A fractured stratum modeling method based on GBM model to simulate mineral crystals

Abstract

The invention discloses a broken stratum modeling method for simulating mineral crystals based on a GBM model, comprising the following steps: step S1, selecting a rock sample of collapsed blocks of a broken stratum, testing the internal mineral structure and composition of the rock sample by using a polarizing microscope and X-ray diffraction, respectively establishing a GBM model template for each crystal block, and then importing the template into a discrete element simulation software to establish a wall, and filling particles into the wall to establish a basic model structure; step S2, testing the mechanical property data of the rock sample by a rock mechanics experiment; step S3, establishing a GBM parameter calibration model, and calibrating the model mesoscopic parameters based on the mechanical property data, so that the model can characterize the macroscopic mechanical properties of the rock; step S4, based on the result of the calibration of the mesoscopic parameters, correcting the GBM model template so that its mechanical behavior is close to the macroscopic result, and obtaining a GBM real broken stratum model; finally, observing the stress of the well wall rock, the expansion of cracks and the change of the force chain grid structure through the GBM real broken stratum model, and analyzing the instability phenomenon of the well wall of the broken stratum.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas drilling, and in particular to a broken stratum modeling method for simulating mineral crystals based on a GBM model. Background Art

[0002] As an unconventional oil and gas energy source, shale gas has attracted widespread attention due to its huge resources. Shale gas reservoirs are mud shale formations with well-developed stratification. During the drilling process, it is very easy to have downhole failures such as stuck drill and buried drill caused by wellbore instability. In severe cases, the wellbore can be scrapped, causing huge economic losses. The study of wellbore stability has a history of several decades. After the middle of the last century, it gradually formed quantitative research from mechanism analysis, and with the development of computer technology, it gradually evolved into a more in-depth numerical simulation study. At present, a large number of studies are mainly based on continuous medium models, which cannot effectively simulate the heterogeneity of rocks and rock fracture behavior. The discrete element model has the characteristics of intuitively displaying defects such as internal cracks and weak surface stratification in rocks, which is helpful for analyzing the damage and destruction process of rocks.

[0003] The idea of ​​building models through discrete element method is to reflect macroscopic mechanical behavior from microscopic mechanical behavior. If more realistic factors can be realized in this process, the mechanical behavior will be more accurate, and the established model will be more realistic. However, the parameters in discrete element method are all microscopic parameters, which cannot be directly connected with macroscopic mechanical structure. Summary of the invention

[0004] In view of the problem that the parameters used in the existing discrete element method for establishing models are all microscopic parameters and cannot be directly linked to the macroscopic mechanical structure, the present invention provides a broken stratum modeling method for simulating mineral crystals based on the GBM model.

[0005] The method for modeling broken strata based on GBM model to simulate mineral crystals provided by the present invention comprises the following steps:

[0006] S1. Combined with the core library, downhole return samples and outcrop observations, statistical analysis of typical broken stratum collapsed blocks, select representative rock blocks as real rock samples for slicing, make thin sections, use polarizing microscope and X-ray diffraction to test the internal mineral structure and composition of the rock samples, establish the GBM model template for each crystal block, and then import it into the discrete element simulation software (PFC simulation software) to build the wall, fill the particles into the wall, and establish the basic model structure.

[0007] S2. Test the mechanical properties of rock samples through conventional triaxial compression and other rock mechanics experiments, obtain the mechanical properties of rocks, clarify the macroscopic mechanical properties and numerical curves of rocks, and select representative experimental data.

[0008] S3. Establish a GBM parameter calibration model and calibrate the model's microscopic parameters based on the mechanical property data of the rock sample, so that the model can characterize the macroscopic mechanical properties of the rock. The specific method is as follows:

[0009] First, a GBM parameter calibration model is established to simulate the real core and assign initial values ​​to each parameter. Then, the size of each parameter is adjusted by the control variable method. The parameters include elastic modulus, stiffness ratio, cohesion, tensile strength, friction coefficient, etc. The stress-strain curve and key mechanical parameters (such as compressive strength and elastic modulus) output by the model are compared with the representative stress-strain curve and key mechanical parameters of the rock obtained in step S2. The size of each parameter is continuously adjusted until the absolute value of the error is less than or equal to 5%. That is, the microscopic parameters of the calibration model are achieved, so that the model can characterize the macroscopic mechanical properties of the rock.

