Quantitative evaluation method for brittleness of deep coal rock gas reservoir based on fracture distribution state

Abstract

The invention relates to the field of petroleum and natural gas engineering, in particular to a quantitative evaluation method for the brittleness of a deep coal-rock gas reservoir based on a fracture distribution state, which comprises the following steps: 1) coring the reservoir, making the reservoir into a standard rock sample, and drying the standard rock sample to constant weight; 2) acquiring the core cross section fracture distribution characteristics and the number of fractures of the standard rock sample, and calculating a natural brittleness index; 3) carrying out transverse and vertical region division on the cross section of the rock core, calculating a region crack complex coefficient for each divided region, and calculating a crack communication effective index according to the region crack complex coefficient; and 4) combining the natural brittleness index and the fracture communication effective index, calculating the comprehensive brittleness index of the coal bed gas reservoir, and performing compressibility evaluation. The coal seam crack distribution state is considered, and based on the natural brittleness index of the rock and the potential coefficient of the complex crack network formed by the regional cracks, the compressibility quantitative evaluation is realized from the static and dynamic double brittleness evaluation of the rock.

Description

Technical Field

[0001] The invention relates to the field of petroleum and natural gas engineering, and in particular to a quantitative evaluation method for the brittleness of deep coal-rock gas reservoirs based on the distribution state of fractures during the exploration and development process. Background Art

[0002] The geological conditions of coal-rock gas reservoirs are complex and changeable. The coal rock is plastic, the reservoir-cap combination varies greatly, and it is difficult to extract adsorbed gas. The production is not ideal and it is difficult to maintain stable production. The initial production after compression is 50,000-120,000 cubic meters per day, and the decline rate in the first year after compression exceeds 60%, which is not ideal. Large-scale volume fracturing forms a complex fracture network in the reservoir, which is the core of efficient development of coal-rock gas. Among them, the brittleness of rock is the essential factor in the formation of complex fracture networks in coal-rock gas, and plays a key role in the optimization and improvement of volume fracturing technology. The brittleness of rock is affected by the mineral composition of the rock, the mechanical properties of the rock, the initial degree of microcrack development, the burial depth of the reservoir, etc. The relationship between different factors is complex, and the fractures are extremely developed, which increases the difficulty of evaluating the brittleness of rocks in deep coal-rock gas reservoirs.

[0003] In the prior art, there are mainly the following methods for evaluating and characterizing the brittleness of conventional reservoir rocks: (1) Patent application No. CN201811398934.X discloses a method for evaluating the fracturability of coal reservoirs. This method calculates the evaluation score and grade classification of the fracturability of coal reservoirs by obtaining the brittleness index of the coal reservoir to be evaluated and the basic parameters such as the fracture toughness, coal seam thickness, elastic modulus difference, and net pressure of the coal reservoir under confining pressure. (2) Patent application No. CN201910757761.4 discloses a method for evaluating the brittleness of shale based on mechanical heterogeneity. This method determines the mineral component deviation coefficient according to the mineral composition of the shale; determines the dynamic Young's modulus and dynamic Poisson's ratio of various minerals; and 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. Through linear fitting, the linear fitting formula of the fitted Young's modulus and fitted Poisson's ratio corresponding to the dynamic Young's modulus and dynamic Poisson's ratio is obtained to calculate the brittleness coefficient. (3) Patent application No. CN202210135674.7 discloses a method for evaluating the brittleness of continental shale oil reservoirs. The method mainly involves collecting rock samples from the target layer, conducting core mineral composition and composition tests, and mineral crystal morphology analysis, and further defining a brittleness index to comprehensively consider factors such as mineral content, crystal shape, and occurrence to quantitatively evaluate the brittleness of shale oil reservoirs.

[0004] Through investigation, it is known that the evaluation of rock brittleness of coal-rock reservoirs and conventional reservoirs focuses on rock mechanical parameters and mineral composition to quantitatively evaluate the brittleness of coal-rock reservoirs. However, neither of them takes into account the development and distribution characteristics of coal-rock reservoir fractures, which are the key factors affecting rock brittleness. At the same time, neither of them considers the evaluation from the perspective of the potential for the formation of complex fracture networks by reservoir microfractures, that is, the comprehensive evaluation of rock brittleness by the two factors of coal-rock reservoir brittleness itself and fracture communication capacity potential meets the objective requirements of coalbed methane reservoir compressibility evaluation. Summary of the invention

