subject here. Simplified model. The hypothesis that input flow rate is always equal to output flow rate implies that: zone B(r,l) <() is only obtained with a certain rate of weight increase. Sensitivity to initial conditions has also been tested, in the same spirit (Fig. A-4). The results show considerable discrepancies in the evolution of the state for a fixed form of the command.

Full simplified model with three variabl es (I , r , axial , v, rotational )

The same kind of simulation can be used to illustrate the response of the simplified model to different independant weight W" and torque r histories. We notice, as in the preceding model with two variables, that stagnation points of the functions B, W, T play an important role in the evolution of the system and that these points are determined by the global command of the system. .. --: .. ': .. ;;;.;.;. -,---....---.J,---,----1--,---J 0.0 0.5 1.0 1.5 2.0 2.5 r-rate of penetration (non dimensional) r-rate of penetration (non dimensional) !-time

:::. • • •' • ;-,, • 1• it 'I• • ••l • • t• .. , • ., . •I• • 1 ' 1 yl • ' \ i\. • •• r• • -. . • • 1"1 'I l /( o s .. � ,'l .� •• .� �- �• • :,. ! .. i' , 1 :1 .r l •. , • • 1 :• •! 1 • :I •• r� �.
2.5 •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 1S 2.
6.0 •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 1S 5.
• • • • • • • •• • •• • • • • • • • • •• •••• • • • .. .
:E 3 C) •a; � � 2 0 •• •• •• ••••+ •••• • •• •• •••••••• •• • ••••• ••••• • ••••••••• ••• ••••••

  Fig. A-1 through A-4 described in the Appendix). Mechanical structure of the simplified version CUTCLEAN 2. The model proposed below is based on: (i) Interpretation and quantification of the forces generated at the bit by cutting, sliding friction and cleaning mechanisms. (ii) Compatibility equations of forces and moment components expressed in a dynamic form. (iii ) Balance equations concerning "conservation" (production and evacuation) of rock cuttings and mud flow. The model relies on ordinary differential equations and is described elsewhere in detail (Ref. 14). Its major features are outlined in the appendix. Drilling is essentially controlled by the three parameters weight on bit, rotary speed and hydraulic energy. In the simplified version of our model, three state variables are naturally introduced and matched with these control parameters. They are �. axial position of the bit; 8, angular position, and /, level of cuttings at the head. These equations are: dl d� dt = Sc dt -xQM (la) d 2 � M dt 2 = Mg -WM + K�� -W8 -We -WF (lb) d20 d (J J dt z + V e dt + Ke (J = Ke (J o -T c -TF (lc) Solving evolution equations numerically. The practical numerical application involves the following variables: /, the d� volume of cuttings at the front, r = M -;;{ , the rate of d(Jpenetration, v = J dt , the instantaneous rotary speed at the bit. A general form of the balance equations is written (Fig.4):

  dl

  15 and 16). In parallel, a phenomenological analytical model, involving elementary forces on the scale of the cutting, has been developed in detail. This work was carried out when building the quasi-static CUTCLEAN model, in order to describe more precisely the hydrodynamic action of mud on particles on the basis of Morison equations. This formulation is inspired by the work of Eronini (Ref. 17). See Fig. Sa , b and c for a brief summary of results using the quasi-static model CUTCLEAN. Later on, when building CUTCLEAN 2, a global mathematical expression has been applied to the results in order to simplify the writing. The general form of the model brings out the importance of the coupling between dynamic equations and mass balance equations, translated by the explicit dependence of weight and torque on l. Criteria for balling: The criterion for balling could be either r-70 or, l�l critical• The results are in agreement with experimental facts. For a constant control parameter W'", we notice three regimes (see Fig. A-1 and A-2, for example):

( i )

 i The relevance of the classic linear type I representation, in most cases.(ii) The frequent occurrence of sequences for which the representation no longer applies. The general form of the results obtained with CUTCLEAN is used to confirm information collected from field data. The overall conclusion of this work is the fact that type II is linked to slow accumulation of fine particles generated by ineffective and insufficient weight, whereas type III is connected with balling obtained by rapid piling up of coarse material produced by excessive weight. Types II and III are linked respectively to high and low values of non dimensional torque, which is similar to a coefficient of friction between the bit and the formation. According to Pessier and Fear (Ref. 1 0) , the most critical reason for the destruction of roller cone bits is bearing or seal failures. It is associated, and occurs simultaneously, with an increase in the coefficient of friction, To. between the bit and the bad bearing (from 0.2 -0.25 reaching 0.55), higher than normal values for drilling efficiency but relatively low values of weight on bit. The bit gradually becomes a low efficiency drag bit as is demonstrated by the trend toward reduced observation variables Es, m or, equivalently, T0 , R0.

  Global parameters D = bit diameter W, WOB = weight on bit To= T/WD = non dimensional torque T, TOB = torque on bit N = rotary speed Q = hydraulics R, ROP = rate of penetration Ro= RIND = non dimensional rate of penetration Es = efficiency m = friction coefficient Parameters and variables specific to the numerical model � = axial coordinate e = angular coordinate 1 = cutting accumulation h = residual mud layer () = equivalent depth of cut v = angular velocity r = rate of penetration M=mass J = inertia term (rotational) V = viscous term (rotational) K = rigidity term (rotational) B = balance of solid mass Subscripts D = dimensionless (To and R0) M= mass C = cutting force or moment F = friction force or moment H = hydraulic force (A-4)

T 0

 0 not significantly affected Strong dependency ofT 0 with Ro values; by Ro values strong departures from mean T 0 average values High T D (0.25) Low T 0 (0.05) around 0.10-0.15 Low Ro (0.05) Low Ro (0.05) LowWOB HighWOB Mauvaise expulsion des cl9blais en roches !andres � :�� t • : � � : , • l : �-� -� -�}� ; : . u l '" ' " J i � - -�,ll\ • tr� Lt i . � , p� �,�� r\ � • . " t f : r � �� i � (', r. l !\ � l l .' 1.. : . . . . :• . j" " il " . . . ! !l� �. , • . . . !1 , I. ' I ,.: .... ._: ..• • • ••1--t • •--•"--" "-1 • '' "-• , . . . . . ...... .. . . . l"o!I!C)Aebond d'outil (bit bouncing) en roches duras.� 11> • • • • • • • • • • • • • • • • -�f.;". I -� I ' ,I I t I . . . . . . . . . . . . . ;!:;:10 o �A ,� J'n -• � 1 •; • lf o l:t.A 1 1

