Numerical simulation method for migration of proppant and fracture supporting property

Abstract

The invention discloses a numerical simulation method for proppant migration and fracture supporting performance, which comprehensively considers the migration characteristics of a proppant and the influence of the migration characteristics on fracture compressibility in a mode of establishing a model for numerical simulation, and comprises the following steps of: firstly, calculating the fracture compressibility based on a discrete element and computational fluid mechanics coupling method; the method comprises the following steps: acquiring migration characteristics and distribution characteristics of a proppant in fractures under given conditions, then according to a distribution result of the proppant in the fractures, further establishing a fractured rock model containing the proppant on the basis of a discrete element and finite difference coupling scheme, and carrying out research on the supporting property of the proppant on the fractures under the action of stress. Therefore, the method overcomes the defects of propping agent migration and propping agent stress compression cutting research in the current research, greatly improves the systematicness, comprehensiveness and reliability of the propping agent migration and fracture support research process, and provides data support for subsequent hydraulic fracturing propping agent injection.

Description

Technical Field

[0001] The invention discloses a numerical simulation method for proppant migration and crack support, belonging to the technical field of numerical simulation. Background Art

[0002] Coalbed methane is an unconventional natural gas and a clean energy source. Its main component is methane. Coalbed methane mainly exists in the coal seam in an adsorbed state (more than 90%). However, at present, coalbed methane generally has the characteristics of deep burial, low permeability, high density, high pressure, and strong adsorption, resulting in low efficiency of coalbed methane extraction. In order to more effectively mine coalbed methane, it is necessary to transform the reservoir to improve the permeability of the coal seam. At present, the commonly used reservoir transformation method is mainly hydraulic fracturing, which creates complex cracks inside the coal seam through hydraulic fracturing, thereby improving the permeability of the coal seam and facilitating the subsequent mining of coalbed methane. However, due to the effect of ground stress, the cracks generated by hydraulic fracturing of coal seams will close and block the extraction channel of coalbed methane; in order to reduce the occurrence of this situation, the method of injecting proppants during hydraulic fracturing is currently adopted. When cracks are generated by hydraulic fracturing, the proppants can enter the cracks to play a supporting role, thereby preventing or delaying the possibility of crack closure as much as possible during the subsequent extraction process.

[0003] In order to improve the supporting effect of proppants, during the hydraulic fracturing process, determining the distribution of proppants and the supporting performance of proppants on fractures has an important impact on the production of oil and gas wells. However, due to the complexity of the current experimental system, the experimental process often separates the migration of proppants from the stress compressibility of fractures, resulting in large errors in the results.

[0004] Therefore, how to provide a new method that can simultaneously analyze the migration characteristics of the proppant and the compressibility of the cracks after the proppant migrates, thereby effectively improving the accuracy of the results and providing data support for the subsequent hydraulic fracturing proppant injection, is the research direction required by the present invention. Summary of the invention

[0005] In view of the problems existing in the above-mentioned prior art, the present invention provides a numerical simulation method for proppant migration and fracture support. Through numerical simulation, the migration characteristics of the proppant and the compressibility of the fracture after the proppant migration are simultaneously simulated and analyzed, thereby effectively improving the accuracy of the results and providing data support for the subsequent hydraulic fracturing proppant injection.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: a numerical simulation method of proppant migration and fracture support, the specific steps are:

[0007] Step A, obtaining a fracture surface geometric model and a proppant geometric model by three-dimensional contour scanning of the rough fracture sample and the proppant sample to be studied;

[0008] Step B, determining proppant parameters and fracturing fluid parameters that need to be studied;

[0009] Step C, determining the fracture opening parameters based on the fracture surface geometric model obtained in step A, and then establishing a discrete element and computational fluid dynamics coupling model, and assigning corresponding parameters;

[0010] Step D, using the proppant geometric model obtained in step A and the discrete element and computational fluid dynamics coupling model established in step C to carry out numerical simulation of proppant migration, and obtain the proppant distribution in the rough fracture under the action of the fracturing fluid;

[0011] Step E, exporting a proppant characteristic file according to the proppant distribution result;

[0012] Step F, establishing a rock model of the fracture filled with proppant according to the fracture surface geometric model obtained in step A and the proppant characteristic file obtained in step E;

[0013] Step G, assigning corresponding parameters to the rock model established in step F and conducting a simulation of the compressibility of cracks under stress;

[0014] Step H, obtaining the compression properties of the crack surface under stress;

[0015] Step I: Integrate the data obtained in steps D and H to finally obtain the proppant migration characteristics and the proppant support properties for the fracture.

