An apparatus and method for testing core permeability with biaxial stress difference

Abstract

This invention relates to a testing device and method for core permeability with biaxial stress difference. The testing device includes: a hollow rubber sleeve with a cavity extending through both ends of the sleeve at its center, the central axis of which coincides with the central axis of the cavity, the cavity being used to accommodate a core conforming to its shape; a confining pressure device for applying confining pressure to the core to achieve a biaxial stress difference; and a core holder for holding the hollow rubber sleeve containing the core. This testing device can generate different biaxial stresses even under the same confining pressure and measure permeability, and has a simple structure.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas field development, specifically a testing device and method for core permeability with biaxial stress difference. Background Technology

[0002] As oil resources are being exploited, the reserves of more and more oil and gas wells are approaching their limits, and the locations of oil reservoirs are relatively fixed. Therefore, maximizing the exploitation of crude oil resources is an urgent problem to be solved, and the determination of rock permeability is an extremely important part of this research. Data on reservoir rock permeability helps researchers better understand the properties of crude oil reservoirs, enabling them to maximize the exploitation and utilization of oil and gas resources using existing technologies.

[0003] According to existing literature, the process of measuring reservoir permeability is usually as follows: first, core samples are taken from the well section that has penetrated the reservoir, and the cores are taken to the laboratory for secondary drilling, usually into a cylindrical shape, and then placed in a specific permeability measuring instrument for measurement. According to Lu Deyi's paper from Xi'an Petroleum University (Lu Deyi. Current Status of Indoor Core Permeability Measurement Methods. [J] Petrochemical Application, 2017, 36(02), 1-3+16), there are several common testing methods: (1) steady-state measurement method; (2) instantaneous pressure pulse method; (3) pore pressure oscillation method; (4) nuclear magnetic resonance method; (5) degassing test method; (6) mercury intrusion porosimetry; (7) rock complex resistivity method; (8) CMS-300 automatic core measurement system.

[0004] However, most of the methods mentioned above measure cylindrical cores, simulating uniform confining pressure in downhole geological conditions. But the actual stress field conditions in a core are not simply biaxial stresses of equal magnitude; they are more complex. The stress field generated by applying confining pressure cannot accurately simulate actual downhole conditions, at least not biaxial stresses. To simulate biaxial stresses in a core, it is necessary to process the core into other shapes, and there are also dimensional requirements, which may lead to insufficient core samples or the inability to process the collected cores into the required experimental shape. Summary of the Invention

[0005] To address the aforementioned problems, one objective of this invention is to provide a testing device for core permeability with biaxial stress difference. Even under the same confining pressure, different biaxial stresses can be generated, allowing for the measurement of permeability. Furthermore, the core sample can be processed and tested using core fragments. Another objective of this invention is to provide a method for testing core permeability with biaxial stress difference.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] This invention provides a testing device for core permeability with biaxial stress difference, comprising:

[0008] A hollow rubber sleeve, wherein a cylindrical cavity is provided in the center of the hollow rubber sleeve, penetrating both ends of the hollow rubber sleeve, the central axis of the cavity coincides with the central axis of the cavity, and the cavity is used to accommodate a rock core that is adapted to the shape of the cavity;

[0009] A confining pressure device is used to apply confining pressure to the hollow rubber sleeve containing the rock core.

[0010] A core holder, which is used to hold a hollow rubber sleeve on which the core is fitted.

[0011] Furthermore, the cross-section of the cavity is square, triangular, or elliptical.

[0012] Furthermore, the testing device also includes a pressure device and a flow meter. The pressure device is used to introduce airflow into the pores formed in the core, and the flow meter is used to display the airflow rate after the airflow stabilizes.

[0013] Furthermore, it also includes strain gauges and a stress display device. The strain gauges are attached to the outer wall of the rock core and are electrically connected to the stress display device, which is used to display the biaxial stress.

[0014] Furthermore, the confining pressure device is a sealed fluororubber pressure-resistant sleeve, which is installed inside the core holder.

