A fault stability evaluation method considering pressure-stress coupling

Abstract

The present application relates to a kind of fault stability evaluation method considering pressure-stress coupling, the method comprises: according to the rock mechanics parameters of target horizon or field measured pressure and ground stress data, the pressure-stress coupling ratio is calculated;Based on seismic interpretation results, three-dimensional structure modeling of fault is carried out, and fault occurrence distribution is simulated and calculated;Initial stress and coupling stress analysis and calculation of section are carried out in combination with three principal stresses, including shear stress and effective normal stress;Monomial quadratic equation about fault instability critical pressure is built, and the risk type of fault instability is judged in combination with the pressure limit of oilfield production practice;The critical pressure of fault instability under different risk types is calculated.The present application can consider the influence of pressure-stress coupling on fault stability, so that the fault stability evaluation result is more in line with actual.

Description

Technical Field

[0001] This invention relates to the field of oil and gas reservoir geological exploration and development technology, specifically to a fault stability evaluation method that considers pressure-stress coupling. Background Technology

[0002] Fault instability is a critical issue in oil and gas exploration, production, and storage due to its destructive effects on traps and reservoirs. Fault instability is characterized by high strain and high permeability, leading to wellbore instability and casing shear failure. It can also cause oil and gas to migrate along the fault, potentially escaping to the surface or seabed. Exploration practice shows that both fluid injection and production can induce fault instability, but the mechanisms differ. Fluid injection-induced fault instability primarily occurs because the high-pressure fluid entering the fault zone reduces the effective normal stress on the fault, thereby decreasing the maximum frictional resistance and leading to instability. In contrast, fluid production-induced fault instability mainly involves pressure-stress coupling. Under the normal fault stress mechanism, the decrease in formation pressure increases the differential stress, thus inducing fault instability.

[0003] Extensive field measurement data reveals the existence of pressure-stress coupling, which not only increases the critical pressure for injection-type fault instability but is also a major mechanism for production-type fault instability. Currently, methods for evaluating fault stability can be broadly categorized into analytical methods and numerical simulation methods. Analytical methods do not consider pressure-stress coupling, resulting in low accuracy in evaluating the instability risk of injection-type faults and an inability to assess the instability risk of production-type faults. While numerical simulation methods can consider pressure-stress coupling, they typically only determine the critical pressure for instability at a specific location through trial-and-error methods, and cannot accurately define the critical pressure across the entire fault plane. Therefore, this invention proposes a fault stability evaluation method that considers pressure-stress coupling based on the principle of pressure-stress coupling and the fault instability mechanism. Summary of the Invention

[0004] The purpose of this invention is to provide a fault stability evaluation method that considers the pressure-stress coupling effect, in order to solve the problem that existing technologies cannot accurately quantify the critical instability pressure at different locations of a three-dimensional fault plane under pressure-stress coupling.

[0005] The technical solution adopted by this invention to solve its technical problem is: a fault stability evaluation method considering pressure-stress coupling, the method comprising the following steps:

[0006] Step 1: Calculate the pressure-stress coupling ratio based on rock mechanics parameters or measured pressure and in-situ stress data, and determine the relationship between in-situ stress and formation pressure.

[0007] Step 2: Based on the seismic interpretation results, perform three-dimensional structural modeling of the faults, and use geological software to simulate and calculate the fault attitude distribution, including strike and dip angle;

[0008] Step 3: Based on the cross-sectional orientation distribution and the magnitude and orientation of the three principal stresses, perform initial stress calculations on the cross-section, including shear stress and effective normal stress, and establish their expressions after coupling effects;

[0009] Step 4: Based on the Coulomb rupture criterion, construct a univariate quadratic equation for the critical pressure of fault instability, and combine it with the upper and lower limits of the actual production pressure in the oilfield to determine the type of fault instability risk.

[0010] Step 5: Calculate the critical pressure for fault instability under different risk types using the coupling ratio, triaxial principal stress, fault attitude, and initial stress analysis results of the fault surface.

[0011] The calculation method for the pressure-stress coupling ratio in step one of the above scheme is as follows:

[0012] The calculation method based on rock mechanics parameters is: K = α(1-2ⅴ) / (1-ⅴ); the calculation method based on measured formation pressure and 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, and a negative value indicates a decrease in formation pressure, MPa; Δσ is the change in stress, MPa.