[0005] The purpose of the present invention is to provide a quantitative evaluation method for the brittleness of coal-rock gas reservoirs based on the distribution state of fractures. First, coring is performed on the target coal-rock gas reservoir, and the core is made into a standard rock sample; secondly, the natural brittleness index is calculated using the fracture distribution characteristics and the number of fractures in the core cross section to characterize the brittle characteristics of the rock itself in the coal-rock gas reservoir. The core cross section is divided into horizontal and vertical regions, and the regional fracture complexity coefficient is calculated for each region to determine the fracture communication area. On this basis, the fracture communication effectiveness index is calculated to characterize the ability of the coal-rock gas reservoir to transform and form a fracture network; finally, the natural brittleness index and the fracture communication effectiveness index are combined to calculate the brittleness index to characterize the rock brittleness of the target coal-rock gas reservoir, so as to achieve the purpose of quantitative evaluation.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A quantitative evaluation method for deep coal-rock gas reservoir brittleness based on fracture distribution state, characterized by comprising the following steps:

[0008] 1) Take coring work on the target coal-rock gas reservoir, make the core into a standard rock sample, and dry the standard rock sample to a constant weight;

[0009] 2) obtaining the crack distribution characteristics and the number of cracks in the core cross section of the standard rock sample, and calculating the natural brittleness index;

[0010] 3) Divide the core cross section into horizontal and vertical regions, calculate the regional fracture complexity coefficient for each divided region, and calculate the fracture communication effectiveness index based on the regional fracture complexity coefficient;

[0011] 4) The natural brittleness index and the fracture communication effectiveness index are combined to calculate the comprehensive brittleness index of the coalbed methane reservoir and conduct a compressibility evaluation.

[0012] Among them, the natural brittleness index represents the brittle characteristics of the coal-rock gas reservoir rock itself, and the fracture communication effectiveness index represents the ability of the coal-rock gas reservoir to transform and form a fracture network.

[0013] Preferably, the diameter of the standard rock sample in step 1) is 2.5 cm and the length is 5 cm.

[0014] Preferably, the drying method in step 1) comprises:

[0015] The standard rock sample was placed in an oven at 100°C and dried to constant weight.

[0016] Preferably, in step 2), the number of cracks is obtained by scanning electron microscopy.

[0017] Preferably, the step 2) further comprises:

[0018] Taking the core cross-section opening of 5 microns as the boundary, the crack is divided into two parts, and the fractal dimensions of the two parts are calculated respectively. The natural brittleness index is calculated by the fractal dimension:

[0019]

[0020] Where: F a The fractal dimension is greater than 5 microns, dimensionless; H a is a constant of proportionality, dimensionless; V a is the number of contact cracks in the box, pieces; Z a is the length of the box side, dimensionless; F b The fractal dimension is smaller than 5 microns, dimensionless; H b is a constant of proportionality, dimensionless; V b is the number of contact cracks in the box, pieces; Z b is the length of the box side, dimensionless; T a is the number of cracks with an opening greater than 5 microns, T b B is the number of cracks with an opening less than 5 microns, z It is the natural brittleness index and is dimensionless.

[0021] Preferably, the step 3) of dividing the core cross section into horizontal and vertical regions further comprises:

[0022] Take the center of the core circle as the origin of the coordinate axis, make a vertical X-axis and Y-axis coordinate system, with a side length of 2.5 cm / n, and draw long lines of 2.5 cm / n in the positive and negative directions of the X axis and the positive and negative directions of the Y axis from the origin as the starting point. Finally, the core is divided into small square areas with a side length of 2.5 cm / n, where n is an integer;

[0023] The calculation method of the crack complexity coefficient of the divided area is:

[0024]

[0025] Where: F ci H is the crack complexity index of the divided area, dimensionless; ci is a proportional constant, dimensionless; Z a / n is the length of the box side, dimensionless; V ciis the number of contact cracks in the box, pieces.

[0026] In the above technical solution, the regional fracture complexity coefficient is calculated for each area to determine the fracture communication area. On this basis, the fracture communication effectiveness index is calculated to characterize the ability of coal-rock gas reservoir transformation to form a fracture network.

[0027] Preferably, the step 3) further comprises: determining, according to the crack complexity coefficient, dividing the region into a relative effective index region or an absolute effective index region;

[0028] The criteria for judgment are:

[0029]

[0030] F ci ≥1.6 (6)

[0031] Among them, formula (5) is the judgment standard of the relative effective index area; formula (6) is the judgment standard of the absolute effective index area.

[0032] Preferably, the calculation method of the crack communication effectiveness index in step 3) is:

[0033]

[0034] Where: B g is the communication effectiveness index, dimensionless; x qi To calculate the abscissa value of the absolute effective index area, dimensionless; y qi To calculate the ordinate value of the absolute effective index area, dimensionless; x pi y is the horizontal coordinate value for calculating the relative effective index area, dimensionless; pi is the ordinate value of the relative effective index area for calculation, dimensionless; n1 is the number of areas for calculating the absolute effective index area, pieces; n2 is the number of areas for calculating the relative effective index area, pieces.

