CN110821481A - Method for evaluating stability of air drilling well wall - Google Patents
Method for evaluating stability of air drilling well wall Download PDFInfo
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Abstract
The invention discloses an air drilling well wall stability evaluation method, which comprises the following steps: s1, establishing a logging calculation model of the formation pore pressure and the collapse pressure, and calculating the formation pore pressure and the collapse pressure by using different methods according to the formation type; s2, determining the magnitude of the ground stress: determining the minimum horizontal principal stress magnitude and direction of the stratum and the maximum horizontal principal stress magnitude and direction according to the logging information; s3, according to the relation between the stress borne by the well wall and the rock strength, comparing the rock cohesion and the critical cohesion, and establishing a well wall stability discrimination model: the invention is beneficial to protecting the oil-gas layer, eliminating the pressure holding effect, improving the mechanical drilling speed, avoiding the occurrence of drilling fluid leakage, differential pressure stuck drilling accidents and the like, and providing decision basis for realizing safe and rapid drilling.
Description
Technical Field
The invention relates to the technical field of geological exploration, in particular to an air drilling well wall stability evaluation method.
Background
Underbalanced drilling is a drilling technique that allows formation fluid to enter a wellbore and circulate it to a surface device in a controlled manner when the effective pressure of the wellbore fluid is lower than the formation pressure, has important functions of increasing the drilling rate, protecting a reservoir and effectively developing a low-pressure low-permeability oil field, and is widely applied in the drilling industry at home and abroad. The adoption of the underbalanced drilling technology is beneficial to protecting an oil-gas layer, eliminating the 'pressure holding effect', improving the mechanical drilling speed, avoiding drilling fluid leakage, pressure difference stuck drilling accidents and the like.
However, the air drilling well is in a negative pressure difference state during drilling, is limited by a plurality of conditions, has more strict requirements on the stratum than other drilling modes, and causes difficulty to actual construction due to stratum factors unsuitable for air drilling; meanwhile, the pressure of the liquid (gas) column in the well under the negative pressure difference state has weak supporting effect on the well wall, and the well wall is collapsed due to the fact that the rock strength of the well wall is not enough to balance the stress, and complex conditions such as drill sticking and the like occur. Due to unreasonable formation evaluation, underbalanced drilling modes are often selected improperly, and serious reservoir damage and economic loss are caused. Therefore, it is of great practical significance to correctly recognize the adaptability of the underbalanced drilling stratum.
Disclosure of Invention
Aiming at the problems, the invention provides the method for evaluating the stability of the well wall of the air drilling well, which is beneficial to protecting an oil-gas layer, eliminating the 'pressure holding effect' to improve the mechanical drilling speed and avoid the occurrence of drilling fluid leakage, differential pressure stuck accidents and the like.
The invention adopts the following technical scheme:
an air drilling well wall stability evaluation method comprises the following steps:
s1, establishing a logging calculation model of formation pore pressure and collapse pressure, and calculating the formation pressure by using different methods according to the formation type;
s2, determining the magnitude of the ground stress: determining the size and the direction of the minimum horizontal main stress and the size and the direction of the maximum horizontal main stress of the stratum according to the logging information;
s3, according to the lithology of the stratum, comparing the cohesive force of the rock with the critical cohesive force, and establishing a well wall stability discrimination model:
(1) reservoir and other permeable formations, δ ═ 1, Φ ≠ 0, Pp＞0
(2) Shale, mud shale,Tight formation, δ 0, poor pore connectivity, Pp＞0
(3) Shale, tight rock, delta-0, phi-0, Pp＝0
In the formula:
c' is rock critical cohesion, MPa;
is the internal friction angle of the rock, °; sigma is positive stress, MPa; sigmaHMaximum horizontal principal stress, MPa; sigmahMinimum horizontal principal stress (MPa), η is the nonlinear correction coefficient of borehole wall stress, PpIs the formation pore pressure in MPa, phi is the porosity in decimal, α is the Biot coefficient.
Preferably, in step S2, determining the position of the stress collapse borehole and the major axis direction of the elliptical borehole by using the dual borehole diameter, the relative azimuth RB, and the AZIM curve of the borehole inclination angle in the formation dip logging; or identifying the direction of the minimum horizontal principal stress of the well wall by using FMI imaging logging information, and calculating the size of the minimum horizontal principal stress by using the logging information.