[0014] The method for calculating the magnitude of the three-dimensional effective principal stress considering coupling effects in step one of the above scheme is as follows:

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

[0016] Where: σ v σ H and σ h These represent the vertical effective principal stress, maximum horizontal effective principal stress, and minimum horizontal effective principal stress under initial formation pressure conditions, respectively, in MPa; σ' v ,σ' H and σ' h These represent the vertical effective principal stress (unaffected by coupling), the maximum horizontal effective principal stress, and the minimum horizontal effective principal stress, respectively, in MPa; ΔP is the change in formation pressure, in MPa; K is the pressure-stress coupling ratio, dimensionless.

[0017] The calculation method for the initial effective normal stress and shear stress of the interrupted surface in step three of the above scheme is as follows:

[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 These represent the normal stress and effective normal stress on the cross section under initial formation pressure conditions, respectively, in MPa; τ o P represents the initial shear stress, in MPa; p σ is the formation pressure, MPa; v σ H and σ h Let be the vertical principal stress, maximum horizontal principal stress, and minimum horizontal principal stress under initial formation pressure conditions, respectively, in MPa; l, m, and n are the squares of the direction cosines corresponding to the three principal stresses, calculated using 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 The direction of the maximum horizontal principal stress is °; θ strike and θ dip These represent the fault strike and dip angle, respectively, in °.

[0022] The calculation method for the effective normal stress and shear stress of the coupled section in step three of the above scheme is as follows:

[0023] σ n =σ no +ΔP(K-Kl-1),

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

[0025] The specific method for step four in the above scheme is as follows: First, substitute the expressions for the effective normal stress and shear stress of the coupled cross section into the Coulomb fracture criterion to construct a quadratic equation with the formation pressure change ΔP as the variable:

[0026] Coulomb's criterion for failure:

[0027] τ=C+μσ n

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

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

[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 coefficients of the quadratic term, the linear term, and the constant term of the above quadratic equation; other formula symbols are the same as above.

[0035] Secondly, the type of fault risk is determined, using the following methods:

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

[0037] B. When the above quadratic equation has roots (b) 2 -4ac≥0), the two roots are represented by ΔP1 and ΔP2 (which can be calculated using the quadratic formula), and ΔP1≤ΔP2. Meanwhile, the minimum and maximum formation pressures allowed for reservoir production activities are represented by P. p-min and P p-max If this is the case, 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 risk of instability.

[0039] When ΔP1 < 0 and ΔP2 > 0: |ΔP1| ≥ (P p -P p-min A fault indicates a risk of extraction; otherwise, there is no risk of instability.

[0040] The specific method for step five in the above scheme is as follows: Based on the variation law of the stress Mohr's circle in the shear stress-normal stress coordinate system corresponding to the change in formation pressure, and the analysis of the roots of the above equation, it can be seen that when there is an injection risk in the fault, the critical pressure for fault instability is ΔP1; when there is a extraction risk in the fault, the critical pressure for fault instability is |ΔP1|. The formula for finding the roots of the above quadratic equation is:

[0041]

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

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

[0044] 1. Extensive field measurement data reveals the existence of pressure-stress coupling, which affects fault stability. Existing analytical methods for fault stability evaluation do not consider pressure-stress coupling, resulting in low evaluation accuracy and an inability to analyze the impact of fluid extraction on fault stability. This invention effectively overcomes these shortcomings by creatively incorporating coupling into a fault stability evaluation method based on a coupling ratio parameter. This not only improves the accuracy of fault stability evaluation but also enables the assessment of the risk of instability in extraction-induced faults.

