Fault stability evaluation method considering pressure-stress coupling effect

Abstract

The invention relates to a fault stability evaluation method considering a pressure-stress coupling effect. The method comprises the following steps: calculating a pressure-stress coupling ratio according to target layer rock mechanical parameters or field actual measurement pressure and ground stress data; fault three-dimensional structure modeling is carried out based on the seismic interpretation result, and fault occurrence distribution is simulated and calculated; analyzing and calculating the initial stress of the section and the stress after the coupling action by combining the three-way principal stress, wherein the stress comprises shear stress and effective normal stress; a quadratic equation of unary about fault instability critical pressure is constructed, and fault instability risk types are judged in combination with the actual pressure limit of oil field production; and calculating the critical pressure of fault instability under different risk types. According to the method, the influence of the pressure-stress coupling effect on the fault stability can be considered, so that the fault stability evaluation result is more practical.

Description

Technical Field

[0001] The present invention relates to the technical field of geological exploration and development of oil and gas reservoirs, and specifically relates to a method for evaluating fault stability considering pressure-stress coupling. Background Art

[0002] Since fault instability has a destructive effect on traps and oil and gas reservoirs, it has become a focus issue in the fields of oil and gas exploration and production and gas storage caverns. Fault instability will exhibit characteristics of high strain and high permeability. On the one hand, it will lead to wellbore instability and shear failure of the casing, and on the other hand, it will also cause oil and gas to cross strata along the fault and even escape to the surface or seabed. Exploration practices have shown that both fluid injection and production can induce fault instability, but the induction mechanisms are different. The mechanism of fluid injection inducing fault instability is mainly that the high-pressure fluid entering the fracture zone reduces the effective normal stress on the fault, thereby reducing the maximum frictional resistance and causing fault instability. The mechanism of fluid production inducing fault instability mainly involves pressure-stress coupling. Under the normal fault stress mechanism, the reduction of formation pressure will increase the differential stress, thereby inducing fault instability.

[0003] A large number of on-site measured data have revealed the existence of the pressure-stress coupling phenomenon, which can not only increase the critical pressure of injection-type fault instability, but also be the main mechanism of production-type fault instability. At present, the evaluation methods for fault stability can be divided into two categories: analytical solution methods and numerical simulation methods. Among them, the analytical solution methods do not consider the pressure-stress coupling effect, and there are problems of low accuracy not only in evaluating the instability risk of injection-type faults, but also in being unable to evaluate the instability risk of production-type faults; although the numerical simulation methods can consider the pressure-stress coupling effect, they usually can only determine the critical pressure of instability at a certain position of the fault through the trial-and-error method, and it is still impossible to accurately determine the critical pressure of the entire fault plane. In view of this, the present invention proposes a method for evaluating fault stability considering pressure-stress coupling based on the pressure-stress coupling principle and the fault instability mechanism. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for evaluating fault stability considering pressure-stress coupling, which is used to solve the problem that the prior art cannot accurately quantify the critical pressure of instability at different positions of a three-dimensional fault plane under the pressure-stress coupling effect.

[0005] The technical solution adopted by the present invention to solve its technical problems: A method for evaluating fault stability considering pressure-stress coupling, the method comprising the following steps:

[0006] Step 1: Calculate the pressure-stress coupling ratio according to rock mechanics parameters or measured pressure and in-situ stress data, and determine the relationship between the in-situ stress and the formation pressure change;

[0007] Step 2: Based on the seismic interpretation results, conduct 3D fault structure modeling, and simulate and calculate the fault occurrence distribution through geological software, including strike and dip;

[0008] Step 3: Based on the section occurrence distribution, combine the magnitudes and orientations of the three principal stresses to conduct initial stress calculation on the section, including shear stress and effective normal stress, and establish its expression after coupling;

[0009] Step 4: Based on the Coulomb failure criterion, construct a quadratic equation about the critical pressure of fault instability, and combine the upper and lower limits of the actual formation pressure in oilfield production to judge the type of fault instability risk;

[0010] Step 5: Use the coupling ratio, three principal stresses, fault occurrence, and the analysis results of the initial stress on the section to calculate the critical pressure of fault instability under different risk types.