Preferably, in step S2, the FMI imaging log is used to identify the borehole wall fractures, so as to determine the direction of the maximum horizontal principal stress of the borehole wall, and the log is used to calculate the magnitude of the maximum horizontal principal stress.
Preferably, in step S1, the sandstone and carbonate formation pore pressures are calculated by modified eaton method and Bowers method, respectively.
Preferably, the calculation formula of the modified eaton method is as follows:
Pp＝σV-(σV-Pw)*(Δtn/Δt)C+0.00981*TVD*Δρm(34)
in the formula, σVOverburden pressure, MPa; pw is the hydrostatic column pressure of the formation water, MPa; ppIs the formation pore pressure, MPa; delta t is the difference value us/ft when the observation point actually measures the sound wave; Δ tnThe difference value of sound wave time on the normal compaction trend line with the same depth as the observation point is us/ft; c is the formation compaction index, typically taken as 0.914; TVD is the vertical depth of the stratum, m; Δ ρmAdding correction values to the formation pore pressure equivalent density.
The invention has the beneficial effects that:
the invention is beneficial to protecting the oil-gas layer, eliminating the pressure holding effect, improving the mechanical drilling speed, avoiding the occurrence of drilling fluid leakage, differential pressure stuck drilling accidents and the like, and providing decision basis for realizing safe and rapid drilling.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings of the embodiments will be briefly described below, and it is apparent that the drawings in the following description only relate to some embodiments of the present invention and are not limiting on the present invention.
FIG. 1 is a schematic diagram of the three pressure gradient trends of a formation of a target well according to the present invention;
FIG. 2 is a schematic diagram of the results of the present invention explaining the rock mechanics of the target well and the stability of the air drilling well wall;
FIG. 3 is a diagram (900-1900 m) illustrating the results of stratum rock mechanics and air drilling well wall stability of the target well section;
FIG. 4 is a diagram (2000-3000 m) of the interpretation result of stratum rock mechanics and air drilling well wall stability of a target well section;
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the drawings of the embodiments of the present invention. It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the described embodiments of the invention without any inventive step, are within the scope of protection of the invention.
Unless otherwise defined, technical or scientific terms used herein shall have the ordinary meaning as understood by one of ordinary skill in the art to which this disclosure belongs. The use of the word "comprising" or "comprises", and the like, in this disclosure is intended to mean that the elements or items listed before that word, include the elements or items listed after that word, and their equivalents, without excluding other elements or items. "upper", "lower", "left", "right", and the like are used merely to indicate relative positional relationships, and when the absolute position of the object being described is changed, the relative positional relationships may also be changed accordingly.
The invention is further illustrated with reference to the following figures and examples.
As shown in fig. 1 to 4, a method for evaluating the stability of an air drilling well wall comprises the following steps:
s1, establishing a logging calculation model of formation pore pressure and collapse pressure, and calculating the formation pressure by using different methods according to the formation type;
and calculating the pore pressure of the sand shale formation by using an improved Eton method, and calculating the pore pressure of the carbonate formation by using a Bowers method.
The method for calculating the formation pressure mainly comprises an equivalent depth method, an Eaton method (Eaton) and a zone velocity method, wherein the equivalent depth method and the Eaton method are more accurate in calculating the pore pressure of the sandstone-shale section stratum, but for the carbonate stratum, the mudstone is less, a normal compaction trend equation is difficult to construct, and the accuracy of calculating the pore pressure of the carbonate section stratum by the equivalent depth method and the Eaton method is poor. Therefore, the sand shale stratum adopts an improved Eton method to calculate the stratum pore pressure, and the carbonate stratum adopts a Bowers method to calculate the stratum pore pressure.
The eaton method is mainly used for the formations with abnormal high pressure mainly based on the undercompression effect, the relationships between the drilling well and the actually measured pressure values and various well logging and geological information are considered, and as shown in formula 33, the ratio of the effective stress (the ratio of the vertical effective stress of the actually measured point to the vertical effective stress of the depth point under the normal compaction condition) and the acoustic wave time difference ratio (the ratio of the acoustic wave time difference on the normal compaction trend line at the same depth point to the actually measured acoustic wave time difference) are in exponential correlation. The method is a practical method.
