Method for predicting interaction between hydraulic fracture and multistage structural surface

Abstract

The invention relates to a method for predicting interaction between a hydraulic fracture and a multi-stage structural surface. The method comprises the following steps: calculating primary and secondary hydraulic fracture net pressures; calculating induced stress generated by the first-stage hydraulic fracture based on the net pressure of the first-stage hydraulic fracture; second-stage hydraulic fracture flow distribution is calculated based on the induced stress generated by the first-stage hydraulic fracture; calculating induced stress generated by the secondary hydraulic fracture based on the net pressure of the secondary hydraulic fracture; and judging the intersection behavior of the second-stage hydraulic fracture and the natural fracture according to the condition that the second-stage hydraulic fracture passes through the second-stage structural surface. According to the method, the intersection behavior of the hydraulic fracture and the multi-stage structural plane in the toughness and viscosity dominant mode can be accurately predicted, and the unconventional oil and gas reservoir transformation effect can be optimized; according to the method, the influence factors influencing the interaction between the hydraulic fracture and the structural surface in different dominant modes are analyzed, the research result can provide theoretical support for the hydraulic fracture expansion condition and the expansion mode under the influence of the multi-stage structural surface, and a thought is provided for optimization of a fractured reservoir efficient fracturing process.

Description

Technical Field

[0001] The invention relates to the technical field of reservoir exploration, and in particular to a method for predicting the interaction between hydraulic fractures and multi-level structural surfaces. Background Art

[0002] With the continuous exploration and development of fossil resources, unconventional oil and gas has become one of the world's major energy sources. At present, unconventional hydraulic fracturing technology is one of the main means to transform reservoirs and improve oil and gas recovery. Fractured reservoirs may develop multi-level structural planes, and the morphology and distribution of hydraulic fractures are significantly affected by the structural planes. There are differences in the transformation concepts of different types of reservoirs. For example, tight sandstone reservoirs with developed fractures need to increase SRV, while low-permeability coal reservoirs need to increase the volume of hydraulic fractures.

[0003] At present, theoretical research on the interaction between hydraulic fractures and structural surfaces mainly focuses on the interaction between single structural surfaces. In unconventional reservoirs, structural surfaces (joint surfaces, natural fractures, and cleats) are relatively developed, and multi-level structural surfaces together form a fracture system. Hydraulic fractures intersect with the fracture system many times during the expansion process, and the hydraulic fracture network morphology is complex. However, the expansion law of hydraulic fractures under the influence of multi-level structural surfaces is currently unclear, and it is difficult to achieve economical and efficient development for different types of fractured reservoirs. Therefore, it is urgent to construct the interaction mechanism between hydraulic fractures and multi-level structural surfaces, clarify the expansion characteristics of hydraulic fractures under the influence of multi-level structural surfaces, and provide a theoretical basis for the efficient transformation of fractured reservoirs. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a method for predicting the interaction between hydraulic fractures and multi-level structural surfaces.

[0005] The present invention is achieved through the following technical solutions:

[0006] The method for predicting the interaction between hydraulic fractures and multi-level structural surfaces includes the following steps:

[0007] Calculate the net pressure of the first-level hydraulic fracture;

[0008] Calculating the induced stress generated by the primary hydraulic fracture based on the relationship between the net pressure and the geometric position of the primary hydraulic fracture;

[0009] Calculating a flow distribution ratio of a secondary hydraulic fracture based on the induced stress generated by the primary hydraulic fracture;

[0010] Calculate the secondary hydraulic fracture flow rate based on the secondary hydraulic fracture flow distribution ratio;

[0011] Based on the secondary hydraulic fracture flow rate, the net pressure of the secondary hydraulic fracture is calculated;

[0012] Based on the relationship between the net pressure and geometric position of the secondary hydraulic fracture, the induced stress generated by the secondary hydraulic fracture is calculated; and the intersection behavior of the secondary hydraulic fracture and the natural fracture is determined by the condition that the secondary hydraulic fracture passes through the secondary structural surface.

