EP0308327B1 - Method for determining the wear of a tool cutter while drilling rock formations - Google Patents

Description

The present invention relates to a method for determining the wear of cutting elements of a tool during drilling of a rock formation.

Oil well drilling techniques have developed considerably in recent decades and have therefore led to an evolution in the drilling tools used to cut rocks.

The most commonly used drilling tools in this field are drill bits, especially those of the wheel type which shear, fragment and grind the rock when the drill string is rotated. However, these tools have relatively short lifetimes of 15 to 20 hours and it is necessary to check their wear to plan their replacement. Wear problems also arise in the case of P.D.C. (Polycrystalline Diamond Compact), but less critically.

US-A-3,782,190 deals with the detection of faults on the rolling bearings of a tricone bit tool. For this, surface sensors (33 and 36, Fig. 1) measure the torque at the rotation table (T) and the weight suspended from the drilling hook. The method consists in using an analog module to calculate the T / WD ratio. W being the weight on the tool, deducted from the hook load in a conventional manner, D being the diameter of the drilling tool.

We already know from US-A-4,627,276 a method in which average values of the advancement speed of a drilling tool, the speed of rotation and the weight on the tool are determined, to deduce the efficiency of drilling, as well as the shear strength of the rock in which drilling is carried out.

Patent EP-A-0.168.996 presents a device for detecting events during drilling, such as in particular ruptures on the drilling tools.

However, none of these documents presents a method for determining the progressive wear of the cutting members, such as in particular the teeth of a rotary cutter bit. The method of the present invention, simple and convenient, uses the measurement of two background parameters, the weight on the tool W and the torque on the tool T; the degree of wear can be obtained at any time by the operator when he wishes.

• on applique une succession de valeurs de poids sur l'outil en cours de forage pendant sur une première période de test,
• on effectue une première série de mesures, au niveau de l'outil, sur le couple et le poids pendant la première période de test,
• on établit à partir de ladite première série de mesures une portion de courbe représentative des variations du couple en fonction des variations du poids,
• on garde en mémoire ladite portion de courbe établie pendant la première période de test,
• on réalise au cours de l'avance du forage une succession de périodes de test, pour chacune desquelles on établit ladite portion de courbe représentative correspondante,
• on compare la portion de courbe obtenue pour une période de test donnée avec au moins une portion de courbe précédemment obtenue,
• on en déduit le degré d'usure des organes de découpe en fonction de la variation d'au moins une grandeur liée à la concavité desdites portions de courbe, par exemple le rayon de courbure moyen desdites portions de courbe, la diminution de la concavité de la courbe indiquant une augmentation de l'usure de l'outil, toutes conditions égales par ailleurs.

The present invention therefore relates to a method for determining the wear of cutting elements of a tool during drilling of a rock formation, by which the weight W applied to the tool and the torque T are measured. necessary for the rotation of said tool, the weight on the tool W and the torque T being linked by a relation of the type T = uW + vW ^α in which u and v are parameters and α is a coefficient depending among other things on the formation , characterized in that

• a series of weight values are applied to the tool being drilled during a first test period,
• a first series of measurements is made, at the tool level, on the torque and the weight during the first test period,
• a portion of the curve representative of the variations in torque as a function of the variations in weight is established from said first series of measurements,
• said portion of the curve established during the first test period is kept in memory,
• a series of test periods are carried out during the advance of the drilling, for each of which said portion of the corresponding representative curve is established,
• the portion of the curve obtained for a given test period is compared with at least one portion of the curve previously obtained,
• the degree of wear of the cutting members is deduced therefrom as a function of the variation of at least one quantity linked to the concavity of said curve portions, for example the mean radius of curvature of said curve portions, the decrease in the concavity of the curve indicating an increase in tool wear, all conditions being equal.

According to a preferred embodiment of the present invention, said first series of measurements from which the first portion of the curve is obtained, is carried out on a new tool practically without wear.