[0015] Another aspect of the present invention provides a method for testing the permeability of a core sample with a biaxial stress difference, based on the testing device for the core sample with a biaxial stress difference, characterized by comprising the following steps:

[0016] The Young's modulus E of the hollow rubber sleeve was tested. 胶套 Compared to Poisson's ratio v 胶套 ;

[0017] The processed rock core with strain gauges attached is placed into the cavity of the hollow rubber sleeve. The hollow rubber sleeve with the rock core is fixed in a specially made rock core holder. The confining pressure device applies confining pressure to the rock core to simulate the stress state of the rock core in the formation. The stress display device is used to display the biaxial stress value.

[0018] According to the Young's modulus E 胶套 Poisson's ratio v 胶套 The biaxial stress difference experienced by the rock core was calculated using the strain gauge readings.

[0019] The permeability of the core was calculated based on the biaxial stress difference.

[0020] Furthermore, the calculation of the core permeability based on the biaxial stress difference also includes the following steps:

[0021] An airflow is introduced into the pores formed in the core through a pressure device until the airflow stabilizes, and the flow rate and pressure values ​​of the airflow at both ends of the core are read.

[0022] The permeability of the core was calculated using Darcy's law by combining the flow rate, pressure value, ground standard pressure, average gas viscosity, core end face area, and core length.

[0023] Furthermore, the permeability of the core is calculated according to formula (1):

[0024]

[0025] In equation (1), k is the permeability of the sample, Q is the volumetric flow rate at steady state, μ is the viscosity of the fluid, L is the length of the sample in the test direction, x is the distance from any point in the sample to the starting point along the test direction, A(x) is the cross-sectional area of ​​the sample at position x perpendicular to the flow direction, P1 is the fluid pressure at the inlet end, and P2 is the fluid pressure at the outlet end.

[0026] Furthermore, the Young's modulus E of the rubber sleeve was tested. 胶套 Compared to Poisson's ratio v 胶套 Specifically, this includes attaching strain gauges to designated positions on the hollow rubber sleeve and calculating the Young's modulus and Poisson's ratio of the sleeve using the elastic-plastic mechanical displacement, stress, and strain relationship.

[0027] Furthermore, the pressure device is a piston container filled with gas.

[0028] The present invention has the following advantages due to the adoption of the above technical solutions:

[0029] The present invention provides a core permeability testing device with biaxial stress difference. Through the design of a hollow rubber sleeve and the placement of the core inside the hollow rubber sleeve, it is possible to generate different biaxial stresses even under the same confining pressure and measure the permeability. The device has a simple structure.

[0030] The core sample can also be a fragment, making it easier to obtain.

[0031] The core permeability testing device with biaxial stress difference provided by this invention has low cost, convenient material sourcing, and simple operation. Attached Figure Description

[0032] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0033] In the attached diagram:

[0034] Figure 1 This is a schematic diagram of a core permeability testing device with biaxial stress difference;

[0035] Figure 2 This is a structural schematic diagram of the first embodiment of the hollow rubber sleeve;

[0036] Figure 3 This is a schematic diagram of the second embodiment of the hollow rubber sleeve;

[0037] Figure 4 This is a structural schematic diagram of the third embodiment of the hollow rubber sleeve;

[0038] Figure 5 It is a three-dimensional simulation of the pressure stress difference cloud map.

[0039] The markings in the attached diagram are as follows:

[0040] 1-Pressure device, 2-Stress display device, 3-Containing pressure device, 4-Flow meter, 5-Core holder, 6-Hollow rubber sleeve, 7-Core, 8-Cavity. Detailed Implementation

[0041] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0042] Embodiments of the present invention provide a testing device and method for core permeability with biaxial stress difference. By designing a hollow rubber sleeve and placing the core within the sleeve, different biaxial stresses can be generated even under the same confining pressure, allowing for the measurement of permeability. The testing method is implemented based on the testing device and is characterized by low cost, readily available materials, and simple operation.

[0043] It should be noted that the biaxial stress difference in this invention refers to the stress difference along the circumferential and radial directions of the core sample. The main purpose of this invention is to simulate the stress state of a core sample in the formation by generating different biaxial stresses under the same confining pressure.