[0035] Preferably, the method for calculating the comprehensive brittleness index of the coalbed methane reservoir in step 4) is:

[0036] BU=B z ·B g (8)

[0037] Where: BU is the comprehensive brittleness index of coalbed methane reservoir, dimensionless.

[0038] On the other hand, the present invention provides a quantitative evaluation device for the brittleness of deep coal-rock gas reservoirs, characterized in that it includes: at least one processor and a memory communicatively connected to the processor; the memory stores instructions executable by the processor, and the instructions are executed by at least one processor so that at least one processor can execute any of the above-mentioned methods for quantitatively evaluating the brittleness of deep coal-rock gas reservoirs.

[0039] On the other hand, the present invention provides a computer-readable storage medium having a computer program stored thereon, characterized in that when the computer program is executed by a processor, the steps of the method for quantitatively evaluating the brittleness of deep coal gas reservoirs as described in any of the above items are implemented.

[0040] In summary, due to the adoption of the above technical scheme, the beneficial effect of the quantitative evaluation method of coal-rock gas reservoir brittleness based on the fracture distribution state of the present invention is as follows: the present invention proposes a quantitative evaluation method of coal-rock gas reservoir brittleness based on the fracture distribution state, by considering the fracture distribution state, the natural brittleness index of the rock itself and the potential coefficient of the complex fracture network formed by regional fractures, from the static and dynamic dual brittleness evaluation of the rock, and the quantitative compressibility evaluation is realized through the comprehensive brittleness index of the rock. It overcomes the defect of simply evaluating from the mechanical parameters of the rock itself in the past, while the distribution state of the fractures in the coal-rock gas reservoir rock is not considered. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 A diagram showing the division of rock sample regions in an embodiment of the present invention;

[0042] Figure 2 This is a comparison chart of the brittleness index of rock samples N1 to N3 in the embodiment of the present invention. DETAILED DESCRIPTION

[0043] The present invention will be described in detail below in conjunction with the accompanying drawings.

[0044] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0045] Example 1

[0046] This example provides a quantitative evaluation method for the brittleness of deep coal-rock gas reservoirs based on the distribution of fractures, as follows:

[0047] Well HY1-1 is a horizontal well in a coal-rock gas region, with a horizontal section length of 2,100 meters and a designed volume fracturing of 30 sections. Three sections Fr1 to Fr3 are taken as examples to illustrate the applicability of the invention of this application. Downhole coring work is carried out, and one rock sample is taken from each of Fr1 to Fr3, numbered N1, N2 and N3. The obtained rock is made into a standard rock sample with a diameter of 2.5 cm and a length of 5 cm, and the standard rock sample is placed in a 100°C oven to dry to constant weight.

[0048] The natural brittleness index is calculated by using the crack distribution characteristics and the number of cracks in the core cross section to characterize the brittle characteristics of the coal-rock gas reservoir rock itself. The core cross section is divided into horizontal and vertical regions, and the regional fracture complexity coefficient is calculated for each region to determine the fracture communication area. On this basis, the fracture communication effectiveness index is calculated to characterize the ability of coal-rock gas reservoir transformation to form a fracture network. The specific contents are as follows:

[0049] Calculation of natural brittleness index: Taking the opening of 5 μm as the boundary, the crack is divided into two parts. Combined with the natural brittleness index, the formula (1), formula (2) and formula (3) are used to solve. The side lengths of the box are 3 mm and 5 mm respectively. The function is solved using the side length of the box and the number of cracks in contact with the box. The fractal dimension is the slope value of the function. The calculation results are shown in Table 1.

[0050] Table 1 Natural brittleness index table

[0051]

[0052]

[0053] Core area division: Take the center of the core circle as the origin of the coordinate axis, make a vertical X-axis Y-axis coordinate system, with a side length of 2.5 cm / n, and draw long lines with a length of 2.5 cm / n from the origin to the positive and negative directions of the X axis and the positive and negative directions of the Y axis, and finally divide the core into small square areas with a side length of 2.5 cm / n. The coordinates of the small square area are mainly based on the coordinates of the center point of the square. In Example 1, n is taken as 6, and the labels and coordinates are as follows Figure 1 shown.

[0054] Calculate the regional fracture complexity index: Calculate the fracture complexity index of the small square area that completely or almost completely covers the core area, such as Figure 1 The box border is marked in black. The calculation results are shown in Table 2.

[0055] Table 2 Regional crack complexity index table

[0056]

[0057]

[0058] Calculate the effective fracture communication index: The effective fracture communication index is divided into the absolute effective communication index and the relative effective communication index. Combining the two can calculate the effective communication index. Among them, formula (5) is the regional selection criterion for the relative effective communication index, formula (6) is the regional selection criterion for the absolute effective communication index, and the calculation is carried out by formula (7). The calculation results are shown in Table 3.

