Hydrofracture-induced fault activation tendency discrimination method considering rock damage

Abstract

The invention relates to a hydraulic fracturing induced fault activation tendency discrimination method considering rock damage, which comprises the following steps of: 1) adopting a triaxial compression experiment for basic parameters, respectively determining hydraulic fracturing stratum cohesion c, an internal friction angle phi and pore water pressure p0, and determining first, second and third principal stresses sigma1, sigma2 and sigma3 of a fault according to logging and seismic inversion data; the length of the hydraulic fracturing crack which flows in a layered mode and is closest to the fault is L, and the fault inclination angle is beta; 2) establishing a fault activation tendency state function M (D) expressed by a damage variable D based on the Mohr-Coulomb's law; and 3) establishing a fault activation tendency judgment criterion, and evaluating the fault activation tendency and risk according to the discriminant. By means of the method, whether the fault is activated or not can be predicted more accurately, the safety of hydraulic fracturing operation is effectively improved, and possible earthquake accidents are avoided.

Description

Technical Field

[0001] The invention relates to the field of petroleum engineering, and in particular to a method for distinguishing the tendency of hydraulic fracturing-induced fault activation taking rock damage into consideration. Background Art

[0002] With the continuous growth of global energy demand, traditional oil and gas resources are gradually depleted, and the development of unconventional oil and gas resources, such as shale gas, oil sands and coalbed methane, has become an important energy strategy. Compared with traditional oil and gas, unconventional reservoirs have the characteristics of low permeability and low porosity. Therefore, hydraulic fracturing technology is needed to improve reservoir permeability and oil and gas production during development. However, the injection of high-pressure liquid during hydraulic fracturing may cause fault activation, leading to geological disasters such as earthquakes, posing a threat to the environment and safety, and restricting the exploitation of unconventional oil and gas. Therefore, how to accurately determine whether there is a risk of induced fault activation during hydraulic fracturing has become an urgent problem to be solved in the development of unconventional oil and gas reservoirs.

[0003] Previous identification methods usually calculated the changes in ground stress caused by hydraulic fracturing to determine whether the critical stress of the fault was exceeded, thereby inducing fault slip. This method is simple and easy to implement, but it ignores the important impact of rock damage on fault stability during the fracturing process. Rocks undergo a complex damage evolution process during hydraulic fracturing, and the accumulation of damage will significantly change the mechanical properties of the rock mass, thereby affecting the stability of the fault. Therefore, the existing identification methods fail to fully consider the rock damage effect, and the identification results may have large uncertainties, making it difficult to accurately assess the risk of hydraulic fracturing-induced fault activation. A more accurate identification method is needed to conduct reasonable fracturing operations. Summary of the invention

[0004] The purpose of the present invention is to provide a method for distinguishing the tendency of hydraulic fracturing-induced fault activation taking into account rock damage. The rock damage model is combined with the fault stability analysis, and a method for distinguishing hydraulic fracturing-induced fault activation taking into account rock damage is proposed. This method can more accurately predict whether a fault will be activated by simulating the damage evolution process and stress redistribution of the rock, thereby effectively improving the safety of hydraulic fracturing operations and avoiding possible earthquake accidents.

[0005] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a method for judging the tendency of hydraulic fracturing-induced fault activation taking into account rock damage:

[0006] 1) Basic parameters: triaxial compression test is used to determine the cohesion c and internal friction angle of hydraulic fracturing formation. The pore water pressure p0, well logging and seismic inversion data determine the first, second and third principal stresses σ1, σ2, σ3 of the fault, the length L of the hydraulic fracturing crack with layered flow closest to the fault, the fault dip angle β and its strike.