• 1 :Fig. 1 -�Fig. 2 - 3 -

 1123 Fig. 1-Two examples of axial dynamics obtained with Trafor data at the bit (courtesy of J. Guesnon) Upper part: chaotic axial dynamics in soft rock drilling Bottom part: bit bouncing with a three-lobed pattern in hard rock drilling

Fig. 4 - 4 5Fig. 6 -

 446 Fig. 4 -Conceptual view of drilling system with : balance pf mass B = B+-s• input flux s+: o : equivalent depth of cut d Cj de output flux b. : I : crushed rock material volume (cutting bed) h : volume of current mud and cutting reaction forces W-vertical forces equilibrium w = w+ -w• W c : Vertical component of cutting forces W F : Vertical component of friction forces WH : Hydraulic lift-off

Fig. 7 •Fig. 8 -Fig. 9 -

 789 Fig. 7 • Determining the zone of maximal efficiency. The sensitivity of the response, R (rate of penetration), to partial control variables, either WorN, the other factor remaining constant. Range of use of bits for the commands weight and rotary speed.Fig. 7a-When choice of W precedes that of N. Fig. 7b-When choice of N precedes that of W.

Fig. A- 1

 1 Fig. A-1 Evolution of the system for a constant weight, W (lower weights) .

  . .. ..... ..... ... ... ....

5

 5

Fig. A- 2

 2 Fig. A-2 Evolution of the system for a constant weight, W (higher weights).

.

  .... ... .. � •

Fig. A- 3

 3 Fig. A-3 Influence of the slope of weight increase, w+• on the response of the system.

5 •

 5 •••• ••••• <( ••••••••• ••••• •••• ••• •• ... , ---.---.--.... ._ __:: ... ..:•:;----1-' of penetration (non dimensional) Fig. A-4 Sensitivity to initial conditions.

  5.0 ------------------= • •"' ••• "' ••"'"' ••"' ••• "' ••"'"' ••"' ••• "' • • "'"' ••"' ••• "' •• "' ••"' ••• "' •• -••-•••"' • • -••-•••"' • • -• • -•••"' •• "' ••-••• "'

TABLE 1 -

 1 SUMMARY

		OF TYPE CHARACTERISTICS
	TYPE I	TYPE II	TYPE III

  •••• •• , .. . .

	-• .. 0.0 ..J...f: :	. .. .. . ......... . •

  •• .............................. , ........ 7 .................... .

	I I I I / I I I I I /I I I I '
	1/ 0	2	4	6		8	10	12
			!-time
	, ' , / , , ' / , / ' / , ' / // ' / ,/' 0 2 4	/ 6	/	8	10	12
			t-time

.

Acknowledgments

We thank our colleagues Maurice Bouteca and Jean Guesnon for valuable discussions on the interpretation of the tests, Isabelle King for valuable improvement of CUTCLEAN quasi-static model.

The authors would like to give credit to the field engineers who have made the data acquisition Trafor a success at Gaz de France well sites. They also wish to thank the management of Geoservices for permission to present the paper. The helpful discussions and comments of Hedi Sellami and Christophe Simon of the Centre de Geotechnique et Exploitation du Sous-Sol of the Ecole des Mines de Paris have been appreciated.

The authors wish to acknowledge the Fonds de Soutien des Hydrocarbures for sponsoring the program.

Appendix -Drilling Model Based on Balance of Mass, Forces and Moments, Taking Account of Coupled Effects.

Variables

In order to optimize control of the operation and to predict balling, a "drilling model" has been developed, that includes large scale phenomena, such as the dynamics of the drillstring, and small scale phenomena, such as the rock cutting mechanism and the circulation of the cuttings. Different hypotheses have been formulated, and a numerical simulation in a simplified configuration that illustrates the responses of the system to different command schemes is presented.

In the most general case, we suppose that the position of the bit is characterized by the following variables:

�

where Qd_t) is the instantaneous mud flow given by the pumps, recorded at the bit.

•

Velocity vR is entirely determined by input flow (vRSR=Q M ).

• h is no longer an independent variable, so that the balance of solid mass can be written in the following simplified form:

since the quantity xh of cuttings is no longer part of the system. A convenient simplification of the model is obtained with the assumption that d� de

is a given function of the prescribed variables, which corresponds to a direct integration of the relation describing the drag effect of mud on the particles. The preceding remarks lead to a new simplified model, with variables g,o,l, described by the following set of equations: • On the l=O axis, with B(r,l)<O, which corresponds to excess values of hydraulics (perfect cleaning) (W=2).

•

On the line B(r,l)=O, which corresponds to singular points of the dynamic system, solutions of the system (see


Marion Desquilbet
email: marion.desquilbet@inra.fr

David S Bullock
email: dsbulloc@illinois.edu

Maria Filippo

D'arcangelo

A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops

Keywords: herbicide-tolerant crops, integrated weed management, health and environmental externalities, weed resistance, lock-in

Weed control in the U.S. Midwest has become increasingly herbicide-centric due to the adoption of herbicide-tolerant (HT) crops in the 1990s. That integrated weed management (IWM) practices, including ecological and mechanical controls, are scarcely used is concerning. IWM would be a more sustainable form of farming for two reasons. First, it would reduce the negative health and environmental externalities associated with herbicide use. Second, it would reduce the selection pressure on weed populations and the development of weed resistance to some herbicides, thereby reducing the uncertainty of the long-term effectiveness of herbicidal weed control. In this context, we develop an economic framework to clarify the interplay among the different market failures that either contribute to the herbicidal "lock-in" or make it problematic. We then analyse the evidence for and perceptions of these market failures based on twenty-four semi-structured interviews with farmers and experts conducted in 2017, as well as on discussions in the academic literature. To this end, we put into perspective the possible self-reinforcing effects in the adoption path of HT crops, such as increasing farm size, changes in farm equipment, increasing incentives for simplified crop rotations, and the loss of practical knowledge of IWM practices.

Introduction

Weeds in farm fields can deprive crops of water, nutrients, and light, and so lower crop yields. In the U.S. Midwest, the region of the world studied in this article, chemical herbicide application rapidly became the central method of weed control after it was developed in the 1930s. Up through the 1990s, weed control relied on selective herbicides, each working on particular groups of weeds. These herbicides were used in combination with mechanical methods for weed control, consisting of tillage and cultivation. In the late 1990s, the advent of herbicide-tolerant (HT) crops, mainly with tolerance to glyphosate, further simplified weed control. Glyphosate is a nonselective herbicide, controlling many different types of plant species by inhibiting an enzyme necessary for plant growth. Glyphosate-tolerant (GT) crops, developed through genetic modification, carry the gene coding for a glyphosate-insensitive form of this enzyme, obtained from a soil bacterium, and conferring crop tolerance to glyphosate once incorporated into the plant genome. They allowed farmers to spray fields with glyphosate after crop emergence, and so kill weeds without killing the crop, notably reducing the need for mechanical weed control. In 2017, GT crops covered 94% and 89% of U.S. soybean and corn areas, respectively, almost entirely replacing previous conventional crop varieties treated with selective herbicides.

Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191

3 Two sustainability issues confront herbicide-centric methods of weed control. First, the scientific literature shows that pesticides, among them herbicides, often have harmful health and environmental effects [START_REF] Pimentel | Environmental and Economic Costs of the Application of Pesticides Primarily in the United States[END_REF]. Second, the repeated use of herbicide-centric methods often leads to weed resistance to the herbicides used [START_REF] Duke | Herbicide-resistant crops: Agricultural, economic, environmental, regulatory, and technological aspects Boca Raton[END_REF], reducing their effectiveness over time, and indeed weed resistance to glyphosate is rapidly increasing [START_REF] Heap | Overview of glyphosate-resistant weeds worldwide[END_REF]. Thus far, farmers have mainly controlled resistant weeds by applying selective herbicides in addition to glyphosate on GT crops [START_REF] Bonny | Genetically modified herbicide-tolerant crops, weeds, and herbicides: Overview and impact[END_REF]. Agrochemical companies have also recently developed HT crop varieties that are tolerant to selective herbicides in addition to glyphosate; crops tolerant to dicamba herbicide were commercialized in 2017 and crops tolerant to 2,4-D were commercialized in 2018. However, while increasing resistance to the selective herbicides used to supplement glyphosate seems inevitable, no new herbicide modes of action have been discovered for more than thirty years [START_REF] Heap | Overview of glyphosate-resistant weeds worldwide[END_REF]. It is not certain that herbicide manufacturers will be able to develop new synthetic products with new modes of action that can effectively control weeds and pass the increasingly demanding regulatory requirements aimed at controlling their effects on human health and the environment [START_REF] Duke | Why have no new herbicide modes of action appeared in recent years?[END_REF].

Integrated weed management (IWM) is an alternative to herbicide-centric weed control, and it at least partially addresses both sustainability issues. IWM integrates multiple weed management methods, including not only chemical and mechanical weed control, but also ecological methods such as crop rotation, cover-cropping, intercropping, planting of competitive cultivars, and the biological control of weeds with insects and pathogens. However, wide-scale IWM implementation would require major changes from current cultivation practices and likely worsen farmers' complexity and costs of weed management. Indeed, ecological weed control has diminished over time; currently, it is used only marginally. The great majority of Midwestern farmers today rotate corn and soybeans; longer rotations involving wheat, oats and alfalfa are extremely rare.

In this context, our aim is to describe the factors that may hinder widespread movement away from the use of HT technology for weed management towards a more sustainable form of agriculture relying on IWM. More precisely, we seek to analyse the extent to which the increased reliance on HT technologies may have generated self-reinforcing effects, making the adoption of IWM practices more difficult and costly than would have been the case had herbicide-resistant crops never been developed. [START_REF] Mortensen | Navigating a critical juncture for sustainable weed management[END_REF] proposed that the development of crops tolerant to multiple herbicides leads to a decline in the science and practice of IWM and that the increased consolidation of the seed and chemical industries is making it more difficult for growers to find high-yielding varieties that do not also contain transgenic herbicide resistance traits. They also argue that the marketing of cultivars tolerant to 2,4-D and dicamba may compel nearly all growers in a region to adopt them once some critical mass of growers in the region adopts them. In fact, soybeans and corn are extremely sensitive to 2,4-D and dicamba, and non-tolerant soybeans and corn could be injured by the application drift of 2,4-D and dicamba from other fields. This "lock-in" of HT technology has not been considered in the more general literature attempting to gauge the welfare impacts The concept of lock-in has been used in several discussions related to agriculture [START_REF] Cowan | Sprayed to death: Path dependence, lock-in and pest control strategies[END_REF][START_REF] Magrini | Why are grain-legumes rarely present in cropping systems despite their environmental and nutritional benefits? Analyzing lock-in in the french agrifood system[END_REF][START_REF] Vanloqueren | Why are ecological, low-input, multi-resistant wheat cultivars slow to develop commercially? A Belgian agricultural 'lock-in' case study[END_REF][START_REF] Wilson | Why farmers continue to use pesticides despite environmental, health and sustainability costs[END_REF]. We complement this literature by presenting a framework for the discussion of the market and policy failures associated with weed control, including the common-pool nature of weed susceptibility to herbicides and the environmental and health externalities of herbicides. We also introduce a framework for examining lock-in, different from those presented in previous articles. We base our framework on the concepts of switching costs, network effects, path dependence in innovation possibilities, and policy persistence. We then use this framework in a discussion of the evidence for and perceptions of the market and policy failures that accompany the use of HT technologies and the barriers to IWM. We base the discussion on evidence from academic articles, public information, and 24 interviews conducted with university experts, regulators, and farmers in 2017.

Weed-Control Market and Policy Failures

Previous Analyses of HT Crop Adoption Neglect Certain Types of Failure

An abundance of the economic literature examines the effects of adopting HT crops in terms of changes in production and management costs, farm profits, the profits of upstream seed and herbicide suppliers, and consumer surplus. For the U.S., empirical studies have mainly focused on HT soybeans, and used market equilibrium models to analyse the welfare effects of adoption (e.g., [START_REF] Price | A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops[END_REF]. This literature concludes that the production cost savings from HT crops lead to aggregate welfare gains.

Though not always accounted for in empirical studies, two HT-market failures have been studied thoroughly in the theoretical agricultural economics literature. First, the monopoly rights granted for GT seeds have allowed the innovator to extract monopoly rents [START_REF] Moschini | Intellectual property rights and the welfare effects of agricultural r&d[END_REF]. Second, some perceive foods derived from GMOs to be of inferior quality, and the costs of segregation and identity preservation of non-GMOs create a market failure [START_REF] Desquilbet | Who pays the costs of non-GMO segregation and identity preservation[END_REF].

