A method for judging whether the pore structure of a rock changes with the particle size

Abstract

The present invention provides a method for determining whether the pore structure of a rock changes with the particle size, which includes the following steps: gradually crushing the same rock sample from coarse to fine, and obtaining particle samples within the mesh number range at each level; screening out particle samples of the same mass at each level and conducting permeability tests under different pressures to obtain the apparent permeability values of the particle samples under different pressures and the permeability ratios of the particle samples at each level; comparing the change differences in the permeability ratios of adjacent-level particle samples under the same pressure parameters to identify whether the pore structure of the rock basically does not change with the particle size or changes significantly with the particle size. Using the determination method provided by the present invention, by comparing the change characteristic differences of the apparent permeability values of particles with different particle sizes with pressure, it is possible to identify that the pore structure basically does not change with the particle size and changes significantly with the particle size, which has the advantages of being easy to operate and convenient, and solves the problem of being difficult to effectively determine whether the pore structure of a large number of particle samples changes with the particle size.

Description

Technical Field

[0001] The present invention belongs to the technical field of petroleum scientific research, relates to the testing technology for characterizing the pore structure of rocks and the fluid flow ability, and particularly relates to a method for judging whether the pore structure of rocks changes with the particle size. Background Art

[0002] The rocks in oil and gas reservoirs are typical porous media. The ability of rocks to allow fluid flow, generally expressed by permeability, is an important quality evaluation index. In engineering practice, due to various factors, it is often impossible to use samples with regular shapes for permeability testing, and thus particle samples are used for testing.

[0003] However, some existing studies have shown that there are significant differences in the permeabilities obtained by testing particle samples with different particle sizes. Generally, as the particle size increases, the permeability value gradually increases. This has brought confusion to the engineering department as to which particle size sample's test result is more reliable and representative. In response to this phenomenon, an easy explanation is that the pore structure is damaged to varying degrees during the process of crushing the sample into particles of different sizes. For example, when large particles are further crushed into smaller particles, larger pores are easily damaged, resulting in a decrease in their flow ability and permeability. This explanation requires observational evidence and a method to judge whether the pore structure changes with the particle size.

[0004] Such methods mainly include: the image method, which is the most direct, but has problems such as high cost and limited applicability; the porosity method, where porosity is only the percentage of the total pore volume in the rock volume and has no one-to-one correspondence with the pore structure; the pore size distribution method, where the pore size distribution determined by low-temperature gas adsorption (such as nitrogen adsorption) or high-pressure mercury injection only includes the quantity of pore volume corresponding to different pore sizes and does not include characteristics such as the connectivity of the pore space. Moreover, low-temperature gas adsorption takes a long time, and high-pressure mercury injection has the problem that the sample is contaminated by mercury and cannot be reused; the permeability method, where the change in the permeability of particle samples with particle size may be due to changes in the pore structure or may also be due to deviations between the determined particle size and the true value.

[0005] Generally speaking, there is currently a lack of an easy-to-operate, convenient method that can effectively judge whether the pore structure of a large number of particle samples changes with the particle size, making it difficult to obtain a comprehensive understanding of the characteristics of the change in the pore structure of rocks with size and its representative scale, and lacking sufficient basis for answering questions such as which particle size particle permeability should represent the matrix flow ability. Summary of the Invention

[0006] To solve the problems in the prior art, the present invention provides a method for judging whether the pore structure of rocks changes with the particle size.

[0007] A method for judging whether the pore structure of a rock changes with the particle size, comprising the following steps:

[0008] Gradually crush the same rock sample from coarse to fine, and obtain particle samples within the mesh range for each level;

[0009] Screen out particle samples of the same mass for each level and conduct permeability tests under different pressures to obtain the apparent permeability values of the particle samples under different pressures and the permeability ratios of the particle samples for each level;

[0010] Compare the change differences of the permeability ratios of adjacent-level particle samples under the same pressure parameters to identify whether the pore structure of the rock basically does not change with the particle size or changes significantly with the particle size.