[0059] Table 3 Calculation Table of Effective Fracture Communication Index

[0060]

[0061]

[0062] Combining the natural brittleness index and the effective fracture communication index, calculate the coal seam brittleness index to characterize the rock brittleness of the target coalbed methane reservoir, so as to achieve the purpose of quantitative evaluation. Specifically, the larger the value of the brittleness index, the stronger the brittleness of the rock in the evaluated reservoir section. According to its value, it is divided into three grades. When the comprehensive brittleness index 0 < BU <= 0.30, it is a poor brittle reservoir; when the comprehensive brittleness index 0.30 < BU <= 0.50, it is a medium brittle reservoir; when the comprehensive brittleness index 0.50 < BU <= 1.0, it is a strong brittle reservoir. Specifically in Example 1:

[0063] Combining the natural brittleness of the coal rock reservoir with the effective communication index can obtain its comprehensive brittleness index, which is solved by formula (8). The comprehensive brittleness indexes of rock samples N1 to N3 are 0.6720, 0.4039, and 0.2703 respectively. It can be seen that the brittleness strength ranking is N1 > N2 > N3. According to the grading results in step 3), the reservoir where the fracturing section Fr1 corresponding to rock sample N1 is located is a strong brittle reservoir, the reservoir where the fracturing section Fr2 corresponding to rock sample N2 is located is a medium brittle reservoir, and the reservoir where the fracturing section Fr3 corresponding to rock sample N3 is located is a weak brittle reservoir. See Appendix Figure 2 .

[0064] The above are only the preferred embodiments 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 in the protection scope of the present invention.

Claims

1. A quantitative evaluation method for the brittleness of deep coal-rock gas reservoirs based on fracture distribution, characterized in that: The following steps are involved: 1) Take coring work on the target coal-rock gas reservoir, make the core into a standard rock sample, and dry the standard rock sample to a constant weight; 2) obtaining the crack distribution characteristics and the number of cracks in the core cross section of the standard rock sample, and calculating the natural brittleness index; 3) Divide the core cross section into horizontal and vertical regions, calculate the regional fracture complexity coefficient for each divided region, and calculate the fracture communication effectiveness index based on the regional fracture complexity coefficient; 4) The natural brittleness index and the fracture communication effectiveness index are combined to calculate the comprehensive brittleness index of the coalbed methane reservoir and conduct a compressibility evaluation.

2. The quantitative evaluation method for the brittleness of deep coal-rock gas reservoirs as claimed in claim 1, wherein the diameter of the standard rock sample in step 1) is 2.5 cm and the length is 5 cm.

3. The quantitative evaluation method for deep coal-rock gas reservoir brittleness according to claim 1, wherein the drying method in step 1) comprises: The standard rock sample was placed in an oven at 100°C and dried to constant weight.

4. The quantitative evaluation method for the brittleness of deep coal-rock gas reservoirs as claimed in claim 1, wherein the number of cracks is obtained by scanning electron microscopy in step 2).

5. The method for quantitatively evaluating the brittleness of deep coal-rock gas reservoirs according to claim 1, wherein the step 2) further comprises: The crack is divided into two parts with the core cross-section opening of 5 microns as the boundary. The fractal dimensions of the two parts are calculated separately, and the natural brittleness index is calculated through the fractal dimension.

6. The method for quantitatively evaluating the brittleness of deep coal-rock gas reservoirs according to claim 1, wherein the step 3) of dividing the core cross section into horizontal and vertical regions further comprises: Take the center of the core circle as the origin of the coordinate axis, make a vertical X-axis and Y-axis coordinate system, with a side length of 2.5 cm / n, and draw long lines of 2.5 cm / n in the positive and negative directions of the X-axis and the positive and negative directions of the Y-axis from the origin as the starting point. Finally, the core is divided into small square areas with a side length of 2.5 cm / n, where n is an integer.

7. The method for quantitatively evaluating the brittleness of deep coal-rock gas reservoirs according to claim 1, wherein the step 3) further comprises: According to the crack complexity coefficient, the area is divided into a relative effective index area or an absolute effective index area.

8. The quantitative evaluation method for deep coal-rock gas reservoir brittleness according to claim 1, wherein the calculation method for the comprehensive brittleness index of the coal-bed methane reservoir in step 4) is: BU=B z ·B g (8) Where: BU is the comprehensive brittleness index of coalbed methane reservoir, dimensionless.

9. A quantitative evaluation device for deep coal-rock gas reservoir brittleness, characterized in that: include: at least one processor and a memory in communication with the processor; The memory stores instructions executable by the processor, and the instructions are executed by at least one processor so that the at least one processor can execute the quantitative evaluation method for the brittleness of deep coal-rock gas reservoirs as described in any one of claims 1-8.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method for quantitatively evaluating the brittleness of deep coal-rock gas reservoirs as described in any one of claims 1 to 8 are implemented.

Images