[0045] 2. While existing numerical simulation methods for fault stability evaluation can consider pressure-stress coupling, they typically only determine the critical instability pressure at a specific location within the fault using trial-and-error methods. This not only fails to accurately determine the critical pressure across the entire fault plane but also suffers from extremely low efficiency. Combining the coupling ratio obtained from numerical simulation with this invention can effectively solve the aforementioned problems, thereby rapidly and accurately determining the critical instability pressure at different locations within the fault. This has significant theoretical guiding significance for the safe and efficient development of fault-block oil and gas reservoirs and the safe operation of gas storage facilities. Attached Figure Description

[0046] Figure 1 This is a flowchart of the steps of the present invention;

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

[0048] Figure 3 This is a geostress profile of an embodiment;

[0049] Figure 4 These are the calculation results of the initial effective normal stress and shear stress of the cross section in the embodiment;

[0050] Figure 5 This is the result of the fault instability risk type assessment in the embodiment;

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

[0052] Figure 7 The results are calculated using traditional fault stability evaluation methods (without considering coupling effects). Detailed Implementation

[0053] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings to clearly and completely illustrate the technical solution of the present invention. These embodiments are only some examples of the present invention and do not constitute a limitation on the scope of protection of the present invention. For those skilled in the art, various modifications or alternative solutions made without creative effort are all covered within the scope of protection of the present invention.

[0054] This application uses the technical solution of the present invention to determine the instability risk type of the fault in the Penglai M oilfield, and evaluates the critical pressure for fault instability under different risk types. The specific process is as follows:

[0055] The example given is "Fault Stability Evaluation for Safe Development of Shallow Gas in Penglai M Oilfield". The research object is located in the northeastern part of the Bohai Bay Basin's southern uplift tectonic belt. The shallow gas reservoirs in the study area are distributed at the top of a faulted anticline controlled by two near-north-south trending strike-slip faults, with a depth range of 500–1200 m. Multiple secondary near-northeast trending normal faults also develop within the reservoir area. This invention uses the Penglai M oilfield as an example to conduct a fault stability study considering the pressure-stress coupling effect, to clarify the types of fault instability risks and their critical pressures during shallow gas development, and to compare the results with conventional fault stability evaluations (without considering the pressure-stress coupling effect) to further illustrate the innovation and applicability of this invention. The target faults evaluated include two strike-slip faults and seven normal faults.

[0056] Basic conditions for the embodiment: The study area has good three-dimensional seismic interpretation data, geostress and rock mechanics parameters and other related data, which provide good basic data for the application of the present invention.

[0057] Implementation process: According to Figure 1 The steps are as follows:

[0058] (1) Calculation method for effective principal stress in three directions considering pressure-stress coupling:

[0059] ① The pressure-stress coupling ratio can be calculated using Poisson's ratio and Biot coefficient obtained from well logging interpretation or rock mechanics experiments. Alternatively, it can be converted from measured values ​​of formation pressure and geostress at different times. This embodiment will use the former method, 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] Rock mechanics experimental data from the Penglai M oilfield indicate that the reservoir's Poisson's ratio is 0.22 and the Biot coefficient is 0.95. The calculated coupling ratio K = 0.68.

[0063] ② The vertical principal stress originates from the gravity of the overlying layer and the 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 for the magnitude of the three-dimensional effective principal stress caused by changes 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] Where: σ v σ H and σ h The effective vertical principal stress, maximum and minimum effective horizontal principal stress, respectively, under initial conditions, in MPa; σ' v ,σ' H and σ' h ΔP represents the vertical effective principal stress, maximum and minimum horizontal effective principal stress after 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 attitude distribution: Three-dimensional seismic interpretation data related to the fault were imported into geological software for three-dimensional fault modeling, simulating the fault attitude distribution, including strike and dip. The results are as follows: Figure 2 As shown.

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

[0068] The geostress used for initial stress calculation of the fault plane originates from comprehensive well logging interpretation. Figure 3 To facilitate simulation calculations, the interpretation results of the triaxial principal stresses were fitted into a linear relationship (Table 1).

[0069] Table 1. Statistical table of geostress data used for fault stability evaluation

[0070]

[0071] Using the ground stress data in Table 1, combined with the fault strike and dip angle, the initial effective normal stress and shear stress of the cross section are calculated using the following formulas:

[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 These are the initial normal stress and initial effective normal stress on the cross section, respectively, in MPa; τ o P represents the initial shear stress, in MPa; p σ is the formation pressure, MPa; v σ H and σ h Let be the vertical principal stress, maximum horizontal principal stress, and minimum horizontal principal stress under the initial conditions, respectively, in MPa; l, m, and n are the squares of the direction cosines corresponding to the three principal stresses, calculated using 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 The direction of the maximum horizontal principal stress is °; θ strike and θ dip These represent the cross-sectional orientation and dip angle, respectively, in degrees.