In order to overcome the inaccuracies of the measurements having an effect on the concavity, the quantity linked to said portions of curves depends on the value of the mean radius of curvature of said portion of curves. By using an average radius of curvature, the portion of the curve obtained can be considered to be more representative of actual wear.

According to a particular embodiment of the invention, at least one function of the parameter v is determined for a given test period from, on the one hand, said portions of curves linked to the periods of previous tests and on the other hand from the relation T = uW + vW ^α , and the degree of wear of the cutting members of the tool is deduced by the reduction of said function of the parameter v.

Advantageously, the weight value is scanned in stages in the vicinity of its target value and the corresponding values of W and T are determined during this sweep.

• la figure 1 illustre schématiquement une méthode d'obtention du critère d'usure du trépan en fonction des paramètres poids exercé sur l'outil W et couple de rotation exercé sur l'outil T,
• la figure 2 est une représentation schématique d'enregistrements effectués en fonction des mesures des paramètres T et W, et
• les figures 3 et 4 montrent des courbes représentatives du couple (T) de rotation en fonction du poids exercé sur l'outil (W).

The invention will be better understood by referring to the description corresponding to the appended figures, among which:

• FIG. 1 schematically illustrates a method for obtaining the drill bit wear criterion as a function of the parameters of the weight exerted on the tool W and the torque exerted on the tool T,
• FIG. 2 is a schematic representation of recordings made as a function of the measurements of the parameters T and W, and
• Figures 3 and 4 show representative curves of the torque (T) of rotation as a function of the weight exerted on the tool (W).

The invention makes it possible to obtain a determination of the wear of cutting members such as, in particular, the teeth of a tool. It therefore applies perfectly to tools with knurled teeth with countersunk teeth and spikes, or to drilling tools having a relationship of the same type of the applied torque (T) as a function of the weight (W), which is the case, for example. example, PDC tools (Polycrystalline Diamond Compact).

The present invention makes it possible to determine the wear of a tool from the measurement of the torque on the tool and of the weight on the tool.

We have already used in several methods of determining events, such as for example faults in the bearings, the variation of the torque on the tool

De manière plus générale, cette équation peut s'écrire sous la forme : $${\text{T = uW + vW}}^{\text{α}}$$

avec α coefficient dépendant entre autre paramètre ,de la formation rocheuse et dont la valeur peut être prise égale à 2.This is notably the case in patent EP-A-0.168.996 where it is indicated that these two parameters can depend on each other according to the relationship: $$\text{T = uW + vW²}$$

More generally, this equation can be written in the form: $${\text{T = uW + vW}}^{\text{α}}$$

with α coefficient depending among other parameters, on the rock formation and whose value can be taken equal to 2.

However, the developments of this method in the document considered are interested in a calculation on a limited set of the recorded values.

The present invention is concerned with the determination of the parameters u and v.

In order to carry out this approach, we first consider a theoretical approach.

• H.W.R. Wardlaw - Optimization of Rotary Drilling Parameters - Dissertation - University of Texas, 1971.
• T.M. Warren - Factors Affecting Torque for a Tricone Bit - S.P.E. 11 994 - 1983.
• P.F. Gnirk and J.B. Cheatham - A Theoretical Description of Rotary Drilling for Idealized Down-Hole Bit-Rock Conditions - S.P.E. Journal, Dec. 1969.

Refer to the following publications:

• HWR Wardlaw - Optimization of Rotary Drilling Parameters - Dissertation - University of Texas, 1971.
• TM Warren - Factors Affecting Torque for a Tricone Bit - SPE 11 994 - 1983.
• PF Gnirk and JB Cheatham - A Theoretical Description of Rotary Drilling for Idealized Down-Hole Bit-Rock Conditions - SPE Journal, Dec. 1969.