[0013] Compared with the prior art, this application has at least the following beneficial effects:

[0014] This application can accurately predict the intersection behavior of hydraulic fractures and multi-level structural surfaces in toughness and viscosity-dominated modes by coupling fluid flow, stress shadow effect and poroelastic effect, and calculating the flow distribution of secondary hydraulic fractures, which is conducive to optimizing the transformation effect of unconventional oil and gas reservoirs;

[0015] In addition, this application analyzes the factors affecting the interaction between hydraulic fractures and structural surfaces in different dominant modes, including structural surface spacing, interface strength, construction factors, and rock mechanics parameters. The research results can provide theoretical support for the expansion conditions and expansion modes of hydraulic fractures under the influence of multi-level structural surfaces, and provide ideas for optimizing the efficient fracturing process of fractured reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0017] Figure 1 It is the interaction diagram between the first-level hydraulic fracture and the first-level structural surface;

[0018] Figure 2 is the interaction diagram between the secondary hydraulic fracture and the secondary structural surface;

[0019] Figure 3 Schematic diagram of the interaction between the secondary hydraulic fracture and the secondary structural surface

[0020] Figure 4 It is a flow chart of the method for predicting the interaction between hydraulic fractures and multi-level structural surfaces in the embodiment;

[0021] Figure 5 Schematic diagram of the total stress superposition principle in the embodiment;

[0022] Figure 6 This is a schematic diagram of the induced stress superposition principle in the embodiment;

[0023] Figure 7 Schematic diagram of the plastic yield range of the hydraulic fracture tip in the embodiment

[0024] Figure 8 A comparison diagram between the new method in the embodiment and the Zhou experiment;

[0025] Fig. 9 A comparison diagram between the new method in the embodiment and the Blanton experiment;

[0026] Fig.10 It is a comparison diagram between the new method in the embodiment and the experiment of Sarmadivaleh;

[0027] Fig.11 A comparison diagram of the new method and the R&P method in the embodiment;

[0028] Fig.12 A schematic diagram of a plane strain hydraulic fracture propagation numerical model in an embodiment;

[0029] Fig.13 is a distribution diagram along path A in the embodiment;

[0030] Fig.14 It is a comparison diagram of the theoretical solution and the numerical solution in the embodiment. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0032] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments of the present invention can be combined with each other. It should be noted that the various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments, and the same and similar parts between the various embodiments can be referred to each other.

[0033] First, clarifying the interaction between hydraulic fractures and single structural planes is the basis for understanding the interaction between hydraulic fractures and multi-level structural planes. When the primary hydraulic fracture (FHF) approaches the primary structural plane (FSP) infinitely, they will interact with each other. Figure 1 As shown in Figure 1, the interaction between the primary hydraulic fracture and the primary structural surface can be divided into four types: (b) the primary hydraulic fracture does not pass through the primary structural surface, and the hydraulic fracture tip is blunted; (c) the primary hydraulic fracture does not pass through the primary structural surface and turns along the primary structural surface; (d) the primary hydraulic fracture directly passes through the primary structural surface; (e) the primary hydraulic fracture passes through the primary structural surface and turns along the primary structural surface. Similarly, Figure 2As shown in the figure, the primary hydraulic fracture turns along the primary structural surface to form two secondary hydraulic fractures (SHF). When the secondary hydraulic fracture approaches the secondary structural surface (SSP) infinitely, their interactions can be divided into four types: (b) the secondary hydraulic fracture tip is passivated at the secondary structural surface; (c) the secondary hydraulic fracture does not pass through the secondary structural surface and turns to expand; (d) the secondary hydraulic fracture directly passes through the secondary structural surface; (e) the secondary hydraulic fracture passes through the secondary structural surface and turns to expand.

[0034] This embodiment studies the interaction between secondary hydraulic fractures and secondary structural surfaces, but the method of this embodiment is also applicable to the interaction between primary hydraulic fractures and primary structural surfaces.

[0035] It is assumed that hydraulic fractures propagate in uniform linear elastic and impermeable rock, that the fluid pressure inside the hydraulic fracture is uniformly applied on the fracture surface, that the fracturing fluid is incompressible, and that hydraulic fracture propagation is continuous and quasi-static. The problems involved are limited to two-dimensional plane strain conditions.

