Multi-cluster hydraulic fracturing fracture form prediction method based on fluid-solid coupling and application

Abstract

The invention belongs to the technical field of shale hydraulic fracturing fracture prediction, and discloses a multi-cluster hydraulic fracturing fracture form prediction method based on fluid-solid coupling, and the method comprises the steps: S1, determining model parameters, fluid data and boundary conditions of hydraulic fracturing fractures; s2, establishing a crack width model; s3, calculating fluid pressure according to the crack width model in the S2; s4, calculating the water head pressure load of the fluid in the crack to the two sides of the crack; s5, predicting the form of the gap according to the relation between the water head pressure load and the ultimate pressure; according to the method, the crack deformation condition at the initial stage of the time node is obtained through calculation, then the flow between cracks during seepage is calculated and considered according to the crack deformation, then the pressure load of fluid to the cracks is obtained according to the flow, and finally the shape of the cracks is predicted according to the relation between the pressure load and the ultimate pressure.

Description

Technical Field

[0001] The present invention belongs to the technical field of shale hydraulic fracturing crack prediction, and in particular relates to a prediction method and application of multi-cluster hydraulic fracturing crack morphology based on fluid-solid coupling. Background Art

[0002] With the breakthrough of exploration technology and the deepening of commercial development, the development of shale oil and gas reservoirs has gradually moved to deeper layers. However, the development of deep shale oil and gas is a world-class problem. Deep shale fracturing faces many difficulties: as the well depth increases, the friction resistance of the fracturing fluid increases, the construction displacement is limited, the net pressure in the fracture is low, the fracture-making ability is weakened, and the sand-liquid ratio is difficult to increase; as the temperature and pressure increase, the plastic characteristics of the rock are enhanced, and the fracture initiation and extension are difficult; the closing pressure is high, the proppant is severely embedded, the fractures of various scales are narrow, the fracture conductivity decreases rapidly, and the long-term conductivity is difficult to maintain.

[0003] The process of inducing the generation and expansion of cracks in rocks by injecting pressurized fluid into the internal space of rocks is called hydraulic fracturing, which has gradually become an important means of exploiting shale oil and gas. At present, the simulation and prediction research on hydraulic fracturing crack morphology has comprehensively considered factors such as crack height growth, fluid rheology related to time and temperature, various filtration mechanisms, bridge plugging and sand screening, pressure drop caused by limited crack extension in the near-wellbore zone, pore erosion, two-dimensional proppant migration, and heat transfer, from simple to complex, from two-dimensional to three-dimensional. However, the existing hydraulic fracturing crack initiation and expansion criteria are mainly aimed at sandstone reservoirs, while shale reservoirs are heterogeneous materials with narrow pores, prominent anisotropy, and complex natural fractures, joints and other weak surfaces. Therefore, the existing calculation or prediction methods of hydraulic fracturing cracks are difficult to apply to the prediction of hydraulic fracturing crack morphology in shale reservoirs. Summary of the invention

[0004] The present invention solves the technical problem by adopting the following technical solutions:

[0005] The method for predicting the fracture morphology of multi-cluster hydraulic fracturing based on fluid-solid coupling includes the following steps:

[0006] S1. Determine the model parameters, fluid data and boundary conditions of hydraulic fracturing fractures;

[0007] S2, establish crack width model;

[0008] S3, calculating the fluid pressure according to the fracture width model in S2;

[0009] S4. Calculate the hydraulic pressure load of the fluid in the fracture on both sides of the fracture;

[0010] S5. Based on the relationship between the head pressure load and the ultimate pressure, the shape of the gap is predicted.

[0011] Furthermore, the method for establishing the gap width model is:

[0012] Assume that the two sides M and N of the crack between blocks i and j are non-parallel, and the coordinates of side M are A(x1, y1) and B(x2, y2); the coordinates of side N are C(x3, y3) and D(x4, y4), then the distance between points A and C is b i , the distance between points B and D is b j The expressions are:

[0013]

[0014]

[0015] According to the equivalent hydraulic opening calculation formula:

[0016]

[0017] In the formula, q is the fracture seepage flow rate, q m is the average hydraulic opening b m The corresponding seepage volume;

[0018] and Therefore, the crack width calculation model is:

[0019]

[0020] Furthermore, the method for calculating the fluid pressure according to the fracture width model in S2 is:

[0021] Let the velocity of the fluid from point i to point j be v ij , whose expression is:

