Simulation method and processor for true triaxial hydraulic fracturing experiment

文档序号：191203 发布日期：2021-11-02 浏览：37次中文

阅读说明：本技术 真三轴水力压裂实验的仿真方法及处理器 (Simulation method and processor for true triaxial hydraulic fracturing experiment ) 是由林伯韬申屠俊杰史璨金衍侯冰于 2021-09-08 设计创作，主要内容包括：本发明涉及油气田开发技术领域,公开了一种真三轴水力压裂实验的仿真方法及处理器。仿真方法包括：根据实际岩心的尺寸确定用于模拟实际岩心的岩样模型的尺寸,在岩样模型的尺寸对应的计算空间内生成颗粒群；在颗粒群的颗粒间施加胶结,并根据实际岩心的强度调整胶结的强度；在计算空间的外围设置墙体以将计算空间围成封闭区域,并通过墙体向岩样模型施加三向的分层的地应力；在岩样模型上开展压裂注液模拟,进行注液点附近预设范围内的孔隙骨架由注液引发的孔隙压力升高并向四周传递孔隙压力的过程；控制孔隙压力增加而导致颗粒间的胶结断裂,直至岩样模型发生内部破裂。本仿真的研究成果可以为现场压裂施工提供更科学的指导,提升油气产量。(The invention relates to the technical field of oil and gas field development and discloses a simulation method and a processor for a true triaxial hydraulic fracturing experiment. The simulation method comprises the following steps: determining the size of a rock sample model for simulating the actual rock core according to the size of the actual rock core, and generating a particle group in a calculation space corresponding to the size of the rock sample model; applying cementation between particles of the particle group, and adjusting the strength of the cementation according to the strength of the actual rock core; arranging a wall body at the periphery of the calculation space to enclose the calculation space into a closed area, and applying three-dimensional layered ground stress to the rock sample model through the wall body; carrying out fracturing liquid injection simulation on the rock sample model, and carrying out the process that the pore pressure of a pore framework in a preset range near a liquid injection point is increased by liquid injection and is transferred to the periphery; the pore pressure increase is controlled to cause interparticle cementitious fracture until internal fracture of the rock sample model occurs. The research result of the simulation can provide more scientific guidance for on-site fracturing construction and improve the oil and gas yield.)

1. A simulation method of a true triaxial hydraulic fracturing experiment is characterized by comprising the following steps:

determining the size of a rock sample model for simulating the actual rock core according to the size of the actual rock core, and generating a particle group in a calculation space corresponding to the size of the rock sample model;

applying cementation between particles of the particle group, and adjusting the strength of the cementation according to the strength of the actual rock core;

arranging a wall body on the periphery of the calculation space to enclose the calculation space into a closed area, and applying three-way layered ground stress to the rock sample model through the wall body;

constructing a fluid computing network on the basis of the rock sample model so as to carry out the evolution of fluid pressure in rock sample pores along with injection liquid and the process of transferring the fluid pressure among the pores;

carrying out fracturing liquid injection simulation on the rock sample model, and carrying out the process that the pore pressure of a pore framework in a preset range near a liquid injection point is increased by liquid injection and is transferred to the periphery;

controlling the pore pressure increase to cause a fracture of the cement between the particles until internal fracture of the rock sample model occurs.

2. The simulation method of claim 1, further comprising:

adjusting the properties of the particles and the strength of the cementation to reduce the gravel particle properties in the actual core; or

Adjusting a contact model between a portion of the particles of the population of particles such that a mechanical property of the portion of inter-particle contact approximates a property of a first natural fracture in the actual core.

3. The simulation method of claim 2, wherein the adjusting the contact model between the portion of the population of particles such that the mechanical characteristics of the portion of the inter-particle contacts approximate the first natural fracture in the actual core comprises:

applying a smooth joint contact model to form a second natural fracture in the rock sample model;

setting a dip angle and a dip tendency of the second natural fracture;

setting first parameters of the smooth joint contact model, the first parameters including: cell normal stiffness, cell tangential stiffness, coefficient of friction, expansion angle, tensile strength, cohesion, and joint friction angle;

adjusting a permeability of the second natural fracture.

4. The simulation method of claim 1, further comprising:

setting control volumes and fluid parameters of the fluid computing network, the fluid parameters including: fluid viscosity, bulk modulus and displacement;

setting permeability parameters of the rock sample model, wherein the permeability parameters comprise relaxation permeability when the rock sample model is not under the action of external stress, limit permeability under the action of infinite external stress and fracture permeability after the rock sample model is fractured;

setting a second parameter of the bond, the second parameter comprising: effective modulus, normal-to-tangential stiffness ratio, tensile strength, cohesion, internal friction angle, and coefficient of friction.

5. The simulation method of claim 1, further comprising:

controlling the bond fracture between the particles to form microcracks and releasing strain energy to form acoustic emission event points;

counting the number of the microcracks generated in the rock sample model, the morphology of the microcracks, and the location of an acoustic emission event point;

forming a fracture morphology, the pore pressure, a pumping pressure curve, and a spatial distribution of the acoustic emission event points;

determining a moment magnitude of the acoustic emission event point and a corresponding type of failure of the microcracks.

6. The simulation method of claim 5, wherein the determining the magnitude of the moment of the acoustic emission event point comprises:

calculating the moment tensor of the acoustic emission event point according to the position and the variation of the contact force around the microcrack;

obtaining a moment tensor matrix according to the moment tensor;

calculating to obtain a scalar moment according to the moment tensor matrix;

and calculating the moment magnitude of the acoustic emission event point according to the scalar moment.

