Object of the Invention

The present invention relates to a drilling method suitable for wells for the injection and production of a gas or oil reservoir. Other uses can be found in mining and civil engineering work.

This method is characterized by the use of the estimation of damage in a plurality of sections along the well to establish the optimal parameters assuring the discharge of caving material without ever reaching the maximum discharge capacity.

According to one embodiment, the drilling parameters are determined through an estimation of damage using a set of analytical steps in which not only is the caving angle taken into account, but also the area of damage in each of the sections of the well .

This analytical method also allows assessing both the width and depth of damage in the wall of the well.

Background of the Invention

Drilling a well for operating gas or oil reservoirs is a very costly project economic wise and the operating conditions for drilling depend on many variables with respect to which there is not always enough data, giving rise to very high values of uncertainty.

This uncertainty is much greater in reservoirs in which drilling is performed for the first time where there is usually no data or physical sample that allows increasing knowledge about the geology in the area.

Drilling generates an empty tubular space obtained after removing rock that occupied that space. Taking the stress state before drilling and the in situ stress as a reference, drilling operations modify the stress state based primarily on two reasons: rock removal eliminates the structural element compensating for the stress state of the free surface of the generated well; and during drilling, the drilling fluid forms a column that exerts pressure on the wall of the well depending primarily on the height to the surface and on the density of said drilling fluid, without taking into account dynamic effects .

In fact, one of the parameters to be modified which are used in the drilling operation is the change in density of the drilling fluid in order to change the pressure that is exerted on the wall of the well.

There are other causes generating stresses such as the bit in charge of drilling the rock, but they are understood as being required for breaking up and removing rock in the space generated in the well.

When the drilling operation uses a drilling fluid, said drilling fluid is usually injected through an inner conduit of the drilling tool. The drilling tool breaks up the rock at the bottom of the borehole generating material having a diverse grain-size distribution that must be removed. The drilling fluid flow injected at the end of the drilling tool entrains this material obtained by drilling rock, flowing upward primarily through the annular space demarcated between the drilling tool and the already generated wall of the well, until reaching the surface where this material is discharged.

The drilling fluid rising up through the annular space exerts pressure against the generated wall of the well. The pressure depends on the weight of the drilling fluid column existing up to the upper surface and also on the speed of the upward flow. The weight of the column is therefore a first estimation of the pressure exerted on the free surface of the wall of the well. A second estimation takes into account the dynamic stresses of the drilling fluid column according to flow conditions .

This pressure may be excessive, exceeding the maximum acceptable stress of the rock generating fractures, for example. Likewise, this pressure may be insufficient and may not compensate for the resistance forces of the removed material to give rise to the well. In this case, the stresses of the rock can exceed the acceptable or yielding stress of the rock, causing the material of the wall to break and cave into the well where drilling is being performed.

If this caving occurs during drilling, the drilling fluid must be capable of discharging the material generated by the drilling tool plus the caving material. The amount of the caving material primarily depends on the volume of rock material that has sustained damage.

Caving occurs in almost all wells. When establishing the design of the drilling conditions, it is important to quantify the caving in order to assess whether there are drilling parameters that render drilling feasible even if this caving occurs .

After having established well viability, knowing the optimal drilling parameters under conditions with caving is also of interest.

Analytical modes for assessing damage in rock almost exclusively considering the value of the angle measuring the width of damage in the wall of the well are known in the state of the art. These analytical techniques use the stress state established by the structure of the rocks forming the reservoir taking into account the in situ stresses. These analytical techniques use Kirsch equations, which allow describing the stress state around a well on an infinite plane. There are also analytical and numerical techniques which allow estimating in situ stresses.

The analytical techniques known up until now have considered, hypothetically, that the medium is isotropic and linear, perform all calculations using the original cylindrical geometry and as a result do not allow correctly calculating the stress around the well and therefore do not allow calculating the depth of damage.

Criteria based solely on the angle covered by the damage in the wall of the well have been developed with techniques of this type. There is a maximum angle referred to as angle of collapse, the value of which is established by each of the companies dedicated to drilling. With this criterion, it is possible to establish under which conditions there is considered to be excessive damage: when at a given depth, the angle of damage is found to be greater than the angle of collapse. In this case, it is established that the damage in the wall of the well prevents discharging the caving material during the drilling operation, the well being determined to be unviable.

