CN114776286A - Borehole wall stability evaluation method, device and equipment and drilling fluid treatment agent optimization method - Google Patents
Borehole wall stability evaluation method, device and equipment and drilling fluid treatment agent optimization method Download PDFInfo
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Abstract
The invention provides a borehole wall stability evaluation method, a device and equipment and a drilling fluid treatment agent optimization method, wherein the borehole wall stability evaluation method comprises the following steps: s1, determining the pore pressure of the drilled stratum; s2, determining rock mechanical parameters; s3, determining the magnitude and the horizontal ground stress direction of the in-situ stress; s4, establishing a borehole wall collapse pressure calculation model, and calculating the collapse pressure equivalent density of the rock before and after soaking respectively; s5, determining the collapse pressure equivalent density increment delta rho and the difference delta rho between the density of the drilling fluid and the collapse pressure equivalent density before soakingDrill(ii) a S6, comparing the delta rho with the delta rhoDrillThe size between the two sections realizes the evaluation of well wall stability. The method improves the precision of borehole wall stability evaluation, solves the problem of borehole wall stability of a drilled shale stratum, can judge the optimization and dosage of the core treating agent in the drilling fluid through the collapse pressure equivalent density increment, and provides a new design idea for the design of the drilling fluid.
Description
Technical Field
The invention relates to the technical field of drilling of shale gas complex strata, in particular to a borehole wall stability evaluation method for drilling a shale stratum, a borehole wall stability evaluation device for drilling the shale stratum, computer equipment for realizing the borehole wall stability evaluation method, a computer readable storage medium storing a computer program and a drilling fluid treating agent optimization method.
Background
The well drilling engineering is underground engineering, relates to a complex and changeable underground situation, causes a series of underground accidents due to the instability of the well wall, has great harm to the well drilling engineering, causes the problem of well wall stability to be a problem which troubles well drilling engineering technicians, and receives wide attention from the world well drilling engineering world. Well drilling engineers have long been working on the research and exploration of well wall stabilization techniques and have made great progress. However, due to a plurality of influencing factors of the well wall stability problem and a plurality of technical problems to be solved, the obtained research results cannot fundamentally solve the well wall stability problem. Therefore, the problem of borehole wall stability needs to be studied more intensively.
The borehole wall instability mainly shows two basic types of compression shear failure and tensile failure of the surrounding rock of the borehole wall. The compressive shear failure is due to the cobalt well fluid density being too low and the stress level at the well wall exceeding the compressive strength of the rock. The stretching damage or the fracturing crack is the main reason of well leakage, and the well leakage can cause the annular pressure to be rapidly reduced and well blowout is easy to occur. For this reason, a large number of models and methods have been proposed and established by scholars at home and abroad to solve the problem of borehole wall instability, and the most common rock strength failure criteria include the Mohr-Coulomb criterion, the Drucker-Prager criterion, the homere-Brown criterion, and the Griffith criterion. However, the above classical criteria still have some defects, the vertical stress can be accurately obtained from the rock density, while the accuracy of the horizontal stress measured by a direct or indirect method is still worth discussing and researching, so each criterion can be used only under certain specific constraint conditions without universality, and the condition limit is added to the borehole wall stability evaluation method.
Some scholars also build a numerical model by combining with field engineering data, and analyze the stability of the well wall by a finite element method, thereby realizing quantitative evaluation of the stability of the well wall. For example, the invention name disclosed in 2019, 2 month and 22 days is a quantitative evaluation method for borehole wall stability based on a risk control model, and patent document with publication number CN109377101A describes a quantitative evaluation method for borehole wall stability based on a risk control model, which includes the following steps: s1, acquiring pore pressure of the drilled stratum by using the logging information and the stratum testing information; s2, obtaining rock mechanics parameters of the drilled stratum by using logging information and an indoor rock mechanics experiment; s3, acquiring the in-situ stress magnitude and the horizontal in-situ stress direction of the drilled stratum by using logging information and indoor experiments; s4, establishing a borehole wall collapse pressure calculation model based on the risk control model, including establishing a straight borehole wall collapse pressure calculation model based on the risk control model and establishing an inclined borehole wall collapse pressure calculation model based on the risk control model. The invention discloses a quantitative evaluation method suitable for the stability of a deep shale gas horizontal section well wall in 2021, 6 months and 25 days, and a patent document with the publication number of CN113033935A discloses a quantitative evaluation method suitable for the stability of the deep shale gas horizontal section well wall, which comprises the following steps: step 1, quantitatively calculating stratum collapse and fracture pressure; step 2, quantitatively representing uncertainty of stratum collapse and fracture pressure; step 3, calculating the drilling ECD of the shale gas horizontal section, and quantitatively representing the uncertainty of the drilling ECD of the shale gas horizontal section; and 4, quantitatively evaluating the stability of the well wall of the shale gas horizontal section based on the quantitative characterization of uncertainty of formation collapse and fracture pressure in the step 2 and the quantitative characterization of uncertainty of drilling ECD of the shale gas horizontal section in the step 3. Although the models overcome the problem of overhigh collapse pressure predicted by the traditional borehole wall stability quantitative evaluation method, theoretical guidance and basis can be provided for drilling of special complex-structure wells such as vertical wells, directional wells, extended reach wells and the like, the calculation is too complex, and the general applicability is not realized.