Other possible types of market failure associated with HT crop adoption have received less attention in the economic literature. First, most of the said literature predated significant weed resistance and considered only the short-term cost savings. Second, the negative health and environmental externalities of herbicides were ignored. Third, possible dynamic inefficiencies caused by self-reinforcing effects in an ex-post inefficient direction were underconsidered. These market failures are discussed below. [START_REF] Hueth | Optimal agricultural pest management with increasing pest resistance[END_REF] initiated the economic literature on pest and weed resistance management. In economic terms, herbicide use involves joint management of two natural resources: the weed population, a detrimental renewable resource, and the stock of susceptibility (non-resistance) of the weed population to herbicides, a beneficial resource. Optimal herbicide application presents an intertemporal trade-off between controlling the present weed population and preserving future herbicide susceptibility. Because weed seeds and pollens escape their fields of origin, farmers enjoy the immediate benefit of herbicidal weed control but share weed susceptibility and future weed populations as common-pool resources. In economic terms, the lack of property responsibility for weed populations and property rights to weed susceptibility creates a market failure: the market cannot optimally allocate, spatially or temporally, the two resources [START_REF] Ambec | Regulation of a spatial externality: Refuges versus tax for managing pest resistance[END_REF]. Coordination or regulation may be warranted to provide individual farmers with incentives to apply resistance management strategies best-suited for the farm community. However, no U.S. laws or regulations currently attempt to offset the market failures just described. We examine causes of the dearth of regulations in section 3.

3. Herbicides Create Negative Health and Environmental Externalities

Herbicides are detrimental to human health and to the environment, although the extent of this externality is hard to quantify, varies by herbicide, and depends on the interactive effects of the different chemicals in the applied herbicide "cocktail" [START_REF] Bourguet | The hidden and external costs of pesticide use[END_REF]. The adoption of IWM practices would reduce herbicide use and therefore these negative externalities [START_REF] Davis | Increasing cropping system diversity balances productivity, profitability and environmental health[END_REF].

More evidence exists of the health effects of pesticides for agricultural workers than for the general population. Occupational exposures to pesticides are linked to a number of different cancers [START_REF] Alavanja | Occupational pesticide exposures and cancer risk: a review[END_REF]. Herbicides may also be associated with health outcomes other than cancer for agricultural workers: for example, [START_REF] Shrestha | Pesticide use and incident hypothyroidism in pesticide applicators in the agricultural health study[END_REF] found evidence of an increased risk of hypothyroidism associated with the 2,4-D, dicamba and glyphosate herbicides. Herbicides also impact the aquatic life, affecting the biological processes and biodiversity in water [START_REF] Davis | Increasing cropping system diversity balances productivity, profitability and environmental health[END_REF], and may be deleterious to soil microbial communities, soil biochemical reactions and soil enzymes [START_REF] Hussain | Impact of pesticides on soil microbial diversity, enzymes, and biochemical reactions[END_REF].

The academic literature on the economic impacts of HT crops usually considers glyphosate to be more environmentally benign than the herbicides it replaces, thereby reasoning that the adoption of HT crops has provided environmental benefits (e.g., [START_REF] Nelson | Simulating a relative environmental effect of glyphosate-resistant soybeans[END_REF]. However, the environmental and health impacts of glyphosate-based herbicides are currently subject to controversy, notably sparked after the International Agency for Research on Cancer (2017) classified glyphosate as a probable human carcinogen. The main points of controversy pertain to the greater effect of glyphosate and co-formulants in glyphosate-based herbicide formulations than of glyphosate alone and to the influence of the datasets and methods used to evaluate the toxicological effects [START_REF] Benbrook | How did the US EPA and IARC reach diametrically opposed conclusions on the genotoxicity of glyphosate-based herbicides?[END_REF]. In addition, in a context where limitations in the number of agricultural workers included in cohort or control-case studies and 6 in the measure of their exposures make it difficult to document the effect of particular pesticides, recent articles in epidemiology concluded that glyphosate was associated with an elevated risk of some types of lymphoma [START_REF] Leon | Pesticide use and risk of non-Hodgkin lymphoid malignancies in agricultural cohorts from France, Norway and the USA: a pooled analysis from the AGRICOH consortium[END_REF][START_REF] Zhang | Exposure to glyphosatebased herbicides and risk for non-Hodgkin lymphoma: A meta-analysis and supporting evidence[END_REF].

A Dynamic Market Failure: Lock-in and Its

Causes [START_REF] Arthur | Competing technologies, increasing returns, and lock-in by historical events[END_REF] defines "lock-in" as occurring when a technology gains, even if by chance, a market advantage over competing technologies, and ends up dominating the market, even though it might not be more efficient. In our context, when herbicides or HT technology were first commercialized, both consistently provided higher farm profits than could IWM. However, this accounting neglects the costs of the negative externalities of herbicide use. Additionally, the spread of weed resistance lowers the profitability of herbicidal weed control. Our focus below is on the self-reinforcing effects ("lock-in") of the herbicide and HT technologies, which may have contributed to the dominance of herbicidal weed control.

Below, we present a framework featuring four causes of lock-in: switching costs, network effects, dependence of the innovation possibilities frontier, and policy persistence.

Switching Costs and Network Effects

The industrial organization literature discusses switching costs and network effects as the principal causes of customer lock-in to early choices. As defined by [START_REF] Klemperer | Competition when consumers have switching costs: An overview with applications to industrial organization, macroeconomics, and international trade[END_REF], "a switching cost results from a consumer's desire for compatibility between his current purchase and a previous investment", independent of other customers' choices. In contrast, network effects are self-reinforcing effects coming from the influence of each customer's decisions on other customers: "network effects arise where current users of a good gain when additional users adopt it" [START_REF] Farrell | Coordination and lock-in: Competition with switching costs and network effects Handbook of industrial organization[END_REF]. There are two types of network effects. Direct network effects arise if adoption by different users is complementary. A typical example is when users of a communication network directly benefit from its adoption by others because they have more opportunities for interactions with peers. Indirect network effects arise through improved opportunities to trade with the other side of a market.

Switching costs are socially detrimental when the tie between consumer transactions obstructs efficient buyer-seller matching. Switching costs and network effects may also limit the entry of new suppliers, creating a social cost of incompatibility [START_REF] Farrell | Coordination and lock-in: Competition with switching costs and network effects Handbook of industrial organization[END_REF]. In addition, the negative externalities of herbicides imply a welfare loss if the switching costs and network effects hamper the adoption of IWM practices.

Path Dependence in Innovation Possibilities

Lock-in can also result from the path dependence of innovation possibilities, which arises if the existing stock of knowledge influences the type of knowledge developed. This happens when past technological advances make future advances more profitable, as current researchers "stand 7 on the shoulders" of their predecessors [START_REF] Acemoglu | The environment and directed technical change[END_REF].