[0011] In a preferred embodiment, the method for obtaining the apparent permeability value is as follows:

[0012] Screen out particle samples of the same mass for each level and conduct permeability tests under different pressures to obtain the apparent permeability values k1, k2, k3, ki of the particle samples under different pressures, and record the corresponding pressure values as p1, p2, p3, pi, where i represents the number of value-taking times of different pressures;

[0013] There is a linear relationship between the i data points composed of the apparent permeability value and the pressure value, and the apparent permeability value kn_q at different pressure values is obtained according to the linear relationship.

[0014] In a preferred embodiment, in the permeability tests under different pressures for each level, at least 3 different pressure points are selected for the permeability tests.

[0015] In a preferred embodiment, the calculation method of the permeability ratio is: the ratio of the apparent permeability value at each pressure value to the apparent permeability value at the minimum pressure value among the apparent permeability values of the particle samples under different pressures within each level.

[0016] In a preferred embodiment, the content of comparing the change differences of the permeability ratios of adjacent-level particle samples under the same pressure parameters is: according to the permeability ratios of the particle samples for each level, take the difference value of the permeability ratios of adjacent-level particle samples under the same pressure parameters. If the difference values of the permeability ratios of adjacent-level particle samples are all less than 0.1, it indicates that the pore structures of adjacent-level particle samples basically do not change with the particle size, otherwise the pore structures of adjacent-level particle samples change significantly with the particle size.

[0017] In a preferred embodiment, the rock sample is a rock type of porous medium.

[0018] In a preferred embodiment, the same rock sample is crushed step by step from coarse to fine, and at least three different levels are selected for crushing to obtain corresponding particle samples of each level.

[0019] In a preferred embodiment, in the step-by-step crushing, the rock sample remaining after the rock sample at the previous stage is crushed to obtain the corresponding particle sample is used for crushing the rock sample at the next stage.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. In the matrix permeability test of shale conducted by the applicant, the judgment method provided by the present invention is used to crush the same sample from coarse to fine step by step, screen out a certain mass of particle samples and conduct permeability tests at different pressures, compare the difference in the change characteristics of the apparent permeability values ​​of particles of different particle sizes with pressure, and identify two types of samples whose pore structure basically does not change with particle size and whose pore structure changes significantly with particle size.

[0022] 2. The present invention has found that for samples whose pore structure does not change substantially with particle size, when the particle size is large, the permeability value calculated based on the particle radius determined by the sieve aperture is too large. Based on the judgment that the pore structure does not change substantially with particle size, the particle radius can be corrected to obtain an equivalent particle radius that is closer to the true value. For samples whose pore structure changes significantly with particle size, the permeability changes significantly with particle size, which further reflects the change of pore structure with particle size, prompting researchers and engineers to conduct a more in-depth analysis of the representativeness of this type of sample structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. Obviously, the drawings in the following description are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0024] Figure 1 Schematic diagram of the apparent permeability value and absolute permeability ratio λ of blocks with different pore structures changing with pressure;

[0025] Figure 2 A flow chart of the method for determining whether the rock pore structure changes with particle size in the present invention;

[0026] Figure 3 The graph is a graph showing the change in apparent permeability values ​​of rock sample 1 with different particle sizes as a function of pressure in Example 1 of the present invention;

[0027] Figure 4It is a bar chart of the apparent permeability values of Sample 1 with different particle sizes under the condition of 3 bar and the permeability ratios under different pressures in Embodiment 1 of the present invention;

[0028] Figure 5 It is a curve graph of the change of the apparent permeability value of different particle size samples of Rock Sample 2 with the pressure in Embodiment 2 of the present invention;

[0029] Figure 6 It is a bar chart of the apparent permeability values of Sample 2 with different particle sizes under the condition of 3 bar and the permeability ratios under different pressures in Embodiment 2 of the present invention. Detailed implementation manners