[0011] In the above solution, the calculation method of the pressure-stress coupling ratio in Step 1:

[0012] The calculation method through rock mechanics parameters is: K = α(1 - 2ν) / (1 - ν); the calculation method through measured formation pressure and in-situ stress data is: K = Δσ / ΔP.

[0013] In the formula: K is the pressure-stress coupling ratio, dimensionless; α is the Biot coefficient, dimensionless; ν is the Poisson's ratio of the rock, dimensionless; ΔP is the change in formation pressure, a positive value indicates an increase in formation pressure, a negative value indicates a decrease in formation pressure, MPa; Δσ is the stress change, MPa.

[0014] In the above solution, the calculation method of the magnitudes of the three effective principal stresses considering the coupling effect in Step 1:

[0015] σ' v = σ v - ΔP, σ' H = σ H +(K - 1)ΔP, σ' h = σ h +(K - 1)ΔP

[0016] In the formula: σ v 、σ H and σ h are the vertical effective principal stress, the maximum horizontal effective principal stress, and the minimum horizontal effective principal stress under the initial formation pressure condition, respectively, MPa; σ’ v 、σ’ H and σ’ h are the vertical effective principal stress (not affected by the coupling effect), the maximum horizontal effective principal stress, and the minimum horizontal effective principal stress after considering the coupling effect, respectively, MPa; ΔP is the change in formation pressure, MPa; K is the pressure-stress coupling ratio, dimensionless.

[0017] In the above solution, the calculation method of the initial effective normal stress and shear stress of the cross-section in step three:

[0018] S no = σ v l + σ H m + σ h n, σ no = S no - P p ,

[0019] In the formula: S no and σ no are the normal stress and effective normal stress on the cross-section under the initial formation pressure condition, in MPa; τ o is the initial shear stress, in MPa; P p is the formation pressure, in MPa; σ v , σ H and σ h are the vertical principal stress, maximum horizontal principal stress and minimum horizontal principal stress under the initial formation pressure condition, in MPa; l, m and n are the squares of the direction cosines corresponding to the three principal stresses, and are calculated by the following formula:

[0020] l = cos 2 θ dip , m = sin 2 (θ strike - θ Hmax )sin 2 θ dip , n = cos 2 (θ strike - θ Hmax )sin 2 θ dip

[0021] In the formula: θ Hmax is the direction of the maximum horizontal principal stress, in °; θ strike and θ dip are the strike and dip angle of the fault, in °.

[0022] In the above solution, the calculation method of the effective normal stress and shear stress of the cross-section after coupling in step three:

[0023] σ n = σ no + ΔP(K - Kl - 1),

[0024] In the formula: σ n and τ are the effective normal stress and shear stress after the pressure-stress coupling effect, in MPa; other symbols are the same as above.

[0025] Specific method for step four in the above solution: First, substitute the expressions of the effective normal stress and shear stress of the coupled cross-section into the Coulomb failure criterion to construct a quadratic equation in one variable with the formation pressure change ΔP as the variable:

[0026] Coulomb failure criterion:

[0027] τ = C + μσ n

[0028] In the formula: C is the cohesion of the fault, in MPa; μ is the friction coefficient of the fault, dimensionless; other symbols are the same as above.

[0029] The quadratic equation in one variable constructed with the formation pressure change ΔP as the variable:

[0030] aΔP + bΔP + c = 0, where

[0031] a = lK 2 (1 - l) - μ 2 (K(1 - l) - 1) 2 ,

[0032] b = 2lK(σ no -σ v ) - 2μ(C + μσ no )(K(1 - l) - 1),

[0033]

[0034] In the formula: a, b, and c are the quadratic coefficient, linear coefficient, and constant term of the above quadratic equation in one variable respectively; other formula symbols are the same as above.

