EP0816630B1 - Method and system for real time estimation of at least one parameter connected to the performance of a downhole tool - Google Patents

Description

The present invention relates to the field of measurements during drilling, in particular measures concerning the behavior of a drilling tool attached to the end of a drill string. The method according to the invention provides a solution for estimate in particular the amplitude of the vertical displacements of the drilling tool or the effort applied to the tool, said estimates being obtained by means of a calculation taking into account measurements made at the top of the drill string, i.e. substantially on the ground surface, generally by means of sensors or a fitting instrumented located in the vicinity of the means for rotating the lining.

We know measurement techniques for the acquisition of information related to dynamic behavior of the drill string, which use a set of sensors bottom connected to the surface by an electrical conductor. In document FR-2 688 026, it two sets of measurement sensors are used connected by a logging type cable, one being located at the bottom of the well, the other at the top of the drill string. However, the presence of a cable along the drill string is troublesome for the operations of proper drilling.

We know the document EP-A-709546 which describes a predictive method of Downhole measurements following surface measurements, from an equation predictive obtained following collections of background and surface measurements. This method requires the collection of bottom measurements and therefore means of bottomhole collection.

We know from documents FR 2645205 or FR 2666845 of the surface placed at the top of the lining which determine certain malfunctions of drilling according to surface measurements, but without taking into account, so physical, dynamic behavior of the lining and the drilling tool in the well.

Between the bottom of a well and the soil surface, there is a string of rods along from which energy dissipative phenomena take place (friction on the wall, torsional damping, ...), conservative phenomena of flexibility, especially in tension-compression. There is thus a distortion between the measurements of the displacements of bottom and surface which mainly depends on the intrinsic characteristics of the lining (length, stiffness, geometry), friction characteristics at the interface rods / wall and random phenomena.

This is why, the information contained in the surface measurements does not alone are not enough to solve the problem posed, that is to say knowing the instantaneous movements of the tool by knowing the instantaneous movements of the surface trim. The surface measurement information must be supplemented by independent information of another kind which takes into account the structure of the drill string and its behavior between the bottom and the surface: this is the role of the knowledge which establishes the theoretical relationships between the bottom and the surface.

The methodology of the present invention uses the conjunction of such a model, defined a priori, and surface measurements acquired in real time.

• on identifie les paramètres dudit modèle en prenant en compte les paramètres dudit puits et de ladite garniture,
• on linéarise ledit modèle autour d'un point de fonctionnement,
• on réduit ledit modèle linéarisé en ne conservant que certains des modes propres de la matrice d'état dudit modèle,
• on calcule, en temps réel, le déplacement de l'outil de forage ou l'effort appliqué sur l'outil, à l'aide du modèle réduit et d'au moins un paramètre mesuré en surface.

Thus, the present invention relates to a method of estimating the effective behavior of a drilling tool attached to the end of a drilling rig and rotated in a well by drive means located on the surface, in which we use a nonlinear physical model of the drilling process based on general mechanical equations. In the method, the following steps are carried out:

• the parameters of said model are identified by taking into account the parameters of said well and of said lining,
• we linearize said model around an operating point,
• said linearized model is reduced by retaining only some of the eigen modes of the state matrix of said model,
• the displacement of the drilling tool or the force applied to the tool is calculated in real time, using the reduced model and at least one parameter measured at the surface.

The model can mainly take into account displacements and efforts vertical and said reduced model can calculate in real time the movement or the effort drilling tool vertical, said parameter measured at the surface being the vertical acceleration of the filling.

The rotational speed measured on the surface can be a second parameter used in the scale model.

The reduced model can be refined by self-adaptive filtering which minimizes the difference between an actual measurement of a parameter related to the displacement of the lining in surface and the corresponding output obtained by said reduced model.

The filtering can take into account the tension force of the rods.