[0010] S4. Based on the results of calibrated microscopic parameters, the GBM model template established earlier is corrected to make its mechanical behavior close to the macroscopic results, and the GBM real broken formation model is obtained.

[0011] In this step, since the Ball in the discrete element is a rigid whole, different cementation cannot be applied to its individual individuals, so clusters are used to simulate different mineral crystals, thereby simulating the real rock sample structure. First, the size of the crystal is specified, and complex grain grading is simulated through different particle gradings; then the GBM model template is imported, and the particle model is divided into GBM ranges through rblock statements to simulate the agglomeration process of grains in rocks; the rblock boundary is deleted, and the wall is automatically imported according to the shape of the group, and the group name is randomly defined for each shape, and the same prefix is ​​defined for the same type of particles to avoid errors in cementation addition; the sliding surface particles are flattened to reduce the tangential stiffness to simulate the sliding surface.

[0012] When applying different cementations between different grains, special attention should be paid to three points: the same mineral should be given the same properties; particles between groups should be given a relatively small property; and different groups of the same mineral should also be given a relatively weak property.

[0013] The parallel bonding model PBM is used to give the particles bonding, and the servo principle is used to load the rock sample, so that the rock sample is destroyed, that is, the corresponding stress is achieved by controlling the speed of the wall. Therefore, the stress between the particles and the wall needs to be back-calculated in each step:

[0014] σ n =∑F / S (1-1) where σ n is the stress between the particle and the wall, F is the contact force of the particle, and S is the contact area between the wall and the particle;

[0015] At the same time, a servo coefficient G is assumed in the PFC software, and the speed v of the wall is determined by the following formula:

[0016] v=G(σ b -σ i )=GΔσ (1-2)

[0017] In the formula, σ b is the current wall stress value; σ i is the target stress; G is the servo coefficient; Δσ is the difference between the target stress and the current wall stress, i.e., the contact stress increment;

[0018] The change in force on the wall, ΔF, is:

[0019] ΔF=K n NvΔt (1-3)

[0020] Where N is the number of contacts between the calculated wall and the particles; Kn is the stiffness of all contacts; v is the current velocity of the wall, which is multiplied by Δt to get the displacement of the next time step;

[0021] From this, the average contact stress increment generated by the contact particles on the wall unit can be calculated:

[0022]

[0023] Where A is the area of ​​the wall unit (3D).

[0024] Numerical calculation may not reach the accurate target stress value within a limited number of calculation steps, and in order to prevent the specimen from being damaged or to avoid instability caused by other factors, the change in wall stress must be less than the absolute value of the difference between the current stress and the target stress. Therefore, in order to ensure that the wall loading is more stable and to avoid particle dispersion, a relaxation coefficient a is defined to reflect the speed of change (usually 0.5), and the stability condition is expressed as:

[0025]

[0026] Where N c is the current contact number between the wall and the particle, Δσ n is the change in wall stress value in the current calculation step.

[0027] Then, from equations (1-2) and (1-5), we can get:

[0028]

[0029] Through the periodic servo of the above formula, it is ensured that the model stress value can change stably, so that the model stress state matches the real formation.

[0030] S5. Through the GBM real broken formation model, observe the stress of the wellbore rock, crack expansion and force chain grid structure changes, and analyze the instability of the wellbore in the broken formation.

[0031] Based on the real mineral component structure, the GBM real broken formation model is used to effectively simulate the real formation; the wellbore instability model is obtained through simulation calculation, and its mechanical propagation path and distribution are clarified by analyzing the distribution of strong and weak force chains in the model. The fracture network is used to analyze the rock fragmentation and block falling during the instability process, and the fragmentation of various mineral grains is analyzed to clarify the failure mode during the instability process.

[0032] Compared with the prior art, the present invention is beneficial in that:

[0033] (1) The method of the present invention determines the mineral structure based on thin section identification, clarifies the mineral type using the results of X-ray diffraction tests, and simulates the destruction process of the mesoscopic cementation of the wellbore rock in the real broken formation after drilling based on the equivalent crystalline model (GBM) of PFC, that is, a broken formation wellbore model for the formation mineral component structure is established, which can realize the evaluation of the instability characteristics such as wellbore crack propagation, force chain load transfer, and wellbore collapse and block falling under the conditions of different mining area components, different crack development degrees, and different stress conditions.