[0035] Secondly, judge the type of fault risk. The specific method is as follows:

[0036] A. When the above quadratic equation in one variable has no roots (b 2 -4ac < 0), the fault has no instability risk.

[0037] B. When the above quadratic equation in one variable has roots (b 2 -4ac ≥ 0), the two roots are represented by ΔP1 and ΔP2 (which can be calculated according to the quadratic formula), and ΔP1 ≤ ΔP2. At the same time, the minimum and maximum formation pressures allowed by reservoir production activities are represented by P p-min and P p-max respectively. Then the following two situations can be distinguished:

[0038] When ΔP1 > 0 and ΔP2 > 0: ΔP1 ≥ (P p-max -P p ) indicates that the fault has an injection risk, otherwise the fault has no instability risk;

[0039] When ΔP1 < 0 and ΔP2 > 0: |ΔP1| ≥ (P p -P p-min ) indicates that there is a production risk in the fault, otherwise there is no risk of instability in the fault.

[0040] The specific method of Step 5 in the above solution: According to the change law of the stress Mohr circle in the shear stress-normal stress coordinate system corresponding to the formation pressure change and the situation of the roots of the above equation, it can be analyzed that when there is an injection risk in the fault, the critical pressure for fault instability is ΔP1; when there is a production risk in the fault, the critical pressure for fault instability is |ΔP1|. The root formula for the above quadratic equation of one variable is:

[0041]

[0042] Where: the relevant symbols are the same as above.

[0043] The present invention has the following beneficial effects:

[0044] 1. A large number of on-site measured data reveal the existence of the pressure-stress coupling phenomenon, which will affect the stability of the fault. Existing analytical solutions for fault stability evaluation do not consider the pressure-stress coupling effect, not only with low evaluation accuracy, but also unable to analyze the influence of fluid production on fault stability. The present invention effectively overcomes the above defects, and creatively integrates the coupling effect into the fault stability evaluation method based on the coupling ratio parameter, not only improving the accuracy of fault stability evaluation, but also realizing the evaluation of the instability risk of production-type faults.

[0045] 2. Existing numerical simulation methods for fault stability evaluation can consider the pressure-stress coupling effect, but usually can only determine the critical pressure for instability at a certain position of the fault through the trial-and-error method for pressure, not only unable to accurately determine the critical pressure of the entire fault plane, but also with extremely low efficiency. Combining the coupling ratio obtained by the numerical simulation method with the present invention can effectively solve the above problems, thereby quickly and accurately determining the critical pressure for instability at different positions of the fault, which has important theoretical guiding significance for the safe and efficient development of fault-block oil and gas reservoirs and the safe operation of gas storage reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 is the step block diagram of the present invention;

[0047] Figure 2 is the three-dimensional fault plane attitude distribution of the embodiment;

[0048] Figure 3 is the in-situ stress profile of the embodiment;

[0049] Figure 4 is the calculation result of the initial effective normal stress and shear stress of the fault plane in the embodiment;

[0050] Figure 5 is the judgment result of the fault instability risk type of the embodiment;

[0051] Figure 6 is the calculation result of the critical pressure of fault instability in the embodiment;

[0052] Figure 7 is the calculation result using the traditional fault stability evaluation method (without considering the coupling effect). Detailed implementation manners

[0053] The following will combine the accompanying drawings to elaborate in detail on the implementation manners of the present invention, so as to clearly and completely expound the technical solution of the present invention. The embodiments described are only partial embodiments of the present invention and do not constitute a limitation on the protection scope of the present invention. For those skilled in the art, various deformations or alternative solutions made without creative efforts are all covered within the protection scope of the present invention.

