Method for simulating three-dimensional rock fracture based on improved four-parameter random growth method

Abstract

The invention discloses a method for simulating a three-dimensional rock fracture based on an improved four-parameter random growth method, and the method specifically comprises the following steps: 1, setting model parameters, including the shape and size parameters of a rock, and the size parameters and the delivery amount of the rock fracture; 2, establishing a model space according to the shape and size of the rock, and dispersing the model space into fine units; step 3, improving a four-parameter random growth method according to the occurrence, size and delivery quantity parameters of the rock fracture, and determining control parameters; and 4, distributing the initial growth nucleus according to the initial growth nucleus distribution probability Pd, growing to surrounding units according to the growth probability Pi to form fractures, and ending the operation when the content of the growth nucleus reaches the porosity P to obtain a final rock three-dimensional fracture model. The invention provides a thought for modeling based on the morphological characteristic parameters of the rock fractures, parametric modeling of the three-dimensional rock fractures can be realized through the method, and the modeling process is simple and convenient.

Description

Technical Field

[0001] The present invention relates to the field of numerical simulation of rock mechanics, and specifically to a method for simulating three-dimensional rock fractures based on an improved four-parameter random growth method. Background Art

[0002] In nature, there are often a large number of fractures inside rocks, including the original damage and defects in the rock structure, with original fractures, and at the same time fractures formed by geological processes. Under the action of external loads, the fractures expand and penetrate, resulting in a significant reduction in the mechanical strength of the rock and making it more prone to macroscopic failure. Studying the mechanical properties of fractured rocks and the evolution mechanism of fractures has always been a research topic that has received much attention. Numerical simulation, as a means of computer-aided analysis, can more intuitively obtain the development of fractures inside rocks and the change of rock strength, and has great advantages in assisting the analysis of the mechanical properties of fractured rocks.

[0003] At present, there are still problems with inaccurate models in the numerical analysis of fractured rocks. Traditional rock numerical models assume rocks as homogeneous materials and consider the fractures inside rocks by selecting equivalent parameters. Such models cannot simulate the development and penetration of fractures inside rocks and have certain defects. In addition, some current mesoscopic fractured rock models use regular geometric shapes to simulate the fractures of rocks. Such models can initially reflect the development of fractures inside rocks, but there are certain defects in the accuracy of the models compared with the morphological characteristics of real fractures. Summary of the Invention

[0004] Aiming at the problem of inaccurate rock fracture models, the present invention provides a modeling method for three-dimensional rock fractures by improving the four-parameter random growth method, and gives a better solution for the mesoscopic model of rock fractures.

[0005] The technical solution of the present invention: A method for simulating three-dimensional rock fractures based on an improved four-parameter random growth method, specifically including the following steps:

[0006] Step 1, set model parameters, including the shape and size parameters of the rock, as well as the size parameters and placement amount of the rock fractures;

[0007] Step 2, establish a model space according to the shape and size of the rock, and discretize the model space into small units;

[0008] Step 3, improve the four-parameter random growth method according to the occurrence, size and placement amount parameters of the rock fractures, and determine the control parameters (porosity P, initial growth nucleus distribution probability P d , growth probability P i and growth cycle number n);

[0009] Step 4, according to the initial growth nucleus distribution probability Pd Distribute the initial growth nuclei and with a growth probability P i Grow towards the surrounding cells to form fractures. When the content of the growth nuclei reaches the porosity P, the operation ends and the final three-dimensional rock fracture model is obtained.

[0010] Preferably, the size of the cell is less than 1 / 50 of the model space size. A smaller cell size can more finely control the shape of the rock fractures.

[0011] Specifically, the control parameter P is the porosity, which in the present invention is the ratio of the number of growth nuclei to the total number of cells in the model space, and is used to control the amount of rock fractures placed.

[0012] Specifically, the control parameter P d is the initial growth nucleus distribution probability, which can control the density of the rock fracture distribution in the present invention. P d is set to a reasonable value according to the amount of fracture placement. It should be noted that P d cannot be greater than the porosity P. The process of distributing the initial growth nuclei with the initial growth nucleus distribution probability P d is as follows: Generate a random number within the range of [0 - 1] for each cell in the model space. When the random number is less than the set initial growth nucleus distribution probability P d then set this cell as an initial growth nucleus. Similarly, traverse all cells to complete the distribution of the initial growth nuclei.

[0012] Specifically, the control parameter P i is the growth probability, which is used to generate fracture models extending in different directions. Each cubic growth nucleus is adjacent to 26 surrounding cells, and the 26 cells around the growth nucleus respectively represent 26 growth directions. By controlling the growth probability of each direction, fractures with different morphological characteristics can be generated. The growth process of the growth nucleus is: Generate a random number within the range of [0 - 1] when traversing each growth direction of each growth nucleus. When the random number is less than the set growth probability of this growth direction, then add the cell in this growth direction as a new growth nucleus.