It has been numerically proven that variation in caving angle with respect to weight of the drilling fluid is approximately linear; nevertheless, the area of damage measured according to a transverse section, and therefore proportional to the caving volume, increases exponentially with respect to the weight of the fluid. In other words, minor variations in the weight of the drilling fluid gives rise to minor variations in the angle of damage (used as parameter in the state of the art) , and it generates, however, major variations in the volume of failed rock. The result is an inadequate estimation of the caving volume.

While attempting to reduce the angle of damage, the pressure calculated for the drilling fluid may also exceed the pressure established as the upper limit, i.e., the pressure above which a crack is generated.

This criterion based on the angle of damage in the wall of the well does not take into account the depth of the damage. It has been experimentally found that the described criteria applied according to the state of the art rule out certain wells since they consider that there would be caving which would not allow drilling parameters such that said well may be viable when in practice such wells would indeed be viable. This is the case of caving with a large angle of damage but not very deep, generating a reduced caving volume.

When this occurs, i.e., when it is concluded that a given well is not viable when it actually is, one alternative is to search for another well location. Another alternative is to modify the well path and geometry, reservoirs being able to be ruled out due to the high risk suggested by numerical simulation models. Since the first location is usually determined by optimization techniques, the change of well location reduces the optimal character of the initial operating plan, or the change may even require drilling two or more wells in place of the former, significantly increasing costs and reducing production capacity .

The present invention establishes a drilling method optimizing the drilling conditions for each coordinate of the well path. This optimization is based on an estimation of damage for a plurality of sections distributed along the well path.

A specific analytical technique other than that known in the state of the art which allows estimating the region of damage, particularly the width and depth of damage, is of special interest, where this analytical technique allows optimizing the drilling parameters for each coordinate of the well path.

The method of estimating the region of damage according to an example of the invention not only allows calculating with greater precision the optimal drilling parameters taking into account the caving material, but also in those cases where the angle of damage is small but the depth of damage is very large, giving rise to significant caving volumes, the method may consider drilling to be unviable when, by applying techniques based on the state of the art, it would have been accepted, leading to attempts to perform drilling that end in failure and with the loss of resources that this entails.

In wells where the criteria according to the state of the art establish well viability, the parameters obtained according to the invention also allow establishing more favorable drilling conditions, maintaining safety levels that are even higher than those determined in the state of the art.

The determination of the region of damage according to a preferred embodiment allows assessing said region before drilling is performed.

Description of the Invention

The present invention relates to a drilling method suitable for wells using a drilling bit for drilling a well of a well- drilling diameter D at a drilling speed (v) and, injecting a drilling fluid with a density (y) and flow rate ( Q) during the drilling process, for the injection or production of a gas or oil reservoir. The method comprises the steps of:

a) generating a geomechanical model of the domain comprising the path of the well to be drilled at least incorporating rock data and the mechanical properties of said rock for a pre- specified in situ stress field;

b) generating a fluid flow model of the same domain at least incorporating drilling fluid data, wherein said fluid flow model models the rock as a porous medium and comprises the pore pressure in said porous medium.

The domain is a pre-specified region comprising the path of the well to be drilled. Taking gas or oil reservoirs as an example, these reservoirs are formed primarily by porous rocks that store gas or oil trapped in the pores . The domain can contain the reservoir and be more extensive to the point of including the portion of rock reaching the surface of the earth. It can also be smaller than the reservoir, containing only a part of said reservoir, although it must indeed contain the path to be drilled. In applications of another type, the fluid can be water, for example. The mechanical behavior of the rock depends both on the mechanical properties of the rock and on the influence of the fluid trapped in the pores of the rock due to the pressure at which said fluid is found. Particularly, at least the mechanical properties of the rock, the properties of the fluid trapped in the rock and also the drilling fluid which is in contact with the generated surface of the well are relevant .

In one embodiment, steps a) and b) are carried out by means of a computational system.

In order to establish the stress state of the well, it is necessary to generate a geomechanical model of the reservoir incorporating these properties as well as the in situ stress field and the boundary conditions or the boundary conditions and the initial conditions if the model is based on an initial value problem also taking dynamic variables into account.