In the formula, σV-overburden pressure, MPa; pw-formation hydrostatic column pressure, MPa; pp-formation pore pressure, MPa; (sigma)V-PP) Effective stress PeMPa; delta t is the difference value of real measured sound waves of an observation point, us/ft; Δ tn-difference in acoustic wave time, us/ft, on the normal compaction trend line at the same depth as the observation point; c-formation compaction index, typically 0.914.
The compaction index deduced inversely by using different measured pressure point data is not a fixed value. In abnormally high pressure formations, the compaction index of the reverse thrust>0.914 and the formation compaction index increases with increasing formation pore pressure, the pore pressure calculated using the reverse compaction index varies greatly (up to 10MPa or more for adjacent meters of pore pressure variation) longitudinally of the same formation, which is not consistent with the fact that the pore pressure of the same formation varies less. Adding an additional correction quantity deltaC on the basis of the original Eton methodPAnd simultaneously, the compaction index is 0.914, so that a pore pressure calculation method suitable for the sand-shale interval of the well region is obtained, and the calculation formula of the improved Eton method is as follows:
Pp＝σV-(σV-Pw)*(Δtn/Δt)C+0.00981*TVD*Δρm(34)
in the formula, σVOverburden pressure, MPa; pw is the hydrostatic column pressure of the formation water, MPa; ppIs the formation pore pressure, MPa; (sigma)V-PP) Is effective stress PeMPa; delta t is the difference value us/ft when the observation point actually measures the sound wave; Δ tnIs the same as the observation pointThe difference in sound wave times on the normal compaction trend line of a depth, us/ft; c is the formation compaction index, typically taken as 0.914 and TVD is the formation drawdown, m.
The Bowers method first determines the vertical effective stress (σ) from the acoustic velocity and 3 empirical parameters (a, B, U)e) And then from overburden pressure (σ)V) The vertical effective stress is subtracted to obtain the pore pressure (P)p) And can be used to calculate an abnormal increase in pore pressure due to under-compaction or other mechanisms. Under abnormally high pressure conditions, the vertical effective stress of the deposit will be lower than it was in some past times and in a so-called "unloaded" condition. It is desirable to know the historical maximum effective stress value of the deposition layer and establish the unload speed and effective stress state of the deposition layer, which may be specified by the unload parameter U. The value of U is determined empirically; sigmamaxReflection from normal compaction and user-specified VmaxCalculating a value; vmaxMaximum formation sound velocity for the unloaded producing formation; dmaxvA depth corresponding to the maximum formation sound velocity, corresponding to the depth at which unloading occurs; the TVD is vertical deep.
When d ismaxvWhen TVD is not more than equal, unloading occurs, and the abnormal pore pressure is:
and the number of the first and second electrodes,
in the formula, σmax-historical maximum effective stress value of the deposit, MPa; Δ tminWith maximum layer velocity VmaxCorresponding sound wave time difference, mu s/ft; Δ tmaxWith minimum layer velocity VminCorresponding sound wave time difference, mu s/ft; a, B, U-empirical parameters, typically A is-2, B is 2.15 and U is-1.8.
As shown in tables 1 and 2, the data statistics of the ground stress and the formation tri-pressure of the target well are shown, and the average gradient of the maximum horizontal stress and the minimum horizontal stress of the target well are respectively 2.34 MPa and 1.64MPa/100 m.
TABLE 1 geostress statistics table for each formation of target well
TABLE 2 statistics table for three pressure data of each well stratum of target oil reservoir
By analyzing the target borehole pressure as shown in tables 1 and 2, the formation pressure of the target well zone can be divided into 3 pressure zones in the longitudinal direction:
normal pressure zone: group 1 is 1.030MPa/100 m; the 2 groups are 1.030MPa/100 m.
Overpressure pressure band: group 3 is 1.225; group 4 is 1.375; the 26 groups were 1.311MPa/100 m.