Disclosure of Invention
The present invention aims to address at least one of the above-mentioned deficiencies of the prior art. For example, one of the objectives of the present invention is to provide a borehole wall stability evaluation method and apparatus that can improve the precision of borehole wall stability evaluation and solve the problem of borehole wall stability in drilling shale formations.
In order to achieve the above object, in one aspect, the present invention provides a borehole wall stability evaluation method for drilling a shale formation, including the following steps: s1, determining the pore pressure of the drilled stratum; s2, determining rock mechanical parameters of the drilled stratum, wherein the rock mechanical parameters comprise cohesion, internal friction angle, Poisson ratio and Biot effective stress coefficient; s3, determining an in-situ stress magnitude and a horizontal ground stress direction of the drilled stratum, wherein the in-situ stress magnitude comprises a maximum horizontal ground stress and a minimum horizontal ground stress, and the horizontal ground stress direction comprises a maximum horizontal stress direction and a minimum horizontal ground stress direction; s4, establishing a borehole wall collapse pressure calculation model based on the pore pressure, the rock mechanical parameters, the in-situ stress size and the horizontal in-situ stress direction, and calculating the collapse pressure equivalent density of the rock before and after soaking respectively; s5, determiningThe collapse pressure equivalent density increment Deltap and the difference Deltarho between the density of the drilling fluid and the collapse pressure equivalent density before soakingDrill(ii) a S6, comparing the delta rho with the delta rhoDrillIf Δ ρDrillIf the value is greater than delta rho, predicting the stability of the well wall; if Δ ρDrillAnd (4) predicting borehole wall instability if the value is less than delta rho.
In an exemplary embodiment of the method for evaluating borehole wall stability of a drilled shale formation according to the present invention, in step S4, a borehole collapse pressure calculation model may be established using one of Mohr-Coulomb criterion, Drucker-Prager criterion, Hoek-Brown criterion, and Griffith criterion to calculate a collapse pressure value, thereby determining the collapse pressure equivalent density.
In an exemplary embodiment of the method for evaluating borehole wall stability of a shale formation, the expression of the borehole wall collapse pressure calculation model is as follows:
wherein the content of the first and second substances,
the collapse pressure equivalent density is expressed as:
In an exemplary embodiment of the borehole wall stability evaluation method for drilling the shale formation, the Biot effective stress coefficient alpha can be 0.4-0.6.
In an exemplary embodiment of the method for evaluating borehole wall stability of a drilled shale formation according to the present invention, in step S1, the pore pressure of the drilled formation may be obtained using the logging data and the formation testing data.
In an exemplary embodiment of the method for evaluating borehole wall stability of a drilled shale formation according to the present invention, in step S2, the rock mechanics parameters of the drilled formation may be obtained by using well logging information and indoor rock mechanics experiments.
In an exemplary embodiment of the method for evaluating borehole wall stability of a drilled shale formation, rock compressive strength under different confining pressures can be determined by using shale as it is, a molar stress circle is drawn to obtain an internal friction angle, and cohesion is calculated according to the rock compressive strength and the internal friction angle.
Another aspect of the present invention provides a computer apparatus, comprising: a processor; a memory storing a computer program which, when executed by the processor, implements the borehole wall stability evaluation method as described above.
Yet another aspect of the present invention provides a computer readable storage medium storing a computer program, which when executed by a processor, implements the borehole wall stability evaluation method as described above.
The invention further provides a borehole wall stability evaluation device for drilling a shale stratum, which comprises a first acquisition module, a second acquisition module, a third acquisition module, a collapse pressure equivalent density calculation module, a difference module and a prediction module, wherein the first acquisition module is used for acquiring the pore pressure of the drilled stratum according to logging information and stratum test information; the second acquisition module is used for acquiring rock mechanical parameters of the drilled stratum according to the logging information and an indoor rock mechanical experiment; the third acquisition module is used for acquiring the in-situ stress magnitude and the horizontal in-situ stress direction of the drilled stratum according to the logging information and an indoor experiment; the collapse pressure equivalent density calculation module is respectively connected with the first acquisition module, the second acquisition module and the third acquisition module and is used for calculating the collapse pressure equivalent density of the rock before and after soaking according to the borehole wall collapse pressure calculation model; the difference making module is connected with the collapse pressure equivalent density calculation moduleAnd is used for calculating the collapse pressure equivalent density increment delta rho and the difference value delta rho between the drilling fluid density and the collapse pressure equivalent density before rock soakingDrill(ii) a The prediction module is connected with the difference module and is used for comparing the Deltarho with the DeltarhoDrillIf Δ ρDrillIf the value is more than delta rho, outputting a prediction result of borehole wall stability, and if the value is more than delta rhoDrillAnd (4) outputting a prediction result of borehole wall instability if the value is less than delta rho.