[0013] The growth probability P in the present invention i is set as follows: Determine a unit vector for each initial growth nucleus, and consider this vector as the normal vector of the fracture surface to be generated. Among the 26 growth directions, the smaller the angle between the growth direction and the straight line where the normal vector is located, the smaller the set growth probability, and the growth probability in the direction perpendicular to the normal vector is the largest. The rock fracture surface generated in this way can control the occurrence of the internal fractures of the rock, and takes into account the randomness of fracture development, which is consistent with the morphological characteristics of the internal fractures of real rocks.

[0014] Specifically, the control parameter n is the number of growth cycles, which is used to control the number of times each initial growth nucleus grows towards surrounding cells. When an initial growth nucleus reaches the growth times, the initial growth nucleus and all the growth nuclei grown therefrom will stop growing, and new initial growth nuclei will be distributed.

[0015] Compared with the current rock fracture modeling methods, the method of the present invention gets rid of the defect that the fracture model has a regular and single shape. Based on the improved four-parameter random growth method to generate fracture surfaces, it can not only reflect the strike, dip direction, dip angle and size of the internal fractures of the rock, but also consider the irregularity of the growth of rock fractures, and the obtained rock fracture model is more in line with the actual situation.

[0016] The present invention provides an idea for modeling based on the morphological characteristic parameters of rock fractures. Through this method, parametric modeling of three-dimensional rock fractures can be realized, and the modeling process is simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a flowchart of the rock fracture model modeling method;

[0018] Figure 2 is a schematic diagram of the growth direction and growth probability;

[0019] Figure 3 is a rock fracture model diagram. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts shall fall within the protection scope of the present application.

[0021] The traditional four-parameter random growth method has four control parameters, namely porosity, distribution probability of growth nuclei, growth probability and probability density. Porosity is the ratio of the final pore volume reached by the model to the total volume; the distribution probability of growth nuclei is the number of growth nuclei distributed in the model space; the growth probability is the probability of a growth nucleus growing towards the surroundings; the probability density is the interaction between multiple phases. The present invention does not consider the interaction of multiphase media, so the probability density parameter is not considered. The main contribution of the present invention is to give a method for setting the growth probability for generating rock fracture surfaces, and at the same time add the control of the number of growth cycles on this basis. Based on these two points, the modeling method proposed by the present invention can control the strike, dip direction, dip angle and size of the fracture surfaces in the rock model, while not losing randomness. The modeling process of the embodiments of the present invention is as Figure 1 shown, and the specific modeling process is as follows:

[0022] Step 1: Set the shape and size of the rock. Taking the SHPB test rock sample model as an example in the present invention, the model shape is disc-shaped, and the size is 50 mm in diameter and 25 mm in thickness. Subsequently, a model space of corresponding size is generated and discretized into small units. Based on this model size, the unit size should be selected within the range of 0.1 - 1 mm. Too small a unit size will affect the modeling efficiency; too large a unit size will affect the model accuracy. Finally, the unit size set in this example is 0.5 mm.

[0022] Step 2: Determine the control parameters of the improved four-parameter random growth method. Set the porosity P and the initial growth nucleus distribution probability P d , where the porosity P controls the amount of crack placement, and the initial growth nucleus distribution probability P d controls the number of generated cracks. It should be noted that the initial growth nucleus distribution probability P d cannot be greater than the porosity P. In this example, the amount of crack placement in the rock is set to 2%, the porosity P is set to 0.02, and the initial growth nucleus distribution probability P d is 0.00006.

[0023] Growth probability P i is set as shown in Figure 2 . Each growth nucleus and its 26 adjacent surrounding units form 26 growth directions. Among them, the units in 6 directions are in surface contact with the growth nucleus, the units in 12 directions are in line contact with the growth nucleus, and the units in 8 directions are in point contact with the growth nucleus. When setting the growth probability of the rock crack model, first set a unit vector, and consider this vector as the normal vector of the crack surface to be generated. The setting process of the unit vector can accurately set the unit normal vector of each crack surface, or it can be set by random generation. The former can generate a rock crack model with oriented arrangement; the latter can generate a randomly distributed rock crack.

[0024] In this case, to reflect the randomness of the internal cracks of the rock, a random setting method is adopted to set a corresponding unit vector for each initial growth nucleus. Subsequently, according to the unit vector, set the growth probabilities of 26 growth directions. The setting rule is: the smaller the angle between the growth direction and the line where the normal vector is located, the smaller the set growth probability, and the growth probability in the direction perpendicular to the normal vector is the largest. The specific method of the growth probability is as follows: (a) Divide the area around the growth nucleus into 3 types of regions. Among them, the growth directions in the first type of region have a smaller angle with the line where the normal vector is located; the growth directions in the second type of region have a smaller angle with the plane perpendicular to the normal vector; the remaining space is the third type of region. The above 3 types of regions correspond to Figure 2The specific ranges of the blue area, yellow area, and green area shown in (b) can be controlled by the included angle; (b) Subsequently, a growth probability is set for each area, that is, the growth probability in each growth direction within the area. Among them, the growth probability of the blue area is the smallest, the green area is the second, and the growth probability of the yellow area is the largest. In this example, the range of the blue area is the area where the included angle with the line where the normal vector is located is less than 40°; the range of the yellow area is the area where the included angle with the plane perpendicular to the normal vector is less than 15°; the remaining part is the green area. The growth probabilities corresponding to the blue area, yellow area, and green area are 0.008, 0.0003, and 0.00005 respectively.