Overpressure pressure band: group 6 is 1.563; group 10 is 1.731; group 13 is 1.747; 20 groups are 1.747; group 24 is 1.866; set 25 is 1.851; group 29 is 1.469; group 31 was 1.528; group 32 is 1.505; group 33 is 1.530; 34 groups of 1.590MPa/100 m.
S2, determining the magnitude of the ground stress: determining the size and the direction of the minimum horizontal main stress and the size and the direction of the maximum horizontal main stress of the stratum according to the logging information;
when a vertical well is drilled in a non-uniform earth stress field, the well wall will always collapse along the direction of minimum horizontal principal stress when the drilling fluid column pressure is too low, forming an elliptical well bore. Therefore, the direction of the major axis of the elliptical borehole in the vertical well and the ground stress have a better corresponding relation, and the direction of the horizontal principal stress in situ can be indicated.
Determining the position of a stress collapse borehole and the major axis direction of an elliptical borehole by using double borehole diameters, a relative azimuth RB and a well inclination azimuth AZIM curve in formation inclination logging; or identifying the direction of the minimum horizontal main stress of the well wall by utilizing FMI imaging logging information;
and identifying the borehole wall pressure crack by using FMI imaging logging information so as to determine the direction of the maximum horizontal main stress of the borehole wall. The trend of the borehole wall pressure crack indicates the direction of the maximum horizontal main stress. When the drilling fluid column is over-pressurized, the well wall will always be fractured along the direction of the maximum horizontal principal stress, forming a fracture.
Identifying borehole wall fractures by utilizing FMI imaging logging information, wherein the response characteristics of the FMI are as follows: the fractures extend longitudinally, generally parallel to the well axis, occur in pairs, and are symmetrically distributed at 180 °. Two black strips are symmetrically distributed on an imaging graph, are parallel to a well axis, extend for a long time, have basically stable orientation, have small change of width, and do not have the erosion and expansion phenomenon of a natural crack.
S3, comparing the rock cohesion with the critical cohesion according to the relation between the stress borne by the well wall and the rock strength, and establishing a well wall stability judgment model;
according to the theory of elastic mechanics, the stress analysis of the rock around the vertical borehole can be simplified into that a round hole is provided with uniform internal pressure P on an infinite planemWhile the plane at infinity is subjected to a maximum horizontal principal stress sigmaHMinimum horizontal principal stress σhIs vertically subjected to overburden pressure σVThe function of (1).
The stress state of the well wall rock can be applied by radial stress sigma in a cylindrical surface coordinate systemrCircumferential stress σθVertical stress σzAnd shear stress τrθTo indicate.
(1) From drilling fluid column pressure PmInduced well-to-well stress distribution
From the solution of the densification (only the inner pressure, and the outer radius being much smaller than the inner radius), the value obtained at the inner pressure PmWell-to-well stress distribution induced by the action:
(2) from maximum horizontal principal stress σHInduced well-to-well stress distribution
(3) From the minimum level principal stress σhInduced well-to-well stress distribution
(4) From overburden pressure σVInduced well-to-well stress distribution
(5) Well-to-well stress distribution caused by drilling fluid seepage effect
When the fluid pressure in the well is increased or the wall building performance of the drilling fluid is poor, a part of the drilling fluid infiltrates into the stratum around the well, the stratum around the well is a pore medium, and the fluid flow in the medium meets Darcy's law, so that the additional stress field generated around the well wall by the radial seepage of the drilling fluid filtrate in the pores of the stratum is as follows:
(6) borehole wall formation pressure distribution
Under the combined action of the drilling fluid column pressure and the ground stress and considering the action of the formation pressure, the stress distribution of the formation around the well is obtained by superposing the following partial stresses:
when the well wall has infiltration, delta is 1; when the well wall is not permeable, delta is 0.
When R is R, the radial, tangential and vertical stresses on the borehole wall surface are:
in the formula: r is the borehole radius, m; r is the polar radius, m; pmThe drilling fluid column pressure is MPa; ppThe method is characterized in that the method comprises the following steps of (1) measuring the formation pore pressure in MPa, mu is Poisson's ratio, phi is porosity and decimal, delta is a permeability coefficient which is 0 when the well wall is impermeable and is 1 when the well wall is permeable, α is an effective stress coefficient, and theta is an included angle formed between a certain point on the periphery of the well and the horizontal maximum principal stress direction.