[0016] Furthermore, the proppant parameters in step B include proppant size, proppant shape, proppant mechanical properties, proppant injection amount and proppant ratio of different particle sizes; the fracturing fluid parameters include fracturing fluid viscosity and fracturing fluid flow rate.

[0017] Further, the step C is specifically:

[0018] Step C1, determining the crack opening parameter, and copying and translating the crack surface geometric model obtained by scanning according to the opening parameter, to obtain the upper and lower crack surfaces corresponding to the opening parameter;

[0019] Step C2: Based on the upper and lower fracture surfaces obtained by copying and translating, the remaining four surfaces are filled with planes to form a fracture model; then the fracture model is meshed to generate a discrete element and computational fluid dynamics coupling model.

[0020] Further, the step D is specifically as follows:

[0021] Step D1, based on the proppant geometric model and its parameters obtained in step A, generating proppant particles at the inlet boundary of the discrete element and computational fluid dynamics coupling model according to the corresponding parameters, and the proppant particles migrate under the action of the simulated fluid;

[0022] Step D2: Carry out a numerical simulation of proppant migration under given parameters, and obtain the proppant distribution results in the fracture after the simulation is completed.

[0023] Furthermore, the step E of exporting the proppant feature file refers to exporting a file containing coordinate information and geometric features of the proppant model from the numerical calculation software.

[0024] Further, the step F is specifically:

[0025] Step F1, according to the crack opening parameter determined in step C1, the crack surface geometric model obtained by scanning is copied and translated according to the opening parameter to obtain the upper and lower crack surfaces corresponding to the opening parameter;

[0026] Step F2: Generate rock block models according to the two fracture surfaces respectively, and divide the rock block models into finite difference grids, thereby forming a fractured rock mass model;

[0027] Step F3: According to the proppant characteristic file of step E, the proppant with corresponding parameters is introduced into the fractured rock model to establish a rock model in which the proppant fills the fracture.

[0028] Compared with the prior art, the present invention comprehensively considers the proppant migration characteristics and its influence on the compressibility of the fracture by establishing a model for numerical simulation. First, based on the method of coupling discrete element and computational fluid dynamics, the migration characteristics and distribution characteristics of the proppant in the fracture under given conditions are obtained. Then, according to the distribution results of the proppant in the fracture, a fractured rock model containing proppant is further established based on the discrete element and finite difference coupling scheme, and the research on the proppant's support for the fracture under stress is carried out. Therefore, the present invention overcomes the drawbacks of the current research that separates the study of proppant migration and proppant stress compression, greatly improves the systematic, comprehensive and reliable research process of proppant migration and fracture support, and provides data support for the subsequent hydraulic fracturing proppant injection. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is the geometric model of proppant particles and the geometric model of fracture surface obtained by scanning in the present invention;

[0030] Where (a) is the geometric model of proppant particles; (b) is the geometric model of fracture surface;

[0031] Figure 2 It is the fracture surface replication translation and discrete element and computational fluid dynamics coupling model in the present invention;

[0032] Among them, (a) is the copy translation of the fracture surface; (b) is the coupling model of discrete element and computational fluid dynamics;

[0033] Figure 3 It is the final distribution result of the proppant in the fracture after the numerical simulation of proppant migration under the given conditions of the present invention;

[0034] Figure 4 It is a rock mass model in which the proppant is filled in the cracks established by the present invention;

[0035] Figure 5 It is a crack opening diagram after the present invention carries out the crack compressibility simulation under stress;

[0036] Figure 6 This is a diagram showing the crack surface damage results after the present invention carries out a crack compressibility simulation under stress. DETAILED DESCRIPTION

[0037] The present invention will be further described below.