[0022]

[0023] In the formula, J ij is the hydraulic gradient, b is the fracture width, v is the fluid kinematic viscosity, and g is the gravitational acceleration;

[0024] Get the flow rate q between the cracks ij for:

[0025]

[0026] Fluid pressure: P = ρ w HkDJ

[0027] In the formula, ρ wis the fluid density, H is the hydraulic head height of the fluid in the fracture, and υ=νρ w ;

[0028] Then the flow rate q ij Further expressed as:

[0029] In the formula, Δp ij =ρ w g(H j -H i )=P j -P i +ρ w g(y j -y i ), y j and i Respectively represent the horizontal heights corresponding to point j and point i; H j H is the water head height of the fluid at point j of the fracture; i is the water head height of the fluid at point i of the fracture; P j is the pressure of the fluid at point j; P i is the pressure of the fluid at point i;

[0030] When there are N' cracks at point j, the total flow rate is Q j for:

[0031]

[0032] Fluid pressure P j for:

[0033]

[0034] In the formula, p j0 The compressive stress value at the initial stage of this time step at time j, K w is the bulk modulus of the fluid, Δt is the time step interval, ΔA j =A j -A j0 , A j and A j0 Represent the new and old basin areas respectively.

[0035] Furthermore, in step S4, the method of applying the hydraulic head pressure load of the fluid in the crack to both sides of the crack is:

[0036] Get the head pressure values ​​corresponding to point (x1, y1) and point (x2, y2):

[0037]

[0038] In the formula, γ wis the water capacity; h1 is the horizontal height of the point (x1, y1); h2 is the horizontal height of (x2, y2).

[0039] Furthermore, in step S5, according to the relationship between the water head pressure load and the ultimate pressure, the method for predicting the shape of the gap is:

[0040] If the water head pressure of the block point is greater than the limit pressure, the crack will be concave toward the inside of the block point; conversely, if the water head pressure of the block point is less than the limit pressure, the crack will be convex toward the outside of the block point. This process is repeated along the edge of the crack to obtain the final crack shape.

[0041] The above-mentioned method for predicting the morphology of multi-cluster hydraulic fracturing fractures based on fluid-solid coupling is applied to the prediction of the morphology of hydraulic fracturing fractures in shale reservoirs.

[0042] The advantages and positive effects of the present invention are:

[0043] 1. The present invention provides a method for predicting the morphology of multi-cluster hydraulic fracturing cracks based on fluid-solid coupling, which can be applied to the prediction of the morphology of hydraulic fracturing cracks in shale reservoirs, and provides a new idea for predicting the morphology of hydraulic fracturing cracks in the field;

[0044] 2. The method for predicting the morphology of multi-cluster hydraulic fracturing cracks based on fluid-solid coupling provided by the present invention first constructs a crack width model, then calculates the fluid flow rate and fluid pressure, and finally obtains the water head pressure according to the fluid pressure, and compares the water head pressure with the limit pressure. The logic is clear and easy to practice and apply;

[0045] 3. The method for predicting the morphology of multi-cluster hydraulic fracturing cracks based on fluid-solid coupling provided by the present invention has simple steps and improves the calculation efficiency while ensuring the calculation accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The technical solution of the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments, but it should be understood that these drawings are designed only for explanation purposes and are not intended to limit the scope of the present invention. In addition, unless otherwise specified, these drawings are intended only to conceptually illustrate the structural configurations described herein and are not necessarily drawn to scale.

[0047] Figure 1 A flow chart of a method for predicting the morphology of multi-cluster hydraulic fracturing cracks based on fluid-solid coupling provided in Example 1 of the present invention;

[0048] FIG2( a ) is a comparison diagram of the calculation results of the method of the present invention and the high-precision boundary element model in terms of crack radius provided in Example 2 of the present invention;

[0049] FIG2( b ) is a comparison diagram of calculation results of the method of the present invention and the high-precision boundary element model in terms of injection rate provided in Example 2 of the present invention;

[0050] FIG2( c ) is a comparison diagram of the calculation results of the method of the present invention and the high-precision boundary element model in terms of crack width provided in Example 2 of the present invention;

[0051] FIG2( d ) is a comparison diagram of the calculation results of the method of the present invention and the high-precision boundary element model in terms of crack area provided in Example 2 of the present invention; DETAILED DESCRIPTION

[0052] First of all, it should be noted that the specific structure, features and advantages of the present invention will be specifically described below by way of example, but all descriptions are only for illustration and should not be understood as limiting the present invention in any way. In addition, any single technical feature described or implied in the embodiments mentioned herein, or any single technical feature displayed or implied in the drawings, can still be combined or deleted between these technical features (or their equivalents) to obtain more other embodiments of the present invention that may not be directly mentioned in this document. In addition, in order to simplify the drawings, the same or similar technical features may be marked only in one place in the same drawing.