[0044] like Figure 1 As shown, a core permeability testing device with biaxial stress difference includes a hollow rubber sleeve 6, a confining pressure device 3, and a core holder 5. The hollow rubber sleeve 6 has a cavity 2 extending through both ends of the hollow rubber sleeve 6. The central axis of the cavity 2 coincides with the central axis of the core 7. The cavity 2 is used to accommodate a core 7 whose shape is adapted to the cavity 8. The cross-section of the cavity 8 is rectangular.

[0045] The confining pressure device 3 is used to apply confining pressure to the core 7 so that the core 7 obtains a biaxial stress difference. The core holder 5 is used to hold and fix the hollow rubber sleeve 6 on which the core 7 is fitted.

[0046] During testing, the core 7 is placed inside the cavity 8 of the hollow rubber sleeve 6, and the core holder 5 is used to hold the hollow rubber sleeve 6 containing the core 7 inside the confining pressure device 3, and confining pressure is applied to the rubber sleeve 6 and the core 7 through the confining pressure device 3.

[0047] It should be noted that the confining pressure here refers to radial stress. Since the cavity cross-section and the core are not cylindrical, when confining pressure is applied only to the hollow rubber sleeve 6, different biaxial stresses can be generated due to the special structure of the core 7, thus better simulating the formation stress situation.

[0048] like Figures 2 to 4 As shown, the cross-section of the cavity 8 can be square, triangular, or elliptical.

[0049] The testing device also includes a pressure device 1 and a flow meter 4. The pressure device 1 is used to introduce airflow into the pores formed in the core 7, and the flow meter 4 is used to display the airflow rate after the airflow stabilizes.

[0050] The testing device also includes a strain gauge and a stress display device 2. The strain gauge is attached to the outer wall of the core 7 and is electrically connected to the stress display device 2. The stress display device is used to display the biaxial stress.

[0051] The confining pressure device 3 is a sealed fluororubber pressure-resistant sleeve, which is installed inside the core holder 5. The sealed fluororubber pressure-resistant sleeve is used to apply confining pressure to the core.

[0052] The testing method for the above-mentioned core permeability testing device with biaxial stress difference includes the following steps:

[0053] S1. Test the Young's modulus E of the hollow rubber sleeve 6. 胶套 And Poisson ratio V 胶套 ;

[0054] S2. Place the processed rock core 7 with strain gauges attached into the cavity of the hollow rubber sleeve 6, fix the hollow rubber sleeve 6 with rock core 7 in the specially made rock core holder 5, apply confining pressure to the rock core 7 through the confining pressure device 3 to simulate the stress state of the rock core 7 in the formation, and read the strain gauge readings.

[0055] S3, based on the Young's modulus E 胶套 Poisson's ratio V 胶套 The biaxial stress difference received by the core 7 was calculated using the strain gauge readings;

[0056] S4. Calculate the permeability of the core sample based on the biaxial stress difference. The calculation of the core sample permeability based on the biaxial stress difference further includes the following steps:

[0057] S4-1. Pressurize the piston container containing gas connected to both ends of the core column, and use the gas to establish a suitable pressure difference between the two ends of the core until the gas flow is stable. Then, read the pressure and gas flow rate at both ends of the core 7 by the pressure gauge and gas flow meter.

[0058] S4-2. Combining the flow rate, pressure value, ground standard pressure, average gas viscosity, core end face area, and core length, calculate the permeability of the core using Darcy's law.

[0059] The permeability of the core is calculated using Darcy's law, taking into account the flow rate, pressure value, ground standard pressure, average gas viscosity, core end face area, and core length. Specifically, the permeability of the core is calculated according to formula (1):

[0060]

[0061] In equation (1), k is the permeability of the sample, Q is the volumetric flow rate at steady state, μ is the viscosity of the fluid, L is the length of the sample in the test direction, x is the distance from any point in the sample to the starting point along the test direction, A(x) is the cross-sectional area of ​​the sample at position x perpendicular to the flow direction, P1 is the fluid pressure at the inlet end, and P2 is the fluid pressure at the outlet end.