[0025] Step 3, determine the number of growth cycles n according to the size of the crack. In this example, the number of growth cycles is selected to be 50 times. Finally, the rock crack model is obtained as Figure 3 shown.

[0026] The core of the present invention is to improve the four-parameter random growth method to propose a method for three-dimensional rock cracks. The improvement of the algorithm includes giving a method for setting the growth probability for generating the rock crack surface and adding a parameter for the number of growth cycles. Based on these two points, the modeling method proposed by the present invention can control the strike, dip, dip angle, and size of the crack surface in the rock model. At the same time, compared with the original modeling method, the model generated by the method of the present invention considers the randomness and irregularity of the development of rock cracks in real situations, providing a better solution for the numerical simulation analysis model.

[0027] The specific implementation methods described above have elaborated in detail the purpose, technical solutions, and beneficial effects of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for simulating three-dimensional rock fractures based on an improved four-parameter random growth method, characterized in that: The specific steps include: Step 1, setting model parameters, including the shape and size parameters of the rock and the size parameters and injection amount of the rock cracks; Step 2: Establish a model space according to the shape and size of the rock, and discretize the model space into small units; Step 3: Improve the four-parameter random growth method according to the occurrence, size and injection parameters of rock fractures, and determine the control parameters, including porosity P, initial growth core distribution probability P d , growth probability P i and the number of growth cycles n; Step 4: According to the initial growth kernel distribution probability P d Distribute the initial growth kernel and grow it with probability P i The cracks grow toward the surrounding units. When the content of the growth core reaches the porosity P, the operation ends and the final three-dimensional rock crack model is obtained.

2. The method for simulating three-dimensional rock fractures based on an improved four-parameter random growth method according to claim 1, characterized in that: The cell size is less than 1 / 50 of the model space size; smaller cell size can better control the shape of rock fractures.

3. The method for simulating three-dimensional rock fractures based on an improved four-parameter random growth method according to claim 1, characterized in that: The control parameter P is the porosity, which is the ratio of the number of growth nuclei to the total number of cells in the model space and is used to control the amount of placement of rock fractures.

4. The method for simulating three-dimensional rock fractures based on an improved four-parameter random growth method according to claim 1, characterized in that: Control parameter P d is the probability of initial growth nucleus distribution, which controls the density of rock crack distribution; P d The value of is set to a reasonable value according to the amount of crack placement, P d Not greater than the porosity P; with the initial growth nucleus distribution probability P d The process of distributing the initial growth kernel is as follows: a random number in the range of [0-1] is generated for each unit in the model space. When the random number is less than the set initial growth kernel distribution probability P d Then set the unit as an initial growth core, and traverse all units in the same way to complete the distribution of the initial growth core.

5. The method for simulating three-dimensional rock fractures based on an improved four-parameter random growth method according to claim 1, characterized in that: Control parameter P i is the growth probability, which is used to generate crack models extending in different directions; each cubic growth core is adjacent to 26 surrounding units, and the 26 units around the growth core represent 26 growth directions respectively; by controlling the growth probability in each direction, cracks with different morphological characteristics can be generated; the growth process of the growth core is: when traversing each growth direction of each growth core, a random number in the range of [0-1] is generated, and when the random number is less than the growth probability set for the growth direction, the unit in the growth direction is added as a new growth core.

6. The method for simulating three-dimensional rock fractures based on an improved four-parameter random growth method according to claim 1, characterized in that: Growth probability P i The setting process is as follows: a unit vector is determined for each initial growth kernel, and the vector is considered to be the normal vector of the fracture surface to be generated; among the 26 growth directions, the smaller the angle between the growth direction and the straight line where the normal vector is located, the smaller the set growth probability is, and the growth probability in the direction perpendicular to the normal vector is the largest; the generated rock fracture surface can control the occurrence of fractures inside the rock, and considers the randomness of fracture development, which is consistent with the fracture morphology characteristics inside the real rock.

7. The method for simulating three-dimensional rock fractures based on an improved four-parameter random growth method according to claim 1, characterized in that: The control parameter n is the number of growth cycles, which is used to control the number of times each initial growth nucleus grows toward the surrounding units; when an initial growth nucleus reaches the growth number, the initial growth nucleus and all the growth nuclei grown from it stop growing, and new initial growth nuclei are distributed.

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