[0036] like Figure 3 As shown, the maximum horizontal principal stress σ H and the horizontal minimum principal stress σ h The angle between the primary structure surface and the y-axis is α, and the angle between the secondary structure surface and the x-axis is The blue part represents the hydraulic fracture. xy ), the primary hydraulic fracture first expands along the positive direction of the y-axis close to the primary structural surface, and then turns to expand at the origin O after being blocked by the primary structural surface to form a secondary hydraulic fracture, which is divided into two secondary hydraulic fractures on the left and right. For the convenience of calculation, a local coordinate system (O'θ x θ y ), where θ x Along the secondary structure plane, θ y Perpendicular to the secondary structure plane, σ xx , σ yy and τ xy They are local coordinate systems O'θ x θ y The stress state at a point (x, y) on the surface.

[0037] like Figure 4 As shown, the method for predicting the interaction between hydraulic fractures and multi-level structural surfaces disclosed in this embodiment includes the following steps:

[0038] 1. Calculate the net pressure of primary and secondary hydraulic fractures through the plane strain hydraulic fracturing theory solution

[0039] The expansion mode of hydraulic fractures is significantly affected by the displacement and viscosity of the fracturing fluid. The energy dissipation of the fluid is mainly concentrated in two aspects: breaking the rock and overcoming the flow resistance. Therefore, it can be divided into two dominant modes: toughness and viscosity. For example, when using a large displacement of slick water or supercritical CO 2 When fracturing fluid is used, the energy consumed by the flow of fracturing fluid is very small, and the expansion mode of hydraulic fracture is dominated by toughness. When high-viscosity guar fracturing fluid is used, the energy of the fracturing fluid is mainly dissipated in overcoming flow resistance, and this is the viscosity-dominated stage. Detournay proposed the length l, width w and net pressure P of hydraulic fractures in impermeable rocks. net The propagation mechanism is controlled by the dimensionless constant k:

[0040]

[0041] In the above formula, k', E', and μ' are three material parameters respectively; Q is the injection rate, m 3 / s.

[0042]

[0043] In the above formula, K IC is the fracture toughness of rock, MPa.m 1 / 2 , E is the Young's modulus of rock, MPa, v is Poisson's ratio, and μ represents the viscosity of the fracturing fluid, mPa·s.

[0044] As shown in Table 1, hydraulic fracture length l, width w and net pressure P net It can be expressed as:

[0045] Table 1: KGD model analytical solution

[0046]

[0047] The net pressure of primary and secondary hydraulic fractures are calculated using the formula in Table 1, and different k values ​​represent different hydraulic fracturing conditions.

[0048] 2. Based on the mathematical model of structural surface stress field, calculate the induced stress generated by the first-level hydraulic fracture

[0049] The stress superposition principle is used to analyze the total stress acting on the secondary structure surface, and this principle is still valid in linear elastic fracture mechanics. Figure 5 , Figure 6 As shown in the figure, the total stress on the secondary structural surface is equivalent to the superposition of far-field stress and induced stress. The far-field stress is uniformly distributed, and the induced stress can be divided into stress shadow, poroelasticity and crack tip induced stress. Among them, stress shadow and poroelasticity induced stress come from the net pressure in the hydraulic fracture and are controlled by the fluid pressure. The crack tip induced stress is the stress in the crack tip area of ​​the I-type crack extension and is controlled by the stress intensity factor.

[0050] During the process of reservoir fracturing, the expansion of hydraulic fractures will significantly disturb the in-situ stress field in the formation, and the stress magnitude and direction around the hydraulic fractures will change. This effect is called the stress shadow effect. According to Llanos, the stress shadow induced stress field caused by hydraulic fractures is expressed as follows:

[0051]

[0052] In the above formula, σ sx1 is the stress shadow induced stress in the x direction caused by the primary crack, σ sy1 is the stress shadow induced stress in the y direction caused by the primary crack, τ s1 is the stress shadow induced shear stress generated by the primary crack, P net1 is the net pressure in the first-level hydraulic fracture, l i (i=0,1,2) is the geometric position relationship, θ i (i=0,1,2) is the geometric angle; Figure 4 shown.