In yet another aspect, the present invention provides a method for optimizing a drilling fluid treatment agent, the method comprising: soaking the rock sample by using different treating agents and/or treating agent combinations, and performing borehole wall stability evaluation on the rock sample soaked by using the different treating agents and/or treating agent combinations by adopting the borehole wall stability evaluation method, wherein the treating agent or treating agent combination with the best borehole wall stability evaluation result is preferably used as the drilling fluid core treating agent.
Compared with the prior art, the invention has the beneficial effects of at least one of the following contents:
(1) the method can accurately describe the rock strength failure condition only by calculating the collapse pressure equivalent density increment change trend according to the rock strength failure criterion without considering the limit of the rock self condition, and has simple calculation method, universal applicability and wide application prospect;
(2) the method is favorable for overcoming the limitations of the traditional rock strength criterion and the limitations of conditions, improving the precision of borehole wall stability evaluation, solving the problem of borehole wall stability of a drilling mud shale stratum, and providing a new research direction for borehole wall stability evaluation;
(3) the optimization and the addition of the core treating agent in the drilling fluid can be judged through the collapse pressure equivalent density increment, so that a new design idea is provided for the design of the drilling fluid.
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The above and other objects and/or features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings, in which:
fig. 1 shows a schematic flow diagram of an exemplary embodiment of the method for borehole wall stability evaluation of a drilled shale formation according to the present invention.
FIG. 2 shows a graphical representation of a Moore stress circle for a Lormaxi downhole rock sample in a region in accordance with an exemplary embodiment of the method of the present invention for borehole wall stability evaluation of a drilled shale formation.
FIG. 3 shows a triaxial stress diagram of shale as is downhole in a Longmaxi group in a region according to an exemplary embodiment of the method for evaluating borehole wall stability of a drilled shale formation of the present invention.
Detailed Description
Hereinafter, the borehole wall stability evaluation method, apparatus, device and drilling fluid treatment agent preferred method of the present invention will be described in detail with reference to exemplary embodiments.
It should be noted that "first," "second," "third," and the like are merely for convenience of description and for ease of distinction, and are not to be construed as indicating or implying relative importance. For those of ordinary skill in the art, the term "pressure" in part herein corresponds to pressure.
The most common rock strength failure criteria comprise the Mohr-Coulomb criterion, the Drucker-Prager criterion, the Hoek-Brown criterion and the Griffith criterion, which can be used for solving the problem of borehole wall instability, but each criterion can be used under a certain specific constraint condition, is not universal, and adds condition limitation to the borehole wall stability evaluation method. For example, the Mohr-Coulomb strength criterion is one of the most popular and widely applied criteria in the geotechnical engineering and rock engineering field, can reflect the influence of hydrostatic pressure three-dimensional isobaric pressure, and is simple and practical, and the parameters are simple and easy to measure.
Therefore, the method and the device utilize a rock strength failure criterion (such as a Mohr-Coulomb strength criterion) to calculate the collapse pressure before and after the rock is soaked, calculate the difference between the collapse pressure and the collapse pressure, establish a borehole wall stability analysis model and analyze the influence of the ground stress on the borehole wall stability. The particularity of the evaluation method is that the collapse pressure increment obtained under the same strength criterion can be obtained without considering the limitation of the conditions of the rock, and the obtained increment change trend can accurately describe the rock strength failure condition, so that a new research direction can be provided for the borehole wall stability evaluation. Meanwhile, the method can provide a new idea for the optimization and design of the drilling fluid core treating agent and the drilling fluid system.
In order to achieve the purpose, the invention provides a borehole wall stability evaluation method for drilling a shale formation on one hand.
In an exemplary embodiment of the invention, a method for evaluating borehole wall stability of a drilled shale formation comprises the following steps.
Step S1, according to the logging information and the formation testing information, determining the pore pressure P of the drilled formationp。
And step S2, determining rock mechanics parameters of the drilled stratum according to the logging information and the indoor rock mechanics experiment. Rock mechanics parameters include cohesion C, internal friction angle φ, Poisson's ratio v, and Biot effective stress coefficient α.