[0034] (2) The method of the present invention can effectively simulate the mineral crystals and cements in the rock, accurately restore the real composition of the rock, and more accurately display the macroscopic characteristics of the microscopic display. It can clarify the mechanical properties and component structure of the formation rock, establish a real physical model after drilling, and provide a basis for the site.

[0035] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The figure is a flow chart of the modeling method of the present invention.

[0037] Figure 2 This is a schematic diagram of the process of simulating shale formations in Example 1 of the present invention.

[0038] Figure 3 This is a schematic diagram of the model parameter calibration principle for simulating shale formations in Example 1 of the present invention.

[0039] Figure 4 This is a diagram of the broken stratum model established in Example 2 of the present invention.

[0040] Figure 5 This is a cementation structure diagram of the broken formation wellbore instability model of Example 2 of the present invention.

[0041] Figure 6 This is a force chain network diagram of the broken formation wellbore instability model of Example 2 of the present invention.

[0042] Figure 7 This is a fracture structure diagram of the broken formation wellbore instability model of Example 2 of the present invention. DETAILED DESCRIPTION

[0043] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0044] Example 1

[0045] The modeling method process of the present invention is as follows Figure 1 In this embodiment, a shale formation is selected for simulation, thin sections are made based on the obtained cores, and a GBM model template for each type of crystal block is established using a polarizing microscope and X-ray diffraction test. The model is imported into the PFC simulation software, and the model is randomly combined and assigned to form a model, as shown in FIG. Figure 2 The mechanical parameters of the selected samples obtained through conventional rock mechanics experiments are as follows: compressive strength 126.2MPa, elastic modulus 45.58GPa.

[0046] Based on the rock mechanics experimental results, the model micro-parameters are calibrated. The calibration results are as follows: Figure 3 As shown in the figure. The rock sample-stress-strain curve is obtained by testing real rock samples through conventional triaxial compression and other rock mechanics experiments. The model-stress-strain curve is simulated by the established GBM model. The compressive strength simulated by the model is 126.6MPa and the elastic modulus is 44.66GPa; the errors are less than 5%, indicating that the model can truly reflect the mechanical properties of the rock sample.

[0047] Example 2

[0048] On the basis of Example 1, a GBM mineral grain model was established. First, a 1*1m square stratum was established using discrete elements, and then the particle model was divided into GBM ranges using rblock statements (rblock statements are built-in operations in the PFC simulation software) to simulate the agglomeration process of grains in rocks. Then, the rblock boundaries were deleted, and the groups were automatically named and grouped according to their shapes. Then, the cementation of each group was performed, and a confining pressure of 10MPa was applied. Figure 4 Then a wellbore with a diameter of 0.31m was dug in the center of the model, and the formation fracture network and hydraulic coupling were given to simulate the original formation, that is, a shale formation wellbore model based on GBM simulated mineral grains was established. Figure 5It can be seen from the model cementation network that different types of rock blocks are endowed with different cementation contacts, which indicates that the model of the present invention can calibrate different types of contacts with corresponding microscopic parameters, thereby truly realizing the reconstruction of the multi-tropic structure inside the shale formation.

[0049] like Figure 6 As shown, the GBM simulates the contact force chain network of the mineral grains in the shale formation and shows that there are two force chain structures in the model: compression and tension. And the size of the force is determined by the thickness of the force chain, and the force transmission and structural changes are analyzed. In the near-well area, there are more compression force chains than tension force chains, and the overall number of force chains is much higher than that in the far-well area, indicating that stress concentration occurs in the near-well area. In the overall model, there are more tension and pressure chains coexisting, and the number is similar, indicating that in this case, the wellbore wall is relatively stable. Figure 7 From the analysis of the fracture network (the red area in the figure is the fracture) and the force chain network, it can be obtained that the fracture network of this model is not well developed. The fracture structure is formed around the well wall due to excavation, and the well wall block falling exists, but the wellbore does not suffer from serious instability such as deformation and damage; and except for the partial expansion of natural fractures, there are no obvious cracks in the overall model.