7. The simulation method of claim 1, wherein applying a bond between particles of the population of particles comprises:

applying a plain joint contact model to a matrix portion of the rock sample model;

applying a bond to the interparticle contacts using the flat joint contact pattern.

8. The simulation method of claim 1, wherein the building a fluid computation network based on the rock sample model comprises:

interconnecting the spherical centers of a plurality of said particles in contact with one another to form a tetrahedral structure, said tetrahedral structure being a fundamental unit of said fluid computational network;

determining a pore having a preset pressure in each of the basic cells;

defining a fluid channel between two adjacent said apertures;

and controlling the injection of the hydraulic fracturing, wherein the pore pressure in the basic unit rises, and simultaneously, the pore pressure is transmitted to the pores in the adjacent basic units through the fluid channels.

9. The simulation method of claim 1, further comprising:

setting properties of the particles, the properties including particle density and damping coefficient;

setting a resolution of the population of particles to control a number of the particles simultaneously generated on a preset edge within the enclosed area;

setting a ground stress accuracy coefficient;

and adjusting the ground stress precision coefficient to enable the simulated value of the ground stress to be consistent with the target value.

10. A processor configured to perform the simulation method of a true triaxial hydraulic fracturing experiment according to any one of claims 1 to 9.

Technical Field

The invention relates to the technical field of oil and gas field development, in particular to a simulation method and a processor for a true triaxial hydraulic fracturing experiment.

Background

With the continuous development of the world economy and industry, the production of conventional oil and gas reservoirs cannot meet the increasing oil and gas resource demand. Therefore, exploration and development work for unconventional oil and gas reservoirs (such as heavy oil, super heavy oil, tight sandstone and shale oil and gas reservoirs and the like) is continuously carried out and deepened. The oil gas occurrence mode, the reservoir physical properties and the stratum fluid properties of the unconventional oil and gas reservoir are obviously different from those of the conventional oil and gas reservoir, the yield is limited in the conventional development mode, and efficient production increasing measures are urgently needed. Practice proves that the large-scale hydraulic fracturing technology is the most effective and reliable means for improving the oil and gas yield of the unconventional oil and gas reservoir, and is widely applied to the development operation of the unconventional oil and gas reservoir at home and abroad.

The indoor true triaxial hydraulic fracturing experiment is an effective means for researching the fracture initiation and extension characteristics of the artificial fracture, and can accurately capture the morphological characteristics of the artificial fracture under the simulated formation condition on the basis of utilizing the in-situ formation core or outcrop. However, the current true triaxial hydraulic fracturing experiment has many defects, for example, the time, personnel and economic cost for developing a large number of true triaxial hydraulic fracturing experiments considering multi-factor variables are extremely high due to the difficulty in obtaining the exposed head and the long time consumption of the experiments.

Disclosure of Invention

In order to overcome the defects of the prior art, the embodiment of the invention provides a simulation method and a processor for a true triaxial hydraulic fracturing experiment.

In order to achieve the above object, a first aspect of the present invention provides a simulation method for a true triaxial hydraulic fracturing experiment, including:

applying cementation between particles of the particle group, and adjusting the strength of the cementation according to the strength of the actual rock core;

arranging a wall body at the periphery of the calculation space to enclose the calculation space into a closed area, and applying three-dimensional layered ground stress to the rock sample model through the wall body;

the pore pressure increase is controlled to cause interparticle cementitious fracture until internal fracture of the rock sample model occurs.

In the embodiment of the present invention, the simulation method further includes:

adjusting the properties of the particles and the strength of the cementation to restore the gravel particle characteristics in the actual core; or

The contact model between some of the particles of the particle population is adjusted so that the mechanical properties of the contact between some of the particles approximate the properties of the first natural fracture in the actual core.

In an embodiment of the present invention, adjusting the contact model between the partial particles of the particle group such that the mechanical characteristics of the partial inter-particle contact are close to the first natural fracture in the actual core comprises:

applying a smooth joint contact model to form a second natural fracture in the rock sample model;

setting the inclination and dip angle of the second natural fracture;

setting first parameters of a smooth joint contact model, wherein the first parameters comprise: cell normal stiffness, cell tangential stiffness, coefficient of friction, expansion angle, tensile strength, cohesion, and joint friction angle;

the permeability of the second natural fracture is adjusted.

In the embodiment of the present invention, the simulation method further includes:

setting control volumes and fluid parameters of a fluid computing network, the fluid parameters including: fluid viscosity, bulk modulus and displacement;

In the embodiment of the present invention, the simulation method further includes:

controlling the cementation and fracture among the particles to form microcracks and releasing strain energy to form acoustic emission event points;

counting the number of micro cracks generated in the rock sample model, the micro crack form and the position of an acoustic emission event point;

forming a fracture morphology, the pore pressure, a pumping pressure curve, and a spatial distribution of acoustic emission event points;

the moment magnitude of the acoustic emission event point and the corresponding type of disruption of the microcracks are determined.

In an embodiment of the invention, determining the moment magnitude of an acoustic emission event point comprises:

calculating a moment tensor of an acoustic emission event point according to the position and the variation of the contact force around the microcrack;

obtaining a moment tensor matrix according to the moment tensor;

calculating to obtain a scalar moment according to the moment tensor matrix;

and calculating the moment magnitude of the acoustic emission event point according to the scalar moment.

In an embodiment of the invention, applying the bond between the particles of the population of particles comprises:

applying a plain joint contact model to a matrix portion of the rock sample model;

and applying the cement at the contact position between the particles by using a plain joint contact model.