[0054] This application uses the technical solution of the present invention to judge the instability risk type of the faults in Penglai M Oilfield and evaluate the critical pressure of fault instability under different risk types. The specific process is as follows:

[0055] The embodiment is "Fault Stability Evaluation for Safe Development of Shallow Gas in Penglai M Oilfield". The research object involved is located in the northeast of the Bonan Low Uplift Structural Belt in the Bohai Bay Basin. The shallow gas reservoirs in the study area are distributed at the top of a faulted anticline controlled by two nearly north-south strike-slip faults, with a depth range of 500 - 1200 m. Multiple secondary nearly northeast-trending normal faults are developed within the gas reservoir range. The present invention takes Penglai M Oilfield as an example to carry out research on fault stability considering the pressure-stress coupling effect, in order to clarify the fault instability risk type and its critical pressure during the development of shallow gas, and compare with the conventional fault stability evaluation results (without considering the pressure-stress coupling effect) to further illustrate the innovation and applicability of the present invention. The target faults to be evaluated include 2 strike-slip faults and 7 normal faults.

[0056] Basic conditions of the embodiment: The study area has good 3D seismic interpretation data, in-situ stress, rock mechanics parameters and other relevant data, providing good basic data for the application of the present invention.

[0057] Implementation process: According to Figure 1 the step block diagram for:

[0058] (1) Triaxial effective principal stress calculation method considering the pressure-stress coupling effect:

[0059] ① The pressure-stress coupling ratio can be calculated on the one hand through the Poisson's ratio and Biot coefficient obtained from well logging interpretation or rock mechanics experiments, and on the other hand through the measured values of formation pressure and in-situ stress at different times. In this embodiment, the former method will be used, and the calculation method is as follows:

[0060]

[0061] In the formula: K is the pressure-stress coupling ratio, dimensionless; α is the Biot coefficient, dimensionless; ν is the Poisson's ratio of the rock, dimensionless.

[0062] The rock mechanics experimental data of Penglai M Oilfield show that the Poisson's ratio of the reservoir is 0.22 and the Biot coefficient is 0.95. After calculation, the coupling ratio K = 0.68.

[0063] ② The vertical principal stress originates from the gravity of the overlying layer and the ground surface is a free surface, so the coupling effect is small and can be ignored. The horizontal principal stress is greatly affected by the coupling effect. The calculation method of the magnitudes of the three-dimensional effective principal stresses caused by the change in formation pressure is as follows:

[0064] σ' v = σ v - ΔP, σ' H = σ H +(K - 1)ΔP, σ' h = σ h +(K - 1)ΔP

[0065] In the formula: σ v 、σ H and σ h are the vertical effective principal stress, the maximum and minimum horizontal effective principal stresses under initial conditions, respectively, in MPa; σ’ v 、σ’ H and σ’ h are the vertical effective principal stress, the maximum and minimum horizontal effective principal stresses considering the coupling effect, respectively, in MPa; ΔP is the change in formation pressure, in MPa; K is the pressure-stress coupling ratio, dimensionless, and its value is 0.68 according to the above calculation results.

[0066] (2) Three-dimensional fault plane attitude distribution: Import the three-dimensional seismic interpretation data related to the fault into the geological software for three-dimensional fault modeling to simulate the fault attitude distribution, including strike and dip, and the results are as Figure 2 shown.

[0067] (3) Calculation of the initial effective normal stress and shear stress of the fault plane:

[0068] The in-situ stress used for the initial stress calculation of the fault plane is from well logging comprehensive interpretation ( Figure 3 ), and for the convenience of simulation calculation, the interpretation results of the three principal stresses are fitted into a linear relationship (Table 1).