[0007] Assuming that the upper and lower plates of the fault are homogeneous elastic bodies, the upper and lower plates are in contact through the fault plane, and are subjected to the maximum principal stress in the vertical direction and the minimum principal stress in the horizontal direction. Shear movement of the upper and lower plates along the fault plane is regarded as fault activation. A three-dimensional fault structure diagram is established, and a two-dimensional geomechanical model for fault activation analysis is drawn. The model satisfies that the normal direction of the fault plane is contained in the plane determined by the directions of the first principal stress σ1 and the third principal stress σ3, and the direction of the second principal stress σ2 is contained in the fault plane, σ1>σ2>σ3, and is perpendicular to the normal direction n of the fault plane. Therefore, σ2 will not affect the normal stress and shear stress of the fault. The fault dip angle is β. According to the established model, the normal stress σ n , shear stress τ n The relationship between the maximum and minimum principal stresses is:

[0008]

[0009] Where: σ1 is the maximum principal stress in the vertical direction; β is the angle between the fault normal and the maximum principal stress. When σ1 is in the vertical direction, β is the fault dip; σ3 is the minimum principal stress in the horizontal direction.

[0010] 2) Based on the Moore-Coulomb law, a fault activation tendency state function M(D) expressed by the damage variable D is established.

[0011] 2.1) According to the specific theory of Mohr-Coulomb failure criterion: Under a certain stress state, if the maximum shear stress is greater than or equal to the shear strength of the rock, it can be determined that the fault may be activated. And the relationship between the fault activation tendency and the normal and shear stress of the fault plane and the friction strength of the fault plane is established, which is specifically expressed as follows:

[0012] τ max ≥τ f =μ(σ n -αp)+C (3)

[0013] In formula (3), μ is the fault friction coefficient, p is the fluid penetration pressure of the damaged fault zone, C is the cohesion of the fault, and α is the Biot coefficient.

[0014] Calculate rock porosity based on average effective stress:

[0015]

[0016] In formula (4), is the porosity at zero stress state; is the stress sensitivity coefficient of porosity, which can be taken as 5.0×10-8Pa-1; is the limit value of porosity under high compressive stress state. is the average effective stress, which can be calculated according to formula (5):

[0017]

[0018] Where: σ2 is the second principal stress; p0 is the pore water pressure when the formation is initially undamaged.

[0019] The effect of damage on permeability is shown in formula (6):

[0020]

[0021] Where: k0 is the permeability under zero stress state, D is the damage variable, α D is the damage sensitivity coefficient. According to Darcy's law, the water pressure difference between undamaged and damaged rock can be expressed by equations (7) and (8):

[0022]

[0023] p=p0+Δp-Δp0 (9)

[0024] In formula (8), Δp0 and Δp are the water pressure differences between the undamaged and damaged rocks, respectively; Q is the amount of fluid passing through the pores of this rock per unit time; μ0 is the fluid viscosity; A is the cross-sectional area of ​​the rock pores at the fault;

[0025] As the damage variable D increases, the microcracks and defects inside the rock gradually expand, resulting in a decrease in the cohesion C. The cohesion C can be expressed as a function of the initial cohesion C0 and the damage variable D, as shown in formula (10):

[0026] C=C0(1-D) (10)

[0027] Where: C0 is the cohesive force in the initial damage-free state.

[0028] 2.2) Calculate the damage variable D

[0029] Calculate the failure criteria F1 and F2 based on the maximum principal stress σ1 and the minimum principal stress σ3

[0030] F1=σ1-f t (11)

[0031]

[0032] Determine the injury status

[0033]

[0034] ε t0 is the initial tensile strain of the rock; ε1 is the current maximum principal strain; ε c0 is the initial compressive strain of the rock; ε3 is the current minimum principal strain; dF1 and dF2 represent the rates of change of F1 and F2, respectively;

[0035] If F1<0 and F2<0, the rock has not reached the tensile or shear failure condition and is in a non-destructive state, and the damage variable D=0;

[0036] If F1 = 0 and dF1 > 0, the rock reaches the tensile failure condition, and the tensile stress is still increasing, the damage variable

[0037] If F2 = 0 and dF2 > 0, the rock reaches shear failure condition, and the compressive stress is still increasing, the damage variable

[0038]

[0039] 2.3) Substitute equations (1)(2)(7)(8)(9)(10) into equation (14) to construct the fault activation tendency state function M(D) expressed by the damage variable D, as shown in equation (15):

[0040] M(D)=τ max -τ f (14)

[0041]

[0042]

[0043] Finally, the fault activation tendency state function M(D) is obtained.

[0044] 3) Establish a criterion for fault activation tendency and evaluate the tendency and risk of fault activation based on the discriminant.