The geomechanical and fluid flow models allow reproducing the behaviour of the rock in the drilling conditions for determining the stresses causing damage under the simplifying hypotheses of the models used.

c) establishing a drilling diameter D and a well path through the geomechanical model and the fluid flow model of the domain ;

d) establishing a discretization t _i = l..N of the coordinate (t) along the well path.

The boreholes do not necessarily have to be vertical and straight. For example, the search for hydrocarbon sources may require tracing paths that navigate through very hard rocks, regions with rather unstable sand, or horizontal paths in the final segments making easy subsequent harnessing of those resources .

After having established the geomechanical and fluid flow models, the path and a discretization along the path are determined in such models. A specific way of defining the path is by means of a piecewise or non-piecewise parametric curve. The points t _i are specific points of the curve where damage in the rock causing caving will be assessed and thereby estimating at those points the volume of caved rock which increases the drilling fluid discharge requirements. The higher the density of points is, the greater the number of evaluations of damage there will be, but precision in the calculation of the caving volume according to a coordinate or parameter (t) which follows along the path will also be greater.

e) determining, for eacht _i , a transverse section S _i of diameter D, and in said section S _i , for one or more values of density of the drilling fluid

i. determining the pore pressure p _p , vertical height Z _i , maximum stress σ _max , minimum stress σ _min and the mechanical properties of the rock in the section based on the geomechanical model;

ii . determining the stress state of the rock in said section S _i at least according to the data from the preceding step;

ill. determining in section S _i the region of damage

under a pre-established rock failure criterion and the value of the area | of said region

A section is determined for each of the points t _i = l..N of the discretization of the well path. The damage for different conditions of the pressure fluid is analyzed in this section, which allows generating a discrete function which in turn allows determining under which conditions drilling can be performed.

For this reason, a discrete set M of densities of the drilling fluid is established.

In each section, the forces acting on the rock (pore pressure p _p , maximum stress σ _max , and minimum stress σ _min ) and also the strength properties thereof are evaluated for determining the stress state of the rock in said section S _i .

Damage is calculated with the resistance capacity of the rock and the forces acting thereon once a failure criterion of the rock has been established.

A damaged area, identified as region within

section S _i , is established with the failure criterion.

The more precise the method of determining the region of damage is, the better the estimation of the rock caving volume will be.

According to a preferred example, the damage is determined by hypothetically assuming that the damaged area is in an elliptical sector and is evaluated by establishing an analytical method that will be described below for determining the dimensions of the ellipse that penetrates the rock.

f) for each coordinate (t) of the discretization t _i = l..N establishing: iv. a correspondence between the discrete values of

density of the drilling fluid and the value of

the area j j the latter being interpreted as the detachment volume (V) of the wall of the well per unit of drilled length;

v. a combination of drilling speed (v) , density (7) of the injected drilling fluid and flow rate (Q) thereof, such that it establishes a volume of material to be

discharged, material being cut by the bit plus the caving material less than that determined by the

drilling system, so as to allow removing the volume of material from the well without the drilling bit collapsing;

g) drilling the well according to the values of drilling speed (v) , density (7) of the injected drilling fluid and flow rate (Q) thereof for each coordinate of the well path.

The value of the area A = \\R \\ of said region R of damage is interpreted as the additional volume per unit of length that must be displaced or cleared during drilling. Obtaining different regions of damage for each of the discretization points allows determining a caving volume according to each point of the path. This variable function gives rise to a different clearing volume which can affect the drilling speed or the pressure within the system (pressure of the fluid in the annular) . This variable (caving volume) can then be used for adjusting the drilling speed, weight of the flows and/or pressure of the drilling fluid, being adjusted to the optimal conditions at all times.

Given that a calculation of the value of the area

of the region R of damage is established for each discrete value of the density of the drilling fluid a function V(y),

the caving volume, is established where the dependence has been obtained in a set of discrete samples.