In an embodiment of the present invention, constructing a fluid computation network on the basis of a rock sample model includes:

interconnecting the centers of a plurality of particles in contact with one another to form a tetrahedral structure, the tetrahedral structure being a basic unit of the fluid computational network;

determining a pore having a preset pressure in each of the basic cells;

defining a fluid channel between two adjacent apertures;

in the injection liquid for controlling the hydraulic fracturing, the pore pressure in the basic unit is increased, and simultaneously, the pore pressure is transmitted to the pores in the adjacent basic unit through the fluid channel.

In the embodiment of the present invention, the simulation method further includes:

setting the properties of the particles, wherein the properties comprise particle density and damping coefficient;

setting the resolution of the particle group to control the number of particles simultaneously generated on a preset edge in the closed area;

setting a ground stress accuracy coefficient;

the ground stress accuracy coefficient is adjusted so that the simulated value of the ground stress is consistent with the target value.

A second aspect of the invention provides a processor configured to perform the simulation method of the true triaxial hydraulic fracturing experiment described above.

A third aspect of the invention provides a machine-readable storage medium having instructions stored thereon for causing a machine to perform the simulation method of a true triaxial hydraulic fracturing experiment described above.

A fourth aspect of the invention provides a computer program product comprising a computer program which, when executed by a processor, implements the simulation method for a true triaxial hydraulic fracturing experiment as described above.

The simulation method of the embodiment of the invention can realize that: (1) carrying out fluid-solid coupling simulation aiming at the whole hydraulic fracturing process of the rock sample model with heterogeneity; (2) applying layered ground stress to the rock sample model, and reducing complex boundary stress on an actual rock core; (3) acoustic emission event points are generated based on fracture dynamics simulations.

The simulation method provided by the embodiment of the invention can realize simulation of a true triaxial hydraulic fracturing experiment, has low time, personnel and economic cost for developing a large amount of true triaxial hydraulic fracturing experiment simulation considering multi-factor variables, can develop refined digital representation aiming at the true triaxial hydraulic fracturing experiment, and contrasts and verifies the indoor experiment result, and perfects the crack initiation and extension mechanism research of artificial cracks in a complex stratum, thereby providing theoretical guidance for hydraulic fracturing construction and seam network construction optimization design. The research result of simulation can provide more scientific theoretical guidance for the on-site fracturing construction design, and further improve the oil and gas yield of the unconventional oil and gas reservoir.

Drawings

The accompanying drawings, which are included to provide a further understanding of the embodiments of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the embodiments of the invention without limiting the embodiments of the invention. In the drawings:

FIG. 1 schematically illustrates a flow diagram of a simulation method of a true triaxial hydraulic fracturing experiment according to an embodiment of the present invention;

FIG. 2 schematically illustrates a real conglomerate pattern containing gravel particles according to an embodiment of the invention;

FIG. 3 schematically illustrates a side view of a rock sample model containing gravel particles generated by a simulation method according to an embodiment of the invention;

FIG. 4 schematically illustrates a front view of a rock sample model containing gravel particles generated by a simulation method according to an embodiment of the invention;

FIG. 5 schematically shows a schematic view of a real rock containing natural fractures according to an embodiment of the invention;

FIG. 6 schematically shows a diagram of a model of a rock sample containing natural fractures generated by a simulation method according to an embodiment of the invention;

FIG. 7 schematically illustrates a ground stress profile of rock for a true triaxial hydraulic fracturing experiment according to an embodiment of the present invention;

FIG. 8 schematically shows a geostress profile of a rock sample model of a simulation method according to an embodiment of the invention;

FIG. 9 schematically illustrates a schematic diagram of the basic elements of a fluid computing network, in accordance with an embodiment of the present invention;

FIG. 10 schematically illustrates one of the schematic diagrams of fracture morphology after simulation of a rock sample model containing natural fractures according to an embodiment of the invention;

FIG. 11 schematically illustrates a second schematic diagram of fracture morphology after simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the present invention;

FIG. 12 schematically illustrates a third schematic diagram of fracture morphology after simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the present invention;

FIG. 13 schematically illustrates one of the pore pressure profiles after a simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the invention;

FIG. 14 schematically shows a second distribution of pore pressure after simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the present invention;

FIG. 15 schematically shows a third distribution of pore pressure after simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the present invention;

FIG. 17 schematically illustrates a second distribution of acoustic emission event points after simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the present invention;

FIG. 18 schematically illustrates a third distribution of acoustic emission event points after simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the invention;

FIG. 19 schematically illustrates a bottom hole pressure change profile of a rock sample model containing natural fractures during a simulation process, in accordance with an embodiment of the present invention;

FIG. 20 schematically illustrates one of the schematic representations of fracture morphology after simulation of a rock sample model containing conglomerate particles (at lower strength) in accordance with an embodiment of the invention;

FIG. 21 schematically illustrates a second schematic representation of fracture morphology after simulation of a rock sample model containing conglomerate particles (at lower strength) in accordance with an embodiment of the invention;

FIG. 22 schematically illustrates one of the schematic representations of fracture morphology after simulation of a rock sample model containing conglomerate particles (at higher strength) in accordance with an embodiment of the invention;

FIG. 24 schematically illustrates a bottom hole pressure variation graph of a conglomerate particle-containing rock sample model during simulation, in accordance with an embodiment of the present invention;

FIG. 25 schematically illustrates a flow diagram for a digital simulation of a true triaxial hydraulic fracturing experiment according to an embodiment of the present invention.

Description of the reference numerals

11 gravel particle 12 first natural fracture

13 second Natural fracture 14 particles

15 pore 16 flow field

17 flow tube 18 microcracks

19 acoustic emission event point

Detailed Description

The following detailed description of embodiments of the invention refers to the accompanying drawings. It should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration and explanation only, not limitation.