[0076] The calculation results of the initial effective normal stress and shear stress of the cross section in the embodiment are as follows: Figure 4 As shown, the effective normal stress is between 2.5 and 15.0 MPa, and the shear stress is between 0 and 4.0 MPa, with the stress increasing with depth.

[0077] Based on the above method for calculating the effective principal stress of the three-dimensional axis after considering coupling, a method for calculating the effective normal stress and shear stress of the coupled section is established:

[0078] σ n =σ no +ΔP(K-Kl-1),

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

[0080] (4) Determining the type of instability risk at different locations of the fault:

[0081] ① Substituting the expressions for the effective normal stress and shear stress of the coupled fracture surface into the Coulomb fracture criterion, a quadratic equation in univariate form with the formation pressure change ΔP as the variable is constructed:

[0082] Coulomb's criterion for failure:

[0083] τ=C+μσ n

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

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

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

[0091] In this embodiment, the fault is shallow and the cohesion is negligible, i.e., C = 0; in order to fully demonstrate all risk types in this embodiment, the friction coefficient μ is taken as 0.4.

[0092] ② To determine the type of fault instability risk, the specific methods are as follows:

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

[0094] B. When the above quadratic equation has roots (b) 2 -4ac≥0), the two roots are represented by ΔP1 and ΔP2 (which can be calculated using the quadratic formula), and ΔP1≤ΔP2. Meanwhile, the minimum and maximum formation pressures allowed for reservoir production activities are represented by P. p-min and P p-max If this is the case, then the following two situations can be distinguished:

[0095] 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 risk of instability.

[0096] When ΔP1 < 0 and ΔP2 > 0: |ΔP1| ≥ (P p -P p-min A fault indicates a risk of extraction; otherwise, there is no risk of instability.

[0097] Assuming the maximum allowable formation pressure P during gas reservoir development in this embodiment. p-max The rupture pressure of the caprock (σ) h +KΔP), minimum value P p-min The stress was 3.0 MPa. The fault instability risk type was determined by combining the coupling ratio, initial effective normal stress and shear stress of the fault, fault attitude, and fault strength-related data. The results are as follows: Figure 5 As shown, most parts of the north-south strike-slip fault do not pose a risk of fault instability. However, due to fluctuations in local attitude, an injection-type risk has emerged, mainly manifested in the shallow part of the western strike-slip fault and the central and northern parts of the eastern strike-slip fault. Figure 5 The northeast-trending normal faults are mainly of two types: risk-free and extraction-risk. The extraction-risk type mainly appears in the southernmost normal fault, which is also caused by its sudden change in trend (from northeast to northeast-east).

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

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

[0100]

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

[0102] Simulation results are as follows Figure 6 As shown, the critical pressure values ​​for instability at injection-type risk sites in strike-slip faults range from 4.9 to 15.0 MPa, while the critical pressure values ​​for instability at production-type risk sites in normal faults range from -5.0 to -1.4 MPa (the negative sign only indicates that the risk type is production-type). Simultaneously, with other parameters remaining constant, simulation calculations of the critical pressure for fault stability were performed without considering pressure-stress coupling. Figure 7 It can only be used to characterize the injection-type risk type, 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] A comparison of the two scenarios reveals that whether or not the pressure-stress coupling effect is considered not only affects the magnitude of the critical pressure for fault instability but also the location of the risk factor for fault instability. This is crucial for formulating oil and gas field development plans and designing gas storage operation parameters. Therefore, it is essential to consider the pressure-stress coupling effect during fault stability evaluation to ensure more accurate evaluation results that better reflect reality.

Claims

1. A method for evaluating fault stability considering pressure-stress coupling, 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. Step 2: Based on the seismic interpretation results, perform three-dimensional structural modeling of the faults, and use geological software to simulate and calculate the fault attitude distribution, including strike and dip angle; Step 3: Based on the cross-sectional orientation distribution and the magnitude and orientation of the three principal stresses, perform initial stress analysis calculations on the cross-section, including shear stress and effective normal stress, and establish their expressions after coupling effects; Step 4: Based on the Coulomb rupture criterion, construct a univariate quadratic equation for the critical pressure of fault instability, and combine it with the upper and lower limits of the actual production pressure of the oilfield to determine the type of fault instability risk. Step 5: Calculate the critical pressure for fault instability under different risk types using the coupling ratio, triaxial principal stress, fault attitude, and initial stress analysis results of the fault surface.