And step S3, determining the in-situ stress magnitude and the horizontal in-situ stress direction of the drilled stratum according to the logging information and indoor experiments. The in-situ stress magnitude comprises a maximum horizontal ground stress and a minimum horizontal ground stress, and the horizontal ground stress direction comprises a maximum horizontal ground stress direction and a minimum horizontal ground stress direction. The angle between the direction of maximum horizontal stress and the direction of minimum horizontal stress is 90 deg.. It should be noted that there is no strict sequence among steps S1, S2 and S3, and it is only necessary to ensure that the pore pressure, the rock mechanical parameters, the magnitude of the in-situ stress and the horizontal direction of the in-situ stress are finally completed.
Step S4, building a borehole wall collapse pressure calculation model based on pore pressure, rock mechanical parameters, in-situ stress and horizontal in-situ stress direction, and respectively calculating the collapse pressure P before rock soakingi frontAnd collapse pressure P after soakingi rear ofAnd further calculating the collapse pressure equivalent density rho before rock soakingi frontAnd the collapse pressure equivalent density p after soakingi rear of。
It should be noted that, according to the parameters obtained in steps S1 to S3, one of the Mohr-Coulomb criterion, Drucker-Prager criterion, Hoek-Brown criterion, and Griffith criterion may be selected to establish a borehole wall collapse pressure calculation model to calculate a collapse pressure value, and further determine the collapse pressure equivalent density.
For example, HoeThe k-Brown and Griffith guidelines, based on their formation type, use the Moore-Coulomb failure criterion to calculate the collapse pressure Pi。
The expression for the mole-coulomb failure criterion is as follows:
borehole wall buckling typically occurs at borehole circumferential angles θ of 90 ° and 270 ° because the effective differential stress is greatest there.
Substituting the maximum and minimum effective stresses here into the strength criterion yields:
the expression of the well wall collapse pressure calculation model obtained by sorting can be as follows:
wherein, the first and the second end of the pipe are connected with each other,
the borehole wall collapse pressure equivalent drilling fluid density expression may be:
in the formula, PiCollapse pressure, Mpa; ρ is a unit of a gradientiIn terms of collapse pressure equivalent drilling fluid density, g/cm3(ii) a Eta is a nonlinear coefficient and has no dimension; sigmaHMaximum horizontal ground stress, MPa; sigmahIs the minimum horizontal ground stress, MPa; c is cohesion, MPa; k is a friction angle related parameter and is dimensionless; alpha is Biot effective stress coefficient, and is dimensionless; pp is the formation pore pressure, MPa; h is well depth m; phi is a friction angle, and phi is a friction angle,and (4) degree.
Wherein the effective stress coefficient alpha of Biot is 0.4-0.6.
Step S5, calculating collapse pressure equivalent density rho before rock soakingi frontAnd collapse pressure equivalent density p after soakingi rear ofObtaining a collapse pressure equivalent density increase Δ ρ ═ ρi front-ρi rear partAnd obtaining a model for evaluating the stability of the well wall, and realizing the quantitative evaluation of the stability of the rock well wall. Meanwhile, calculating the difference between the density of the drilling fluid and the equivalent density of the collapse pressure before soaking to obtain an equivalent density threshold value delta rhoDrill。
Step S6, comparing the Δ ρ and the Δ ρDrillIf Δ ρDrillIf the value is more than delta rho, the borehole wall stability is predicted; if Δ ρDrillIf the value is less than delta rho, the borehole wall instability is predicted.
When the density of the liquid used for soaking is a definite value, namely the density of the drilling fluid if the density is at the oilfield site, the density is rhoDrillThen at this time the drilling fluid density is compared to the collapse pressure equivalent density p before soakingi frontDifference between the two is represented by Δ ρDrillIs expressed as Δ ρDrill＝ρDrill-ρi frontThen comparing Δ ρ with Δ ρDrillThe magnitude between the two when Δ ρDrillWhen the value is larger, the borehole wall is in a stable state, and when the value is larger, the borehole wall is more stable, and when the value is larger, the borehole wall is in a stable stateDrillWhen the absolute value of the difference value between the two is larger, the borehole wall collapse phenomenon is more serious because the density rho of the drilling fluid is largerDrillIs determined, the density of the drilling fluid used in the field is different, then rhoDrillThe values are different, i.e., the size of the equivalent density threshold varies from block to block, depending on the drilling fluid density used.
The criterion for determining the stability of the well wall is that when the value of the delta rho is equal to the value of the standardDrillWhen the value is greater than Delta rho, the well wall is in a stable state, when the value is larger, the well wall is more stable, and when the value is larger, the well wall is in a stable stateDrillWhen the absolute value of the difference value between the two is larger, the borehole wall collapse phenomenon is more serious, and the collapse phenomenon is easy to occur.