The combination of drilling speed (ν), density (y) of the injected drilling fluid and flow rate (Q) thereof can be established at each discretization point of the path such that it establishes a volume of material to be discharged, material being cut by the bit plus the caving material

less than that determined by the drilling system, so as to allow removing the volume of material without the drilling bit collapsing. A person skilled in drilling knows how much material can be removed by the drilling system depending on the density (7) of the injected drilling fluid and the flow rate (Q) . According to the invention, the skilled person can determine the drilling speed (v), the density (7) of the injected drilling fluid and flow rate (Q) for each coordinate of the well path as for each density (7) he knows the volume of the caving material

The values of the area of damage, and therefore the

caving volume, can be determined at points not within the discretization, for example, by interpolating the (scalar) values obtained at adjacent points.

The material being cut by the bit is determined by the drilling speed (v) and the section of well being drilled.

The drilling parameters used along the pre-established path are obtained with these values.

Description of the Drawings

The foregoing and other features and advantages of the invention will be more clearly understood based on the following detailed description of a preferred embodiment, provided only by way of illustrative and non-limiting example in reference to the attached drawings.

Figure 1 shows a diagram for making a well in an oil reservoir, defined by a well path, where a section S on which the region of damage is determined is established at a given point of the path.

Figure 2 shows a diagram for making a well in a sectional view, as well as a pair of ellipses with different eccentricity used in the calculation steps according to an example of the invention .

Figure 3 shows a graph of stresses on the periphery of a family of ellipses. The family of ellipses is represented by means of a plurality of curves identified with an arrow in which the direction in which the eccentricity increases is shown. The abscissas show the angle in the section taking the point of minimum stress as a reference.

Figure 4 shows an image of a vertical segment of the well in which the width of the damage has been measured according to the vertical height. Regions with damage are darker.

Figure 5 shows two graphs related to one another. The graph on the left shows a drawing with the factor of safety F according to the eccentricity e with a value of one to consider the factor of equilibrium between the external forces and resistance forces. After having determined the eccentricity, the graph on the right shows the ellipse with said eccentricity determining the transverse area of the damage.

Figure 6 partially reproduces Figure 1 as an embodiment where the determination of damage is carried out in a discrete set of the well path for subsequently evaluating the caving volume, and therefore for performing drilling according to the parameters estimated according to an embodiment.

Detailed Description of the Invention

According to the first inventive aspect, the present invention relates to a drilling method suitable for wells for the injection or production of a gas or oil reservoir. A preferred example proposes a method which allows estimating the region of damage due to caving in the wall of a well during the drilling operation for the injection or production of a gas or oil reservoir.

Figure 1 schematically shows the section of a reservoir with oil reserves, where the upper line represents the surface of the reservoir and the volume of the reservoir identified by the lower line (Rs), a well (P) being demarcated therein.

The well (P) is a well having a circular section S extending along a path depicted by a curve. The curve is shown in Figure 1 beginning at the surface, descending in an almost vertical path, and after increasing its inclination ending in an almost horizontal segment.

The drilling parameters which allow drilling in optimal conditions are to be determined along this path.

In a first step, a geomechanical model of the reservoir incorporating at least rock data and the mechanical properties of said rock under the conditions determining a pre-specified in situ stress field is generated in a computational system.

The stresses in the rock are not determined only by the in situ stresses and the properties at each point of the domain of the geomechanical model, but they also depend on the dynamic pressure and fluid conditions of the fluids stored in the rock. These variables are established by generating a fluid flow model of the same reservoir in the computational system. A preferred way of reservoir modeling includes a geomechanical model and a fluid flow model coupled to one another where changes in the pressure field in the fluid flow model are taken into account in the geomechanical model and deformations and changes in porosity and permeability properties are taken into account in the fluid flow model.

In addition to the path, other parameters, such as the drilling diameter, are also determined.

Once the path has been determined by means of a curve contained in the domain of the geomechanical and fluid flow models, a discretization t _i = l. . N of the coordinate ( t ) along the well path is established. The coordinate t is a coordinate parametrizing the course of the well path.

The values of pore pressure p _p , vertical height Z _i , maximum stress σ _max , minimum stress σ _min and the mechanical properties of the rock in the section based on the geomechanical model are determined at each of the discretization points.

These values allow calculating the stress state at each discretization point of the path.