The invention also relates to a system for estimating effective behavior. of a drilling tool attached to the end of a drill string and rotated in a well by means of drive located on the surface, in which an installation of calculation includes means for non-linear physical modeling of the drilling process based on general mechanical equations. The parameters of said means modeling are identified by taking into account the parameters of said well and said lining, and the calculation installation includes means for linearizing said model around an operating point, means for reducing said linearized model so to keep only some of the eigen modes of the state matrix of said model, means of calculating, in real time, the displacement of the drilling tool or the applied force on the tool, using the modeling means once linearized and reduced and the means measuring at least one parameter related to the displacement of the lining on the surface.

The modeling means may only take into account the traction-compression, and the parameter can be one of the following: the rotation speed, vertical acceleration and packing tension.

• la figure 1 représente schématiquement les moyens mis en oeuvre pour une opération de forage,
• la figure 2 représente un exemple de diagramme d'un modèle physique en traction-compression,
• la figure 3 représente un diagramme d'un estimateur en boucle ouverte,
• la figure 4 représente un diagramme d'un estimateur avec recalage,
• la figure 5 représente schématiquement la méthodologie de la constitution de l'estimateur selon l'invention.

The present invention will be better understood and its advantages will become clear on reading the description of an example, which is in no way limitative, illustrated by the figures below appended, among which:

• FIG. 1 schematically represents the means used for a drilling operation,
• FIG. 2 represents an example of a diagram of a physical model in traction and compression,
• FIG. 3 represents a diagram of an open loop estimator,
• FIG. 4 represents a diagram of an estimator with registration,
• FIG. 5 schematically represents the methodology of the constitution of the estimator according to the invention.

Figure 1 illustrates a drilling rig on which we will operate the invention. The surface installation comprises a lifting device 1 comprising a lifting tower 2, a winch 3 which allow the displacement of a drilling hook 4. Under the drill hook are suspended drive means 5 for rotating the assembly of the drill string 6 placed in the well 7. These drive means can be of the drive rod or kelly type coupled to a rotation table 8 and the mechanical motorizations, or of the type motorized drive head or "power swivel" suspended directly from the hook and guided longitudinally in the tower.

The drill string 6 is conventionally constituted by rods of drilling 10, of part 11 commonly called BHA for "Bottom Hole Assembly" mainly comprising drill rods, a drilling tool 12 in contact with the land being drilled. The well 7 is filled with a fluid, called a drilling fluid, which circulates from the surface at the bottom through the inner channel of the drill string and rises to the surface by the annular space between the walls of the well and the drill string.

For the implementation of the invention, an instrumented fitting 13 is inserted between the drive means and the top of the lining. This fitting allows measure the speed of rotation, the tensile force and the longitudinal vibrations of the top of the trim, and incidentally the couple. These so-called surface measurements are transmitted by cable or radio to an electronic recording, processing, display, not shown here. Instead of fitting 13, we can use other sensors such as a tachometer on the rotation table to measure the speed of rotation, a measurement of tension on the dead strand of hauling and possibly a device for measuring the torque on the motorized device, if the accuracy of the measurements obtained is sufficient.

Part 11 of the BHA may more specifically include, drill collars, stabilizers, and a second instrumented fitting 14 which will only be used to control experimentally the present invention by allowing the comparison between the displacement of the drilling tool 12 actually measured by the instrumented fitting 14 and the estimated displacement thanks to the implementation of the present invention. So it is clear that the application of the present invention does not use an instrument fitting placed at the bottom of Wells.

The driller who conducts a drilling operation with the devices described in the Figure 1 has three possible actions, which are therefore the possible control variables for driving, the weight on the tool which is adjusted by the winch which controls the hook position, rotation speed of the rotary table or equivalent, the flow of injected drilling fluid.

• un appareil de forage comprenant une installation de levage,
• un ensemble d'entraínement: organe de régulation et motorisation,
• un ensemble de tiges,
• un ensemble de masses-tiges,
• un outil,
• un terrain représentant le contact outil/roche.