: Fraction de la surface du front de taille enlevée par les dents de l'outil
δ : Pénétration des dents dans la formation
η : Facteur d'efficacité de la boue au niveau du front de taille
$$\frac{\text{λ}}{\text{2π}}$$

: Fraction de la surface du front de taille supportant le poids W
µ : Viscosité dynamique de la boue
ϑ : Angle définissant la géométrie d'un copeau
We first define a number of notations used below:
C₁: Half-width of the teeth of the drilling tool (when driving in)
D: Tool diameter
E: Young module of training
m: Dimensionless constant (depending on the type of training)
N: Tool rotation speed
n: Flat of worn teeth
P: Differential pressure on the working face
Q: Sludge flow
r: Distance of a rock chip from the tool axis
R: Radius of a dial
S: Shear stress of the rock at break
T: Torque on the tool
V: Speed of the mud at the cutting face
V _A : Instantaneous tool forward speed
W: Weight on the tool
Z: Specific mass of the mud
α: Exponent of the weight in the relation T = uW + vW ^α
$$\frac{\text{γ}}{\text{2π}}$$

: Fraction of the face of the face removed by the teeth of the tool
δ: Penetration of teeth in formation
η: Factor of mud efficiency at the cutting face
$$\frac{\text{λ}}{\text{2π}}$$

: Fraction of the surface of the working face supporting the weight W
µ: Dynamic viscosity of the mud
ϑ: Angle defining the geometry of a chip

Using Wardlaw's hypotheses and integrating the elementary couple along the radius r, the expression of the global couple T is written: $$\text{T =}\frac{\text{sδ}}{\text{Cosϑ Sinϑ}}\frac{\text{<figure-callout id="8" label="D2" filenames="imgf0001.png" state="{{state}}">D²</figure-callout>}}{\text{8}}$$

la fraction de surface du front de taille correspondant à des éclats de roche se libérant de la pression différentielle P et en appelant de même celle correspondant à des éclats broyés sous équipression, soit $$\frac{\text{γ3}}{\text{2π}}$$

dans le cas où l'influence du débit de la boue et son efficacité de la boue η au niveau du front de taille sont prépondérants, soit $$\frac{\text{γ₂}}{\text{2π}}$$

, la pénétration moyenne des dents δ est donnée par : $$\text{δ =}\frac{\text{2π}}{{\text{γ₁ + γ}}_{\text{i}}}\frac{{\text{V}}_{\text{A}}}{\text{N}}$$

avec i = 2,3 suivant le mode de fonctionnement considéré, d'où : $$\text{T =}\frac{\text{2π}}{{\text{γ₁ + γ}}_{\text{i}}}\frac{\text{1}}{\text{Sin ϑ Cos ϑ}}\frac{\text{SD²}}{\text{8}}\frac{{\text{V}}_{\text{A}}}{\text{N}}$$

et en remplaçant V_A par son expression en fonction du mode de forage considéré par Wardlaw, les expressions du couple s'écrivent :

dans l'équation 7, m est une constante sans dimension, dépendant de la nature de la formation.By the way, by calling $$\frac{\text{γ1}}{\text{2π}}$$

the fraction of the surface area of the face corresponding to fragments of rock freeing from the differential pressure P and similarly calling that corresponding to fragments crushed under equipression, that is to say $$\frac{\text{γ3}}{\text{2π}}$$

in the case where the influence of the mud flow and its mud efficiency η at forehead level are preponderant, that is $$\frac{\text{γ₂}}{\text{2π}}$$

, the average penetration of the teeth δ is given by: $$\text{δ =}\frac{\text{2π}}{{\text{γ₁ + γ}}_{\text{i}}}\frac{{\text{V}}_{\text{AT}}}{\text{NOT}}$$

with i = 2.3 depending on the operating mode considered, hence: $$\text{T =}\frac{\text{2π}}{{\text{γ₁ + γ}}_{\text{i}}}\frac{\text{1}}{\text{Sin ϑ Cos ϑ}}\frac{\text{SD²}}{\text{8}}\frac{{\text{V}}_{\text{AT}}}{\text{NOT}}$$

and replacing V _A by its expression according to the drilling mode considered by Wardlaw, the expressions of the couple are written:

in equation 7, m is a dimensionless constant, depending on the nature of the formation.