(7) Nonlinear correction of borehole wall stress
The above analysis is obtained assuming that the wall surrounding rock is a linear elastomer, and actually, the elastic modulus of the shale is related to the confining pressure, and generally, the elastic modulus E is obviously increased along with the increase of the confining pressure PC, and is in a nonlinear relationship. The density value of the drilling fluid required for keeping the well wall stable calculated by the linear elasticity theory is larger than the actual value. Therefore, in order to obtain reasonable results, the influence of the change of the elastic modulus of the surrounding rock on the stress of the well wall, namely the influence of the nonlinear characteristic of the rock on the stress, needs to be considered.
The stress calculation formula of the rock around the well under the action of the non-uniform earth stress is modified as follows:
wherein η is the nonlinear correction coefficient of the borehole wall stress, η is 0.95 generally.
From the above equation analysis, when θ is 90 ° and θ is 270 °, the stress difference σ is foundθ-σrThere is a maximum where the borehole wall is most prone to collapse and destabilize. The stress on the well wall at the position of theta 90 degrees and theta 270 degrees is as follows:
(8) formula of well wall stress of permeable and impermeable stratum
The rock shearing failure is mainly controlled by the maximum and minimum principal stresses, and the maximum principal stress is the circumferential stress sigma which can be obtained by analyzing according to a well wall stress formulaθThe minimum principal stress is the radial stress sigmarTherefore, the vertical stress σ can be disregardedZ。
① when δ is 1, the effective stress formula of the permeable borehole wall rock is:
② when δ is 0, for P ≠ 0pFor > 0, the stress formula is:
③ when δ is 0, P for Φ is 0pIn the case of 0, the stress formula is:
in gas drilling, due to the very low permeability of shale (δ is considered to be 0), under conditions of poor pore connectivity, and PpIf the stress equation of the well wall is more than 0, the stress equation of the well wall can be processed according to the formula 47; for shale with very low permeability (almost δ ═ 0) and porosity equal to zero (Φ ═ 0), the formation pore pressure P is considered to bep0, the well wall stress formula is 48; the reservoir should be treated as in equation 46.
And principal stress sigma1The shear stress and the normal stress on any oblique section forming an angle of β have the following relationship:
the above equation is a parameter equation of a Moire stress circle, on the circleEither point corresponds to a set of shear and normal stress conditions on the bevel. Obviously, when the principal stress σ is reduced1And σ3Defined stress versus ultimate shear strength envelope curve
When tangent, the rock mass is in a critical yield failure state. The shear yield surface orientation corresponding to the tangent point A
Substituting formula 50, obtaining the ultimate shear stress and the positive stress on the shear yield surface, and respectively bringing the ultimate shear stress and the positive stress into a shear strength envelope curve, namely a specific expression of the Mohr-Coulomb strength criterion:
whether the well wall is stable or not is finally shown as the stress state of the surrounding rock of the well hole is compared with the rock failure criterion. If the borehole wall stress exceeds the strength envelope, the borehole wall will fail, otherwise the borehole wall will be stable.
From the above analysis, borehole wall collapse instability occurs at 90 ° and 270 ° for θ, when σ is presentθ、σrMaximum and minimum principal stresses, respectively. Therefore, equations 46 and 47 and (equation 48) are substituted into the formula 50 for the molar-coulomb strength criterion expressed by the principal stress, and the collapse pressure BP is made equal to PmThen, a calculation model of the borehole collapse pressure BP under the above conditions can be obtained:
① reservoir and other permeable rock formations (δ ═ 1, Φ ≠ 0, Pp＞0)
② shale, tight rock (delta 0, poor pore connectivity, P)p＞0)
③ shale, tight rock (δ 0, Φ 0, P)p＝0)
In the formula:
for gas drilling, there is no liquid column pressure in the wellbore, i.e., the liquid column pressure is approximately equal to zero. In this case, the mechanical environment of the rock around the borehole is already different from that of the drilling fluid, and the stability of the borehole wall is mainly determined by the critical value of the cohesion of the formation rock. The collapse pressure calculated by the formulas 52-54 is the lowest value for ensuring the stability of the well wall, the collapse pressure of the gas drilling stratum is approximate to zero, and if the collapse pressure is zero, the balanced stratum stress is mainly the cohesion of the rock, and the cohesion of the rock is a critical value substantially.