[0053]

[0054] In the above formula, σ sx2 is the stress shadow induced stress in the x direction caused by the secondary crack, σ sy2 is the stress shadow induced stress in the y direction caused by the secondary crack, τ s2 is the stress shadow induced shear stress generated by the secondary crack, P net2 is the net pressure in the secondary hydraulic fracture, r i (i=0,1,2) is the geometric position relationship, β i (i=0,1,2) is the geometric angle, such as Figure 4 shown.

[0055] The change in pore pressure caused by injection and production of fluids will have a significant impact on the magnitude and direction of ground stress. Figure 4 As shown in the figure, when the tip of the secondary hydraulic fracture is infinitely close to the secondary structural surface, the radius of the plastic zone at the tip of the secondary hydraulic fracture is very small. Therefore, it can be considered that the seepage field at the intersection of the tip of the secondary hydraulic fracture and the secondary structural surface is in a stable state. Figure 3 The net pressure is applied at the O' point, producing a poroelastic effect σ px and σ py As shown in formula (6), its value depends on the net pressure and Biot coefficient. Wright expressed the poroelastic response during hydraulic fracturing as:

[0056]

[0057] In the above formula, σ px1 is the poroelastic induced stress in the x direction, σ py1 is the poroelastic induced stress in the y direction, ε is the Biot coefficient, and v is the Poisson's ratio.

[0058]

[0059] In the above formula, σ px2 is the poroelastic induced stress in the x direction, σ py2 is the poroelastic induced stress in the y direction, ε is the Biot coefficient, and v is the Poisson's ratio.

[0060] From linear elastic fracture mechanics, we know that the stress solution at the tip of a type I crack is approximately:

[0061]

[0062] In the above formula, σ fx is the type I crack stress in the x direction, σ fy is the type I crack stress in the y direction, τ fxy is the shear stress of mode I crack, K I is the stress intensity factor, r is the length from the tip of the hydraulic fracture, in meters.

[0063] The far-field stress component on the secondary structure surface can be expressed as:

[0064]

[0065] In the above formula, and τ ixy They are respectively the normal stress and shear stress on the failure path of the secondary structure surface unit body under the action of far-field stress.

[0066] The stress component induced by stress shadow on the secondary structure surface can be expressed as:

[0067]

[0068] In the above formula, and τ sxy They are respectively the normal stress and shear stress on the failure path of the secondary structural surface unit body affected by the stress shadow.

[0069] The poroelastic induced stress component on the secondary structural surface can be expressed as:

[0070]

[0071] In the above formula, and τ pxy are the normal stress and shear stress on the failure path of the secondary structural surface unit body under the action of poroelasticity, respectively.

[0072] The stress component of type I crack on the secondary structural surface can be expressed as:

[0073]

[0074] In the above formula, and τ fxy They are the normal stress and shear stress on the failure path of the secondary structural surface unit body under the action of mode I crack.

[0075] The stress on the unit cell on the secondary structure surface can be obtained by superimposing the equation (12):

[0076]

[0077] In the above formula, σ xx is the x-direction stress of the unit cell on the secondary structure surface, σ yy is the y-direction stress of the unit cell on the secondary structure surface, τ xy is the combined tangential stress of the unit cell on the secondary structure surface.

[0078] The normal stress and shear stress on the failure path of the secondary structure surface unit can be obtained by superposition equation (13):

[0079]

[0080] In the above formula, and is the combined normal and tangential stress on the unit cell.

[0081] 3. Calculate the flow distribution of secondary hydraulic fractures

[0082] When the primary hydraulic fracture turns and expands along the primary structural surface, two secondary hydraulic fractures are formed. Due to the difference in crack width between the secondary hydraulic fracture and the primary hydraulic fracture, the hydraulic fracture can be regarded as a sudden contraction pipeline model. Wang et al. proposed that the pressure drop caused by the flow of fluids with different flow rates in the pipeline is:

[0083]

[0084] In the above formula, ΔP i is the pressure drop of the secondary hydraulic fracture, M is the pressure drop constant, ρ is the fluid density, Q i is the displacement of the secondary hydraulic fracture.

[0085] Under the condition of short hydraulic fracture length, the fracturing loss caused by the flow of fracturing fluid can be ignored, so the pressure loss of secondary hydraulic fracture is only related to the normal stress on the fracture surface, which can be expressed as:

[0086]

[0087] In the above formula, Ptotal is the pressure inside the crack, MPa; R Q is the flow ratio of the two secondary hydraulic fractures.