According to the stress state, for each discretization point the perpendicular section S _i is determined, and in said section, the region of damage under a pre-established

rock failure criterion and the value of the area A = \\R\\ of said region R are determined.

According to a preferred example, the region of damage is calculated by proposing a configuration of the damage in the section according to an elliptical shape which is determined as described below.

Figure 1 shows a path where the region of damage in a section S located at a height z is to be calculated. At this height z, the tangent n to the path coincides with the normal to the transverse plane of section where the region of damage is to be determined. The subscript "i" has been eliminated given that a specific point is considered. The path is contained in a

domain R _s demarcated in the upper portion by the surface

This same drawing depicts by means of a dashed line the plane transverse to the drilling path of the well at a pre- established point.

Figure 2 schematically shows a circumference in a thick line representing the theoretical wall of the well in the plane of section S .

Given the direction normal to the plane of section S, by means of rotation about said normal a direction where stress is minimum and a direction which is perpendicular to the preceding direction, where stress is maximum are

established. These directions are those used as the axes of reference for establishing the site where damage occurs and its extent .

After having established the axes, the stress state in the rock along the curve defined by the circumference corresponding to the wall of the well is determined based on the geomechanical model. The value of the equivalent stress is calculated based on the stress state determining, from this calculation, the arc of a curve where said equivalent stress is greater than the acceptable stress of the rock. This arc is centered in π/2 due to the way of constructing the axes of reference and the width thereof is the caving angle

Figure 2 shows both axes which are axes that will correspond to the major and minor sides of a family of ellipses. This family of ellipses is parameterized by means of the eccentricity e defined as a/b. For a value of eccentricity equal to 1, the ellipse is the circumference having a radius R corresponding to the circumference representing the wall of the well according to section S. For increasing values of eccentricity e , ellipses having one end of the major side penetrating the rock while the minor side being smaller than the radius of the well R, are obtained. Of the ellipses thus obtained, the part of the ellipse which penetrates the rock and which will be the curve defining the region of damage will be of special interest.

The points where the caving angle starts and ends are the points where the intersection between the circumference and any of the ellipses of the parameterized family in e is established.

The values of a and b in Figure 2 identify the length and width of a given ellipse. Two ellipses of eccentricity e and e ₀ are also shown.

The factor of safety is used to determine the ellipse defining the region of damage

where is the sum of external forces on the rock at a given

point of the rock, depending at least on the in situ stresses, on the density (y) of the drilling fluid, on the elastic properties of the rock and on the pore pressure p _p ; and

where is the sum of resistance forces of the rock at the

same point, depending on the stress tensor, on the strength properties of the rock and on the angle of internal friction of the rock. This factor of safety depends on the angle and on the factor of eccentricity where the value of one identifies the equilibrium between the forces and the resistance capacity. Damage is deemed to exist when this equilibrium is broken. Nevertheless, it is possible for a person skilled in the art to select values f ₀ other than but close to one as the factor of safety, for example. Valid f ₀ values are those comprised in the [0.7, 1.3] range, and more preferably in the [0.8, 1.2] range, and more preferably in the [0.9, 1.1] range and more preferably in the [0.95, 1.05] range.

Figure 3 shows a graph of stress according to the angle Θ where for values close to π/2, identified in the drawing as close to 90 given that it is expressed in degrees instead of radians, the stress acquires asymptotically high values as eccentricity increases.

This fact renders the approach according to the state of the art for the estimation of damage useless since in no case would it consider that a safe situation exists.

With this hypothesis, the area of the end of the ellipse reaches unacceptable values in almost any case which would invalidate this method of determining the region of damage. Nevertheless, it has been found that if this bias is overcome by eliminating values above the pre-specified value θ ₀ < π/2, then the method predicts with great precision the region of damage.

After having established θ ₀ < π/2, the value of eccentricity e ₀ closest to one is determined, verifying where f ₀

is the pre-established reference value close to one.

Figure 4 shows an image of the perforated wall in a well, showing the areas where damage has occurred. The letters N, E, S and W identify North, East, South and West, respectively, and correspond to a perimetral development of 360 degrees ( 2π radians) .