[0069] Table 1 Statistical Table of In-situ Stress Data for Fault Stability Evaluation

[0070]

[0071] Calculate the initial effective normal stress and shear stress of the fault plane by combining the in-situ stress data in Table 1 with the strike and dip of the fault. The calculation formulas are as follows:

[0072] S no = σ v l + σ H m + σ h n, σ no = S no - P p ,

[0073] In the formula: S no and σ no are the initial normal stress and initial effective normal stress on the fault plane, respectively, in MPa; τ o is the initial shear stress, in MPa; P p is the formation pressure, in MPa; σ v , σ H and σ h are the vertical principal stress, maximum horizontal principal stress and minimum horizontal principal stress under initial conditions, respectively, in MPa; l, m and n are the squares of the direction cosines corresponding to the three principal stresses, and are calculated by the following formula:

[0074] l = cos 2 θ dip , m = sin 2 (θ strike - θ Hmax )sin 2 θ dip , n = cos 2 (θ strike - θ Hmax )sin 2 θ dip

[0075] In the formula: θ Hmax is the direction of the maximum horizontal principal stress, in °; θ strike and θ dip are the strike and dip of the fault plane, respectively, in °.

[0076] The calculation results of the initial effective normal stress and shear stress of the fault plane in the embodiment are as Figure 4 shown. The effective normal stress is between 2.5 and 15.0 MPa, and the shear stress is between 0 and 4.0 MPa, and the stress increases with the increase of depth.

[0077] Combined with the above calculation method of the three-way effective principal stress considering the coupling effect, a calculation method for the effective normal stress and shear stress of the section after coupling is established:

[0078] σ n = σ no + ΔP(K - Kl - 1),

[0079] where: σ n and τ are the effective normal stress and shear stress after the pressure-stress coupling effect, respectively, in MPa; other symbols are the same as above.

[0080] (4) Judgment of the instability risk types at different parts of the fault:

[0081] ① Substitute the expressions of the effective normal stress and shear stress of the section after coupling into the Coulomb failure criterion to construct a quadratic equation with one variable with the formation pressure change ΔP as the variable:

[0082] Coulomb failure criterion:

[0083] τ = C + μσ n

[0084] where: C is the cohesion of the fault, in MPa; μ is the friction coefficient of the fault, dimensionless; other symbols are the same as above.

[0085] The constructed quadratic equation with one variable with the formation pressure change ΔP as the variable:

[0086] aΔP 2 + bΔP + c = 0, where,

[0087] a = lK 2 (1 - l) - μ 2 (K(1 - l) - 1) 2 ,

[0088] b = 2lK(σ no - σ v ) - 2μ(C + μσ no )(K(1 - l) - 1),

[0089]

[0090] where: a, b, and c are the quadratic term coefficient, linear term coefficient, and constant term of the above quadratic equation, respectively; other formula symbols are the same as above.

[0091] In the embodiment, it is a shallow fault, and the cohesion is ignored, that is, C = 0; in order to fully display all risk types in the embodiment, the friction coefficient μ is taken as 0.4.

[0092] ② Judge the instability risk type of the fault. The specific method is as follows:

[0093] A. When the above quadratic equation has no real roots (b 2 −4ac < 0), there is no risk of fault instability.

[0094] B. When the above quadratic equation has real roots (b 2 −4ac ≥ 0), the two roots are represented by ΔP1 and ΔP2 (which can be calculated according to the quadratic formula), and ΔP1 ≤ ΔP2. At the same time, the minimum and maximum formation pressures allowed by reservoir production activities are represented by P p-min and P p-max respectively. Then the following two cases can be distinguished:

[0095] When ΔP1 > 0 and ΔP2 > 0: ΔP1 ≥ (P p-max −P p ) indicates that there is an injection risk in the fault, otherwise there is no risk of fault instability;

[0096] When ΔP1 < 0 and ΔP2 > 0: |ΔP1| ≥ (P p −P p-min ) indicates that there is a production risk in the fault, otherwise there is no risk of fault instability.