[0050] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A method for modeling broken strata based on GBM model to simulate mineral crystals, characterized in that: Here are the steps: S1. Select the collapsed blocks of broken strata as real rock samples, use polarizing microscope and X-ray diffraction to test the internal mineral structure and composition of the rock samples, establish the GBM model template of each crystal block, and then import it into the discrete element simulation software to establish the wall, and fill the particles into the wall to establish the basic model structure; S2. Test the mechanical properties of real rock samples through rock mechanics experiments including triaxial compression experiments, obtain the macroscopic mechanical properties and numerical curves of rocks, and select representative experimental data from them; S3. Establish a GBM parameter calibration model and calibrate the model's microscopic parameters based on the mechanical property data of the rock sample so that the model can characterize the macroscopic mechanical properties of the rock; the details are as follows: Establish a GBM parameter calibration model, simulate the real rock sample and assign initial values ​​to each parameter first, then continuously adjust the size of each parameter through the control variable method, compare the stress-strain curve and key mechanical parameters output by the model with the representative stress-strain curve and key mechanical parameters of the rock obtained in step S2, until the absolute value of the error is less than or equal to 5%, that is, the microscopic parameters of the calibration model are achieved, so that the model can characterize the macroscopic mechanical properties of the rock; S4. Based on the results of calibrated microscopic parameters, the GBM model template is corrected to make its mechanical behavior close to the macroscopic results, and the GBM real broken formation model is obtained; In this step, clusters are used to simulate different mineral crystals, thereby simulating the real rock sample structure. First, the size of the crystal is specified, and complex grain grading is simulated through different particle gradings. Then the GBM model template is imported, and the particle model is divided into GBM ranges through rblock statements to simulate the agglomeration process of grains in rocks. The rblock boundary is deleted, and the wall is automatically imported according to the shape of the group. The group name is randomly defined for each shape, and the same prefix is ​​defined for the same kind of particles to avoid errors in cementation addition. The sliding surface particles are flattened to reduce the tangential stiffness to simulate the sliding surface. The parallel bonding model (PBM) is used to give the particles a bond, and the stress state of the model is made to match the real formation through periodic servo. The servo principle is also used to load and cause the rock sample to be destroyed. S5. Through the GBM real broken formation model, observe the stress of the wellbore rock, crack expansion and force chain grid structure changes, and analyze the instability of the wellbore in the broken formation.

2. The broken stratum modeling method for simulating mineral crystals based on the GBM model according to claim 1, characterized in that: In step S1, the core library, downhole return samples and outcrop observations are combined to statistically analyze the rock blocks of typical broken strata collapse, and representative rock blocks are selected as real rock samples for slicing, making thin sections, and testing using a polarizing microscope and X-ray diffraction.

3. The broken stratum modeling method based on GBM model simulating mineral crystals according to claim 1, characterized in that: In step S4, the stress state of the model is made to match the real formation through periodic servo, and the specific method is as follows: Inversely calculate the particle and wall stresses at each step: s n =∑F / S (1-1) In the formula, σ n is the stress between the particle and the wall, F is the contact force of the particle, and S is the contact area between the wall and the particle; Assuming a servo coefficient G, the velocity v of the wall is determined by the following formula: v=G(σ b -s i )=GΔσ (1-2) In the formula, σ b is the current wall stress value; σ i is the target stress; G is the servo coefficient, Δσ is the difference between the target stress and the current wall stress, i.e., the contact stress increment; The change in force on the wall, ΔF, is: ΔF=K n NvΔt (1-3) Where N is the number of contacts between the calculation wall and the particles; K n is the stiffness of all contacts; v is the current velocity of the wall, which is multiplied by Δt to get the displacement of the next time step; The contact stress increment generated by the contact particles on the wall elements is: Where A is the area of ​​the wall unit; Define a relaxation coefficient a to reflect the speed of change, then the stability condition is expressed as: |Boss n |<a|Dσ|or Where N c is the current contact number between the wall and the particle, Δσ n is the change in the wall stress value in the current calculation step; Then we get: Through the periodic servo of the above formula, it is ensured that the model stress value can change stably, so that the model stress state matches the real formation.

4. The broken stratum modeling method based on GBM model simulating mineral crystals according to claim 1, characterized in that: Step S5 is specifically as follows: based on the real mineral component structure, the GBM real broken formation model is used to effectively simulate the real formation; the wellbore instability model is obtained through simulation calculation, and its mechanical propagation path and distribution are clarified by analyzing the distribution of strong and weak force chains of the model, and the rock mass breakage and block falling during the instability process are analyzed by using the fracture network, and the breakage of various mineral grains is analyzed to clarify the failure mode during the instability process.

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