2. The fault stability evaluation method considering pressure-stress coupling effect according to claim 1, characterized in that: The specific method of step one is as follows: ① The calculation method for the pressure-stress coupling ratio is as follows: Calculated using rock mechanics parameters, or K = Δσ / ΔP calculated from 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 change in stress, MPa. ②The calculation method for the magnitude of the triaxial effective principal stress caused by changes in formation pressure is as follows: in v =s v -ΔP, in H =s H +(K-1)ΔP, σ' h =s h +(K-1)ΔP Where: σ v σ H and σ h These represent the vertical effective principal stress, maximum horizontal effective principal stress, and minimum horizontal effective principal stress under initial formation pressure conditions, respectively, in MPa; σ' v ,σ' H and σ' h These represent the vertical effective principal stress, the maximum horizontal effective principal stress, and the minimum horizontal effective principal stress, respectively, after considering the coupling effect, in MPa; Other formula symbols are the same as above.

3. The fault stability evaluation method considering pressure-stress coupling as described in claim 1, characterized in that: The specific method for step three is as follows: The initial effective normal stress and shear stress of the fault section are calculated using the triaxial principal stresses, fault strike, and fault dip angle. The calculation formulas are as follows: S no =s v l+s H m+s h n, s no =S no -P p , In the formula: S no and σ no These represent the normal stress and effective normal stress on the cross section under initial formation pressure conditions, respectively, in MPa; τ o P represents the initial shear stress, in MPa; p σ is the formation pressure, MPa; v σ H and σ h Let be the vertical principal stress, maximum horizontal principal stress, and minimum horizontal principal stress under initial formation pressure conditions, respectively, in MPa; l, m, and n are the squares of the direction cosines corresponding to the three principal stresses, calculated using the following formula: l=cos 2 i dip ,m=sin 2 (i strike -θ Hmax )sin 2 i dip ,n=cos 2 (i strike -θ Hmax )sin 2 i dip In the formula: θ Hmax The direction of the maximum horizontal principal stress is °; θ strike and θ dip These represent the fault strike and dip angle, in °; The formulas for calculating the effective normal stress and shear stress of the cross section after coupling are as follows: σ n =σ no +ΔP(K-Kl-1), Where: σ n τ and τ are the effective normal stress and shear stress after pressure-stress coupling, respectively, in MPa; other symbols are the same as above.

4. The fault stability evaluation method considering pressure-stress coupling as described in claim 1, characterized in that: The specific method for step four is as follows: ① Based on the Coulomb fracture criterion, and combined with the expressions for the effective normal stress and shear stress of the coupled fracture surface, a quadratic equation in univariate form with the formation pressure change ΔP as the variable is constructed: aΔP 2 +bΔP+c=0, where... a=lK 2 (1-l)-μ 2 (K(1-1)-1) 2 , b=2lK(σ no -s v )-2μ(C+μσ no )(K(1-l)-1), In the formula: C is the fault cohesion, MPa; μ is the fault friction coefficient, dimensionless; a, b, and c are the quadratic coefficient, linear coefficient, and constant term of the above quadratic equation, respectively; other formula symbols are the same as above. ② Determine the type of fault instability risk based on the condition of the equation roots and the upper and lower limits of actual production pressure. The specific method is as follows: A. When the equation has no roots (b) 2 -4ac<0), the fault has no risk of instability; 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. Meanwhile, the minimum and maximum formation pressures allowed for reservoir production activities are represented by P. p-min and P p-max If this is the case, then the following two situations can be distinguished: 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 risk of instability. When ΔP1 < 0 and ΔP2 > 0: |ΔP1| ≥ (P p -P p-min A fault indicates a risk of extraction; otherwise, there is no risk of instability.

5. The fault stability evaluation method considering pressure-stress coupling as described in claim 1, characterized in that: The specific method for step five is as follows: When there is a risk of injection in the fault, the critical pressure for fault instability is ΔP1; when there is a risk of extraction in the fault, the critical pressure for fault instability is |ΔP1|.

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