[0097] Assume that the maximum formation pressure P p-max allowed during the gas reservoir development process in the embodiment is the fracture pressure of the caprock (σ h +KΔP), and the minimum value P p-min is 3.0 MPa. Combine the coupling ratio, the initial effective normal stress and shear stress of the fault, the fault attitude, and the fault strength-related data to judge the type of fault instability risk. The results are as Figure 5 shown. Most of the fault parts of the north-south strike-slip fault have no risk of fault instability, but due to the fluctuation of the local attitude, an injection-type risk type appears, mainly in the shallow part of the western strike-slip fault and the middle and northern parts of the eastern strike-slip fault ( Figure 5 ). The north-east trending normal fault mainly has two types: no risk and production-type risk. The production-type risk type mainly appears in the southernmost normal fault, also due to the sudden change in its strike (from north-east to north-north-east).

[0098] (5) Calculation of the critical pressure of fault instability under different risk types:

[0099] When there is an injection risk in the fault, the critical pressure of fault instability is ΔP1; when there is a production risk in the fault, the critical pressure of fault instability is |ΔP1|. The quadratic formula for the above quadratic equation is:

[0100]

[0101] In the formula: the relevant symbols are the same as above.

[0102] The simulation results are asFigure 6 As shown, the critical pressure value for the instability of the strike-slip fault injection type risk area is between 4.9 and 15.0 MPa, and the critical pressure value for the instability of the normal fault production type risk area is between -5.0 and -1.4 MPa (the negative sign only represents that its risk type is the production type). At the same time, under the condition that other parameters remain the same, the simulation calculation of the critical pressure of fault stability without considering the pressure-stress coupling effect was carried out ( Figure 7 ), which can only be used to characterize the injection type risk, and the minimum critical pressure (southern normal fault) is 0.3 MPa, which is 4.6 MPa different from 4.9 MPa when considering the coupling effect.

[0103] Through the comparison of the above two situations, it is found that whether to consider the pressure-stress coupling effect not only affects the magnitude of the critical pressure of fault instability, but also affects the risk location of fault instability, which is crucial for the formulation of oil and gas field development plans and the design of gas storage operation parameters. Therefore, it is very necessary to consider the pressure-stress coupling effect in the process of fault stability evaluation, so as to make the evaluation results more accurate and more in line with the actual situation.

Claims

1. A method for evaluating the stability of a fault considering the coupling effect of pressure-stress, characterized in that, The method includes the following steps: Step 1: Calculate the pressure-stress coupling ratio based on rock mechanics parameters or measured pressure and stress data, and determine the relationship between in-situ stress and formation pressure change; Step 2: Conduct 3D fault structure modeling based on seismic interpretation results, and simulate and calculate the fault occurrence distribution through geological software, including strike and dip; Step 3: Conduct initial stress analysis and calculation of the fault plane, including shear stress and effective normal stress, based on the fault occurrence distribution in combination with the magnitudes and orientations of the three principal stresses, and establish its expression after coupling; Step 4: Based on the Coulomb failure criterion, construct a quadratic equation about the critical pressure of fault instability, and judge the type of fault instability risk in combination with the upper and lower limits of the actual production pressure in the oilfield; Step 5: Calculate the critical pressure of fault instability under different risk types by using the coupling ratio, three principal stresses, fault occurrence, and the results of initial stress analysis of the fault plane.

2. The fault stability evaluation method considering the pressure-stress coupling effect according to claim 1, characterized in that: The specific method of Step 1: ① The calculation method of the pressure-stress coupling ratio is: Calculated through rock mechanics parameters, or K = Δσ / ΔP calculated through measured data In the formula: K is the pressure-stress coupling ratio, dimensionless; α is the Biot coefficient, dimensionless; ν is the Poisson's ratio of the rock, dimensionless; ΔP is the change in formation pressure, a positive value indicates an increase in formation pressure, and a negative value indicates a decrease in formation pressure, MPa; Δσ is the stress change, MPa; ② The calculation method of the magnitudes of the three effective principal stresses caused by the change in formation pressure is as follows: σ' v = σ v - ΔP, σ' H = σ H + (K - 1)ΔP, σ' h = σ h + (K - 1)ΔP Where: σ v , σ H and σ h are the vertical effective principal stress, the maximum horizontal effective principal stress, and the minimum horizontal effective principal stress under the initial formation pressure condition, respectively, in MPa; σ’ v , σ’ H and σ’ h are the vertical effective principal stress, the maximum horizontal effective principal stress, and the minimum horizontal effective principal stress considering the coupling effect, respectively, in MPa; Other formula symbols are the same as above.