Policy Persistence

Policy persistence is the tendency of a policy to be maintained or renewed, even when its original rationale no longer justifies continuance. [START_REF] Coate | A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops[END_REF] explain this bias towards the status quo as being the result of the adjustment costs that economic agents undertake to benefit from a policy. This costly response incentivises agents to persuade policymakers to maintain or renew an existing policy, even when the agents are ex-ante indifferent to competing policies.

Evidence and Perceptions of the Market and Policy Failures

Methodology

We used the concepts developed above about market and policy failures associated with herbicidal weed control to design questions for one-hour, in-person, and semi-structured interviews. We conducted 24 interviews in 2017 with ten university experts (agricultural economists, weed and crop scientists, and extension specialists), five regulators from the EPA and USDA, the representative of a producer association, a crop consultant, and seven farmers.

An extension specialist of University of Illinois helped to set up interviews with farmers of heterogenous farm sizes and farm ownership structures, and with the crop consultant. We initially sampled a group of fifteen university experts and six regulators or former regulators with recorded publications on the topic of HT crops, herbicide resistance, agricultural technology and IWM, as well as two environmental advocacy groups. Other prospective participants were selected through snowball sampling by asking interviewees to suggest, either in interview requests or in person, additional participants. In total, we contacted 25 experts from universities, three environmental advocacy organizations and ten regulators. Seventeen experts were finally interviewed, thirteen did not return emails, and eight were not available or declined. Most of those who declined suggested alternative contacts in the same department or institution. Most of the interviews, lasting between 45 minutes and an hour and a half, were conducted in person, and three were conducted via telephone or teleconference. All interviews were recorded with the permission of the participants.

In this section, we analyse the evidence and perceptions of these market and policy failures in light of the interview responses, academic literature, and other available public information.

Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191 8

The Adoption Path of HT Crops

Rapid and Widespread Adoption of GT Crops

GT crops were commercialized during a period of increasing weed resistance to existing selective herbicides for Midwestern soybean and corn production-with increased resistance to ALS-inhibiting herbicides being a case in point [START_REF] Shaner | Lessons learned from the history of herbicide resistance[END_REF]. This initial weed resistance fuelled GT crop adoption [START_REF] Owen | Herbicide-resistant crops and weed resistance to herbicides[END_REF] but appears not to be the main driver of GT crop adoption, as illustrated by this answer given by a farmer when asked whether he experienced weed resistance to herbicides prior to the commercialization of GT technology:

I don't think so, I really don't think so. I mean, it's just that those other chemicals didn't work as well. It was not resistance. It was just, "well, they may work, they may not," depending on how big the weed was, when you sprayed, whether it cooled off that night, did you get a rain within a certain amount of hours, or they weren't actively growing, they were partially drought-affected. And then Roundup TM came, and if you got part of that plant sprayed with Roundup, that baby was dead. It just worked.

Indeed, the academic literature cites several initial economic advantages that fuelled the rapid adoption of GT crops [START_REF] Fernandez-Cornejo | Adoption of bioengineered crops[END_REF]. First, GT crops lowered weed-control costs. In addition to glyphosate being cheaper than the cocktail of selective herbicides it replaced, the costs of operator time and fuel were lower, as fewer tractor passes were needed in the fields each year. Weed identification ("scouting") costs were also much lower because glyphosate controlled such a broad spectrum of weeds. Second, controlling weeds with glyphosate provided farmers with important time flexibility: glyphosate could kill larger weeds than selective herbicides, widening the time window for spraying, and thus lowering the risk of rain and mud keeping heavy machinery out of the fields while weeds grew too large to kill. Third, because glyphosate controlled such a wide range of weeds, GT technology greatly simplified weed management, saving valuable management time for other activities (Gardner, Nehring, & Neson, 2009). Fourth, GT technology made the mechanical control of weeds (with interrow cultivators) unnecessary. Fifth, the development of HT crops reduced the use of pre-plant tillage techniques, again saving operators time and money. Using 1988-2006 state-level panel data, Fernandez-Cornejo, Hallahan, Nehring, Wechsler, and Grube (2012) found that HT adoption reinforced the adoption of conservation tillage practices. [START_REF] Perry | Testing for complementarity: Glyphosate tolerant soybeans and conservation tillage[END_REF] reported that GT soybeans and conservation tillage are complementary practices. Finally, using data from a nationwide, early-2000s survey of soybean farmers, Fernandez-Cornejo, Hendricks, and Mishra (2005) found, after controlling for other factors, that adoption of HT soybeans was positively and significantly correlated with off-farm household income.

Our interviewees discussed all aspects of the cost reductions mentioned above. One farmer summarized the general tenor of the conversations about incentives for GT technology adoption as follows: It was a win-win situation that you didn't even have to think about. You just filled the sprayer with Roundup, and you went to spray whatever needed to be sprayed. Simplicity was the greatest thing about it. Didn't even have to think.

A father and son collaborating on an Illinois farm made the significant savings of managerial resources allowed by HT technology abundantly clear: Son: I hate to make this statement-take it for what it is-but Roundup made the farmers that weren't necessarily good on the things they should have been doing, it made them look like good farmers.

Father: It forgave a lot of mistakes. Son: That's a good way to put it.

Changes in Fixed Capital: Farm Size and Equipment

The interviewees made it clear that GT technology's management resource savings and increased time flexibility not only provided the seasonal crop benefits described above, but also triggered changes over time in farmers' decisions about fixed assets, lowering the demand for mechanical weed control equipment. In addition, almost all of the interviewees speculated that the flexibility and simplicity of using GT crops permitted farmers to expand their acreage and manage larger farms. One farmer stated, Yeah, I think that has definitely allowed the big guys to get bigger, sure. Because they go in and they plant, and then spray, and then harvest.

However, the causality between GT crop adoption and increases in farm size is difficult to determine; many factors influence farm size. In theory, technological change may affect farm size through two channels: by allowing economies of scale of crop production (that is, by lowering the per-unit costs of output expansion) or through labour saving-that is, by reducing the labour needed for any given production quantity. Agricultural economists tend to judge the labour-saving aspects of technological change to be the more important. MacDonald, Korb, & Hoppe (2013) presented empirical evidence of time savings from GT technology, which they cite as playing an important role in the consolidation of crop farms since 1995 along with other technological changes such as larger farm equipment and faster tractors, as well as costs savings from new information technologies.