To illustrate an example of the present invention, a model of the mechanical system composed of the following technological elements will be used:

• a drilling rig comprising a lifting installation,
• a set of drive: regulator and motor,
• a set of rods,
• a set of drill sticks,
• a tool,
• a terrain representing the tool / rock contact.

The described model will treat the drill string as a one-dimensional element vertical. Displacements in vertical translation will be considered, displacements being neglected.

FIG. 2 represents the block diagram of the traction-compression model. It is a classic model with finite differences which includes several meshes represented by blocks 20. Each mesh represents a part of the drill string, drill rods and drill collars. These are mass-spring-damping triples shown in the diagrams referenced 21, 22, 23. Each block is provided with two inputs and outputs represented by the pairs of arrows 24 and 25 which represent the input and output voltages and the vertical movement speeds of inputs and outputs. This representation shows the way to digitally connect several rods (or meshes) as we connect physically the packing rods.

Block 26 represents the drilling rig. It is a collection of masses, of springs and friction.

Block 27 represents the tool in its longitudinal behavior.

Block 28 represents the law relating the movements of the drilling tool to the shape of the working face and the compressive strength of the rock. According to a position instantaneous vertical of the tool and the shape of the working face, the weight is determined acting on the tool.

This model is validated using data recorded on site using the instrumented bottom and surface fittings.

The drilling fluid and the well walls only intervene insofar as they generate a resistant friction torque. From experience, and using background measurements and surface, we can establish a friction law along the linear rods as a function of rotational speed and longitudinal speed.

The tensile-compression model thus obtained is generally of high order, that is to say of the order of 50 to 100 to reproduce reality with sufficient finesse.

To obtain a quickly executable and robust model when changing drilling conditions, for example the change of land crossed, we proceed to steps below described.

We linearize the generally non-linear model. In the example described above, we linearize the model by choosing an operating point (a rotation speed and a weight on the tool) representative of the actual drilling conditions. We can verify that the behavior of the knowledge traction-compression model, once linearized, is correct in the vicinity of the operating point.

x = X-X ₀

X ₀= valeurs des états au point de fonctionnement

e = E-E ₀

E ₀= valeurs des entrées au point de fonctionnement

s = S-S ₀

S ₀= valeurs des sorties au point de fonctionnement

Linearization around an operating point consists in calculating the Jacobian of the non-linear state system. The linear state system obtained is then of the form: x = A. x + B. e s = C. x + D. e with:

x = X - X ₀

X ₀ = values of the states at the operating point

e = E - E ₀

E ₀ = values of the inputs at the operating point

s = S - S ₀

S ₀ = values of the outputs at the operating point

The pseudo-modal form is first done by a change of base: z = P. x z = P. x

P est la matrice des vecteurs propres

A est la matrice diagonale des valeurs propres.

After resolution we get: z = P -1 .AP z + P -1 .B. e z = Λ. z + B Λ . e s = CP z + D. e s = C Λ . z + D. e

P is the matrix of eigenvectors

A is the diagonal matrix of eigenvalues.

After linearization, the traction-compression model retains an order Student. The analysis of the eigen modes of the traction-compression model allows to quantify the contribution of each mode to the outputs of interest. Onne then keeps only the relevant modes; that is to say those who have an influence notable on the dynamic behavior represented by said outputs.

• suppression des modes non ou peu observables sur les sorties mesurées,
• suppression des modes hautes fréquences, n'entrant pas dans la bande de fréquence de la commande ou de l'estimateur.

The scale model must reproduce the phenomena in a certain frequency band. The criteria for mode selection are therefore twofold and are based on concepts of observability:

• deletion of the modes not or hardly observable on the measured outputs,
• suppression of the high frequency modes, not entering the frequency band of the control or the estimator.

The reduction method used is the singular disturbance method. It consists in keeping from the state matrix and from the command matrix, the lines and the columns corresponding to the modes to keep. To keep static gains, the modes Rapids are replaced by their static value, which has the consequence of introducing a direct matrix.

The method assumes that the fast modes take their equilibrium in a time negligible, that is to say that they are established instantaneously (quasi-static hypothesis).