[0062] Example 1

[0063] The hollow rubber sleeve has an outer diameter of 30 mm and a height of 40 mm. It is made of rubber with a Young's modulus E = 7.84 MPa, Poisson's ratio of 0.43, and a density of 0.93 kg / m³. The cavity of the hollow rubber sleeve and the cross-section of the rock core are square. The cross-sectional dimensions of the rock core are shown in Table 1 below.

[0064] The core sample is shale with a Young's modulus E = 4.9 GPa, Poisson's ratio 0.3, and density 2600 kg / m³. 3The simulation was performed using COMSOL's solid mechanics module, with two linear elastic materials, one for the rubber sleeve and one for the core. Transient structural characteristics were analyzed using elements including inertia terms. Discretization employed quadratic coincident edge-point elements, with the default mesh size used and a relatively fine mesh size. The biaxial stress difference was described by its maximum value. Figure 5 As shown in (1), (2) and (3), Figure 5 (1) The stress cloud diagram obtained from numerical simulation of a core with dimensions of 20*6*40 mm. Figure 5 (2) The stress cloud diagram obtained from numerical simulation of a core with dimensions of 20*8*40 mm. Figure 5 (3) The stress cloud diagram obtained from numerical simulation for a core with dimensions of 20*10*40 mm is shown in the figure. It can be seen from the figure that there is a stress difference between the long and short sides of the core, thus generating a biaxial stress difference. Therefore, the testing device provided by this invention can be used to test the permeability of cores with different biaxial stresses.

[0065] Table 1. Biaxial stress difference in rectangular core samples

[0066] Core dimensions (mm*mm*mm) X-direction stress (MPa) Y-direction stress (MPa) Biaxial stress difference MPa 20*6*40 194.21 130.42 63.79 20*8*40 192.68 97.093 95.587 20*10*40 143.42 90.565 52.855

[0067] Example 2

[0068] The difference between Example 2 and Example 1 lies in the cross-sectional shape of the hollow rubber sleeve cavity, which is triangular, and the shape of the rock core that mates with the cavity, which is a cylindrical structure with a triangular cross-section. It should be noted that in the table, X-direction refers to the axial direction, and Y-direction refers to the forward direction. After applying confining pressure to rock cores of different sizes, the X-direction stress and Y-direction stress, and the corresponding stress difference between the two directions, are obtained as shown in Table 2 below:

[0069] like Figure 5 As shown in (4), (5) and (6), Figure 5 (4) The stress cloud diagram obtained from numerical simulation of core dimensions a=6b=10c=10h=40 Figure 5 (5) The stress cloud diagram obtained from numerical simulation of core dimensions a=16b=16c=16h=40 Figure 5 (6) The stress cloud diagram obtained from numerical simulation for core dimensions a = 17.32b = 17.32c = 17.32h = 40 is shown in the figure. It can be seen from the figure that there is a stress difference between the X and Y directions of the core, thus generating a biaxial stress difference. Therefore, the testing device provided by this invention can be used to test the permeability of cores with different biaxial stresses.

[0070] Table 2. Biaxial stress difference in triangular core samples

[0071]

[0072] Example 3

[0073] The difference between Example 3 and Example 1 lies in the cross-sectional shape of the hollow rubber sleeve cavity, which is elliptical, and the shape of the rock core that mates with the cavity, which is a cylindrical structure with an elliptical cross-section. It should be noted that in the table, X-direction refers to the axial direction, and Y-direction refers to the forward direction. After applying confining pressure to rock cores of different sizes, the X-direction stress and Y-direction stress, and the corresponding stress difference between the two directions, are obtained as shown in Table 3 below:

[0074] like Figure 5 As shown in (7), (8) and (9), Figure 5 (7) is the stress contour map obtained from numerical simulation of core dimensions a=12b=7h=40. Figure 5 (8) The stress contour map is obtained from numerical simulation of core dimensions a=10b=6h=40. Figure 5 (9) The stress cloud diagram obtained from numerical simulation of a core with dimensions a = 12, b = 5, and h = 40 is shown, where a refers to the major axis of the ellipse, b refers to the minor axis of the ellipse, and h refers to the length of the core. The diagram shows a stress difference between the X and Y directions of the core, resulting in a biaxial stress difference. Therefore, the testing device provided by this invention can be used to test the permeability of cores with different biaxial stresses.