[0030] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0031] When gas flows in a porous medium, especially a porous medium dominated by micro-nano pores, in addition to the viscous flow mechanism consistent with liquid Newtonian fluid, due to the existence of various non-linear flow mechanisms such as slip and transition, at different average pressures, the calculated permeability based on Darcy's law is different, showing non-Darcy characteristics (Darcy permeability or absolute permeability is only related to the pore structure of the porous medium, and has nothing to do with the flowing fluid, the applied pressure gradient, etc., corresponding to the viscous flow situation of Newtonian fluid), which is generally also called apparent permeability. As the pressure increases, the non-Darcy flow mechanism weakens, approaching Darcy flow, and the apparent permeability gradually decreases. Figure 1 Shows the change of the ratio λ of the apparent permeability to the absolute permeability of blocks with different pore structures calculated in previous studies with pressure. This feature is mainly used to determine the change of the corresponding permeability value with the decrease of the fluid pressure in the reservoir during oil and gas production.

[0032] Through Figure 1 It can be seen that for different pore structures, the changing trend of λ with pressure is different, which is directly related to the difference in their pore structures. It can be imagined that for the same sample, during the process of gradually crushing to make the particle size smaller, if the pore structure changes significantly, the corresponding change characteristics of the apparent permeability with pressure will also change significantly.

[0033] Therefore, the present invention proposes a method for judging whether the pore structure of a rock changes with the particle size, and its process is as Figure 2 shown, including the following steps:

[0034] Step 100: In a rock sample of porous medium, the same rock sample is crushed step by step from coarse to fine, and particle samples within the mesh number range are obtained at each level. During the crushing process, at least three different levels of crushing are selected to obtain corresponding particle samples with different mesh number particle sizes to ensure the validity of the judgment results. If the number of crushing levels is small and the particle size parameters are less than three, it is difficult to evaluate the change trend and relative change range.

[0035] Step 200: Screen out particle samples of the same mass at each level and conduct permeability tests under different pressures to obtain the apparent permeability values of the particle samples under different pressures and the permeability ratios of the particle samples at each level.

[0036] In some specific embodiments, the method for obtaining the apparent permeability value is as follows:

[0037] Screen out particle samples of the same mass at each level and conduct permeability tests under different pressures to obtain the permeability values k1, k2, k3, ki of the particle samples under different pressures, and the corresponding pressure values are recorded as p1, p2, p3, pi. i represents the number of times of pressure values. There is a linear relationship between the i data points composed of the apparent permeability value and the pressure value. According to the linear relationship, the apparent permeability value kn_q at different pressure values is obtained. In the permeability test, at least three or more pressure points are selected for the permeability test to ensure the linear correlation of the data. It is preferred to select three pressure points for the test, which is the simplest and fastest.

[0038] For the i data points composed of the apparent permeability value and the pressure value, the equation can be specifically used to fit and determine the numerical values of a and b, and then the apparent permeability value kn_q at different pressures is calculated according to this equation. n represents the larger value of the mesh number of the particle samples in each level, and q represents the pressure value under different pressures.

[0039] Since it is difficult to ensure that the equilibrium pressure exactly falls on the set pressure point position during the test process, it is necessary to determine the apparent permeability value at the set pressure point by the method of interpolation after fitting. The above permeability value is the apparent permeability value. For the convenience of describing the process of obtaining the apparent permeability value, it is written as permeability and apparent permeability value respectively.

[0040] In some specific embodiments, the calculation method of the permeability ratio is: the ratio of the apparent permeability value of the particle sample at each pressure value to the apparent permeability value at the minimum pressure value among the apparent permeability values of the particle samples under different pressures within each level.

[0041] Step 300: Compare the change differences of the permeability ratios of adjacent-level particle samples under the same pressure parameters to identify whether the rock pore structure basically does not change with the particle size or changes significantly with the particle size.