By taking the example of the first operating mode (equation 5), it is possible to show that the radius of curvature of the representative curve increases when the tool wears, and therefore that the term v of equation 1 decreases .

et si l'on se place sur cette courbe en dessous du point d'inflexion, c'est-à-dire pour $$\text{W <}\frac{\text{1}}{\sqrt{\text{3j}}}$$

le rayon de courbure est donné par :

Indeed equation 5 can be written: $$\text{T = I}\frac{\text{W²}}{\text{1 + j W²}}$$

and if we place ourselves on this curve below the inflection point, that is to say for $$\text{W <}\frac{\text{1}}{\sqrt{\text{3d}}}$$

the radius of curvature is given by:

est en général bien inférieure à 1, la variation de ρ lorsque l'outil s'use dépend principalement de celle de I. Or : $$\text{I =}\frac{\text{8}}{\text{λ²}}\frac{\text{C₁}}{\text{D}}\frac{\text{γ₁}}{\text{γ₁ + γ₂}}\text{sin²β/2 sinβ}\frac{\text{1}}{\text{DP}}$$

Knowing that the value of the report $$\frac{\text{T}}{\text{W}}$$

is generally much less than 1, the variation of ρ when the tool wears depends mainly on that of I. Now: $$\text{I =}\frac{\text{8}}{\text{λ²}}\frac{\text{C₁}}{\text{D}}\frac{\text{γ₁}}{\text{γ₁ + γ₂}}\text{sin²β / 2 sinβ}\frac{\text{1}}{\text{DP}}$$

par ailleurs la valeur de λ augmentant, I ne peut que décroître, ρ croître et donc v diminuer corrélativement.As the teeth wear δ decreases (equation 14), so C₁ also decreases since: $$\text{C₁ = δtg ß / 2}$$

moreover, the value of λ increasing, I can only decrease, ρ increase and therefore v decrease correspondingly.

K₁ étant une constante, pour un outil donné, fonction du décentrement de l'axe des molettes par rapport à l'axe de l'outil, R le rayon courant d'une molette et ϑ′ l'angle complémentaire du demi-angle au sommet de la même molette.Similarly Warren proposes as an equation for the torque on the tool:

K₁ being a constant, for a given tool, function of the decentering of the axis of the wheels relative to the axis of the tool, R the current radius of a wheel and ϑ ′ the complementary angle of the half-angle at top of the same wheel.

sachant que la pénétration δ est liée au poids sur l'outil d'après Cheatham et Gnirk par la relation :

avec :
K₂ et K₃, paramètres fonction des caractéristiques géométriques des dents, de l'angle de frottement interne de la formation et, dans une faible mesure, de la vitesse de rotation N.
σ, contrainte de compression à la pression différentielle P.
η , largeur du méplat des dents usées.The integration of equation (12) leads to equation (13):

knowing that the penetration δ is related to the weight on the tool according to Cheatham and Gnirk by relationship:

with:
K₂ and K₃, parameters depending on the geometrical characteristics of the teeth, the internal friction angle of the formation and, to a small extent, the speed of rotation N.
σ, compressive stress at differential pressure P.
η, width of the flat surface of the worn teeth.

Equation (14) is written:

Donc pour un outil, une formation, des conditions hydrauliques et de forage donnés, lorsque l'outil s'use η augmente et, l'équation (15) montrant que la concavité de la courbe représentative diminue dans ce cas, la valeur du paramètre v de l'équation : $${\text{T = uW + vW}}^{\text{α.}}$$

diminue elle aussi.