[0075] Table 3 Biaxial Stress Difference of Elliptical Cores

[0076]

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for testing the permeability of a core sample with biaxial stress difference, based on a testing device for the permeability of a core sample with biaxial stress difference, characterized in that, include: A hollow rubber sleeve, wherein a cylindrical cavity is provided in the center of the hollow rubber sleeve, penetrating both ends of the hollow rubber sleeve, the central axis of the cavity coincides with the central axis of the cavity, and the cavity is used to accommodate a rock core that is adapted to the shape of the cavity; A confining pressure device is used to apply confining pressure to the hollow rubber sleeve on which the rock core is fitted, wherein the confining pressure refers to radial stress; A core holder, the core holder being used to hold a hollow rubber sleeve on which the core is fitted; The cross-section of the cavity is square, triangular or elliptical. Since the cross-section of the cavity and the rock core are not cylindrical, when only the confining pressure is applied to the hollow rubber sleeve, different biaxial stresses can be generated due to the special structure of the rock core. After applying confining pressure to rock cores of different sizes, X-direction stress and Y-direction stress are obtained respectively. The testing device also includes a pressure device and a flow meter. The pressure device is used to introduce airflow into the pores formed in the core, and the flow meter is used to display the airflow rate after the airflow stabilizes. It also includes strain gauges and a stress display device. The strain gauges are attached to the outer wall of the rock core and are electrically connected to the stress display device, which is used to display the biaxial stress. The testing device for core permeability with two-direction stress difference tests the Young's modulus E of the hollow rubber sleeve 胶套 and Poisson's ratio v 胶套 The processed core with strain gauges is put into the cavity of the hollow rubber sleeve, the hollow rubber sleeve with the core is fixed in a special core holder, the confining pressure device is used to apply confining pressure to the core to simulate the stress state of the core in the formation, the stress display device is used to display two-direction stress values, the two-direction stress difference of the core is calculated according to the Young's modulus E 胶套 , Poisson's ratio v 胶套 and the strain gauge readings, and the permeability of the core is calculated according to the two-direction stress difference. Test the Young's modulus E of the rubber sleeve 胶套 Compared to Poisson's ratio v 胶套 Specifically, this includes attaching strain gauges to designated positions on the hollow rubber sleeve and calculating the Young's modulus and Poisson's ratio of the sleeve using the elastic-plastic mechanical displacement, stress, and strain relationship.

2. The method for testing the permeability of a core with biaxial stress difference according to claim 1, characterized in that, The confining pressure device is a sealed fluororubber pressure-resistant sleeve, which is installed inside the core holder.

3. The method for testing core permeability according to claim 1, characterized in that, The calculation of the core permeability based on the biaxial stress difference also includes the following steps: An airflow is introduced into the pores formed in the core through a pressure device until the airflow stabilizes, and the flow rate and pressure values ​​of the airflow at both ends of the core are read. The permeability of the core was calculated using Darcy's law by combining the flow rate, pressure value, ground standard pressure, average gas viscosity, core end face area, and core length.

4. The method for testing core permeability according to claim 3, characterized in that, Calculate the permeability of the core using formula (1): （1） In equation (1), k is the permeability of the sample, Q is the volumetric flow rate at steady state, μ is the viscosity of the fluid, L is the length of the sample in the test direction, x is the distance from any point in the sample to the starting point along the test direction, A(x) is the cross-sectional area of ​​the sample at position x perpendicular to the flow direction, P1 is the fluid pressure at the inlet end, and P2 is the fluid pressure at the outlet end.

5. The method for testing core permeability according to claim 1, characterized in that, The pressure device is a piston container filled with gas.

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