A quantitative evaluation method for shale brittleness based on bedding characteristics

Abstract

The application relates to the field of petroleum and natural gas engineering, in particular to a quantitative evaluation method of shale brittleness based on bedding characteristics in the process of exploration and development, which comprises the following steps: taking a rock sample of target layer shale to obtain the maximum Young's modulus of a block, and calculating a macroscopic brittleness index; processing the rock sample into a cuboid rock block to perform a nanoindentation experiment, calculating an interlayer heterogeneity coefficient between two adjacent probes according to mechanical parameters of the same horizontal position between beddings, and then obtaining a microscopic brittleness index, and calculating a total brittleness index of the rock by combining the macroscopic brittleness index with the microscopic brittleness index; the application firstly innovatively tests microcosmic rock mechanical experimental data between different beddings of shale based on nanoindentation, quantitatively calculates the heterogeneity between different beddings, realizes quantitative evaluation of the microcosmic brittleness of shale rock, and further improves the conventional macroscopic evaluation method.

Description

Technical Field

[0001] This invention relates to the field of oil and gas engineering, and in particular to a quantitative evaluation method for shale brittleness based on bedding characteristics during exploration and development. Background Technology

[0002] Large-scale horizontal well volumetric fracturing is a key technology for achieving large-scale, efficient development of shale gas reservoirs. It creates complex, multi-scale fracture networks, artificially generating gas reservoirs and altering the fluid flow field, which is central to shale gas volumetric development. Rock brittleness is a crucial factor influencing the formation of complex fracture networks through volumetric fracturing and is also the basis for optimized fracturing design. In contrast, foreign shale gas reservoirs, as discovered through downhole coring, exhibit strong heterogeneity, complex and variable mineral composition, and well-developed bedding. Currently, methods for evaluating rock brittleness primarily rely on quantitative calculations based on mineral composition and rock mechanical parameters.

[0003] (1) Patent application number 201910757761.4, a method for evaluating the brittleness of shale based on mechanical heterogeneity. The method determines the mineral component deviation coefficient according to the mineral composition of shale; determines the dynamic Young's modulus and dynamic Poisson's ratio of various minerals; determines the estimated Young's modulus and estimated Poisson's ratio of the reservoir using a component model based on the dynamic Young's modulus and dynamic Poisson's ratio of various minerals; and obtains the corresponding linear fitting formula of fitted Young's modulus and fitted Poisson's ratio by linear fitting, and calculates the brittleness coefficient.

[0004] (2) Patent application No. 202510063185.9, Method, Apparatus, Equipment, Medium and Product for Evaluating the Brittleness of Unconventional Oil and Gas Reservoirs. The method obtains stress-strain data of a core sample of a target unconventional oil and gas reservoir under uniaxial or triaxial compression tests; determines the peak strain of the core sample under uniaxial or triaxial compression based on the stress-strain data; determines the pre-peak total energy of the core sample based on the peak strain; and determines the rock brittleness value of the target unconventional oil and gas reservoir based on the peak strain and the pre-peak total energy.

[0005] (3) Patent application number 202310971924.5, Shale brittleness evaluation method and device based on elemental analysis. This method studies the relationship between mineral and element content, identifies sensitive elements in different minerals, establishes a conversion model between elements and minerals, calculates the brittleness index, and classifies according to the brittleness classification standard of shale gas reservoirs.

[0006] A review of domestic and international literature and patents reveals that current research has not yet comprehensively evaluated the brittleness of shale gas reservoirs from both macroscopic and microscopic perspectives, considering the combined effects of bedding characteristics and the rock mechanics properties of bedding on shale gas reservoir brittleness. Therefore, it is necessary to propose a new method that aligns with the bedding development characteristics of shale gas reservoirs, yields more applicable evaluation results, and further provides a foundational basis for reservoir stimulation scheme design. Summary of the Invention

[0007] The purpose of this invention is to provide a quantitative evaluation method for shale brittleness based on bedding characteristics, which overcomes the shortcomings of evaluating shale solely based on the overall macroscopic mechanical parameters or mineral composition of the rock, without considering the influence of the microscopic mechanical differences between bedding layers, a core factor affecting shale brittleness.