[0088] 4. Determine the plastic yield area at the crack tip and calculate the induced stress generated by the secondary hydraulic fracture

[0089] During the expansion of hydraulic fractures, there is obvious stress concentration at the tip of the hydraulic fracture, and the stress at the tip of the hydraulic fracture approaches infinity, which is enough to cause plastic deformation of the rock. In order to avoid the influence of stress singularity and truly reflect the stress distribution at the tip of the hydraulic fracture, considering the small range of plastic yield at the tip of the hydraulic fracture, linear elastic fracture mechanics is only applicable to the plastic zone (r p ) Figure 7 Similar to other studies analyzing crack intersection problems, it is assumed that the stress in the plastic region of the secondary structural surface is less than or equal to the plastic zone radius (r p ) stress at .

[0090] The Drucker-Prager yield function F is expressed using the stress tensor invariant as:

[0091]

[0092] In the above formula, I 1 is the first invariant of the stress tensor; J 2 is the second invariant of stress deviator; a and b are material parameters; c and δ are rock cohesion and internal friction angle, respectively.

[0093] 5. The intersection behavior of secondary hydraulic fractures and natural fractures is determined by the conditions under which the secondary hydraulic fractures pass through the secondary structural surface.

[0094] Condition 1: When the maximum tensile stress at the tip of the secondary hydraulic fracture reaches the tensile strength of the rock on the other side of the secondary structural surface, the secondary hydraulic fracture will pass through the secondary structural surface. Otherwise, the tip of the secondary hydraulic fracture will be blunted or will turn and expand along the secondary structural surface:

[0095]

[0096] In the above formula, σ 1 is the maximum tensile stress at the tip of the secondary hydraulic fracture, and T is the tensile strength of the rock on the opposite side.

[0097] Condition 2: Formula (19) is derived from linear elasticity, that is, the location of stress calculation should be within the plastic zone (r p ), the formula (20) must be satisfied:

[0098] F>0 (20)

[0099] Condition 3: When the secondary structural surface remains stable and does not slide, the secondary hydraulic fracture may continue to extend through the secondary structural surface. Otherwise, it cannot extend through the layer. When the normal stress on the secondary structural surface is compressive stress, the Moore-Coulomb criterion can be used to determine whether the secondary structural surface has sliding damage:

[0100]

[0101] In the above formula, C and η are the cohesion and friction coefficient of the structural surface.

[0102] If all three conditions are met, it is considered as passing through, otherwise it is considered as not passing through.

[0103] 6. Experimental verification

[0104] In order to verify the applicability of the method of the present invention (hereinafter referred to as the new method), this example verifies the analysis results with the experimental results published by previous researchers. Limited by the experimental conditions and the complexity of model preparation, the indoor experiments published so far are all experiments on the intersection of hydraulic fractures and single natural fractures. The basic experimental parameters are shown in Table 2. Figure 8 and Fig. 9 The comparison results show that 22 of the 25 experimental results are consistent with the results calculated by the new method. The reason for the inconsistency may be related to the properties of the prefabricated crack material or experimental errors. Zhou used different experimental materials to simulate the differences in the properties of natural cracks. The plastic zone at the crack tip and the stress singularity may cause the actual plastic zone to be larger than the theoretical plastic zone radius, which in turn affects the experimental results. In general, the new method has good consistency with the indoor experimental results.

[0105] Table 2: Zhou and Blanton experimental material parameters

[0106]

[0107] In order to verify the applicability of the viscosity-dominated part of the new method, this example verifies the analysis results with the experimental results published by Sarmadivaleh, and the basic experimental parameters are shown in Table 3. Fig.10 From the comparison results, it can be seen that 7 of the 8 experimental groups have consistent results with the calculation results of the new method. The reason for the inconsistency of the remaining 1 group of results may be that during the preparation of the experimental samples, the filling glue was affected by the environment, resulting in differences in the properties of the glue, which in turn affected the experimental results. In general, the viscosity-dominated part of the new method has good consistency with the indoor experimental results.