Development of Weed Resistance to Glyphosate

Soon after the introduction of HT technology, the industry producing glyphosate and HT seed frequently claimed that the HT system was different from selective herbicide systems in that biological resistance to HT technology would not evolve. For example, [START_REF] Bradshaw | Perspectives on glyphosate resistance[END_REF], all employees of Monsanto, claimed that "the complex manipulations that were required for the development of glyphosate-resistant crops are unlikely to be duplicated in nature to evolve glyphosate-resistant weeds" (see [START_REF] Bonny | Genetically modified herbicide-tolerant crops, weeds, and herbicides: Overview and impact[END_REF] 10 scientific promises" [START_REF] Joly | On the economics of techno-scientific promises[END_REF]. As an academic researcher we interviewed opined:

Many people drank the Kool-Aid that Monsanto was serving that said, "Oh, this is a really complex way that the glyphosate kills weeds, so resistance will never evolve." They forgot to read Darwin and Wallace.

Indeed, the initial claims that resistance would not develop were wrong. Currently, three weeds are displaying significant glyphosate resistance in Midwest fields: tall waterhemp (Amaranthus tuberculatus (Moq.) Sauer), giant ragweed (Ambrosia trifida L.), and marestail (Erigeron canadensis L.).

Given that weed susceptibility to glyphosate is a common-property resource among farmers (as weed resistance may diffuse spatially via weed seeds or pollen), even if they had been aware from the start of the risks of resistance, farmers would not have had individual incentives to develop management strategies to delay it. The current aggregate farm structure also makes it difficult for farmers to cooperate with each other; interviewees told us that current farms are sometimes so large, with fields of single farms spread out over as much as one hundred miles, that producers may not even know their neighbours, and compete to rent or buy land. Additionally, most Midwestern farm land is rented, and growers who do not own their land have less incentive to account for the long-term development of resistant weeds. Several interviewees also highlighted farmers' faith in technology and their beliefs that either companies will offer new solutions or old solutions will work again: Farmer. I think there's always going to be a chemical to kill certain things. Interviewer. You think the industry can develop new chemicals if we come to that? Farmer. Oh yeah, I'm sure they can. Some of the older chemicals we may not be able to use anymore because weeds are tolerant to them. If you have a certain chemical that does not kill a weed anymore, you go to a different chemical, and if that one does not kill it anymore, you come back to another chemical. There's always a circle that if you wait long enough it will do it again.

In contrast to the regulation of insect-resistant Bt crops, which imposed a mandatory refuge policy from the start to manage resistance in target pests, no regulation has been established for HT crops even though the topic was briefly considered in House hearings in 2010 (US House Committee on Oversight and Government Reform, 2010). Our interviews highlighted several reasons for regulatory differences among genetically modified crops. First, the regulatory context is different: contrary to Bt crops, GT crops contain no toxin added by genetic engineering at which regulation could be aimed. Furthermore, the EPA determined that because Bt crops lowered pesticide use, Bt susceptibility was a public good. In contrast, the perception was that GT crops were introduced to simplify growers' lives, rather than decrease herbicide use. Additionally, different EPA divisions regulate Bt crops and HT crops, and there was little internal push for regulation of the latter. Furthermore, whereas entomologists were actively pushing for a public strategy to establish refuges, weed scientists were more focused on weed control than weed ecology and, by and large, did not call for HT crop regulation. 11 equivalent simple policy has been identified to manage the development of weed resistance; IWM solutions are more complex and more costly to implement, and they vary depending on the specific context. The EPA has recently developed guidelines on resistance development for pesticide labels and educational materials for farmers (U.S. EPA, 2017a[START_REF] Epa | Office of pesticide programs, pesticide registration notice (prn) 2017-2. Guidance for herbicide-resistance management, labeling, education, training, and stewardship[END_REF]. However, the EPA is not contemplating mandatory policies, which would be more likely to have a sizeable influence on the evolution of weed resistance.

Current Tools to Fight Resistant Weeds

The current strategies for controlling glyphosate-resistant weeds are mainly herbicide-centric, and they include increasing doses of glyphosate, supplementing glyphosate with other herbicides, and using transgenic dicamba-or 2,4-D-resistant crops.

Glyphosate-and dicamba-tolerant Roundup Ready Xtend TM soybeans were commercialized in 2017. Enlist TM or Enlist Duo TM soybeans and corn, tolerant to 2,4-D alone and to 2,4-D and glyphosate, respectively, were commercialized in 2018 (after our interviews). The adoption rate of dicamba-tolerant soybeans in the U.S. was approximately 20% in 2017. 1Our interviews indicated that many farmers judge dicamba to be less effective than glyphosate because dicamba does not control grasses and is ineffective on large weeds. In addition, because of dicamba's tendency to drift to neighbouring farms, in 2017 several of the interviewed farmers avoided these crops for fear of problems with neighbours. As summarized by an interviewed academic, the more fundamental problem is that weeds will evolve resistance to dicamba: Both of those, the 2,4-D and the dicamba, are relatively old products-some of the oldest. They haven't had widespread usage and so the level of evolved resistance to those herbicides is quite low, so they are like maybe a band-aid for a short period of time. But it is still a system based on herbicides.

Application of herbicide mixtures might mitigate the evolution of weed resistance, as long as resistance to the active ingredients has not already evolved, but resistance mitigation would exacerbate health and environmental costs. An academic interviewee stated, I think the industry's approach to it-and I have misgivings about this-is that we throw on more herbicides. If you get a new herbicide coming to the market now, it's always a two-or three-way mixture of different herbicide active ingredients, with the hope that if I have enough tools in the jug, then I'm going to be able to manage that problem. The negative to me from that is that we are using more pesticides than we were before, which has environmental implications. But if you look at the models, it's probably the smartest way to approach the resistance problem because weeds are less likely to survive a mixture than a single active ingredient.

It is possible to complement the chemical control of resistant weeds with mechanical control, including one or possibly two pre-plant tillage passes (one in the early spring and the other immediately before planting, to control emerged weeds). Several farmers had begun using 12 their old interrow cultivators again on localized parts of their farms where resistant weeds were problematic. However, the relatively small sizes of the old cultivators greatly slowed the production process. For example, one farmer explained that he had cultivated a few acres with an old, six-row cultivator even though his newer planter could cultivate 16 rows. Therefore, more time was consumed cultivating six rows up the field, six rows back, and then four rows up again. Contemplating the effort required to cultivate more than the marginal areas he had cultivated that year, he remembered the consequences of cultivating as a younger farmer:

My hearing is bad because of that cultivation. […] It was no fun, and it took the whole summer up. Just a pain.