[0042] Specifically: according to the permeability ratio of each level of particle samples, the difference value of the permeability ratio of adjacent levels of particle samples under the same pressure parameter is taken. If the difference value of the permeability ratio of adjacent levels of particle samples is less than 0.1, it indicates that the pore structure of adjacent levels of particle samples basically does not change with the particle size; otherwise, the pore structure of adjacent levels of particle samples changes significantly with the particle size.

[0043] The judgment method provided by the present invention is used to judge whether the pore structure of rock changes with the particle size by comparing the difference in the characteristics of the apparent permeability value of samples with different particle sizes changing with pressure. It is a method that is easy to operate, convenient, and can effectively judge whether the pore structure of a large number of particle samples changes with the particle size.

[0044] In addition, the present invention provides the following specific test process and calculation steps to further illustrate the judgment method in the present invention.

[0045] 1) The original rock sample (which can be the full diameter taken from the bottom of the well, generally 10 cm or 7 cm in diameter, core, or can also be an irregular block sample or fragment) is dried in an oven at 105 °C for more than 12 hours until the sample mass no longer decreases, ensuring that the free and bound water in the sample is fully removed;

[0046] 2) Use an analytical balance with a precision of one ten-thousandth to weigh the mass m0 of the sample in air and the mass m1 of the sample immersed in anhydrous alcohol with a known density ρ ethanol Based on Archimedes' principle of buoyancy, the external volume V bulk =(m0 - m1) / ρ ethanol of the sample can be obtained, and then the apparent density ρ bulk =m0 / V bulk ;

[0047] 3) Use a jaw crusher or other machinery to crush the sample to obtain as many coarse particles as possible, and screen to obtain about 60 g of the sample between 4 mesh and 8 mesh (denoted as 4 - 8 mesh sample), and dry it in an oven at 105 °C for more than 6 hours until the mass no longer changes;

[0048] 4) Load the dried particle sample into a particle porosity and permeability instrument, and successively measure the apparent permeability values k1, k2, k3 near the final equilibrium pressures of 3 bar, 5 bar, and 7 bar respectively according to the method described in the invention patent (Patent No. ZL201910944457.0) "A method for obtaining the permeability of particle samples", and the corresponding pressures are recorded as p1, p2, p3;

[0049] 5) For the three data points (p1, k1), (p2, k2), (p3, k3), use the equation Determine the numbers a and b by fitting, and then calculate the apparent permeability values kn_3, kn_5, and kn_7 when p is 3 bar, 5 bar, and 7 bar respectively according to this equation. In the variable name, n represents the larger value of the mesh number of the particle sample. For example, for a 4 - 8 mesh sample, n is 8; (Note: Since it is difficult to ensure that the equilibrium pressure exactly falls at the positions of 3 bar, 5 bar, and 7 bar during the test, it is necessary to determine the values of the three points of 3 bar, 5 bar, and 7 bar by interpolation after fitting; Testing 4 or more pressure points is also possible. The inventor has tested and found that there is a good linear correlation between k and 1 / p when it is 3 - 12 bar. Testing 3 points is the simplest and fastest);

[0050] 6) Use a jaw crusher or other machinery to crush and screen the sample after the previous step and the remaining sample to obtain about 60 g of a sample between 8 and 12 meshes (denoted as 8 - 12 mesh sample). Repeat steps 4 and 5 to obtain the apparent permeability values K12_3, K12_5, and K12_7;

[0051] 7) Use a jaw crusher or other machinery to crush and screen the sample after the previous step and the remaining sample to obtain about 60 g of a sample between 12 and 16 meshes (denoted as 12 - 16 mesh sample). Repeat steps 4 and 5 to obtain the apparent permeability values K16_3, K16_5, and K16_7;