[0108] Table 3: Sarmadivaleh experimental material parameters

[0109]

[0110] 7. Theoretical Verification

[0111] From the above analysis, it can be seen that the intersection behavior of hydraulic fractures and multi-level structural surfaces is mainly affected by parameters such as intersection angle, far-field stress and interface strength. The new method is compared with the R&P method. The results are as follows Fig.11 As shown in the stress ratio-friction coefficient diagram, the curve above represents that the hydraulic fracture can pass through the structural surface, otherwise, the hydraulic fracture will be blunted or turned. Compared with the R&P method, the new method is easier for hydraulic fractures to pass through the structural surface. This is because the R&P method only considers the effects of the crack tip induced stress and horizontal stress of type I cracks on crack propagation. Under the condition of constant tensile strength, the critical sliding friction coefficient that affects the intersection behavior in the R&P method (Formula 22) is only related to the horizontal stress ratio, and is negatively correlated.

[0112]

[0113] For the new method, it can be seen from formula (8) that the horizontal stress ratio (σ H / σ h ) only affects the far-field stress component. Because the intersection angle α between the hydraulic fracture and the structural surface is 90°, the effect of the change in the horizontal stress ratio on the shear stress component acting on the structural surface can be ignored. From the stress superposition formula (13), it can be seen that in the new method, the compressive stress acting on the structural surface is It is larger than the R&P method, so when the hydraulic fracture intersects the structural surface, the hydraulic fracture is more likely to extend through the structural surface.

[0114] 8. Numerical verification

[0115] Construct a plane strain hydraulic fracture propagation numerical model such as Fig.12 As shown in Table 4, the rock mechanical parameters and ground stress state are shown in Table 4. The displacement is 0.001m 3 / s, and the viscosity of the fracturing fluid is 0.01mPa·s. The model size is set to 1.2m×1.2m, the fracturing point is located at the center of the lower formation, the hydraulic fracture extends along the horizontal maximum principal stress direction, a primary structural surface is set in the middle of the model, the intersection angle between the primary hydraulic fracture and the structural surface is γ, path A is set along the structural surface direction, and paths B and C are set along the horizontal and vertical directions of the model respectively.

[0116] Table 4: Geological parameters and far-field stress

[0117]

[0118] Set the intersection of the hydraulic fracture and the structural surface as the origin, and extract the displacement ratio R of the secondary hydraulic fractures on both sides along path A. Q , the results are as follows Fig.13 As shown. When the intersection angle γ = 70°, R QThe maximum value of the numerical solution is 2.25, R Q The maximum value of the theoretical solution is 2.04, the relative error is -9.33%, and R Q The minimum value of the numerical solution is 1.34, R Q The minimum theoretical solution is 1.42, and the relative error is 5.97%. When the intersection angle γ = 80°, R Q The maximum value of the numerical solution is 1.6, R Q The maximum value of the theoretical solution is 1.49, the relative error is -6.88%, and R Q The minimum value of the numerical solution is 1.18, R Q The minimum value of the theoretical solution is 1.21, and the relative error is 2.54%. The main reason for the difference is that the numerical solution takes into account the fluid loss problem in the hydraulic fracture, while the theoretical solution assumes that the fluid is an ideal Newtonian fluid and ignores the effect of fluid loss on fracture expansion. Therefore, as the distance increases, the relative error between the numerical solution and the theoretical solution gradually decreases. Overall, the theoretical solution and the numerical solution have good consistency.

[0119] Assuming that the stress along the y-axis is S22, the S22 result is as follows: Fig.14 As shown in (a) and (b), stress follows the rules of elastic mechanics, tensile stress is positive, and compressive stress is negative. Affected by the stress shadow induced stress, the compressive stress on the surface of the primary hydraulic fracture increases significantly. At the tip of the secondary hydraulic fracture, there is a stress concentration phenomenon in the theoretical solution results, which leads to a positive S22 stress, while the numerical solution solves this problem well. Overall, the theoretical solution and the numerical solution have good consistency in stress distribution results. The S22 values ​​are extracted along path B and path C, respectively, as shown in Fig.14 (c) and (d). Fig.14 In (c), both the numerical solution and the theoretical solution S22 reach the minimum value at the intersection of the primary hydraulic fracture and the primary structural surface. The minimum normal stress in the numerical solution is -20.19MPa, and the minimum normal stress in the theoretical solution is -22.14MPa, with a relative error of 9.66%. As the distance increases, the theoretical solution value of S22 gradually increases. The crack of the secondary hydraulic fracture on the right is larger than that of the secondary hydraulic fracture on the left. The maximum value reaches -7.07MPa, and then gradually decreases. At a location far away from the secondary hydraulic fracture, the theoretical solution and the numerical solution results gradually converge and return to the far-field stress magnitude.