It should be noted that, if directional indications (such as up, down, left, right, front, and rear … …) are referred to in the embodiments of the present application, the directional indications are only used to explain the relative positional relationship between the components, the movement situation, and the like in a specific posture (as shown in the drawings), and if the specific posture is changed, the directional indications are changed accordingly.

In addition, if there is a description of "first", "second", etc. in the embodiments of the present application, the description of "first", "second", etc. is for descriptive purposes only and is not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In addition, technical solutions between the various embodiments can be combined with each other, but must be realized by a person skilled in the art, and when the technical solutions are contradictory or cannot be realized, such a combination should not be considered to exist, and is not within the protection scope of the present application.

Fig. 1 schematically shows a flow diagram of a simulation method of a true triaxial hydraulic fracturing experiment according to an embodiment of the present invention.

As shown in fig. 1, a simulation method of a true triaxial hydraulic fracturing experiment is provided, which comprises the following steps:

step 101, determining the size of a rock sample model for simulating an actual rock core according to the size of the actual rock core, and generating a particle group in a calculation space corresponding to the size of the rock sample model;

102, applying cementation among particles of the particle group, and adjusting the strength of the cementation according to the strength of the actual rock core;

103, arranging a wall body at the periphery of the calculation space to enclose the calculation space into a closed area, and applying three-way layered ground stress to the rock sample model through the wall body;

104, constructing a fluid calculation network on the basis of the rock sample model so as to carry out the evolution of fluid pressure in rock sample pores along with injection liquid and the process of transferring the fluid pressure among the pores;

105, carrying out fracturing liquid injection simulation on the rock sample model, and carrying out the process that the pore pressure of a pore framework in a preset range near a liquid injection point is increased by liquid injection and is transferred to the periphery;

and 106, controlling the pore pressure to increase so as to cause the cementation fracture among the particles until the rock sample model is internally cracked.

In step 101, the size of the true triaxial rock sample model in the digital simulation is determined according to the size of the actual rock core selected in the true triaxial fracturing experiment. Considering that the calculation amount of the discrete element particles is large, the calculation object size of the digital simulation method is set according to the calculation resources. Aiming at single computer resources, the simulation method can be suitable for true triaxial fracturing experiment rock sample models with side lengths of dozens of millimeters to several meters.

For most cases of true triaxial fracturing experiments, the rock sample model is generally set to be 300 × 300 × 300mm or 400 × 400 × 400mm, corresponding to the size of the experimental sample commonly used in the room. Meanwhile, a calculation space corresponding to the size of the rock sample is generated in the model and used for generating a particle group, and the calculation space is surrounded into a closed area by a smooth wall body around the calculation space. And selecting a proper resolution Re according to actual conditions and generating a particle group in the closed area. The particles generated at this stage are given properties such as a particle density ρ b and a damping coefficient fb.

Where the resolution Re is specifically characterized by the number of particles that can be generated simultaneously on one side of the enclosed area. The stronger the heterogeneity of the rock sample model, the higher the fineness requirement on the model, and the higher the corresponding resolution Re.

In step 102, a contact pattern is defined where the particles in the population of particles contact each other, thereby applying a bond between the particles, the strength of the bond being dependent on the strength of the actual core. After the cement is applied, the population of particles is referred to as a rock sample model. By adjusting the strength parameters defined in the contact model, the strength of the rock sample model can be corrected to the strength of the actual core.

A flat joint contact model (flat joint model) was applied to the matrix portion of the rock sample model. The contact position of the contact model to the particlesAfter applying the bond, the discrete population of particles will exhibit the mechanical properties of the continuous medium, essentially requiring the following parameters to be set for the bond: effective modulusNormal to tangential stiffness ratioTensile strengthCohesive force cfj, internal friction AngleAnd a coefficient of friction μ fj.

The main methods for adjusting the strength parameters of the rock sample model are classified into a direct definition method and a trial and error method. Direct definition method: and measuring the micromechanics parameters of the actual rock by using rock micromechanics parameter testing equipment, and setting the micromechanics parameters of the contact model according to the measurement result.

The trial and error method is to use the generated rock sample model to carry out numerical simulation of mechanical parameter test (simulation aiming at single-axis or three-axis experiment), compare the macroscopic mechanical parameter values obtained by simulation, such as compressive strength and elastic modulus, with the corresponding parameter values measured by real rock in a laboratory, and adjust the microscopic parameter values of the contact model until the simulation value of the macroscopic mechanical parameter is close to the measured value of the real rock. Because the instrument for directly testing the microscopic parameters of the real rock has high cost, the simulation method can preferentially adopt a trial and error method to adjust the strength parameters of the rock sample model, and compare the simulation result of a single-axis or three-axis experiment with the actual measurement value of the real rock in a laboratory to dynamically adjust the strength parameters of the rock sample model.

And observing and recording heterogeneous characteristics of the actual rock core in the true triaxial fracturing experiment, and representing the heterogeneous characteristics in the rock sample model. Fig. 2 schematically shows a real conglomerate schematic diagram containing gravel particles according to an embodiment of the invention, fig. 3 schematically shows a side view of a rock sample model containing gravel particles generated by a simulation method according to an embodiment of the invention, and fig. 4 schematically shows a front view of the rock sample model containing gravel particles generated by the simulation method according to an embodiment of the invention, so that the rock sample model in simulation restores bedding and gravel particle characteristics developed in real rock by adjusting particle properties of different layers and bond strength between particles.

Fig. 5 schematically shows a diagram of a real rock containing natural fractures according to an embodiment of the present invention, and fig. 6 schematically shows a diagram of a rock sample model containing natural fractures generated by a simulation method according to an embodiment of the present invention, wherein the mechanical properties of the part of inter-particle contact are close to those of the natural fractures by setting the contact model between the particles in a specific range.