So for a given tool, training, hydraulic and drilling conditions, when the tool wears η increases and, equation (15) showing that the concavity of the representative curve decreases in this case, the value of the parameter v of the equation: $${\text{T = uW + vW}}^{\text{α.}}$$

decreases too.

relation dans laquelle u et v sont deux paramètres fonction du type d'outil, de la formation, des conditions hydrauliques, de la vitesse de rotation de l'outil, et où α est un coefficient de valeur numérique en général voisine de 2.Thus, according to the method of the present invention, the degree of wear of a tool is estimated using a criterion or a quantity linked to the mean concavity depending on the value of the parameter v defined by the relation : $${\text{T = uW + vW}}^{\text{α.}}$$

relation in which u and v are two parameters depending on the type of tool, the formation, the hydraulic conditions, the speed of rotation of the tool, and where α is a coefficient of numerical value generally close to 2.

In order to support this theoretical conclusion, experimental studies on a test bench made it possible to confirm and even to quantify the influence of tooth wear on the value of the parameter v.

• diamètre des outils : 0,152 m (6")
• formation : calcaire de Buxy
• vitesse de rotation : 116 tr/min
• pression de confinement : 90 bars
• débit de la boue à l'eau : 400 l/min

Les résultats obtenus ont permis de tracer les courbes $${\text{T = uW + vW}}^{\text{α}}\text{ avec α = 2}$$

par régression en utilisant la méthode des moindres carrés.The test conditions were:

• tool diameter: 0.152 m (6 ")
• formation: Buxy limestone
• rotation speed: 116 rpm
• confinement pressure: 90 bars
• flow of mud to water: 400 l / min

The results obtained made it possible to draw the curves $${\text{T = uW + vW}}^{\text{α}}\text{with α = 2}$$

by regression using the method of least squares.

These curves are shown in Figure 3.

$$\text{v* = 10⁷ v}$$

Thus Figure 3 shows the decrease in the value of v for the same type of tool (J3D), as a function of three different degrees of wear (new tool, wear T4 and complete wear of teeth T8) with, due to the choice of non-standard units: $$\text{u * = 10³ u}$$

$$\text{v * = 10⁷ v}$$

FIG. 3 represents the experimental curves 15, 16, 17 of the torque (T) as a function of the weight on the tool (W). Curve 15 corresponds to a new tool, curve 16 to the same type of tool with a degree of wear T4 (the designation of the tool in this state being: J3DT4).

Curve 17 corresponds to the same tool type with a degree of wear T8 (the tool in this state being designated by J3DT8).

The abscissa of FIG. 3 represents the weight exerted on the tool expressed in tonnes and on the ordinate, the torque T expressed in m.daN.

Table 1 gives for each curve the designation of the tool and the values of u * and v *.

Thus, it clearly appears that v decreases when the wear of the tool increases. An examination of the curves clearly shows that it is curve 15 which has the greatest concavity.

It is therefore verified that, for a borehole whose hydraulic conditions and the speed of rotation of the rod train vary little and whose lithology of the formations crossed is not radically diversified, the value of v decreases as and when tool wears out. It is therefore possible to consider this value, or any other function of v, as representing a tool wear criterion.

If on the other hand the conditions which have just been mentioned vary significantly, it is advisable to assign v a weighting factor as a function of the preceding variations whose influence can be determined by the relationships 5, 6 and 7 and / or by reference tests.

The present invention will now be described in its practical implementation on a drilling site and in particular in the implementation of the determination of the wear of the cutting members.

FIG. 1 diagrams the environment allowing the value of the wear criterion v or f (v) to be obtained, the function f corresponding to an additional processing of the parameter v, for example to present the results in a simple manner to the operator.

The invention mainly requires knowledge of the two background parameters, weight on the tool (W) and torque on the tool (T).

In order to examine the variations in torque on the tool, a weight sweep is carried out in stages on the tool. The scanning range and the number of steps must be sufficient. The duration of each weight step is determined by establishing a stabilized operating regime.

In FIG. 1, the reference 1 represents a well drilled using a drilling tool of the tricone type 2 fixed to the end of a drill string 3.