[0052] 8) Use a jaw crusher or other machinery to crush and screen the sample after the previous step and the remaining sample to obtain about 60 g of a sample between 16 and 20 meshes (denoted as 16 - 20 mesh sample). Repeat steps 4 and 5 to obtain the apparent permeability values K20_3, K20_5, and K20_7;

[0053] 9) Further calculate the apparent permeability values at 3 pressures obtained from the tests of 4 mesh numbers as follows: For each mesh number of the sample, calculate the permeability ratio Rn_53 of the 5 bar apparent permeability value to the 3 bar apparent permeability value and the permeability ratio Rn_73 of the 7 bar apparent permeability value to the 3 bar apparent permeability value respectively. Among them, both Rn_53 and Rn_73 are less than 1, and generally Rn_73 is less than Rn_53. n represents the larger value of the mesh number of the particle sample. For example, for a 4 - 8 mesh sample, n is 8; There are a total of 12 apparent permeability values for 4 mesh numbers. Calculate a set of permeability ratios Rn_53 and Rn_73 for each mesh number. Finally, obtain 4 sets of Rn_53 and Rn_73, specifically: R8_53 and R8_73; R12_53 and R12_73; R16_53 and R16_73; R20_53 and R20_73;

[0054] 10) As the mesh number increases, compare the differences in R53 and R73 between two adjacent mesh number particle samples. If the differences between both are less than 0.1, it indicates that the pore structures of the two mesh number particle samples are basically unchanged; otherwise, it indicates that the pore structures of the two samples have changed significantly. Specifically: compare the differences between R8_53 and R12_53, R8_73 and R12_73; the differences between R12_53 and R16_53, R12_73 and R16_73; the differences between R16_53 and R20_53, R16_73 and R20_73.

[0055] 11) The present invention compares samples of 4 particle sizes. In practice, more can be compared, but it should not be less than 3 particle sizes, otherwise it is difficult to evaluate the change trend and relative change amplitude.

[0056] To further illustrate and verify the method of the present invention for judging whether the pore structure of rock changes with particle size, two embodiments are provided.

[0057] Embodiment 1

[0058] The following is the process and results of using this method for two shale samples. The measured apparent densities of the two rock samples are shown in Table 1.

[0059] Table 1 Basic parameter table of rock samples

[0060]

[0061] Among them, the apparent permeability values of the particles with different particle sizes of Rock Sample 1 under 3 pressure values and the curve fitted with the equation are shown in Figure 3 . The apparent permeability value kn_3 at 3 bar, the permeability ratio Rn_53 of the apparent permeability value at 5 bar to the apparent permeability value at 3 bar, and the permeability ratio Rn_73 of the apparent permeability value at 7 bar to the apparent permeability value at 3 bar calculated based on the equation and fitting parameters are shown in Table 2 and Figure 4 . It can be seen from this that for this sample, from 8 to 16 mesh, the permeability ratios (non-Darcy characteristics, Rn_53, Rn_73) are very close, judging that its pore structure has not changed significantly. The apparent permeability values of the 12-mesh and 16-mesh samples are very close. The increase in the apparent permeability value of the 8-mesh should be due to the relatively large particle radius determined based on the sieve pore size, and its true apparent permeability value should be close to those of 12 and 16 mesh. From 16 to 20 mesh, the change in the permeability ratio increases slightly, and the difference in its apparent permeability value from those of 12 and 16 mesh increases slightly. The differences between the permeability ratios of adjacent mesh numbers are all less than 0.1, indicating that the pore structures of the 8-20 mesh particle samples are basically unchanged, so the pore structure of the rock of Sample 1 does not change with particle size.