[0053] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application may be combined with each other.

[0054] Example 1

[0055] like Figure 1 As shown, the method for predicting the morphology of multi-cluster hydraulic fracturing cracks based on fluid-solid coupling provided in this embodiment includes the following steps:

[0056] S1. Determine the model parameters, fluid data and boundary conditions of hydraulic fracturing fractures;

[0057] S2, establish crack width model;

[0058] S3, calculating the fluid pressure according to the fracture width model in S2;

[0059] S4. Calculate the hydraulic pressure load of the fluid in the fracture on both sides of the fracture;

[0060] S5. Based on the relationship between the head pressure load and the ultimate pressure, the shape of the gap is predicted.

[0061] Specifically, the method for establishing the gap width model is:

[0062] Assume that the two sides M and N of the crack between blocks i and j are non-parallel, and the coordinates of side M are A(x1, y1) and B(x2, y2); the coordinates of side N are C(x3, y3) and D(x4, y4), then the distance between points A and C is b i , the distance between points B and D is b j The expressions are:

[0063]

[0064]

[0065] According to the equivalent hydraulic opening calculation formula:

[0066]

[0067] In the formula, q is the fracture seepage flow rate, q m is the average hydraulic opening b m The corresponding seepage volume;

[0068] and Therefore, the crack width calculation model is:

[0069]

[0070] The method for calculating fluid pressure based on the fracture width model in S2 is:

[0071] Let the velocity of the fluid from point i to point j be v ij , whose expression is:

[0072]

[0073] In the formula, J ij is the hydraulic gradient, b is the fracture width, v is the fluid kinematic viscosity, and g is the gravitational acceleration;

[0074] Get the flow rate q between the cracks ij for:

[0075]

[0076] Fluid pressure: P = ρ w HkDJ

[0077] In the formula, ρ w is the fluid density, H is the hydraulic head height of the fluid in the fracture, and υ=νρ w ;

[0078] Then the flow rate q ij Further expressed as:

[0079] In the formula, Δp ij =ρ w g(H j -H i )=P j -P i +ρ w g(y j -y i ), y j and i Respectively represent the horizontal heights corresponding to point j and point i; H j H is the water head height of the fluid at point j of the fracture; i is the water head height of the fluid at point i of the fracture; P j is the pressure of the fluid at point j; P i is the pressure of the fluid at point i.

[0080] When there are N' cracks at point j, the total flow rate is Q j for:

[0081]

[0082] Fluid pressure P j for:

[0083]

[0084] In the formula, p j0 The compressive stress value at the initial stage of this time step at time j, K w is the bulk modulus of the fluid, Δt is the time step interval, ΔA j =A j -A j0 , A j and A j0 Represent the new and old basin areas respectively.

[0085] In step S4, the method of applying the hydraulic pressure load of the fluid in the crack to both sides of the crack is:

[0086] Get the head pressure values ​​corresponding to point (x1, y1) and point (x2, y2):

[0087]

[0088] In the formula, γ w is the water capacity; h1 is the horizontal height of the point (x1, y1); h2 is the horizontal height of (x2, y2).

[0089] In step S5, according to the relationship between the water head pressure load and the ultimate pressure, the method for predicting the shape of the gap is:

[0090] If the water head pressure of the block point is greater than the limit pressure, the crack will be concave toward the inside of the block point; conversely, if the water head pressure of the block point is less than the limit pressure, the crack will be convex toward the outside of the block point. This process is repeated along the edge of the crack to obtain the final crack shape.

[0091] Example 2

[0092] Compared with the existing hydraulic fracturing fluid-solid coupling model, the present invention adopts a semi-theoretical and semi-empirical method to solve the shale hydraulic fracturing process, avoiding the cumbersome data transmission process between the seepage field and the stress field, and the non-convergence phenomenon of numerical calculation, greatly improving the calculation efficiency and convergence.