[0045] When M(D)<0, it means that the friction resistance of the fault plane is stronger than the shear stress of the fault plane, and the fault has no activation tendency;

[0046] When M(D)>0, it means that the friction resistance of the fault plane is weaker than the shear stress of the fault plane, and the fault has a tendency to activate;

[0047] When M(D) = 0, it means that the friction resistance of the fault plane is equal to the shear stress of the fault plane, and the fault is in a critical activation state.

[0048] The beneficial effects of the present invention are:

[0049] 1. The method for distinguishing hydraulic fracturing fault activation taking into account rock damage provided by the present invention comprehensively considers the influence of rock damage on the stress field, can more accurately evaluate the stability of the fault during hydraulic fracturing, and provide guidance for accurate prediction of fault activation.

[0050] 2. The method for determining hydraulic fracturing fault activation taking into account rock damage provided by the present invention provides safer operating guidelines for hydraulic fracturing design, helps avoid potential risks such as earthquake-induced, geological disasters or fault ruptures, and improves the safety of oil and gas production.

[0051] 3. The method for distinguishing hydraulic fracturing fault activation taking into account rock damage provided by the present invention can more accurately predict the risk of fault activation, take preventive measures in advance, and avoid seismic activities or rupture events caused by improper hydraulic fracturing, thereby reducing environmental pollution and economic losses. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 : Flow chart of the method of the present invention;

[0053] Figure 2 : Three-dimensional fault structure diagram;

[0054] Figure 3 : A geomechanical two-dimensional model for fault activation analysis. DETAILED DESCRIPTION

[0055] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0056] 1) Basic parameters: triaxial compression test is used to determine the cohesion c and internal friction angle of hydraulic fracturing formation. The pore water pressure p0, well logging and seismic inversion data determine the first, second and third principal stresses σ1, σ2, σ3 of the fault, the length L of the hydraulic fracturing crack with layered flow closest to the fault, the fault dip angle β and its strike.

[0057] Assuming that the upper and lower plates of the fault are homogeneous elastic bodies, the upper and lower plates are in contact through the fault plane, and are subjected to the maximum principal stress in the vertical direction and the minimum principal stress in the horizontal direction. Shear movement of the upper and lower plates along the fault plane is regarded as fault activation. A three-dimensional fault structure diagram is established, and a two-dimensional geomechanical model for fault activation analysis is drawn. The model satisfies that the normal direction of the fault plane is contained in the plane determined by the directions of the first principal stress σ1 and the third principal stress σ3, and the direction of the second principal stress σ2 is contained in the fault plane, σ1>σ2>σ3, and is perpendicular to the normal direction n of the fault plane. Therefore, σ2 will not affect the normal stress and shear stress of the fault. The fault dip angle is β. According to the established model, the normal stress σ n , shear stress τ nThe relationship between the maximum and minimum principal stresses is:

[0058]

[0059] Where: σ1 is the maximum principal stress in the vertical direction; β is the angle between the fault normal and the maximum principal stress. When σ1 is in the vertical direction, β is the fault dip; σ3 is the minimum principal stress in the horizontal direction.

[0060] 2) Based on the Moore-Coulomb law, a fault activation tendency state function M(D) expressed by the damage variable D is established.

[0061] 2.1) According to the specific theory of Mohr-Coulomb failure criterion: Under a certain stress state, if the maximum shear stress is greater than or equal to the shear strength of the rock, it can be determined that the fault may be activated. And the relationship between the fault activation tendency and the normal and shear stress of the fault plane and the friction strength of the fault plane is established, which is specifically expressed as follows:

[0062] τ max ≥τ f =μ(σ n -αp)+C (3)

[0063] In formula (3), μ is the fault friction coefficient, p is the fluid penetration pressure of the damaged fault zone, C is the cohesion of the fault, and α is the Biot coefficient.

[0064] Calculate rock porosity based on average effective stress:

[0065]

[0066] In formula (4), is the porosity at zero stress state; is the stress sensitivity coefficient of porosity, which can be taken as 5.0×10-8Pa-1; is the limit value of porosity under high compressive stress state. is the average effective stress, which can be calculated according to formula (5):

[0067]

[0068] Where: σ2 is the second principal stress; p0 is the pore water pressure when the formation is initially undamaged.