The natural fractures in the rock sample model can be realized by a smooth joint contact model (smooth joint model), and the smooth joint contact model can simulate the sliding behavior of a rock body at the natural fracture interface. When applying the contact model to the crack development, the following model parameters are mainly set: normal stiffness of unitUnit tangential stiffnessCoefficient of friction mu_sjExpansion angle psi_sjTensile strength of the steel sheetCohesion c_sjAnd joint angle of friction

Since the permeability of natural fractures is higher than that of rock matrix, the hydraulic properties of the natural fractures also need to be characterized in a rock sample model. The digital simulation method is provided with a crack permeability multiplier Zeta_f＝k_fau/k_mtxTo describe the relationship between the permeability of natural fractures and the permeability of the rock matrix, where k_fauPermeability of natural fractures, k_mtxIs the permeability of the rock matrix.

In step 103, three-dimensional layered ground stress sigma x1, sigma x2 … sigma xn, sigma y1, sigma y2 … sigma yn, sigma z1 and sigma z2 … sigma zn are independently applied to the rock sample model through the walls around the rock sample model, and the complex ground stress state of the rock in the triaxial experiment is restored. Fig. 7 schematically shows a ground stress distribution diagram of a rock of a true triaxial hydraulic fracturing experiment according to an embodiment of the present invention, and fig. 8 schematically shows a ground stress distribution diagram of a rock sample model of a simulation method according to an embodiment of the present invention, see fig. 7 and 8. And controlling the error range between the simulated ground stress value and the target ground stress value by setting a ground stress precision coefficient delta tol, and optimizing the calculation efficiency.

Wherein, the ground stress accuracy coefficient can be determined by the following formula:

σ₀to a target ground stress value, σ_pIs the current ground stress value during loading.

In step 104, a fluid computing network is constructed on the basis of the rock sample model, and a fluid-solid coupling scheme based on a discrete element method is formed, so that the evolution of the fluid pressure in the real rock pore along with the injection liquid and the transmission process of the fluid pressure among the pores are simulated. Meanwhile, permeability parameters of the rock sample model are set, and the permeability parameters mainly comprise: relaxed permeability k in the absence of external stress_iniLimiting permeability k under the action of infinite external stress_strAnd fracture permeability k after fracture of rock sample model_frc。

Introducing the principle and method of generating a fluid computing network, fig. 9 schematically shows a schematic diagram of basic units of a fluid computing network according to an embodiment of the present invention, and as shown in fig. 9, the spherical centers of four particles in contact with each other are connected to form a tetrahedral structure, which is a basic unit of a fluid computing network and is called a watershed. An aperture having a fluid pressure is defined in each flow field and a fluid passage, called a flow tube, is defined between two adjacent apertures. During the injection of hydraulic fracturing, the pore pressure in the flow field is constantly rising, while the pore pressure is transferred through the flow tube to the pores in the adjacent flow field. The flow process of the fluid in the flow tube is set to laminar flow and follows the Hagen-Poiseuille equation, and when there is a pressure differential between adjacent pores, the flow in the flow tube between the pores is determined by the following equation:

wherein mu is fluid viscosity (unit: mPas), delta P is pressure difference (unit: Pa) between adjacent pores, and L is_pIs the length (unit: m) of the flow tube, r_pIs the flow tube radius (in m). The flow tube radius can also be characterized as openness or conductivity, related to permeability of the rock sample model and interparticle contact force. The larger the permeability is, the smaller the contact force between particles is, and the higher the opening degree and the flow conductivity are. Flow tube radius r_pIs determined by the following equation:

wherein r is_p,∞And r_p,0The radius (unit: m) of the flow tube under the action of contact force between infinite particles and contact force between zero particles. Alpha is the sensitivity coefficient (dimensionless) and the sensitivity degree of the radius of the flow pipe which is reduced along with the increase of the contact force between the particles. The increase in fluid pressure within a particular pore, over a time step Δ t, is determined by the following equation:

wherein, K_fIs the volume flow rate (unit: m) of the fluid³/s)，V_dIs the pore volume (unit: m)³) Σ q Δ t is a net volume of the inflowing/outflowing fluid (unit: m is³)，ΔV_dPores caused by deformation of rock sample modelVolume change (unit: m)³)。

The permeability k of the rock sample model is determined by the following formula:

wherein, V_cCalculating the control volume (unit: m) of the network for the fluid³). The fluid parameters include fluid viscosity [ mu ] (unit: mPas), bulk modulus K_b(unit: Pa) and a discharge rate r (unit: m)³S), and the like.

In steps 105 and 106, digital simulation of a true triaxial hydraulic fracturing experiment is performed, fracturing injection simulation is performed on the rock sample model, and a process that pore pressure of a pore framework in a certain range near an injection point is increased by injection and is transferred to the periphery is described. Further, the increase of the control pore pressure value leads to the fracture of the cement among particles until the internal fracture process of the rock sample model occurs. After the injection simulation is carried out for a period of time, the number of microcracks generated in the rock sample model, the microcrack form and the position of the acoustic emission event point are counted, a fine crack form, pore pressure, a pumping pressure curve and the spatial distribution of the acoustic emission event point are formed, and information such as the moment magnitude of the acoustic emission event point, the damage type of the corresponding microcrack and the like can be further acquired.