In the present embodiment, in step S4, the Mohr-Coulomb criterion is used to calculate the rock collapse pressure, which involves two parameters, i.e., cohesion and internal friction angle. The internal friction angle is obtained by measuring the compressive strength of the rock under different confining pressures by using the original rock sample, drawing a molar stress circle, then referring to the obtained internal friction angle, assuming that the internal friction angles of all rock cores are the same value, and obtaining the cohesive force by using the relationship between the compressive strength and the friction angle according to the compressive strength value obtained by the experiment, so as to obtain two parameters by using the method.
In this embodiment, the Mohr-Coulomb criterion, the Drucker-Prager criterion, the Hoek-Brown criterion, and the Griffith criterion are as follows.
(1) Drucker-Prager intensity criteria
J2＝H1+H2J1 (5)
In the formula, J1Is the first constant tensor of stress, J2Is a second constant tensor of stress, H1Is a first rock parameter, H2Is the second rock parameter.
J1、J2Expressed by principal stress:
H1and H2The method can be calculated according to the cohesion and the internal friction angle of the rock, and the expressions are respectively as follows:
wherein C is the cohesion of rock, Mpa;
is the internal friction angle, degree, of the rock.
(2) The Hoek-Brown intensity criterion is as follows:
in formula (II), sigma'1Is the maximum effective principal stress, MPa, when the rock is damaged; sigma'2Is the middle effective main stress in the process of rock block damage, MPa; sigma'3Is the minimum effective principal stress, MPa, when the rock is damaged; sigmaciThe uniaxial compressive strength of the rock mass is MPa; m isbS and a are coefficients reflecting the mechanical properties of the rock mass, and the values of the coefficients depend on the rock strength, the development degree of the rock mass structural plane, the geometrical form, the fluid properties, the filler properties and the like.
(3) Griffith intensity criterion
Maximum tangential stress sigma when the fracture tipbWhen the value reaches a certain critical value, the fracture begins to expand. This critical value is precisely the tensile strength σ of the materialt. Because the local tensile strength of the material around the crack and the axial ratio m of the ellipse of the crack are difficult to measure, a uniaxial tensile test is carried out in the direction vertical to the long axis of the ellipse of the crack to obtain the tensile strength sigmatAs a critical value for tangential stress.
When the material is uniaxially stretched to failure, σy＝σt，τxyWhen the value is equal to 0, then
(4) Griffith Strength criterion
Wherein the content of the first and second substances,
wherein m is b/a; a is a long semi-axis of a fracture ellipse; b is a minor semi-axis of the fracture ellipse; beta is a betax1Is the x-axis and the maximum principal stress sigma1And (4) forming an included angle in the direction.
The fracture initiation pressure calculation method according to the present invention may be programmed as a computer program and corresponding program code or instructions may be stored in a computer readable storage medium, which when executed by a processor causes the processor to perform the above-mentioned borehole wall stability evaluation method, the processor and memory may be comprised in a computer device.
The exemplary embodiments according to still another aspect of the present invention also provide a computer-readable storage medium storing a computer program. The computer readable storage medium stores a computer program that, when executed by a processor, causes the processor to perform a borehole wall stability evaluation method according to the present invention. The computer readable recording medium is any data storage device that can store data read by a computer system. Examples of the computer-readable recording medium include: read-only memory, random access memory, read-only optical disks, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the internet via wired or wireless transmission paths).
Exemplary embodiments according to still another aspect of the present invention also provide a computer apparatus. The computer device includes a processor and a memory. The memory is for storing a computer program. The computer program is executed by the processor such that the processor executes the borehole wall stability evaluation method according to the present invention.
The invention further provides a borehole wall stability evaluation device for drilling a shale stratum.
In another exemplary embodiment of the invention, the device for evaluating the borehole wall stability of the drilled shale formation comprises a first acquisition module, a second acquisition module, a third acquisition module, a collapse pressure equivalent density calculation module, a difference module and a prediction module.
The first acquisition module is used for acquiring the pore pressure of the drilled stratum according to the logging information and the stratum test information.
And the second acquisition module is used for acquiring rock mechanics parameters of the drilled stratum according to the logging information and the indoor rock mechanics experiment.
And the third acquisition module is used for acquiring the in-situ stress magnitude and the horizontal in-situ stress direction of the drilled stratum according to the logging information and the indoor experiment.
And the collapse pressure equivalent density calculation module is respectively connected with the first acquisition module, the second acquisition module and the third acquisition module and is used for calculating the collapse pressure equivalent density of the rock before and after soaking according to the borehole wall collapse pressure calculation model.