[0120] like Fig.14As shown in (d), on path C, the theoretical solution and numerical solution of S22 reach the maximum value at the tip of the lower side of the primary hydraulic fracture, and then along the surface direction of the primary hydraulic fracture, the theoretical solution gradually decreases and reaches the minimum at the intersection, which is -22.14MPa, while the numerical solution reaches the minimum of -21.45MPa at 7mm from the tip of the lower side of the primary hydraulic fracture. As the distance increases, S22 gradually increases and stabilizes at -16.8MPa. The phenomenon of large differences in S22 appears on both paths B and C. The main reason is that in the theoretical solution, the tip of the secondary hydraulic fracture is a stress singularity point, σ sy and σ fy are all positive infinity. When calculating S22 using the stress superposition principle formula (12), the theoretical solution will show an abnormally high S22 result near the tip of the secondary hydraulic fracture. This is unavoidable. The numerical solution solves the problem of stress singularity. Overall, the theoretical solution and the numerical solution are in good agreement.

[0121] This embodiment provides a new method for predicting the interaction between hydraulic fractures and multi-level structural surfaces by coupling fluid flow, stress shadow effect and poroelastic effect, solves the flow distribution problem of secondary hydraulic fractures, and can accurately predict the intersection behavior of hydraulic fractures and multi-level structural surfaces in toughness and viscosity dominant modes, which is conducive to optimizing the transformation effect of unconventional oil and gas reservoirs. In addition, this application analyzes the influencing factors (structural surface spacing, interface strength, construction factors and rock mechanics parameters) that affect the interaction between hydraulic fractures and structural surfaces in different dominant modes. The research results can provide theoretical support for the expansion conditions and expansion modes of hydraulic fractures under the influence of multi-level structural surfaces, and provide ideas for optimizing the efficient fracturing process of fractured reservoirs.

[0122] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for predicting the interaction between hydraulic fractures and multi-level structural surfaces, characterized in that: The following steps are involved: Calculate the net pressure of the first-level hydraulic fracture; Calculating the induced stress generated by the primary hydraulic fracture based on the relationship between the net pressure and the geometric position of the primary hydraulic fracture; Calculating a flow distribution ratio of a secondary hydraulic fracture based on the induced stress generated by the primary hydraulic fracture; Calculate the secondary hydraulic fracture flow rate based on the secondary hydraulic fracture flow distribution ratio; Based on the secondary hydraulic fracture flow rate, the net pressure of the secondary hydraulic fracture is calculated; Calculating the induced stress generated by the secondary hydraulic fracture based on the relationship between the net pressure and the geometric position of the secondary hydraulic fracture; The intersection behavior of secondary hydraulic fractures and natural fractures is determined by the conditions under which the secondary hydraulic fractures pass through the secondary structural surface.

2. The method for predicting the interaction between hydraulic fractures and multi-level structural surfaces according to claim 1, characterized in that: The induced stresses include stress shadow, poroelasticity and mode I crack tip stress: The stress shadow induced stress field generated by the first-level hydraulic fracture is expressed as: In the above formula, σ sx1 is the stress shadow induced stress in the x direction generated by the first-level hydraulic fracture, σ sy1 is the stress shadow induced stress in the y direction generated by the first-level hydraulic fracture, τ s1 is the stress shadow induced shear stress generated by the primary hydraulic fracture, P net1 is the net pressure in the first-level hydraulic fracture, l i (i=0,1,2) is the geometric position relationship, θ i (i=0,1,2) is the geometric angle; The poroelastic induced stress field generated by the first-order hydraulic fracture is expressed as: In the above formula, σ px1 is the poroelastic stress induced in the x direction by the primary hydraulic fracture, σ py1 is the poroelastic induced stress in the y direction generated by the first-order hydraulic fracture, ε is the Biot coefficient, and v is the Poisson's ratio; The mode I fracture stress field at the hydraulic fracture tip is expressed as: In the above formula, σ fx is the type I crack stress in the x direction, σ fy is the type I crack stress in the y direction, τ fxy is the shear stress of mode I crack, K I is the stress intensity factor, and r is the length from the tip of the hydraulic fracture.