And setting the fracture of each interparticle cementation to form a microcrack by adopting a discrete medium-based acoustic emission event point statistical method, so as to release strain energy and form an acoustic emission event point. The particles at the two ends of the broken bond (source particles) are displaced and the contact force between the source particles and the surrounding particles is changed. Therefore, the moment tensor of the acoustic emission event point can be calculated according to the position and the variation of the contact force around the microcrack, and is determined by the following formula:

wherein M is_ijIs thatMoment tensor, Δ F, of acoustic emission event points_iIs the i-th component (unit: N), R, of the variation of the contact force_jIs the jth component (in m) of the distance between the contact point and the center point of the acoustic emission event point, and S is the surface that surrounds the acoustic emission event point. Based on the obtained moment tensor, it can be decomposed into isotropic and biased parts, respectively quantified as a percentage:

where tr (M) is the trace of the moment tensor M,is a bias eigenvalue. The value range of T is-100% to 100%. When the content is 30 percent<T<100% or-100%<T<-at 30%, the micro-cracks corresponding to the acoustic emission event points are tensile cracks; when the content is-30%<T<And when the acoustic emission event point is 30%, the micro-cracks corresponding to the acoustic emission event point are shear cracks. Furthermore, from the moment tensor matrix, a scalar moment can be calculated, determined by the following equation:

wherein M is₀Is a scalar moment, m_jIs the j-th eigenvalue of the moment tensor matrix. From the scalar moment, the moment magnitude (M) of the acoustic emission event point can be further calculated_w) Comprises the following steps:

according to the distribution condition of the moment-vibration number values of the acoustic emission event points obtained through calculation, all the detected acoustic emission event points can be marked in the rock sample model, and the color and the size of the acoustic emission event points can be displayed based on the numerical value.

The embodiment of the invention provides a discrete element based on combination dynamics and fluid-solid coupling, and a digital simulation method suitable for a true triaxial hydraulic fracturing experiment sample, and the simulation method of the embodiment of the invention can realize that: (1) carrying out fluid-solid coupling simulation aiming at the whole hydraulic fracturing process of the rock sample model with heterogeneity; (2) applying layered ground stress to the rock sample model, and reducing complex boundary stress on an actual rock core; (3) acoustic emission event points are generated based on fracture dynamics simulations.

The following two specific embodiments are used to specifically describe the simulation method of the true triaxial hydraulic fracturing experiment according to the embodiment of the present invention. In the first embodiment, the digital simulation is mainly carried out on the true triaxial hydraulic fracturing process in the shale rock sample containing the bedding and natural fractures. In another embodiment, a true triaxial hydraulic fracturing process in a conglomerate rock sample containing gravel particles is primarily simulated digitally.

In one embodiment, a true triaxial hydraulic fracturing process in a shale rock sample containing bedding and natural fractures is digitally simulated, and corresponding fracture morphology, pore pressure, acoustic emission event point distribution, and bottom hole pressure change curve conditions are obtained. Referring to fig. 10-19, fig. 10 schematically illustrates one of the schematic diagrams of fracture morphology after simulation of a rock sample model containing natural fractures according to an embodiment of the present invention; FIG. 11 schematically illustrates a second schematic diagram of fracture morphology after simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the present invention; FIG. 12 schematically illustrates a third schematic diagram of fracture morphology after simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the present invention; FIG. 13 schematically illustrates one of the pore pressure profiles after a simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the invention; FIG. 14 schematically shows a second distribution of pore pressure after simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the present invention; FIG. 15 schematically shows a third distribution of pore pressure after simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the present invention;

FIG. 16 schematically illustrates one of the distributions of acoustic emission event points after a simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the invention; FIG. 17 schematically illustrates a second distribution of acoustic emission event points after simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the present invention; FIG. 18 schematically illustrates a third distribution of acoustic emission event points after simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the invention; FIG. 19 schematically illustrates a bottom hole pressure change profile during simulation of a rock sample model containing natural fractures, in accordance with an embodiment of the invention.

Comparing fig. 10 and 16, fig. 11 and 17, and fig. 12 and 18, it can be seen that the distribution diagram of the fracture morphology and the distribution diagram of the acoustic emission event points after the rock sample model containing the natural fractures is simulated are consistent with a certain degree of matching.

The simulation method of the present embodiment mainly includes the following six steps.

The method comprises the following steps: and determining the size of the rock sample model in the embodiment to be 300 multiplied by 300mm according to the size of the actual rock core selected in the true triaxial fracturing experiment, and corresponding to the indoor actual experiment. And generating a closed area surrounded by walls with corresponding size in the numerical model. Selecting proper resolution ratio R according to actual conditions_e20, and generating a particle group inside the closed region, the particle density ρ of the particles generated at this stage_b＝2500kg/m³Damping coefficient f_b＝0.7。

Step two: and defining a contact model for the mutual contact positions of the particles in the particle group so as to realize the application of the cementation between the particles, wherein the cementation strength is determined according to the strength of the actual rock core. By making contact with a moldThe defined intensity parameters are adjusted, and the intensity parameters of the rock sample model can be corrected to target values. In this embodiment, a plain joint contact model is applied to the matrix part of the rock sample model, uniaxial compression experiment simulation is performed on the rock sample model according to a trial-and-error method, a simulation result is compared with strength parameters measured in a laboratory, the strength parameters of the rock sample model are adjusted, and the following model parameters are set: effective modulusNormal to tangential stiffness ratioTensile strengthCohesion c_fj20MPa, internal friction angle phi_fj40 ° and coefficient of friction μ_fj＝0.5。