3. The fault stability evaluation method considering the pressure-stress coupling effect according to claim 1, characterized in that: The specific method of Step 3: Use the three principal stresses, fault strike, and fault dip to calculate the initial effective normal stress and shear stress of the fault plane. The calculation formulas are as follows: S no = σ v l + σ H m + σ h n, σ no = S no - P p , Where: S no and σ no are the normal stress and effective normal stress on the section under the initial formation pressure condition, MPa; τ o is the initial shear stress, MPa; P p is the formation pressure, MPa; σ v , σ H and σ h are the vertical principal stress, maximum horizontal principal stress and minimum horizontal principal stress under the initial formation pressure condition, MPa; l, m and n are the squares of the direction cosines corresponding to the three principal stresses, and are calculated by the following formula: l = cos 2 θ dip ,m = sin 2 (θ strike -θ Hmax )sin 2 θ dip ,n = cos 2 (θ strike -θ Hmax )sin 2 θ dip Where: θ Hmax is the maximum horizontal principal stress direction, °; θ strike and θ dip are the fault strike and dip respectively, °; The calculation formulas for the effective normal stress and shear stress of the fault plane after coupling are as follows: σ n = σ no + ΔP(K - Kl - 1), In the formula: σ n and τ are the effective normal stress and shear stress respectively after the pressure-stress coupling effect, in MPa; other symbols are the same as above.

4. The fault stability evaluation method considering the pressure-stress coupling effect according to claim 1, characterized in that: The specific method of Step 4: ① Based on the Coulomb failure criterion, in combination with the expressions of the effective normal stress and shear stress of the fault plane after coupling, construct a quadratic equation with the change in formation pressure ΔP as the variable: aΔP 2 +bΔP + c = 0, where, a = lK 2 (1 - l)-μ 2 (K(1 - l)-1) 2 , b = 2lK(σ no - σ v )) - 2μ(C + μσ no )(K(1 - l) - 1), In the formula: C is the cohesion of the fault, MPa, μ is the friction coefficient of the fault, dimensionless; a, b, and c are the quadratic term coefficient, linear term coefficient, and constant term of the above quadratic equation respectively; other formula symbols are the same as above; ② Judge the type of fault instability risk according to the situation of the equation roots and the upper and lower limits of the actual production pressure. The specific method is as follows: A. When the equation has no root (b 2 −4ac < 0), there is no risk of instability of the fault; B. When the above quadratic equation has roots (b 2 −4ac ≥ 0), the two roots are represented by ΔP1 and ΔP2 respectively, and ΔP1 ≤ ΔP2. At the same time, the minimum and maximum formation pressures allowed by reservoir production activities are represented by P p-min and P p-max respectively. Then the following two cases can be distinguished: When ΔP1 > 0 and ΔP2 > 0: ΔP1 ≥ (P p-max − P p ) indicates that there is an injection risk in the fault, otherwise there is no risk of instability in the fault; When ΔP1 < 0 and ΔP2 > 0: |ΔP1| ≥ (P p -P p-min ) indicates that there is a production risk in the fault, otherwise there is no risk of instability in the fault.

5. A method for evaluating the stability of a fault considering the pressure-stress coupling effect according to claim 1, characterized in that: The specific method of Step 5: When there is an injection risk in the fault, the critical pressure of fault instability is ΔP1; when there is a production risk in the fault, the critical pressure of fault instability is |ΔP1|.

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