The weight and torque measurements exerted on the tool 2 can be transmitted to the surface, for example by the drilling fluid channel, or by any other means (electric cable, etc.) and to the computer 4 by a power line symbolized by the arrow 5.

The computer supplies the operator with the value of the wear criterion according to the invention.

Each time the operator wants to know the value of the wear criterion, he initiates from the surface the test procedure shown in Figure 2. The values of T and W are then recorded, then processed and used by the computer.

FIG. 2 shows side by side an evolution curve 6 of the weight W acting on the tool as a function of the depth PR and a curve 7 of evolution of the torque T acting on the tool as a function of the PR depth. The depth scales PR of curves 6 and 7 are identical and correspond to each other.

The weight and torque measurements exerted on the tool can be carried out in a manner known to those skilled in the art, for example using the method known as M.W.D. ("Measurement While Drilling"). The variation of the weight exerted on the tool can be obtained in a conventional manner by supporting more or less the drill string from the surface.

Parameter measurements are made as a function of depth and / or time. It is recommended to be able to perform at least five steps per test, which leads to an overall test duration of around fifteen minutes.

FIG. 2 schematically represents a test comprising five stages.

The level of the bearing 8 corresponds to the weight on the tool being drilled from which the operator decides to trigger the test. The test includes a reduction in the weight on the tool to arrive at level 9 and then an increase in four other levels, respectively 10, 11, 12 and 13 corresponding to an increase in weight on the tool.

During this variation in the weight on the tool, the variations in the torque T acting on the tool are recorded (curve 7) and at the end of the fifth bearing 13, one can return to the level of the bearing 14, which is substantially equal to the weight on the drilling tool before the test is triggered and which therefore corresponds approximately to level 8. Thereafter, it is possible to return to another level corresponding to optimum conditions of use of the drilling tool taking into account its state of wear which has just been determined. The first test must be performed, the tool being at the start of the pass. The number of subsequent tests depends on the operator's choices.

The first series of measurements carried out at the bottom of the well, at the level of the tool, on the torque and the weight corresponds to the beginning of the attack of the rock by the tool, that is to say that its wear can to be considered practically zero.

The series of measurements made during the first test period is transmitted to the surface of the computer, which establishes from this data a portion of the curve representative of the variations in torque as a function of variations in weight. The computer then keeps in memory this curve corresponding to the test period.

When desired, the operator triggers a new test period, causing a step change in the weight applied to the tool, the computer again receives a new series of measurements from which he can obtain the curve representative of the variations of T as a function of W.

• par comparaison du réseau de courbes ; toute diminution de la concavité de la courbe indiquera une augmentation de l'usure de l'outil. Bien entendu, ces courbes pourront être tracées géométriquement et donc transmises à l'opérateur qui en déduira le degré d'usure, mais on peut facilement envisager que la comparaison de la concavité des courbes soit effectuée directement par le calculateur sans qu'il soit nécessaire d'avoir un tracé visualisé,
• par calcul du paramètre v ; les courbes obtenues expérimentalement vérifiant la relation T = uW + vW^α, on peut calculer à partir des mesures réalisées la valeur de v. L'augmentation de l'usure de l'outil est déduite de la diminution de cette valeur de v.

The degree of wear can then be deduced in two ways:

• by comparison of the network of curves; any decrease in the concavity of the curve will indicate an increase in tool wear. Of course, these curves can be traced geometrically and therefore transmitted to the operator who will deduce the degree of wear, but it can easily be envisaged that the comparison of the concavity of the curves is carried out directly by the computer without it being necessary to have a visualized route,
• by calculating the parameter v; the curves obtained experimentally verifying the relation T = uW + vW ^α , the value of v can be calculated from the measurements carried out. The increase in tool wear is deducted from the decrease in this value of v.

Of course, the measurements made at the bottom of the well may present some inaccuracies, it is therefore necessary, from these measurements, to determine an average curve, for example by the method of least squares, and to deduce therefrom the average concavity of this curve represented by the mean radius of curvature, or a quantity linked to this mean value. Then, the determination of the wear can be done, either by controlling the evolution of the average concavity according to the progress of the drilling, or by composing the average concavity with average concavities established during reference tests.