[0038] As shown in the figure, the specific steps of this embodiment are:

[0039] Step A: The fracture surface can be obtained by various methods, including but not limited to artificial splitting of rock samples, fracture surfaces obtained after experimental fracturing experiments, and artificially made fracture surfaces. In this example, the fracture surface is obtained by artificial splitting of shale samples; the proppant can be obtained from actual field engineering; the rough fracture sample (i.e., fracture surface) and proppant sample to be studied are scanned by three-dimensional contour to obtain the fracture surface geometric model and the proppant geometric model as shown in FIG. Figure 1 As shown; the proppant geometric model in this embodiment simplifies the proppant into a spherical shape;

[0040] Step B, determining the proppant parameters and fracturing fluid parameters to be studied; the proppant parameters include proppant size, proppant shape, proppant mechanical properties, proppant injection amount and proppant ratio of different particle sizes; fracturing fluid parameters include fracturing fluid viscosity and fracturing fluid flow rate. In this embodiment, different proppant particle sizes are selected as the research objects, and the proppant particle sizes are 40 mesh, 50 mesh and 60 mesh respectively.

[0041] Step C: determine the fracture opening parameters based on the fracture surface geometric model obtained in step A, and then establish a discrete element and computational fluid dynamics coupling model, and assign corresponding parameters, specifically:

[0042] Step C1, in this embodiment, the crack opening parameter is determined to be 1 mm; according to the opening parameter, the crack surface geometric model obtained by scanning is copied and moved in the modeling software through the copy and move commands to achieve the copy and translation of the crack surface by 1 mm, so as to obtain the upper and lower crack surfaces with an opening of 1 mm;

[0043] Step C2: According to the determined crack opening parameters, the remaining four faces of the crack are filled with planes and meshed to form a crack model. In the modeling software, the crack surface model is filled around to generate a complete crack model. The crack model is imported into the meshing software to generate a discrete element and computational fluid dynamics coupling model, and the boundary conditions are set. In this embodiment, the left side of the model is the velocity inlet boundary, the right side is the free outlet, and the remaining boundaries are wall boundaries. Figure 2 shown.

[0044] Step D: using the proppant geometric model obtained in step A and the discrete element and computational fluid dynamics coupling model established in step C to carry out numerical simulation of proppant migration, and obtain the proppant distribution in the rough fracture under the action of the fracturing fluid, specifically:

[0045] Step D1, based on the proppant geometric model and its parameters obtained in step A, proppant particles are generated according to the corresponding parameters at the inlet boundary of the discrete element and computational fluid dynamics coupling model, and the proppant particles migrate under the action of the simulated fluid; in this embodiment, proppant particles are generated on the velocity inlet side of the fluid model, and the proppant particles move under the action of the fracturing fluid in the fluid model, and proppant migration simulations of 40 mesh, 50 mesh and 60 mesh particle sizes are carried out respectively.

[0046] Step D2, carry out numerical simulation of proppant migration under given parameters, and obtain the proppant distribution results in the fracture after the simulation. In this embodiment, except for the proppant particle size, the other parameters are kept consistent. After the simulation, the proppant distribution of 40 mesh, 50 mesh and 60 mesh particle sizes is obtained as follows: Figure 3 Shown

[0047] Step E: exporting a proppant feature file according to the proppant distribution result; the exporting of the proppant feature file refers to exporting a file containing coordinate information and geometric features of the proppant model from the numerical calculation software. In this embodiment, the exported file is in txt format, containing the position, density, and particle size information of the proppant particle model.

[0048] Step F: Establish a rock model of the proppant-filled fracture according to the fracture surface geometry model obtained in step A and the proppant feature file obtained in step E, specifically:

[0049] Step F1, in this embodiment, the crack opening parameter is determined to be 1 mm; according to the opening parameter, the crack surface geometric model obtained by scanning is copied and moved in the modeling software through the copy and move commands to achieve the copy and translation of the crack surface by 1 mm, so as to obtain the upper and lower crack surfaces with an opening of 1 mm;

[0050] Step F2: Generate rock block models according to the two fracture surfaces respectively, and divide the finite difference grids to form a fractured rock mass model; in this embodiment, the upper fracture surface is used to generate the upper rock block model, and the lower fracture surface is used to generate the lower rock block model, and the fracture space is between the upper and lower rock block models;

[0051] Step F3, according to the proppant characteristic file of step E, the proppant with corresponding parameters is imported into the fractured rock model, and a rock model with proppant-filled fractures is established; in this embodiment, a txt text containing the position, density, and particle size information of the proppant particle model is imported into the software, the proppant particles are regenerated in the fractured rock model, and a rock model with proppant-filled fractures is established.