Figure 3 shows the block diagram of a loop-type estimation system opened. Block 40 shows diagrammatically the means for measuring surface parameters, here the voltage Tms and vertical acceleration Zms, the speed of rotation of the lining Vms measured at the table or at the motorized injection head. Block 41 represents the model which simulates the physical model of non-linear tension-compression by calculating the function transfer between the inputs (Vms, Zms) and the outputs Tes, Tef and Zef representing respectively the estimated tension on the lining at the surface, the estimated tension and the estimated vertical acceleration at the lower end of the packing in the well.

However, the transfer function is always an approximation of reality and any mismatch between the model and the actual drilling process can create a discrepancy between the estimated values and the real values by integrating the differences. Also in the in most cases, it is advantageous to carry out a readjustment, or readjustment, using at least minus a comparison between the value of an estimated output and its value actually measured. Here, the linear estimator is preferably readjusted from the surface tension.

The estimation technique is based on the filtering principles of Luenberger and Kalman ("Automatic linear systems" by P. De Larminat and Y. Thomas-Flammarion science; Paris IV, 1975). The principle of a linear estimator can be illustrated by FIG. 4 where the measurement of the voltage Tms and the estimated value Tes and of the voltage are compared in the means 42, the difference between these two values being injected into an adapter 43 in real time. The objective here is to reconstruct as faithfully as possible the outputs rather than having an exact model. This is why we perform a registration state. As the outputs are directly linked to the states, state registration consists in weighting between the states predicted by the model at time t and the states reconstructed from the only measured outputs. This weighting is not a simple average, but it takes into account the degree of precision of the state estimates obtained by its two independent channels.

Once readjusted the states of the model which represent the dynamics of the process drilling, all outputs, whether measured or not, can be recalculated.

This estimate is not only interesting for unmeasured variables like Tef and Zef it also applies to the measured variables (for example Tms) which were used for registration. The estimated value Tes is the equivalent of a value filtered on the basis of a model: this is why we generally use the term filtering (filtering of Luenberger, Kalman filtering ...).

The state registration technique, as described above, introduces a slaving of Tms measured on Tes estimated.

This closure eliminates the risk of divergence mentioned above, when the model is simulated in open loop (figure 3).

There is thus a desensitization of the estimated variables with respect to imperfections of the model. In this context, we no longer need to have a perfect model: a model approached is sufficient.

Furthermore, only one measurement is available here, the voltage T, for carrying out the readjustment: it does not seem possible to readjust a large number of states from this measured. This is why, the nonlinear traction-compression model is not suitable in despite its greater precision.

There is therefore a compromise to be made between the precision and the order of the system. he look for the minimum order model that meets precision tolerances desirable, and that is also easy to adjust and robust.

• il faut sauvegarder les modes propres de vibration en traction-compression qui sont prépondérants dans les sorties à ré estimer ;
• pour des raisons de cohérence et de stabilité numérique, il faut rejeter les modes de fréquences élevées supérieures à f_max = f_e/2 où fe est la fréquence d'échantillonnage des entrées et des sorties.

The choice of the order of the reduced model depends on the following qualitative criteria:

• it is necessary to save the eigen modes of vibration in traction and compression which are predominant in the outputs to be re-estimated;
• for reasons of consistency and numerical stability, it is necessary to reject the high frequency modes greater than f _max = f _e / 2 where fe is the sampling frequency of the inputs and outputs.

It is therefore superfluous to choose a reduced model of higher order if we want integrate the model at the sampling rate.

In addition, it should not be forgotten that the reduced estimation model must, preferably meet the technological constraints of real time.

• discrétisation du modèle réduit,
• discrétisation des filtres passe-haut,
• agrégation des filtres passe-haut et du modèle réduit, l'ensemble devient le modèle d'estimation,
• calcul des gains de recalage,
• construction de l'estimateur complet.