[0093] This embodiment uses a high-precision boundary element model (ILSA) and the method of the present invention (C2) to compare the prediction methods of multi-cluster hydraulic fracturing crack morphology. It can be seen from Figure 2(a), Figure 2(b), Figure 2(c), and Figure 2(d) that the method of the present invention is basically consistent with the calculation results of the high-precision boundary element model (ILSA) in terms of crack radius, injection rate, crack width and crack area;

[0094] The computational efficiency of the method of the present invention and the high-precision boundary element model are shown in Table 1. It can be seen that the computational efficiency of the present invention is much higher than that of the high-precision boundary element model.

[0095] Table 1

[0096]

[0097] The above embodiments describe the present invention in detail, but the contents are only preferred embodiments of the present invention and cannot be considered to limit the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the present invention.

Claims

1. A method for predicting the morphology of multi-cluster hydraulic fracturing cracks based on fluid-solid coupling, characterized in that: The steps include: S1. Determine the model parameters, fluid data and boundary conditions of hydraulic fracturing fractures; S2, establishing a crack width model; S3, calculating the fluid pressure according to the fracture width model in S2; S4. Calculate the hydraulic pressure load of the fluid in the fracture on both sides of the fracture; S5. Based on the relationship between the head pressure load and the ultimate pressure, the shape of the gap is predicted.

2. The method for predicting the morphology of multi-cluster hydraulic fracturing cracks based on fluid-solid coupling according to claim 1, characterized in that: The method to establish the gap width model is: Assume that the two sides M and N of the crack between blocks i and j are non-parallel, and the coordinates of side M are A(x1, y1) and B(x2, y2); the coordinates of side N are C(x3, y3) and D(x4, y4), then the distance between points A and C is b i , the distance between points B and D is b j The expressions are: According to the equivalent hydraulic opening calculation formula: In the formula, q is the fracture seepage flow rate, q m is the average hydraulic opening b m The corresponding seepage volume; and Therefore, the crack width calculation model is:

3. The method for predicting the morphology of multi-cluster hydraulic fracturing cracks based on fluid-solid coupling according to claim 2, characterized in that: The method for calculating fluid pressure based on the fracture width model in S2 is: Let the velocity of the fluid from point i to point j be v ij , whose expression is: In the formula, J ij is the hydraulic gradient, b is the fracture width, v is the fluid kinematic viscosity, and g is the gravitational acceleration; Get the flow rate q between the cracks ij for: Fluid pressure: P = ρ w HkDJ In the formula, ρ w is the fluid density, H is the hydraulic head height of the fluid in the fracture, and υ=νρ w ; Then the flow rate q ij Further expressed as: In the formula, Δp ij =ρ w g(H j -H i )=P j -P i +ρ w g(y j -y i ), y j and i Respectively represent the horizontal heights corresponding to point j and point i; H j H is the water head height of the fluid at point j of the fracture; i is the water head height of the fluid at point i of the fracture; P j is the pressure of the fluid at point j; P i is the pressure of the fluid at point i; When there are N' cracks at point j, the total flow rate is Q j for: Fluid pressure P j for: In the formula, p j0 The compressive stress value at the initial stage of this time step at time j, K w is the bulk modulus of the fluid, Δt is the time step interval, ΔA j =A j -A j0 , A j and A j0 Represent the new and old basin areas respectively.

4. The method for predicting the morphology of multi-cluster hydraulic fracturing cracks based on fluid-solid coupling according to claim 3 is characterized by: In step S4, the method of applying the hydraulic pressure load of the fluid in the crack to both sides of the crack is: Get the head pressure values ​​corresponding to point (x1, y1) and point (x2, y2): In the formula, γ w is the water capacity; h1 is the horizontal height of the point (x1, y1); h2 is the horizontal height of (x2, y2).

5. The method for predicting the morphology of multi-cluster hydraulic fracturing cracks based on fluid-solid coupling according to claim 1, characterized in that: In step S5, according to the relationship between the water head pressure load and the ultimate pressure, the method for predicting the shape of the gap is: If the water head pressure of the block point is greater than the limit pressure, the crack will be concave toward the inside of the block point; conversely, if the water head pressure of the block point is less than the limit pressure, the crack will be convex toward the outside of the block point. This process is repeated along the edge of the crack to obtain the final crack shape.

6. The method for predicting the morphology of multi-cluster hydraulic fracturing cracks based on fluid-solid coupling as described in any one of claims 1 to 5 is applied to predicting the morphology of hydraulic fracturing cracks in shale reservoirs.

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