[0062] Table 2 Apparent permeability values of Sample 1 with different particle sizes under 3 bar condition and permeability ratios at different pressures

[0063]

[0064] Example 2

[0065] For Sample 2, the apparent permeability values of particles with different particle sizes measured under three pressures and the curve fitted with the equation are shown in Figure 5 . The apparent permeability value kn_3 at 3 bar, the permeability ratio Rn_53 of the apparent permeability value at 5 bar to the apparent permeability value at 3 bar, and the permeability ratio Rn_73 of the apparent permeability value at 7 bar to the apparent permeability value at 3 bar calculated based on the equation and fitting parameters are shown in Table 3 and Figure 6 . It can be seen from this that for this sample, from 8 to 20 mesh, its permeability ratio (non-Darcy characteristic, Rn_53, Rn_73) continuously and significantly decreases, indicating that its pore structure has changed significantly, and correspondingly, its apparent permeability value also continuously changes with the particle size.

[0066] Table 3 Apparent permeability values of Sample 2 with different particle sizes under 3 bar condition and permeability ratios at different pressures

[0067]

[0068]

[0069] Through the application and comparison of the above two samples, it is shown that the pore structure of the rock can be judged whether it changes with the particle size based on this method.

[0070] The above embodiments are only exemplary embodiments of the present application and are not used to limit the present application. The protection scope of the present application is defined by the claims. Those skilled in the art can make various modifications or equivalent substitutions within the essence and protection scope of the present application, and such modifications or equivalent substitutions should also be regarded as falling within the protection scope of the present application.

Claims

1. A method for judging whether the pore structure of a rock changes with the particle size, characterized in that, It includes the following steps: Gradually crush the same rock sample from coarse to fine, and obtain particle samples within the mesh number range for each level; For each level, screen out particle samples of the same mass and conduct permeability tests under different pressures to obtain the apparent permeability values of the particle samples under different pressures and the permeability ratios of the particle samples for each level; Compare the change differences in the permeability ratios of adjacent-level particle samples under the same pressure parameters to identify whether the rock pore structure basically does not change with the particle size or changes significantly with the particle size; The content of comparing the change differences in the permeability ratios of adjacent-level particle samples under the same pressure parameters is as follows: According to the permeability ratios of the particle samples for each level, take the difference value of the permeability ratios of adjacent-level particle samples under the same pressure parameters. If the difference values of the permeability ratios of adjacent-level particle samples are all less than 0.1, it indicates that the pore structures of adjacent-level particle samples basically do not change with the particle size; otherwise, the pore structures of adjacent-level particle samples change significantly with the particle size; Based on the judgment that the pore structure does not change with the particle size, correct the particle radius to obtain an equivalent particle radius closer to the true value; The method for obtaining the apparent permeability value is as follows: For each level, screen out particle samples of the same mass and conduct permeability tests under different pressures to obtain the apparent permeability values k1, k2, k3, ki of the particle samples under different pressures, and record the corresponding pressure values as p1, p2, p3, pi, where i represents the number of times of taking different pressure values; There is a linear relationship among the i data points composed of the apparent permeability values and the pressure values. According to the linear relationship, obtain the apparent permeability value kn_q at different pressure values; The rock sample is a rock type of porous medium.

2. The method for judging whether the pore structure of a rock changes with the particle size according to claim 1, characterized in that, In the permeability tests for each level under different pressures, at least 3 different pressure points are selected for the permeability tests.

3. A method for judging whether the pore structure of a rock changes with the particle size according to claim 1, characterized in that, The calculation method of the permeability ratio is: among the apparent permeability values of the particle samples under different pressures within each level, the ratio of the apparent permeability value at each pressure value to the apparent permeability value at the minimum pressure value.

4. A method for judging whether the pore structure of a rock changes with the particle size according to claim 1, characterized in that, In the process of gradually crushing the same rock sample from coarse to fine, at least 3 different levels are selected for crushing to obtain the corresponding particle samples for each level.

5. A method for determining whether the pore structure of a rock changes with the particle size according to claim 4, characterized in that, In the process of gradual crushing, the remaining rock sample after crushing the previous-level rock sample to obtain the corresponding particle sample is used for crushing the next-level rock sample.

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