The difference making module is connected with the collapse pressure equivalent density calculating module and is used for calculating the collapse pressure equivalent density increment delta rho and the difference value delta rho between the drilling fluid density and the collapse pressure equivalent density before rock soakingDrill。
The prediction module is connected with the difference module and is used for comparing the Deltarho with the DeltarhoDrillIf Δ ρDrillIf the value is more than delta rho, outputting a prediction result of borehole wall stability, and if the value is more than delta rhoDrillAnd (4) outputting a prediction result of borehole wall instability if the value is less than delta rho.
In still another aspect, the invention provides a method for optimizing the drilling fluid treatment agent.
In yet another exemplary embodiment of the present invention, a drilling fluid treatment agent preferred method comprises: soaking the rock sample by using different treating agents and/or treating agent combinations, and performing borehole wall stability evaluation on the rock sample soaked by using the different treating agents and/or treating agent combinations by adopting the borehole wall stability evaluation method, wherein the treating agent or treating agent combination with the best borehole wall stability evaluation result is preferably used as a core treating agent in the drilling fluid.
For a better understanding of the invention, the following further illustrates the invention in connection with the drawings and examples, but the invention is not limited to the following examples.
Example 1
Taking a formation 2500m away from the shale of the Longmaxi group in a certain area as an example, as shown in fig. 1, the borehole wall stability evaluation method for drilling the shale formation is realized through the following steps.
Step one, acquiring pore pressure P of a drilled stratum by utilizing logging information and stratum testing informationpIs 25 MPa.
And step two, obtaining rock mechanical parameters of the drilled stratum by utilizing logging information and an indoor rock mechanical experiment, wherein the rock mechanical parameters comprise that the cohesion C is 15MPa, the internal friction angle phi is 30 degrees, the Poisson ratio v is 0.199, and the Biot coefficient alpha is 0.6.
The rock collapse pressure calculation involves two parameters of cohesion and internal friction angle, the rock compressive strength under different confining pressures is measured by using the original model, the internal friction angle is obtained by drawing a molar stress circle, then the obtained internal friction angle is referred, the internal friction angles of all rock cores are assumed to be the same value, the cohesion is obtained by using the relationship between the compressive strength and the friction angle according to the compressive strength value obtained by the experiment, and the two parameters are obtained by the method.
For example, the rock sample under the Longmaxi group in a certain area is tested under the condition of the confining pressure of 25MPa, the molar stress circle is drawn, and the common tangent line is drawn. FIG. 2 is a schematic diagram of a Moore stress circle of a Longmaxi underground rock sample in a certain area, wherein the abscissa represents sigma and the unit is MPa, and the ordinate represents tau and the unit is MPa. As shown in FIG. 2, the internal friction angles of the original samples of the shale in the Longmaxi group in a certain area are respectively 30.25 degrees, and for the convenience of calculation and comparative analysis, the internal friction angles of all cores are assumed to be 30 degrees, and the well depth is assumed to be 2500 m.
And step three, acquiring the in-situ stress magnitude and the horizontal ground stress direction of the drilled stratum by utilizing logging information and indoor experiments, wherein the horizontal ground stress direction comprises a maximum horizontal stress direction which is 0 degree.
For example, by performing triaxial stress experimental analysis on a Longmaxi group downhole rock sample in a certain region, the results of triaxial stress of the downhole shale as it is are shown in FIG. 3. The abscissa in fig. 3 represents strain, dimensionless, and the ordinate represents stress in units of MPa, the curve pointed to by symbol 1 represents the triaxial stress test result of sample No. 1, and the curve pointed to by symbol 2 represents the triaxial stress test result of sample No. 2. As can be seen from FIG. 3, the Longmaxi group underground shale in a certain area also shows higher compressive strength as it is, and the compressive strength of the Longmaxi group underground shale reaches 146MPa at most. After the triaxial stress experiment, a plurality of obvious cracks appear on the surface of the rock core, and the rock core has a more obvious stripping trend.
And step four, establishing a calculation formula of the borehole wall collapse pressure of the shale according to the basic parameters given in the step one to the step three. And calculating the collapse pressure of the rock before and after soaking through a borehole wall collapse pressure calculation formula, and further calculating the collapse pressure equivalent density of the rock before and after soaking.
The calculation formula of the borehole wall collapse pressure is as follows:
wherein the content of the first and second substances,
further, the borehole wall collapse pressure equivalent drilling fluid density expression may be:
in the formula, PiTo collapse pressure, Mpa；ρiIn terms of collapse pressure equivalent drilling fluid density, g/cm3(ii) a Eta is a nonlinear coefficient which can be generally 0.95 and is dimensionless; sigmaHMaximum horizontal ground stress, MPa; sigmahIs the minimum horizontal ground stress, MPa; c is cohesion, MPa; k is a friction angle related parameter and is dimensionless; alpha is an effective stress coefficient of Biot, can be 0.4-0.6 for the shale, and is dimensionless; p ispIs the formation pore pressure, MPa; h is well depth m; phi is the rubbing angle, degree.