[0069] The effect of damage on permeability is shown in formula (6):

[0070]

[0071] Where: k0 is the permeability under zero stress state, D is the damage variable, α Dis the damage sensitivity coefficient. According to Darcy's law, the water pressure difference between undamaged and damaged rock can be expressed by equations (7) and (8):

[0072]

[0073] p=p0+Δp-Δp0 (9)

[0074] In formula (8), Δp0 and Δp are the water pressure differences between the undamaged and damaged rocks, respectively; Q is the amount of fluid passing through the pores of this rock per unit time; μ0 is the fluid viscosity; A is the cross-sectional area of ​​the rock pores at the fault;

[0075] As the damage variable D increases, the microcracks and defects inside the rock gradually expand, resulting in a decrease in the cohesion C. The cohesion C can be expressed as a function of the initial cohesion C0 and the damage variable D, as shown in formula (10):

[0076] C=C0(1-D) (10)

[0077] Where: C0 is the cohesive force in the initial damage-free state.

[0078] 2.2) Calculate the damage variable D

[0079] Calculate the failure criteria F1 and F2 based on the maximum principal stress σ1 and the minimum principal stress σ3

[0080] F1=σ1-f t (11)

[0081]

[0082] Determine the injury status

[0083]

[0084] ε t0 is the initial tensile strain of the rock; ε1 is the current maximum principal strain; ε c0 is the initial compressive strain of the rock; ε3 is the current minimum principal strain; dF1 and dF2 represent the rates of change of F1 and F2, respectively;

[0085] If F1<0 and F2<0, the rock has not reached the tensile or shear failure condition and is in a non-destructive state, and the damage variable D=0;

[0086] If F1 = 0 and dF1 > 0, the rock reaches the tensile failure condition, and the tensile stress is still increasing, the damage variable

[0087] If F2 = 0 and dF2 > 0, the rock reaches shear failure condition, and the compressive stress is still increasing, the damage variable

[0088]

[0089] 2.3) Substitute equations (1)(2)(7)(8)(9)(10) into equation (14) to construct the fault activation tendency state function M(D) expressed by the damage variable D, as shown in equation (15):

[0090] M(D)=τ max -τ f (14)

[0091]

[0092] Finally, the fault activation tendency state function M(D) is obtained.

[0093] 3) Establish a criterion for fault activation tendency and evaluate the tendency and risk of fault activation based on the discriminant.

[0094] When M(D)<0, it means that the friction resistance of the fault plane is stronger than the shear stress of the fault plane, and the fault has no activation tendency;

[0095] When M(D)>0, it means that the friction resistance of the fault plane is weaker than the shear stress of the fault plane, and the fault has a tendency to activate;

[0096] When M(D) = 0, it means that the friction resistance of the fault plane is equal to the shear stress of the fault plane, and the fault is in a critical activation state.

Claims

1. A method for determining the tendency of hydraulic fracturing-induced fault activation taking into account rock damage, characterized in that: 1) Basic parameters: triaxial compression test is used to determine the cohesion c and internal friction angle of hydraulic fracturing formation. The pore water pressure p0, the first, second and third principal stresses σ1, σ2, σ3 of the fault determined by logging and seismic inversion data, the length L of the hydraulic fracture with layered flow closest to the fault, the fault dip angle β and its strike direction; 2) Based on the Moore-Coulomb law, a fault activation tendency state function M(D) expressed by the damage variable D is established; 3) Establish a criterion for fault activation tendency and evaluate the tendency and risk of fault activation based on the discriminant.

2. A method for determining the tendency of hydraulic fracturing-induced fault activation considering rock damage according to claim 1, characterized in that: In the step 1), the specific method is: Assuming that the upper and lower plates of the fault are homogeneous elastic bodies, the upper and lower plates are in contact through the fault plane, and are subjected to the maximum principal stress in the vertical direction and the minimum principal stress in the horizontal direction. Shear movement of the upper and lower plates along the fault plane is regarded as fault activation. A three-dimensional fault structure diagram is established, and a two-dimensional geomechanical model for fault activation analysis is drawn. The model satisfies that the normal direction of the fault plane is contained in the plane determined by the directions of the first principal stress σ1 and the third principal stress σ3, and the direction of the second principal stress σ2 is contained in the fault plane, σ1>σ2>σ3, and is perpendicular to the normal direction n of the fault plane. Therefore, σ2 will not affect the normal stress and shear stress of the fault. The fault dip angle is β. According to the established model, the normal stress σ n , shear stress τ n The relationship between the maximum and minimum principal stresses is: Where: σ1 is the maximum principal stress in the vertical direction; β is the angle between the fault normal and the maximum principal stress. When σ1 is in the vertical direction, β is the fault dip; σ3 is the minimum principal stress in the horizontal direction.