There is currently no significant IWM adoption in the U.S. Midwest. Farmers overwhelmingly consider IWM to be insufficiently effective, and too complex, time consuming and costly. An academic interviewee provided an apt summary:

Herbicides work, for the most part. They're inexpensive, it's easy. Doing something like rotating crops would help reduce the number of weeds, but it would not completely take care of weeds. You still would need herbicides, probably. "I can rotate and spray, or I can just spray. So I just spray." The other available strategies require multiple strategies to be completely effective. You could do lots of things to get there, or you could do one thing: spraying herbicides.

Barriers to IWM

Given the negative externalities of herbicides, the inevitable development of weed resistance, and that no new herbicide modes of action seem to be in the research and commercial pipelines, the future methods and costs of weed control are worrying. Below, we discuss additional difficulties producers face when transitioning to IWM.

Possible Barriers to Crop Rotation

Although IWM is a combination of several practices, the consensus from our interviews was that [crop rotation is] the single most important strategy that it would be nice if we do more of, and that true IWM has to start with a good crop rotation; otherwise, you're forcing the chemicals to do all the work.

Longer crop rotations would be especially beneficial for corn and soybeans, given that they have similar planting dates, shared weed problems and corresponding shared herbicide strategies. However, there was a consensus among the interviewees that bringing additional crops into the current corn-soy rotation is currently "not really an option," as corn and soybeans thrive in Midwestern growing conditions. As underlined by [START_REF] Liebman | Ecological management of agricultural weeds United Kingdom[END_REF] Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191 (p. 191), larger farms are less suited for ecological weed management. Thus, increasing farm sizes have reinforced the efficiency of herbicidal weed control. Through our framework, we can interpret this as coming from differences in profitability and being exacerbated by lock-in. First, herbicides have made the removal of oats and alfalfa from rotations possible, which has further encouraged farm specialization in field crops and widened the disconnection between crop and animal production. Reintroducing these crops would be difficult in the current situation, as they are no longer compatible with the internal structure of farms without animal production-which is a type of switching cost. The absence of local markets for wheat, oats, and alfalfa also complicates the access to inputs suited for their production, and severely limits the ability of farmers using IWM to deliver their crops to local buyers; this is an indirect network effect. Cover-cropping could complement herbicidal weed control, but it is complicated, and increases weather-related risks.

13

Corn and soybean production are incentivised by two principal government policies. First, the Renewable Fuels Standard mandates that the petroleum industry mix a minimum proportion of crop-based ethanol into the nation's gasoline supply (U.S. EPA, 2018). As a result, approximately one-third of the nation's total corn production went to the production of ethanol in 2016 (U.S. Department of Energy, 2018). This government-mandated demand for corn increases the world price of corn relative to that of other crops and thus encourages corn production. Second, subsidized crop insurance is the principal instrument used by the U.S. government to supplement the incomes of farmers in the corn-soybean belt. To some extent, the programme incentivises monocultural agriculture simply because it increases the already existing competitive advantage of corn and soybean production to that of other crops likely to be used in longer-term rotations in the U.S. Midwest. Crop insurance subsidies cover some small rotation crops-oats, for example-but not others, such as alfalfa [START_REF] Zulauf | Premium subsidy and insured U.S. Acres[END_REF]. We can conjecture about a policy persistence effect: there have been relatively few regulatory obstacles to simplifying crop rotations; farmers' current assets are compatible with herbicidal weed control in corn-soy rotations, making it more difficult politically to increase regulatory obstacles to herbicide development and use.

Possible Barriers to Interrow Cultivation

Our interviews resulted in detailed discussions about the use of interrow cultivation for weed suppression. Cultivation was discussed more as a complement to herbicide-centric weed control than as a component of IWM. The general thought expressed was that weed resistance may now be making such cultivation necessary, much as it was a necessary complement to selective herbicides prior to the adoption of HT crops. However, the barriers to interrow cultivation present a general difficulty for IWM practices beyond this specific case. This definitive answer came from a farmer when asked whether large-scale mechanical weed control would be an option in the case of resistance build-up:

No … [laughs] I don't think we'll ever see that again! This response may be attributed to a switching cost resulting from increases in farm size, 14 which has created incompatibilities with interrow cultivation. Currently, the equipment industry lacks incentives to develop more efficient cultivators than the old ones still owned by some farmers, which creates an indirect network effect for a farmer who would want an efficient cultivator but cannot buy one because of a lack of general market demand. No doubt manufacturers could respond quickly to an increase in cultivator demand. Cultivators are relatively simple machines, manufactured at relatively low costs. However, cultivating 24 rows at a time would be challenging because the wider cultivator makes it more difficult to avoid digging up the crop along with the weeds. Interrow cultivation is also not very compatible with no-till methods, which can complement HT technology.

Our interviewees told us that a sprayer can be driven at fifteen miles per hour across the fields, allowing the farmer to cover a number of acres very quickly and with little difficulty. Field cultivators typically must be driven from five to seven miles per hour, and the work is exacting; the driver must pay constant attention to the action behind him to avoid digging up the crop with the weeds. Of course, the costs of the farmer's cultivation time and efforts might be defrayed by hired labour. However, our interviewees stressed that the pool of labour capable of cultivating efficiently and willing to take on the work is quite limited in rural Midwestern labour markets.

Considering the discussion above, the prohibitive costs of interrow cultivation are not the fixed cost of buying new machinery, but rather the variable costs of the slow task of cultivation. An academic summarized the situation: I can spray a field much faster than I can cultivate it.

While the cost of labour is exogenous to the dynamics of weed control and therefore not a factor in lock-in, the price of herbicides and thus the relative costs of labour and herbicides are possibly affected by the policy persistence of regulations encouraging herbicide use, as discussed before. This topic was beyond the scope of the interviews.

Possible Barriers in Terms of Knowledge

Another important barrier to large-scale IWM is the path dependence of innovation. Almost a generation of farmers have now farmed with GT crops, and know-how about selective herbicide management and field scouting is fading. However, our interviewees believed that knowledge of pre-GT practices has not yet completely disappeared and, though less accessible, is still available from land-grant universities and extension services, via the Internet, and through traditional communication outlets. Of course, the U.S. is reducing its public R&D in the agrifood sector-a general trend that is largely exogenous to the dynamics of weed control. This public investment has decreased both in absolute terms and relative to private research; the expenses of agri-food R&D are now double the expenses of public R&D [START_REF] Pardey | Agricultural r& d is on the move[END_REF]. Farmers have begun substituting public extension services with services from private crop consultants and input retailers.