[0008] This invention provides a quantitative evaluation method for the brittleness of shale based on bedding characteristics, comprising the following steps: 1) Take rock samples from the target shale, core the samples and measure their mechanical parameters. At the same time, collect the basic data of Young's model of rock mechanical parameters of the target stratum to obtain the maximum Young's modulus of the block, and calculate the macroscopic brittleness index accordingly. 2) The rock sample was processed into a cuboid rock block for nanoindentation experiment. The probes for nanoindentation were arranged according to the bedding as the interface to obtain the micromechanical parameters at different positions in different bedding, characterizing the mechanical differences between bedding. Then, the interlayer heterogeneity coefficient between two adjacent probes was calculated based on the mechanical parameters at the same lateral position between bedding. The overall interlayer heterogeneity coefficient between two adjacent layers was further calculated. 3) The overall interlayer heterogeneity coefficient between different bedding planes is obtained from the overall interlayer heterogeneity coefficient of two adjacent layers, and then the micro brittleness index is obtained. The micro brittleness index is combined with the macro brittleness index to calculate the total brittleness index of the rock. The brittleness level of the rock is classified according to its value. When 0 < total brittleness index ≤ 0.30, the brittleness is poor and the level is Class III reservoir; when 0.30 < total brittleness index ≤ 0.60, the brittleness is moderate and the level is Class II reservoir; when 0.60 < total brittleness index ≤ 1, the brittleness is good and the level is Class I reservoir.

[0009] Preferably, in step 1), the obtained rock sample is cored to make a standard cylindrical rock core; Then, the mechanical parameters of the standard cylindrical rock core were measured, specifically: the Young's modulus of the rock core was measured using a triaxial rock mechanics testing system. The macroscopic brittleness index was calculated using the measured Young's modulus of the core sample and the maximum Young's modulus of the block. The calculation formula is as follows: In the formula: E s For the actual measurement of Young's modulus, MPa; E maxB1 is the maximum Young's modulus of the block, in MPa; B1 is the macroscopic brittleness index, dimensionless.

[0010] Preferably, in step 2), the obtained rock sample is taken and made into a rectangular rock block, wherein one side is parallel to the bedding direction and the other side is perpendicular to the bedding direction. Nanoindentation experiments were conducted on a cuboid rock block. In the experiment, multiple probes were arranged at the same spacing within the same bedding layer to measure the Young's modulus at the target location. The number of probes was set according to the number of bedding layers in the direction perpendicular to the bedding layer. The interlayer heterogeneity coefficient is obtained by calculating the Young's modulus at the same transverse position of two adjacent bedding layers, using the following formula: In the formula: E ij Let m be the Young's modulus in the i-th row and j-th column, in MPa, where i is the transverse direction, representing different stratification directions, and j is the longitudinal direction, which is within the same stratification; ij is the interlayer heterogeneity coefficient, which is dimensionless.

[0011] Preferably, the formula for calculating the overall interlayer heterogeneity coefficient is as follows: Where: K j This is the overall interlayer heterogeneity coefficient corresponding to the adjacent layers, and it is dimensionless.

[0012] Preferably, the formula for calculating the micro-brittleness index is as follows: In the formula: G is the interlayer average heterogeneity coefficient, which is dimensionless; B2 is the micro-brittleness index, which is dimensionless.

[0013] Preferably, the formula for calculating the total brittleness index is as follows: In the formula: BI is the total fragility index, which is dimensionless.

[0014] Compared with the prior art, the advantages of the present invention are as follows: This invention is the first to innovatively use nanoindentation testing to measure the microscopic rock mechanics experimental data between different bedding layers of shale, quantitatively calculate the heterogeneity between different bedding layers, and realize the quantitative evaluation of the microscopic brittleness of shale rocks, which is a further improvement compared with conventional macroscopic evaluation methods.

[0015] Compared with conventional evaluation methods, this invention overcomes the drawback of evaluating rocks by only using the overall macroscopic mechanical parameters or mineral composition, without considering the influence of the microscopic mechanical differences between bedding layers, which are the core factors affecting shale brittleness.

[0016] This application innovatively evaluates the overall brittleness characteristics of rocks from the perspective of the combination of differences in micromechanical features between layers and macromechanical features. Attached Figure Description

[0017] Figure 1 A schematic diagram showing the location of the probe used in the nanoindentation experiment to measure Young's modulus.