3. A method for determining the tendency of hydraulic fracturing-induced fault activation considering rock damage according to claim 1, characterized in that: In the step 2), the specific method is: 2.1) According to the specific theory of Mohr-Coulomb failure criterion: Under a certain stress state, if the maximum shear stress is greater than or equal to the shear strength of the rock, it can be determined that the fault may be activated. And the relationship between the fault activation tendency and the normal and shear stress of the fault plane and the friction strength of the fault plane is established, which is specifically expressed as follows: t max ≥τ f =μ(σ n -αp)+C (3) In formula (3), μ is the fault friction coefficient, p is the fluid penetration pressure of the damaged fault zone, C is the cohesion of the fault; α is the Biot coefficient; Calculate rock porosity based on average effective stress: In formula (4), The porosity at zero stress state; is the stress sensitivity coefficient of porosity, which can be taken as 5.0×10-8Pa-1; is the limit value of porosity under high compressive stress state. is the average effective stress, which can be calculated according to formula (5): Where: σ2 is the second principal stress; p0 is the pore water pressure when the formation is initially undamaged; The effect of damage on permeability is shown in formula (6): Where: k0 is the permeability under zero stress state, D is the damage variable, α D is the damage sensitivity coefficient. According to Darcy's law, the water pressure difference between undamaged and damaged rocks can be expressed by equations (7) and (8): p=p0+Δp-Δp0 (9) In formula (8), Δp0 and Δp are the water pressure differences between the undamaged and damaged rocks, respectively; Q is the amount of fluid passing through the pores of this rock per unit time; μ0 is the fluid viscosity; A is the cross-sectional area of ​​the rock pores at the fault; As the damage variable D increases, the microcracks and defects inside the rock gradually expand, resulting in a decrease in the cohesion C. The cohesion C can be expressed as a function of the initial cohesion C0 and the damage variable D, as shown in formula (10): C=C0(1-D) (10) Where: C0 is the cohesive force in the initial damage-free state. 2.2) Calculate the damage variable D Calculate the failure criteria F1 and F2 based on the maximum principal stress σ1 and the minimum principal stress σ3 F1=σ1-f t (11) Determine the injury status ε t0 is the initial tensile strain of the rock; ε1 is the current maximum principal strain; ε c0 is the initial compressive strain of the rock; ε3 is the current minimum principal strain; dF1 and dF2 represent the rates of change of F1 and F2, respectively; If F1<0 and F2<0, the rock has not reached the tensile or shear failure condition and is in a non-destructive state, and the damage variable D=0; If F1 = 0 and dF1 > 0, the rock reaches the tensile failure condition, and the tensile stress is still increasing, the damage variable If F2 = 0 and dF2 > 0, the rock reaches shear failure condition, and the compressive stress is still increasing, the damage variable 2.3) Substitute equations (1)(2)(7)(8)(9)(10) into equation (14) to construct the fault activation tendency state function M(D) expressed by the damage variable D, as shown in equation (15): M(D)=τ max -t f (14) Finally, the fault activation tendency state function M(D) is obtained.

4. A method for determining the tendency of hydraulic fracturing-induced fault activation considering rock damage according to claim 1, characterized in that: The specific method in the step 3) is: When M(D)<0, it means that the friction resistance of the fault plane is stronger than the shear stress of the fault plane, and the fault has no activation tendency; When M(D)>0, it means that the friction resistance of the fault plane is weaker than the shear stress of the fault plane, and the fault has a tendency to activate; When M(D) = 0, it means that the friction resistance of the fault plane is equal to the shear stress of the fault plane, and the fault is in a critical activation state.

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