Step three: and observing and recording heterogeneous characteristics of the actual rock core in the true triaxial fracturing experiment, and representing the heterogeneous characteristics in the rock sample model. In this example, natural fractures developed in shale were characterized by a smooth joint contact model, where natural fractures had both a dip and a dip of 70 ° throughout the entire rock sample model. The smooth joint contact model applied to the crack development was set with the following model parameters: normal stiffness of unitUnit tangential stiffnessCoefficient of friction mu_sj0.6, expansion angle psi_sj3 degree of tensile strengthCohesion c_sj0.5MPa and joint friction angleFurthermore, hydraulic characteristics for natural fracturesDifference, set crack permeability multiplier ζ_f＝10。

Step four: and applying three-dimensional layered ground stress to the rock sample model through the walls around the rock sample model, and restoring the complex ground stress state of the actual rock core in a triaxial experiment. According to the triaxial stress condition in the actual true triaxial fracturing experiment, the embodiment selects the three-way uniform ground stress state sigma_x＝25.0MPa，σ_y＝8.0MPa，σ_z20.0MPa, and setting a ground stress accuracy coefficient delta_tol＝0.005。

Step five: a fluid computing network is constructed on the basis of a rock sample model, and a fluid-solid coupling scheme based on a discrete element method is formed to simulate the evolution of fluid pressure in actual rock core pores along with injection liquid and the transfer process of the fluid pressure among the pores. Setting permeability parameters of the rock sample model, mainly comprising relaxation permeability k under the action of no external stress_ini＝1.0×10^-18m²Limiting permeability k under the action of infinite external stress_str＝1.0×10^-19m²And fracture permeability k after fracture of rock sample model_frc＝1.0×10^-16m². Simultaneously setting the fluid viscosity mu as 200 mPas and the bulk modulus K_b2.2GPa, 5X 10 of discharge capacity r^-7m³/s。

Step six: and (3) carrying out digital simulation of a true triaxial hydraulic fracturing experiment, and carrying out liquid injection simulation on the rock sample model treated by the steps. In the rock sample model, a process that the pore pressure in pores in a certain range near an injection point is increased due to injection is represented, and a process that interparticle cementation is broken due to the increase of the pore pressure value until the rock sample model is broken is further carried out. After the injection simulation is carried out for a period of time, the number of the microcracks generated in the rock sample model, the microcrack form and the positions of the acoustic emission event points are counted to form fine crack form description, pore pressure, acoustic emission event point distribution and bottom hole pressure curve evolution results.

In another embodiment, a true triaxial hydraulic fracturing process in a conglomerate rock sample containing gravel particles is digitally simulated and corresponding fracture morphology and bottom hole pressure profile conditions are obtained. Referring now to fig. 20-24, fig. 20 schematically illustrates one of the schematic views of a fracture morphology after simulation of a rock sample model containing conglomerate particles (when the conglomerate particles are of lower strength) in accordance with an embodiment of the invention; FIG. 21 schematically illustrates a second schematic representation of fracture morphology after simulation of a rock sample model containing conglomerate particles (at lower strength) in accordance with an embodiment of the invention; FIG. 22 schematically shows one of the schematic representations of fracture morphology after simulation of a rock sample model containing conglomerate particles (when their strength is high) in accordance with an embodiment of the invention.

FIG. 23 schematically illustrates a second schematic representation of fracture morphology after simulation of a rock sample model containing conglomerate particles (at higher strength) in accordance with an embodiment of the invention; FIG. 24 schematically shows a bottom hole pressure variation graph of a rock sample model containing conglomerate particles during a simulation process, in accordance with an embodiment of the invention.

The simulation method of the present embodiment mainly includes the following six steps.

The method comprises the following steps: according to the size of the actual core selected in the true triaxial fracturing experiment, the size of the rock sample model in the embodiment is determined to be 300 multiplied by 300mm, the rock sample model corresponds to an indoor actual experiment, and a closed area surrounded by walls in a corresponding size is generated in the numerical model. Selecting proper resolution ratio R according to actual conditions_e25, and generating a particle group in the closed region, and the particle density ρ of the particles generated at this stage_b＝2500kg/m³Damping coefficient f_b＝0.7。

Step two: and defining a contact model for the mutual contact positions of the particles in the particle group so as to realize the application of the cementation between the particles, wherein the cementation strength is determined according to the strength of the actual rock core. By adjusting the intensity parameters defined in the contact model, the intensity parameters of the rock sample model can be corrected to target values. In this embodiment, a plain joint contact model is applied to the matrix part of the rock sample model, uniaxial compression experiment simulation is performed on the rock sample model according to a trial-and-error method, a simulation result is compared with strength parameters measured in a laboratory, the strength parameters of the rock sample model are adjusted, and the following parameters are set: effective modulusNormal to tangential stiffness ratioTensile strengthCohesion c_fj20MPa, internal friction angleAnd coefficient of friction mu_fj＝0.5。

Step three: and observing and recording heterogeneous characteristics of the actual rock core in the true triaxial fracturing experiment, and representing the heterogeneous characteristics in the rock sample model. In the embodiment, the conglomerate particles of the actual rock core are characterized by changing the strength parameters of the contact model among the particles in the partial region. The conglomerate particles are distributed on the Y-Z plane in an axial symmetry way relative to the central plane of the rock sample model (see figure 20); and setting the following model parameters for the contact model within the characterization range of the conglomerate particles: (1) lower strength conglomerate particles: effective modulusNormal to tangential stiffness ratioTensile strengthCohesion c_fj40MPa, internal friction angleCoefficient of friction mu_fj0.5; (2) high-strength conglomerate particles: effective modulusNormal to tangential stiffness ratioTensile strengthCohesion c_fj80MPa, internal friction angleCoefficient of friction mu_fj＝0.5。