① reservoir and other permeable rock formations (δ ═ 1, Φ ≠ 0, Pp＞0)
② shale, tight rock (delta 0, poor pore connectivity, P)p＞0)
③ shale, tight rock (δ 0, Φ 0, P)p＝0)
C' is rock critical cohesion, and the rock critical cohesion is compared with the rock cohesion C calculated by logging, so that the rock critical cohesion C can be used as a judgment criterion for borehole wall stability during shearing failure around a borehole: when C is larger than C', the well wall is stable; and when C is less than C', the well wall is unstable.
Although the present invention has been described with reference to a preferred embodiment, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (5)
1. The method for evaluating the stability of the well wall of the air drilling well is characterized by comprising the following steps of:
s1, establishing a logging calculation model of formation pore pressure and collapse pressure, and calculating the formation pressure by using different methods according to the formation type;
s2, determining the magnitude of the ground stress: determining the size and the direction of the minimum horizontal main stress and the size and the direction of the maximum horizontal main stress of the stratum according to the logging information;
s3, according to the lithology of the stratum, comparing the cohesive force of the rock with the critical cohesive force, and establishing a well wall stability discrimination model:
(1) reservoir and other permeable formations, δ ═ 1, Φ ≠ 0, Pp＞0
(2) Shale, tight rock, delta 0, poor pore connectivity, Pp＞0
(3) Shale, tight rock, delta-0, phi-0, Pp＝0
In the formula:
c' is rock critical cohesion, MPa;
is the internal friction angle of the rock, °; sigmaHMaximum horizontal principal stress, MPa; sigmahIs the minimum horizontal principal stress, MPa, η is the nonlinear correction coefficient of the borehole wall stress, PpIs the formation pore pressure in MPa, phi is the porosity in decimal, α is the Biot coefficient.
2. The method for evaluating the stability of the air drilling well wall according to the claim 1, characterized in that in the step S2, the position of the stress collapse well hole and the direction of the major axis of the elliptical well hole are determined by using the double well diameters, the relative azimuth RB and the well inclination azimuth AZIM curves in the dip angle well logging to determine the direction of the minimum horizontal principal stress of the well wall; or identifying the direction of the minimum horizontal principal stress of the well wall by using FMI imaging logging information, and calculating the size of the minimum horizontal principal stress by using the logging information.
3. The method for evaluating the stability of the borehole wall of the air drilling well according to claim 1, wherein in step S2, the FMI imaging logging data is used to identify the borehole wall pressure crack, so as to determine the direction of the maximum horizontal principal stress of the borehole wall, and the logging data is used to calculate the magnitude of the maximum horizontal principal stress.
4. The method of claim 1, wherein in step S1, the pore pressure of the sandstone formation is calculated by modified eaton method, and the pore pressure of the carbonate formation is calculated by Bowers method.
5. The method for evaluating the stability of the air drilling well wall according to claim 4, wherein the improved Eton method has the calculation formula as follows:
PP＝σV-(σV-PW)×(Δtn/Δt)C+0.00981×TVD×Δρm(34)
in the formula, σVOverburden pressure, MPa; pw is the hydrostatic column pressure of the formation water, MPa; ppIs the formation pore pressure, MPa; delta t is the difference value us/ft when the observation point actually measures the sound wave; Δ tnThe difference value of sound wave time on the normal compaction trend line with the same depth as the observation point is us/ft; c is the formation compaction index, typically taken as 0.914; TVD is vertical depth, m; Δ ρmAdding correction values to the formation pore pressure equivalent density.
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| CN113221347A (en) * | 2021-05-07 | 2021-08-06 | 中国石油大学(北京) | Well wall stability drilling optimization method, device and equipment |
| CN113449353A (en) * | 2021-05-18 | 2021-09-28 | 西南石油大学 | Horizontal well wall stability analysis method considering stratum fluid seepage effect |
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