The estimator is therefore constructed according to the following steps:

• discretization of the reduced model,
• discretization of high-pass filters,
• aggregation of high-pass filters and the reduced model, the whole becomes the estimation model,
• calculation of registration gains,
• construction of the complete estimator.

The methodology for the construction of the estimator according to the invention can be illustrated by FIG. 5. Block 50 represents a physical model representing a rotary drilling process, for example illustrated in Figure 2. This model takes into account account for a determined operating situation, in particular by receiving the mechanical characteristics of the drill string used, symbolization referenced 51, well and surface conditions, symbolization referenced 52, and friction laws, symbolized by reference 53. Block 54 represents the main tension model once linearized and reduced as described above. All these steps gathered under the DF brace run in delayed time compared to the progress of the process rotary drilling, the other steps gathered under the TR brace are executed in time real.

Block 55 is directly what has been called the estimator. Means of measure 56 placed on the top of the drill string give the measurements vertical acceleration, tension and speed of rotation at the top of the rods, i.e. surface. These surface measurements are taken into account in the estimator, as described above, to give an estimate of the displacement values of the tool drilling, in particular the vertical acceleration Zef from which the vertical displacement will be deducted of the drilling tool.

The present invention is advantageously implemented on a construction site drilling in order to have as precise an estimate as possible of the vertical acceleration of the drilling tool in real time, using only surface measurements, in particular the vertical acceleration and the speed of rotation of conventional means of setting rotation of the drill string, and of a surface installation equipped with means electronic and computer. It is very interesting to have an estimate of the parameters background so as to detect, and even prevent known malfunctions, by example the so-called "bit bouncing" behavior characterized by a separation of the tool from the size face although the head of the drill string remains substantially stationary and a force of significant compression is applied to the tool. This can result in harmful effects on the life of the tools, on the increase of the mechanical fatigue of the drill string and the frequency of connection breaks.

Claims (5)

1. A method designed to estimate the effective behaviour of a drill bit (12) fastened to the end of a drill string (6) and driven into rotation in a well (7) by surface driving means (8), wherein a non-linear physical model (50) of the drilling process based on general mechanics equations is used, characterized in that the following stages are carried out :

   the parameters of said model are identified by taking account of the parameters of said well and of said string (51, 52, 53),

   said model is linearized about a working point,

   said linearized model is reduced while keeping only some of the specific modes of the state matrix of said model,

   the displacement of the drill bit or the stress applied to the bit is computed in real time by means of the reduced model (54) and of at least one parameter measured at the surface, and in that said model mainly takes account of the displacements and the vertical stresses, and said reduced model computes in real time the vertical motion or stress of the drill bit, said parameter measured at the surface being the vertical acceleration of the string.
2. A method as claimed in claim 1, wherein the displacement of the drill bit or the stress applied to the bit is computed in real time by means of the reduced model and of at least the two parameters measured at the surface : the vertical acceleration and the rotational speed of the string.
3. A method as claimed in any one of claims 1 to 2, wherein the reduced model is fined down by means of self-adaptive filtering which minimizes the difference between a real measurement of a parameter linked with the displacement of the string at the surface and the corresponding output obtained by said reduced model.
4. A method as claimed in claim 3, wherein said filtering takes account of the tensile stress measured at the surface on the string.
5. A system designed to estimate the effective behaviour of a drill bit (12) fastened to the end of a drill string (6) and driven into rotation in a well (7) by surface driving means (8), wherein a computing unit (9) comprises means intended for non-linear physical modelling of the drilling process based on general mechanics equations, characterized in that parameters of said modelling means are identified by taking account of the parameters of said well and of said string, in that the computing unit comprises means designed for linearization of said model about a working point, means for reducing said linearized model so as to keep only certain specific modes of the state matrix of said model, means for real-time computation of the displacement of the drill bit or of the stress applied to the bit, by means of the modelling means once linearized and reduced and means (13) for measuring at least one parameter linked with the displacement of the string at the surface : the rotational speed, the vertical acceleration and the tension of the string, wherein the modelling means only take account of the traction and the compression.

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