Step four: and applying three-dimensional layered ground stress to the rock sample model through the walls around the rock sample model, and restoring the complex ground stress state of the actual rock core in a triaxial experiment. According to the triaxial stress condition in the actual true triaxial fracturing experiment, the three-direction uniform ground stress state sigma is selected for the rock sample model in the embodiment_x＝4.0MPa，σ_y＝14.0MPa，σ_z20.0MPa, and setting a ground stress accuracy coefficient delta_tol＝0.005。

Step five: a fluid computing network is constructed on the basis of a rock sample model, and a fluid-solid coupling scheme based on a discrete element method is formed to simulate the evolution of fluid pressure in the core pore along with injection liquid and the transfer process of the fluid pressure among pores. Setting permeability parameters of the rock sample model, mainly comprising relaxation permeability k under the action of no external stress_ini＝7.0×10^-15m²Limiting permeability k under the action of infinite external stress_str＝7.0×10^-17m²And fracture permeability k after fracture of rock sample model_frc＝7.0×10^-14m². Simultaneously setting the fluid viscosity mu as 200 mPas and the bulk modulus K_b2.2GPa, 5X 10 of discharge capacity r^-7m³/s。

Step six: and (3) carrying out digital simulation of a true triaxial hydraulic fracturing experiment, and carrying out liquid injection simulation on the rock sample model treated by the steps. In the rock sample model, a process that the pore pressure in pores in a certain range near an injection point is increased due to injection is represented, and the process that the cementation among particles is broken due to the increase of the pore pressure value is further carried out until the rock sample model is broken. After a period of injection simulation, counting the number and the form of the microcracks generated in the rock sample model to form a fine crack form description and a well bottom pressure curve evolution result.

FIG. 25 schematically illustrates a flow diagram for a digital simulation of a true triaxial hydraulic fracturing experiment according to an embodiment of the present invention. As shown in fig. 25, in summary, the digital simulation process of the true triaxial hydraulic fracturing experiment includes: (1) determining the size of the sample to generate a particle group; (2) applying the cementation between the particles to form a rock sample model; (3) characterizing heterogeneous characteristics of the rock sample model; (4) applying complex ground stress boundary conditions; (5) realizing fluid-solid coupling, and constructing a fluid computing network; (6) and performing digital simulation of hydraulic fracture and describing hydraulic fracture morphology.

However, under the influence of complex ground stress and formation heterogeneity, the initiation and extension mechanism of the artificial fracture in the reservoir is very complex, the prediction difficulty of the non-planar fracture path is extremely high, and basic research aiming at the initiation and extension mechanism of the artificial fracture under complex geological and working conditions needs to be developed, so that scientific guidance is provided for the design of field fracturing construction. The indoor true triaxial hydraulic fracturing experiment is an effective means for researching the fracture initiation and extension characteristics of the artificial fracture, and can accurately capture the morphological characteristics of the artificial fracture under the simulated formation condition on the basis of utilizing the in-situ formation core or outcrop. However, the current true triaxial hydraulic fracturing experiment has many defects, for example, due to the limitation of equipment conditions, the loading of complex stress boundary conditions, the application of formation pore pressure, and the simulation of formation geology and mechanical heterogeneity characteristics are extremely difficult, and an equipment solution with high repeatability and easy use is not available up to now; secondly, the indoor true triaxial hydraulic fracturing experiment can only provide a qualitative analysis result, and key engineering factors such as the extension form of the artificial crack, the interaction of discontinuous surfaces such as the artificial crack and a natural crack, and the pore pressure evolution of the crack and the matrix cannot be precisely described; thirdly, because an underground rock sample with a size suitable for the true triaxial fracturing experiment cannot be obtained, the indoor true triaxial fracturing experiment usually adopts outcrop rock samples to replace the underground rock sample, and the geological and mechanical characteristics of a real stratum cannot be reflected; finally, because the outcrop is difficult to obtain and the experiment consumes a long time, the time, personnel and economic cost for developing a large number of true triaxial hydraulic fracturing experiments considering multi-factor variables are extremely high.

The numerical simulation technology for hydraulic fracturing partially makes up the defects of a true triaxial hydraulic fracturing experiment. The numerical simulation scheme based on the continuous medium is widely applied, such as a finite element method, a finite difference method, a phase field method and the like. The numerical model based on the method can accurately reflect the artificial fracture extension path and the evolution of pore pressure in the hydraulic fracturing process, but the heterogeneity of a reservoir cannot be well reflected, and the convergence of the model is poor due to the fact that too many discontinuous surfaces are arranged in the model. In addition, the fracture propagation path of the numerical simulation methods is obviously influenced by modeling, the path is often required to be manually preset, the path is not consistent with the actual fracture propagation path, the non-planar propagation characteristic of the artificial fracture is difficult to depict, and the interaction with the natural fracture often faces the condition that the calculation is difficult to converge. In addition, the numerical simulation method is difficult to characterize the discontinuity of the rock sample, such as the presence of gravel and holes. The discrete element method can be adopted to accurately depict the non-continuous characteristics; however, this method is far more computationally intensive than other numerical methods and increases rapidly with increasing sample size. In addition, the conventional simulation means based on the discrete element method cannot generate acoustic emission event points, so that mutual verification cannot be carried out with event point data measured by an acoustic emission monitoring instrument of an indoor true triaxial mechanical experiment to evaluate the reliability of a simulation result.

Embodiments of the present invention provide a processor configured to perform a simulation method of any one of the above embodiments of a true triaxial hydraulic fracturing experiment.

In particular, the processor may be configured to:

applying cementation between particles of the particle group, and adjusting the strength of the cementation according to the strength of the actual rock core;

the pore pressure increase is controlled to cause interparticle cementitious fracture until internal fracture of the rock sample model occurs.