[0052] Step G, after assigning corresponding parameters to the rock model established in step F, simulation of fracture compressibility under stress is carried out; in this embodiment, the rock model parameters are obtained by the parameter calibration method in numerical simulation, and the parameters are assigned to the rock model, the top of the rock model is set as a stress boundary of 50MPa, the bottom of the rock model is set as a fixed boundary, and the surroundings are set as rolling support boundaries. Figure 4 shown.

[0053] Step H, obtaining the compression properties of the fracture surface under stress; in this embodiment, the fracture opening and fracture surface damage results of the proppant-supported fractures with particle sizes of 40 mesh, 50 mesh and 60 mesh under 50 MPa stress are obtained as follows: Figure 5 and 6 shown.

[0054] Step I: Integrate the data obtained in steps D and H to finally obtain the proppant migration characteristics and the proppant support properties for the fracture.

[0055] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A numerical simulation method for proppant migration and fracture support, characterized in that: The specific steps are: Step A, obtaining a fracture surface geometric model and a proppant geometric model by three-dimensional contour scanning of the rough fracture sample and the proppant sample to be studied; Step B, determining proppant parameters and fracturing fluid parameters that need to be studied; Step C, determining the fracture opening parameters based on the fracture surface geometric model obtained in step A, and then establishing a discrete element and computational fluid dynamics coupling model, and assigning corresponding parameters; Step D, using the proppant geometric model obtained in step A and the discrete element and computational fluid dynamics coupling model established in step C to carry out numerical simulation of proppant migration, and obtain the proppant distribution in the rough fracture under the action of the fracturing fluid; Step E, exporting a proppant characteristic file according to the proppant distribution result; Step F, establishing a rock model of the fracture filled with proppant according to the fracture surface geometric model obtained in step A and the proppant characteristic file obtained in step E; Step G, assigning corresponding parameters to the rock model established in step F and conducting a simulation of the compressibility of cracks under stress; Step H, obtaining the compression properties of the crack surface under stress; Step I: Integrate the data obtained in steps D and H to finally obtain the proppant migration characteristics and the proppant support properties for the fracture.

2. The method for numerical simulation of proppant migration and fracture support according to claim 1, characterized in that: The proppant parameters in step B include proppant size, proppant shape, proppant mechanical properties, proppant injection amount and proppant ratio of different particle sizes; the fracturing fluid parameters include fracturing fluid viscosity and fracturing fluid flow rate.

3. The method for numerical simulation of proppant migration and fracture support according to claim 1, characterized in that: The step C is specifically as follows: Step C1, determining the crack opening parameter, and copying and translating the crack surface geometric model obtained by scanning according to the opening parameter, to obtain the upper and lower crack surfaces corresponding to the opening parameter; Step C2: Based on the upper and lower fracture surfaces obtained by copying and translating, the remaining four surfaces are filled with planes to form a fracture model; then the fracture model is meshed to generate a discrete element and computational fluid dynamics coupling model.

4. The method for numerical simulation of proppant migration and fracture support according to claim 1, characterized in that: The step D is specifically as follows: Step D1, based on the proppant geometric model and its parameters obtained in step A, generating proppant particles at the inlet boundary of the discrete element and computational fluid dynamics coupling model according to the corresponding parameters, and the proppant particles migrate under the action of the simulated fluid; Step D2: Carry out a numerical simulation of proppant migration under given parameters, and obtain the proppant distribution results in the fracture after the simulation is completed.

5. The method for numerical simulation of proppant migration and fracture support according to claim 1, characterized in that: The step E of exporting the proppant feature file refers to exporting a file containing coordinate information and geometric features of the proppant model from the numerical calculation software.

6. The method for numerical simulation of proppant migration and fracture propping according to claim 3, characterized in that: The step F is specifically as follows: Step F1, according to the crack opening parameter determined in step C1, the crack surface geometric model obtained by scanning is copied and translated according to the opening parameter to obtain the upper and lower crack surfaces corresponding to the opening parameter; Step F2: Generate rock block models according to the two fracture surfaces respectively, and divide the rock block models into finite difference grids, thereby forming a fractured rock mass model; Step F3: According to the proppant characteristic file of step E, the proppant with corresponding parameters is introduced into the fractured rock model to establish a rock model in which the proppant fills the fracture.

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