The calculation result of the equivalent density of the shale collapse pressure of the Longmaxi group in a certain area is obtained through the calculation of the calculation formula of the borehole wall collapse pressure and is shown in the table 1.
TABLE 1 calculation of collapse pressure equivalent density of Lomaxi shale as-received downhole in a region
Step five, calculating equivalent density increment delta rho (rho) of borehole wall collapse pressurei front-ρi rear partAnd realizing quantitative evaluation of well wall stability. Meanwhile, calculating the density of the drilling fluid and the collapse pressure equivalent density rho before soakingi frontDifference value Δ ρ between themDrill＝ρDrill-ρi front。
Step six, comparing the delta rho with the delta rhoDrillThe magnitude between the two when Δ ρDrillWhen the value is larger, the borehole wall is in a stable state, and when the value is larger, the borehole wall is more stable, and when the value is larger, the borehole wall is in a stable stateDrillWhen the value is less than delta rho, the well wall is in a destabilization state. When the absolute value of the difference between the two is larger, the borehole wall collapse phenomenon is more serious.
The criterion for determining the stability of the well wall is that when the value of the delta rho is equal to the value of the standardDrillWhen the value is larger, the borehole wall is in a stable state, and when the value is larger, the borehole wall is more stable, and when the value is larger, the borehole wall is in a stable stateDrillWhen the absolute value of the difference value between the two is larger, the borehole wall collapse phenomenon is more serious, and the collapse phenomenon is easy to occur.
In conclusion, the method overcomes the limitation that the traditional rock strength criterion is conditioned, improves the precision of well wall stability evaluation, and can provide theoretical guidance and basis for drilling wells with special complex structures such as vertical wells, directional wells, extended reach wells and the like.
Example 2
The example provides a drilling fluid treatment agent optimization method for a Longmaxi shale well in a certain area, and the specific implementation mode is as follows: soaking the underground rock sample by adopting different treating agents and treating agent combinations, calculating the collapse pressure equivalent density of the underground rock sample before and after soaking by using the borehole wall collapse pressure calculation formula, determining the influence of the treating agent on the collapse pressure equivalent density of the shale outcrop core, and finally preferably selecting the treating agent with the minimum influence on the collapse pressure of the shale outcrop core. And finally, calculating the collapse pressure of the shale underground rock sample in the Longmaxi group in a certain area, wherein the result is shown in a table 2.
TABLE 2 calculation of equivalent density increase for collapse pressure of downhole rock sample
As can be seen from Table 2, the effect of different treatments and combinations of treatments on the collapse pressure equivalent density increase of rock samples in Longmaxi wells in a certain area was different, and the collapse pressure equivalent density increase of rock samples soaked in white oil was the smallest, which was 0.18g/cm3Secondly, the diesel soaked rock sample collapse pressure equivalent density increment value is 0.29g/cm3The collapse pressure equivalent density increment of the rock sample soaked by 3 percent KCl is the largest, and the value is 0.94g/cm3The collapse pressure equivalent density increment of the rock sample soaked by 7 percent UHIB is 0.46g/cm3The collapse pressure equivalent density increment of the rock sample soaked by 7 percent LAT is 0.54g/cm3The collapse pressure equivalent density increment of the rock sample soaked by the 3 percent UHIB and the 3 percent LAT is 0.38g/cm3. The value of the density is close to the value of 0.29g/cm of the increment of the collapse pressure equivalent density after the diesel oil is soaked3。
In summary, the 3% UHIB + 3% LAT has smaller collapse pressure equivalent density increment than other treating agents, which indicates that the rock sample soaked by the 3% UHIB + 3% LAT has stronger borehole wall stabilizing capability. Furthermore, the collapse pressure equivalent density increase of KCl, UHIB and LAT soaked rock samples is positively correlated to concentration.
Therefore, the optimization and dosage of the core treating agent in the drilling fluid can be judged through the collapse pressure equivalent density increment, and a new design concept is provided for the design of the drilling fluid.
Although the present invention has been described above in connection with the exemplary embodiments and the accompanying drawings, it will be apparent to those of ordinary skill in the art that various modifications may be made to the above-described embodiments without departing from the spirit and scope of the claims.