In the case where the value of the weight on the tool at the bottom can be obtained precisely from a surface measurement, in particular in the case of vertical wells, this surface measurement can possibly be used in the context of the present invention.

The characteristics of the formation crossed, of the tool, the values of the hydraulic conditions and of N can be taken into account to normalize the value of the parameter v and / or that of the criterion.

The results obtained on the drilling site are presented in Figure 4. These results correspond to a well drilled for the Société Nationale Elf-Aquitaine off the Netherlands. Unlike the bench tests presented above, the tool was large in diameter (17 "1/2). The wear, after a drill pass from dimension 1306 m to dimension 1673 m, was equal to T2. groups of the measurements carried out were distributed respectively over 27 m, 3.5 m and 7.5 m.

Although in this case the wear of the tool is low, the decrease in the value of v does take place.

The abscissa of FIG. 4 expresses the weight exerted on the tool in tonnes and the ordinate the torque exerted on the tool expressed in m.daN.

Curve 18 is located between dimensions 1320 and 1347 m, curve 19 between dimensions 1460 and 1463.5 m and curve 20 between dimensions 1559 and 1566.5 m.

During this test, the tool went from new condition to T2 wear, ie little wear.

The following table indicates for each curve the depth test depth, the value of u * and the value of v *.

Thus it appears that for low wear the value of v * (and consequently of v) has varied significantly, which indicates that the method according to the invention makes it possible to know the wear of the tool with precision.

The points shown in Figures 3 and 4 correspond to the values used to establish the different curves. It is not necessary to draw the curves during the various tests. It is enough to obtain the value of v or of a function of v to be informed about the state of wear of the tool. Of course, it is desirable that the operator has at his disposal a graphic representation of the representative points and of the corresponding curves.

The various curves as well as the values of u have been given to facilitate the reader understanding of the present application.

The method according to the present invention makes it possible to avoid drilling with worn tools or to reassemble tools too early. The knowledge of the degree of wear of the tools that it allows plays a direct role in reducing the cost of drilling.

Claims (4)

1. A method for determining the wear of cutting members of a tool during drilling in a rock formation, whereby the measurements of the weight W applied to the tool and the torque T required to rotate the tool are taken, the weight on the tool W and the torque T being linked by a ratio of the type T = uW +vWα in which u and v are the parameters and α is a coefficient dependent, among other things, on the formation, characterised in that

- a series of weight values is applied to the tool during drilling during an initial testing period,

- a first series of measurements is taken at the level of the tool on the torque and the weight during an initial testing period,

- on the basis of the first series of measurements, a curve section representing the variations in the torque in relation to the variations in the weight is drawn up,

- the curve section drawn up during the initial testing period is stored,

- during drilling, tests are carried out over a succession of periods and the corresponding representative curve section for each period is drawn up,

- the curve section obtained for a given test period is compared with at least one curve section obtained previously,

- from this, the degree of wear on the cutting members is determined on the basis of the variation in at least one measurement of the degree of concavity of the curve sections, for example the mean radius of curvature of the curve sections, the decrease in concavity of the curve denoting an increase in wear on the tool, all other conditions being equal.

2. A method of determining wear in accordance with claim 1, characterised in that the first series of measurements from which the first curve section is determined, is carried out on a practically new tool showing no signs of wear.

3. A method of determining wear in accordance with claim 1, characterised in that over a given test period, at least one function of the parameter v is determined on the basis of the curve sections relating to the previous test periods on the one hand the and ratio T= uW + vWα on the other and in that the degree of wear on the cutting members of the tool is deduced on the basis of the decrease in the function of the parameter V.

4. A method of determining wear in accordance with any one of claims 1 to 3, characterised in that a step by step scan is taken of the value of the weight on the tool close to the reference value and the corresponding values of W and T are determined during the scan.

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