15

GT technology; very few farmers chose IWM over selective herbicides. Nor has IWM been the focus of academic weed science, which has traditionally been herbicide-centric in its research and funding [START_REF] Davis | Weed science research and funding: A call to action[END_REF][START_REF] Shaner | The future for weed control and technology[END_REF]. Private companies now conduct a larger share of agri-food R&D than do public institutions, and they have greater incentive to investigate herbicide-centric rather than non-herbicidal weed control strategies. For example, current cover-crops research is primarily public; supply companies make scant profits selling cover crop seed. There is also inertia in research: there are sunk costs, and there is an incentive to exploit past investments in specific intellectual capital.

Discussion and Conclusion

Our analysis has highlighted that the current dominance of herbicide-centric weed control in crop farming in the U.S. Midwest is unlikely to fade any time soon in favour of a more sustainable form of agriculture relying on IWM. Indeed, IWM adoption faces high barriers to entry, including barriers to crop rotation, interrow cultivation, and knowledge acquisition. Some of these barriers seem mostly exogenous to the dynamics of weed control. First, corn and soybeans thrive in the agricultural conditions of the U.S. Midwest, reducing private economic incentives for other rotations. Second, the high cost of qualified rural labour hampers the adoption of more time-consuming IWM practices. Third, the increase in the relative share of private agri-food R&D compared with public R&D may decrease the incentives for IWM research because of low appropriability.

Other barriers, however, are at least partly endogenous to the dynamics of weed control; they feature various self-reinforcing effects that contribute to the lock-in of herbicide-centric weed control. These lock-in effects help maintain the herbicide-centric path of weed control dynamics despite another externality at play, which is the common-pool nature of weeds and of weed susceptibility to herbicides. This common-pool nature contributes to the decrease in herbicides' effectiveness over time as weed resistance develops. The negative externalities of herbicides on health and the environment make this path problematic.

We have identified four types of self-reinforcing effects. First, incompatibilities of IWM practices with the current size of farms and farm machinery create switching costs. Existing machinery, such as the interrow cultivators made unnecessary by GT technology, are ill-suited for use with today's larger machinery. Farm size is enhanced by the availability of herbicidal weed control; the large size of farms makes it difficult to adopt IWM practices, which tend to be labour-and management-intensive processes. Second, indirect network effects have resulted as chemical herbicides have contributed to the simplification of crop rotations, which no longer include fodder crops; this simplification has fostered the specialization and separation of field crop farms and livestock farms. The reintroduction of fodder crops on grain farms lacking livestock production would be challenging, and transporting fodder crops to livestock-intensive areas is expensive. Third, there is path dependence in R&D. IWM has never been the main 16 focus of weed science. Fourth, the U.S. biofuels mandate and subsidized crop insurance programmes encourage corn and soybean production, and therefore discourage longer crop rotations. Although it was not specifically addressed in our interviews, the policy persistence of regulations pertaining to the market authorization and use of herbicides probably also plays a role in the current price of herbicides and the high cost-effectiveness of herbicides compared with labour.

The lock-in effects that we have described do not result from GT crop adoption alone but also more generally from the much longer path of herbicidal control. GT crops may have reinforced this herbicide-centric path by encouraging increases in farm acreage and machinery sizes, but the trends of larger farms and equipment were already in play long before the advent of GT crops.

The prospects of whether herbicide-centric weed control will still dominate in the long run may depend on two factors. First, it will depend on whether agrochemical companies manage to develop new herbicide compounds as weed resistance to current herbicides increases. Second, increased knowledge and citizen awareness of the negative health and environmental impacts of herbicides could create political leverage to change farm policies, pesticide authorization procedures, and policies to support the development of alternative cropping systems, thus changing the incentives faced by farmers to so predominately rely on herbicides for weed control.

We have provided a qualitative description of the dynamic effects at play in the path of GT crop adoption in the U.S. Midwest. Our economic analysis of the effects of the adoption of farm innovations has shown that dynamic relationships exist between weed management choices and choices about fixed farm assets such as land and equipment. It is therefore important to account not only for the variable costs at the field scale but also for the marketscale structural transformations brought about by these innovations. This study also underlines the interest in comparing the effect of an innovation (for example, GT crops) not only with the dominant technology in place (in our case, herbicidal control) but also with more sustainable alternatives (such as IWM) that have beneficial effects on health and the environment. 17

Disclosure statement

No potential conflict of interest was reported by the authors.

  Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191 4 of HT technology adoption.

  Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191

  Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191

  Economics. […] It worked, it flat worked. Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191 9 Another farmer told us,

  , for other examples). This claim falls under what sociologists have coined "the economics of techno-Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191

  Finally, providing incentives to farmers to create refuges for Bt crops can be relatively simple. No Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191

  Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191

  Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191

  Midwestern farmers' knowledge of IWM practices was limited even before the advent of Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191

  Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191

  Comment citer ce document : Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and policy failures associated with the adoption of herbicide-tolerant crops. International Journal of Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191

. Weed Susceptibility to Herbicides Is a Common-Pool Resource

	2.2
	5
	Comment citer ce document :
	Desquilbet, M., Bullock, D. S., D'Arcangelo, F. M. (2019). A discussion of the market and
	policy failures associated with the adoption of herbicide-tolerant crops. International Journal of
	Agricultural Sustainability, in Press, 1-12. , DOI : 10.1080/14735903.2019.1655191

See https://extension.psu.edu/looking-back-at-xtend-soybean-and-changes-for-2018.

Acknowledgments

We thank all those who kindly agreed to be interviewed for the purpose of the study. These people were four economists, three agronomists, two weed scientists and an extension specialist from Iowa State University, University of Illinois, University of Minnesota, University of Wisconsin-Madison and Western Illinois University; two employees from the U.S. Environmental Protection Agency; three employees from the USDA Economic Research Service, one member of a state Soybean Association, as well as a crop consultant and seven farmers from Illinois. We thank Gary Letterly for his help with organizing the farmer interviews. We also thank Bruno Chauvel and Frédéric Goulet for their comments. This essay represents only the views of the authors.

Funding

This work was supported by the Institut National de la Recherche Agronomique's SMaCH (sustainable management of crop health) metaprogramme and by the European Union Center at the University of Illinois at Urbana-Champaign through a Jean Monnet Center of Excellence grant. Marion Desquilbet acknowledges funding from ANR under grant ANR-17-EUR-0010 (Investissements d'Avenir program).