Claims (11)
1. The borehole wall stability evaluation method for drilling the shale stratum is characterized by comprising the following steps of:
s1, determining the pore pressure of the drilled stratum;
s2, determining rock mechanical parameters of the drilled stratum, wherein the rock mechanical parameters comprise cohesion, internal friction angle, Poisson' S ratio and Biot effective stress coefficient;
s3, determining an in-situ stress magnitude and a horizontal ground stress direction of the drilled stratum, wherein the in-situ stress magnitude comprises a maximum horizontal ground stress and a minimum horizontal ground stress, and the horizontal ground stress direction comprises a maximum horizontal stress direction and a minimum horizontal ground stress direction;
s4, establishing a borehole wall collapse pressure calculation model based on the pore pressure, the rock mechanical parameters, the in-situ stress size and the horizontal in-situ stress direction, and calculating the collapse pressure equivalent density of the rock before and after soaking respectively;
s5, determining collapse pressure equivalent density increment delta rho and the difference delta rho between the density of the drilling fluid and the collapse pressure equivalent density before soakingDrill；
S6, comparing the delta rho with the delta rhoDrillIf Δ ρDrillIf the value is greater than delta rho, predicting the stability of the well wall; if Δ ρDrillAnd (4) predicting borehole wall instability if the value is less than delta rho.
2. The method of claim 1, wherein in step S4, a borehole wall collapse pressure calculation model is established according to one of Mohr-Coulomb criterion, Drucker-Prager criterion, Hoek-Brown criterion and Griffith criterion, so as to calculate the collapse pressure value and further determine the collapse pressure equivalent density.
3. The method for evaluating borehole wall stability of a drilled shale formation according to claim 2, wherein the expression of the borehole wall collapse pressure calculation model is as follows:
the collapse pressure equivalent density is expressed as:
in the formula, PiCollapse pressure, Mpa; rhoiG/cm for collapse pressure equivalent drilling fluid density3(ii) a Eta is a nonlinear coefficient and has no dimension; sigmaHMaximum horizontal ground stress, MPa; sigmahMinimum horizontal ground stress, MPa; c is cohesion, MPa; k is a friction angle related parameter and is dimensionless; alpha is the Biot effective stress coefficient and is dimensionless; ppIs the formation pore pressure, MPa; h is well depth m; phi is the rubbing angle, degree.
4. The borehole wall stability evaluation method for drilling the shale formation according to claim 3, wherein the Biot effective stress coefficient alpha is 0.4-0.6.
5. The method of claim 1, wherein in step S1, the pore pressure of the drilled formation is obtained using the logging data and the formation testing data.
6. The method for evaluating borehole wall stability of a drilled shale formation according to claim 1, wherein in step S2, the rock mechanics parameters of the drilled formation are obtained by using logging information and indoor rock mechanics experiments.
7. The method of claim 6, wherein the compressive rock strengths at different confining pressures are determined by using the original shale, the internal friction angle is determined by drawing a molar stress circle, and the cohesion is calculated according to the compressive rock strengths and the internal friction angle.
8. A computer device, characterized in that the computer device comprises:
a processor;
a memory storing a computer program which, when executed by the processor, implements the borehole wall stability evaluation method according to any one of claims 1 to 7.
9. A computer-readable storage medium storing a computer program, wherein the computer program, when executed by a processor, implements the borehole wall stability evaluation method according to any one of claims 1 to 7.
10. The device for evaluating the borehole wall stability of the drilled shale stratum is characterized by comprising a first acquisition module, a second acquisition module, a third acquisition module, a collapse pressure equivalent density calculation module, a difference module and a prediction module, wherein,
the first acquisition module is used for acquiring the pore pressure of the drilled stratum according to the logging information and the stratum test information;
the second acquisition module is used for acquiring rock mechanical parameters of the drilled stratum according to the logging information and an indoor rock mechanical experiment;
the third acquisition module is used for acquiring the in-situ stress magnitude and the horizontal in-situ stress direction of the drilled stratum according to the logging information and an indoor experiment;
the collapse pressure equivalent density calculation module is respectively connected with the first acquisition module, the second acquisition module and the third acquisition module and is used for calculating the collapse pressure equivalent density of the rock before and after soaking according to the borehole wall collapse pressure calculation model;
the difference making module is connected with the collapse pressure equivalent density calculating module and is used for calculating the collapse pressure equivalent density increment delta rho and the difference value delta rho between the density of the drilling fluid and the collapse pressure equivalent density before rock soakingDrill；
The prediction module is connected with the difference module and is used for comparing the Deltarho with the DeltarhoDrillIf Δ ρDrillIf the value is more than delta rho, outputting a prediction result of borehole wall stability, and if the value is more than delta rhoDrillIf the value is less than delta rho, outputting a prediction result of borehole wall instability.
11. A preferred method of drilling fluid treatment, the preferred method comprising: soaking a rock sample by using different treating agents and/or treating agent combinations, and performing borehole wall stability evaluation on the rock sample soaked by using the borehole wall stability evaluation method according to any one of claims 1-7, and preferably selecting the treating agent or treating agent combination with the best borehole wall stability evaluation result as the drilling fluid core treating agent.
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