[0018] Figure 2 This is a comparison chart of the interlayer heterogeneity coefficients between different strata. Detailed Implementation

[0019] The following examples illustrate this application in detail. The present invention provides a quantitative evaluation method for shale brittleness based on bedding characteristics, comprising the following steps: 1. W is a shale gas reservoir development block, with W1 as the main development layer. Downhole coring was conducted at the target layer. Mechanical parameters of the core samples were measured, and the macroscopic brittleness index was calculated to characterize its overall brittleness. Details are as follows: (1) Standard core preparation: The obtained rock samples were cored and prepared into standard cylindrical rock cores with a diameter of 2.5cm and a height of 5cm, named XL-1-1; (2) Measurement of mechanical parameters: The Young's modulus of the core was measured using a triaxial rock mechanics testing system, and the Young's modulus of the core was measured to be 34108 MPa. (3) Macroscopic brittleness index: Calculated using the measured Young's modulus of the core and the maximum Young's modulus of the block. The maximum Young's modulus of the block is the largest Young's modulus of the core sample within the block where the core was taken. In the formula: E s For the actual measurement of Young's modulus, MPa; E max B1 is the maximum Young's modulus of the block, in MPa; B1 is the macroscopic brittleness index, dimensionless; given that the maximum Young's modulus of the block rock sample is 47198 MPa, the value of the macroscopic brittleness index is 0.723.

[0020] 2. Rock samples were processed into square blocks for nanoindentation experiments. The probes for nanoindentation were arranged according to bedding planes. The obtained micromechanical parameters were the mechanical parameters of rock samples at the same spacing within the bedding planes, which were used to compare the mechanical properties between layers. Then, the mechanical parameters of adjacent rock samples at the same lateral position between layers were calculated to obtain the small interlayer heterogeneity coefficient, and then the interlayer heterogeneity coefficient between two adjacent layers was obtained. The specific details are as follows: (1) Rock sample preparation: The obtained rock sample was taken and made into a rock block of 6mm×6mm×4mm. The cutting direction was parallel to the bedding direction on one side and perpendicular to the bedding direction on the other side. This rock sample was named XL-2-1. (2) Nanoindentation experiment: In the experiment, a total of 10 probes were arranged at the same spacing within the same stratum to measure the Young's modulus at the target location. The probes were set according to the number of strata in the direction perpendicular to the stratum. Figure 1 As shown, the rock sample prepared in this experiment has 12 bedding layers. (3) Interlayer heterogeneity coefficient: The Young's modulus at the same transverse position of two adjacent bedding layers is calculated. The Young's modulus values ​​at probes 1 to 6 of bedding layers are shown in Table 1, and the Young's modulus values ​​at probes 7 to 12 of bedding layers are shown in Table 2. The interlayer heterogeneity coefficient at each location is calculated according to formula (2) as shown in Table 3. In the formula: E ij Let m be the Young's modulus in the i-th row and j-th column, in MPa, where i is the transverse direction, representing different stratification directions, and j is the longitudinal direction, which is within the same stratification; ij is the interlayer heterogeneity coefficient, which is dimensionless; Table 1. Measurement data of Young's modulus at different locations in strata 1-6. 34576 36272 33056 33054 37713 32170 34534 36072 33039 33613 37697 32169 34291 36053 33147 33664 38059 32269 34629 36202 33470 33840 37650 32420 34043 36023 32537 33953 38048 32540 34935 36251 33388 33456 37647 32239 34212 36040 33439 33548 38178 32855 34332 35563 33362 33254 37992 32878 34392 36022 33268 33628 37847 32993 34972 35786 33492 33140 37772 32800 Table 2. Measurement data of Young's modulus at different locations in strata 7-12. Table 3. Coefficients of interlayer heterogeneity at various locations. 3. Overall interlayer heterogeneity coefficient: Using the interlayer heterogeneity coefficient calculation and combined with formula (3), the interlayer heterogeneity coefficient between each layer is calculated: Where: K j This is the overall interlayer heterogeneity coefficient corresponding to adjacent layers, dimensionless; the calculation results are attached. Figure 2 As shown.

[0021] The micro-brittleness index is obtained from the interlayer heterogeneity coefficient of each adjacent layer, and the total brittleness index is obtained by combining it with the macro-brittleness index, thereby achieving the purpose of quantitative evaluation. The specific content is as follows: (1) Microscopic brittleness index: It is obtained by using the interlayer heterogeneity coefficient and combined with formula (4) to calculate that the average interlayer heterogeneity coefficient is 0.119; G is the interlayer average heterogeneity coefficient, which is dimensionless; Combining formula (5), the micro-brittleness index is calculated to be 0.664; In the formula: B2 is the micro-brittleness index, which is dimensionless.