The image is taken a posteriori , once the well has been drilled or obtained by sensing during drilling. The dark spots are areas of damage in the wall based on which it is possible to determine the width of damage at a given vertical height z but they do not allow establishing the depth in the wall or providing the determination thereof before drilling is performed.

Figure 5 shows a graph of the function with

eccentricity e as a free parameter. It is where the function takes this value i.e., the value which determines the eccentricity e which in turn defines a single ellipse of the family of ellipses defined above.

In this embodiment, the ellipse has an eccentricity of 0.4. The right hand side of the drawing shows a quarter of a circumference, the circumference representing the section of the wall of the well, and also a quarter of the ellipse having an eccentricity of 0.4. The inner area of the ellipse having an eccentricity of 0.4 is established as the region of damage.

Given this region of damage, it is possible to repeat the method for a plurality of sections S distributed along the well path, particularly the discretization t _i = l..N along the well path. By means of interpolating the sections of damage along a segment of the well path, it is possible to determine the caving volume in that segment before drilling is performed.

With the caving volume, it is possible to establish drilling fluid injection parameters which allow discharging the caved volume. Otherwise, the method allows recalculating the caving volume by changing the pressure conditions established by the drilling fluid. For example, if the density γ of the drilling fluid increases, the pressure on the walls of the well increases and compensates for the stresses exerted by the eliminated rock on the well and giving rise to the free surface, given that they now no longer perform a structural function. The region of damage is thereby reduced given that the pressure forces against the wall exert this compensating force.

This force has a limit since an excessive increase in the density of the drilling fluid can cause a pressure that is too high to be withstood by the wall of the well, generating cracks. Therefore, this embodiment shows how the region of damage depends on the parameters used in the calculation of the stress state at the point where the plane of section S has been plotted out, and particularly on the pressure of the drilling fluid.

It is possible to simulate different conditions of damage for a plurality of densities of the drilling fluid and establish those that give rise to a caving volume less than the limit acceptable by the means installed in the well. In other words, a correspondence between the discrete values of density of

the drilling fluid and the value of the area of

damage is determined, the latter being interpreted as the detachment volume (V) of the wall of the well per unit of drilled length.

With this correspondence and for each of the sections along the discretization t _i = l..N, a combination of drilling speed (v), density (7) of the injected drilling fluid and flow rate (Q) thereof is determined, such that it establishes a material to be discharged material being cut by the bit plus the

caving material less than that determined by the drilling

system.

The volume determined by the drilling system is determined by the skilled person needing the density of the drilling fluid (7) and flow rate (Q) for determining said maximum volume of the drilling system so as to allow removing the volume of material detaching from the well without the drilling bit collapsing.

After having determined the drilling parameters, drilling is carried out according to the values of drilling speed (v), density (7) of the injected drilling fluid and flow rate (Q) thereof for each coordinate of the well path.

The drilling operation discharges the sum of two volumes of material, the material being cut by the bit plus the material being caved near the bit because the damaged area corresponds to material showing a stress greater than the allowable stress of the rock in that location and then it further collapses. The sum of the two volumes is discharged by the injected drilling fluid. An skilled person on drilling systems, according to his practice, determines the maximum volume of material to be discharged by the drilling system.

This skilled person is able to determine the volume of material to be discharged depending at least on the density and the flow rate of the drilling fluid being injected.

The invention provides the correspondence the volume

being detached when drilling, to the skilled person and therefore solves the problem determining a volume of material to be discharged being less than that determined by the drilling system. The rest of the feature is just a clarification indicating that the result of using the combination of parameters (drilling speed (v) , density (y) of the injected drilling fluid and flow rate (Q) fulfilling the specified condition allows removing the volume of material from the well without a drilling bit collapsing.

As described, it is possible to use an alternative method as a preferred way of determining the damage in a section of the well before drilling is performed: calculating the stress state by means of a geomechanical and fluid flow model numerical resolution method, for example, by means of finite elements. The region exceeding the acceptable stress is determined with this calculation for each value of the density of the drilling fluid, establishing said region as the region of damage.

In any of the particular ways of calculating the damage, according to another embodiment, the dynamic effects of the drilling fluid flow are taken into account. The dynamic effects can be interpreted in terms of pressure variations in a drilling fluid column with respect to static pressure.