3. The method for predicting the interaction between hydraulic fractures and multi-level structural surfaces according to claim 1 or 2, characterized in that: The stress shadow induced stress field generated by the secondary hydraulic fracture is expressed as: In the above formula, σ sx2 is the x-direction stress shadow induced stress generated by the secondary hydraulic fracture, σ sy2 is the y-direction stress shadow induced stress generated by the secondary hydraulic fracture, τ s2 is the stress shadow induced shear stress generated by the secondary hydraulic fracture, P net2 is the net pressure in the secondary hydraulic fracture, r i (i=0,1,2) is the geometric position relationship, β i (i=0,1,2) is the geometric angle; The poroelastic induced stress field generated by the secondary hydraulic fracture is expressed as: In the above formula, σ px2 is the poroelastic stress induced in the x direction by the secondary hydraulic fracture, σ py2 is the poroelastic induced stress in the y direction generated by the secondary hydraulic fracture, ε is the Biot coefficient, and v is the Poisson’s ratio.

4. The method for predicting the interaction between hydraulic fractures and multi-level structural surfaces according to claim 1, characterized in that: The calculation formula for the flow distribution ratio of the secondary hydraulic fracture is: In the above formula, ΔP i is the pressure drop of the secondary hydraulic fracture, M is the pressure drop constant, ρ is the fluid density, Q i is the secondary hydraulic fracture flow rate, is the normal stress on the secondary hydraulic fracture surface, P total P total is the pressure inside the crack, in MPa; R Q is the flow ratio of the two secondary hydraulic fractures.

5. The method for predicting the interaction between hydraulic fractures and multi-level structural surfaces according to claim 1, characterized in that: The conditions for secondary hydraulic fractures to cross the secondary structural surface include the following three conditions: Condition 1: The maximum tensile stress at the tip of the secondary hydraulic fracture reaches the tensile strength of the rock on the other side of the secondary structural surface; Condition 2, F>0, F is the Drucker-Prager yield function; Condition 3, the secondary structure surface remains stable and does not slide; When conditions 1, 2 and 3 are all met, the secondary hydraulic fracture crosses the secondary structural surface.

6. The method for predicting the interaction between hydraulic fractures and multi-level structural surfaces according to claim 5, characterized in that: The calculation formula for the maximum tensile stress at the tip of the secondary hydraulic fracture is: In the above formula, σ1 is the maximum principal stress at the tip of the secondary hydraulic fracture, σ xx is the x-direction stress of the unit cell on the secondary structure surface, σ yy is the y-direction stress of the unit cell on the secondary structure surface, τ xy is the combined tangential stress of the unit cell on the secondary structure surface.

7. The method for predicting the interaction between hydraulic fractures and multi-level structural surfaces according to claim 5, characterized in that: The following formula is used to determine whether sliding failure occurs on the secondary structural surface: In the above formula, is the combined tangential stress on the unit cell, is the normal combined stress on the unit body, C is the cohesion of the structural surface, and η is the cohesion and friction coefficient of the structural surface.

8. The method for predicting the interaction between hydraulic fractures and multi-level structural surfaces according to claim 1, characterized in that: When k<1, the calculation formula of the net fracture pressure is: When k>4, the calculation formula of the net fracture pressure is: In the above formula, P net is the net fracture pressure, k is a dimensionless constant, E is the Young's modulus of the rock, in MPa, v is the Poisson's ratio, and μ represents the viscosity of the fracturing fluid, in mPa·s.

9. The method for predicting the interaction between hydraulic fractures and multi-level structural surfaces according to claim 8, characterized in that: In the above formula, Q is the injection rate, m 3 / s; K IC is the fracture toughness of rock, in MPa.m 1 / 2 .

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