[0022] (2) Total brittleness index: The total brittleness index is calculated by combining the macro brittleness index and the micro brittleness index. The total brittleness index is 0.693 according to formula (6). In the formula: BI is the total fragility index, which is dimensionless.

[0023] Based on the value, the rock brittleness level is classified. The W1 reservoir, the main development layer in the W shale gas reservoir development block, has good brittleness and is classified as a Class I reservoir.

[0024] The present invention will be specifically described through embodiments. These embodiments are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way, nor are they limited to the forms disclosed herein. They should not be regarded as excluding other embodiments. Modifications and simple changes made by those skilled in the art that do not depart from the technical concept and scope of the present invention shall all fall within the protection scope of the technical solution of the present invention.

Claims

1. A quantitative evaluation method for the brittleness of shale based on bedding characteristics, characterized in that, Includes the following steps: 1) Take rock samples from the target shale, core the samples and measure their mechanical parameters. At the same time, collect the basic data of Young's model of rock mechanical parameters of the target stratum to obtain the maximum Young's modulus of the block, and calculate the macroscopic brittleness index accordingly. 2) The rock sample was processed into a cuboid rock block for nanoindentation experiment. The probes for nanoindentation were arranged according to the bedding as the interface to obtain the micromechanical parameters at different positions in different bedding, characterizing the mechanical differences between bedding. Then, the interlayer heterogeneity coefficient between two adjacent probes was calculated based on the mechanical parameters at the same lateral position between bedding. The overall interlayer heterogeneity coefficient between two adjacent layers was further calculated. 3) The overall interlayer heterogeneity coefficient between different bedding planes is obtained from the overall interlayer heterogeneity coefficient of two adjacent layers, and then the micro brittleness index is obtained. The micro brittleness index is combined with the macro brittleness index to calculate the total brittleness index of the rock. The brittleness level of the rock is classified according to its value. When 0 < total brittleness index ≤ 0.30, the brittleness is poor and the level is Class III reservoir; when 0.30 < total brittleness index ≤ 0.60, the brittleness is moderate and the level is Class II reservoir; when 0.60 < total brittleness index ≤ 1, the brittleness is good and the level is Class I reservoir. In step 2), the obtained rock sample is taken and made into a rectangular rock block, with one side parallel to the bedding direction and the other side perpendicular to the bedding direction. Nanoindentation experiments were conducted on cuboid rock blocks. In the experiment, multiple probes were arranged at the same spacing within the same bedding plane to measure the Young's modulus at the target location. In the direction perpendicular to the bedding plane, the number of probes was set according to the number of bedding planes. The interlayer heterogeneity coefficient is obtained by calculating the Young's modulus at the same transverse position of two adjacent bedding layers, using the following formula: （2） In the formula: E ij Let m be the Young's modulus in the i-th row and j-th column, in MPa, where i is the transverse direction, representing different stratification directions, and j is the longitudinal direction, which is within the same stratification; ij is the interlayer heterogeneity coefficient, which is dimensionless; The formula for calculating the overall interlayer heterogeneity coefficient is as follows: （3） Where: K j This is the overall interlayer heterogeneity coefficient corresponding to adjacent layers, and it is dimensionless. The formula for calculating the micro-brittleness index is as follows: （4） In the formula: G is the interlayer average heterogeneity coefficient, which is dimensionless; （5） In the formula: B2 is the micro-brittleness index, which is dimensionless.

2. The quantitative evaluation method for shale brittleness based on bedding characteristics as described in claim 1, characterized in that, In step 1), the obtained rock samples are cored to make standard cylindrical rock cores; Then, the mechanical parameters of the standard cylindrical rock core were measured, specifically: the Young's modulus of the rock core was measured using a triaxial rock mechanics testing system. The macroscopic brittleness index was calculated using the measured Young's modulus of the core sample and the maximum Young's modulus of the block. The calculation formula is as follows: （1） In the formula: E s For the actual measurement of Young's modulus, MPa; E max B1 is the maximum Young's modulus of the block, in MPa; B1 is the macroscopic brittleness index, dimensionless.

3. The quantitative evaluation method for shale brittleness based on bedding characteristics as described in claim 2, characterized in that, The formula for calculating the total brittleness index is as follows: （6） In the formula: BI is the total fragility index, which is dimensionless.

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