A method for determining effective porosity of shale taking into account both lamellar fractures and matrix pores and its application

Abstract

The present invention discloses a method and application for determining the effective porosity of shale, taking into account both lamellar fractures and matrix pores. The method includes conducting a helium porosity measurement experiment on a shale sample until a first set time is reached, obtaining multiple experimental data comprising pairs of experimental time and porosity. Based on a mathematical model of shale effective porosity and experimental time, the experimental data is fitted to obtain a fitting relationship between the effective porosity of the rock sample and the experimental time. The effective porosity, matrix porosity, and lamellar fracture porosity of the rock sample are then determined based on the rock sample selection direction and the fitting relationship. This method can achieve the separate quantification of matrix porosity and lamellar fracture porosity of shale samples, providing key parameters for pure shale oil resource evaluation, sweet spot prediction, and reserve submission.

Description

Technical Field

[0001] The present invention relates to the technical field of petroleum exploration, and in particular to a method for measuring effective porosity of shale taking both lamellae fractures and matrix pores into consideration and its application. Background Art

[0002] The world is rich in shale oil and gas resources. With the advancement and widespread application of technologies such as horizontal drilling and volumetric fracturing, the vigorous exploration and development of unconventional oil and gas resources has become a major trend in global energy development. Porosity is a key parameter in oil and gas resource evaluation, crucial for determining reservoir space, fluid saturation, and geological reserves. However, due to the extremely low permeability and complex mineral composition of shale, particularly the development of micro- and nano-pores and lamellae fractures, coupled with varying understandings of testing methods among different laboratories, the reproducibility of test results between laboratories is poor, making accurate characterization of shale porosity challenging.

[0003] Pure shales deposited in continental lacustrine environments in China, exemplified by the Gulong Shale, are characterized by high clay content, extensive lamination, and lamellae fractures. These shales possess diverse reservoir spaces, including a unique matrix pore-lamellae fracture complex, which accounts for 70% of the total reservoir space. Pore types in the Gulong Shale are primarily clay mineral intercrystalline pores, detrital mineral intergranular dissolution pores, and organic pores. These pores are small in scale, poorly connected, and exhibit strong heterogeneity. Millimeter-scale lamination structures underpin the extensive development of lamellae fractures. Lamination fractures can reach up to millimeter widths, resulting in an effective porosity of 1.18%, contributing up to 14% of the reservoir space. High-porosity areas connect vertically with fractures up to 250 μm, linking the upper and lower fractures. The abundance of lamellae fractures not only increases the shale's seepage pathways but also represents effective reservoir space, making them of considerable significance. However, evaluating the dual reservoir space of lamellae fractures and pore-fracture complexes is difficult due to limitations in testing methods and complex structures.

[0004] Currently, there are numerous methods for measuring shale porosity, primarily qualitative and quantitative. Qualitative evaluation methods for reservoir space, represented by optical microscopy and scanning electron microscopy, are limited to distinguishing pore types and pore-fracture combinations within a two-dimensional plane. Quantitative volumetric evaluation techniques, represented by saturated liquid methods, gas injection porosimetry (GIP), gas adsorption, high-pressure mercury injection, nuclear magnetic resonance (NMR), and image analysis, enable quantitative evaluation of pore structure and reservoir space, providing information on pore throat systems. Saturated liquid methods are divided into water immersion porosimetry (WIP) and dual liquid porosimetry (DLP). Shale mineral composition and pore wettability are complex and diverse, particularly due to the presence of a large amount of easily swellable clay minerals. Accurately measuring shale porosity using the saturated liquid method is challenging. Furthermore, the DLP method, which is part of the saturated liquid method, requires core sections for measurement, which presents significant limitations. The commonly used GIP method, based on Boyle's law, currently employs helium as a carrier gas. By filling the sample pores at a known pressure and measuring the pressure change, it indirectly measures the effective porosity of shale plug samples. Foreign researchers primarily use the Gas Research Institute (GRI) method to measure total shale porosity. This method is a specialized GIP method primarily used for shale gas reservoir evaluation. The measurement targets crushed particles, resulting in significant errors in apparent volume determination, and the results are significantly affected by sample pretreatment and experimental parameter settings. CO2 adsorption and N2 adsorption methods offer advantages in measuring the pore size of shale micropores (<2 nm) and mesopores (2-50 nm), respectively. However, like liquid-based methods, these methods are not only limited by the relationship between molecular diameter and pore diameter, but also pose difficulties in quantifying the gas adsorption effect of shale pores. Mercury injection testing is primarily used to characterize macropores (>50 nm) in shales. Conventional mercury injection (30 MPa) reveals a theoretical lower limit of effective pore size of approximately 24.5 nm, while high-pressure mercury injection (413 MPa) can reach 1.8 nm. Constant-rate mercury injection (approximately 6 MPa) can simultaneously reveal information about pores and throats. Shales have low porosity and permeability, requiring high displacement pressures to inject mercury into the sample. However, this process is prone to artificial fractures and the overall mercury removal efficiency of the shale is low, making the test destructive and precluding the use of the sample for further testing. Nuclear magnetic resonance (NMR) provides non-destructive information on shale porosity, pore size distribution, pore connectivity, and movable fluids. However, this process requires reservoir-specific parameter evaluation, resulting in significant uncertainty in interpretation. It is also influenced by multiple factors, including the test environment, instrument parameters, sample microporosity, paramagnetic materials, and fluid type. Image analysis methods currently mostly use micro-nano CT and focused ion beam field emission scanning electron microscopy analysis technology to characterize pore structure and its connectivity. However, this method is greatly limited by resolution and can only characterize pore throats of 50nm and above.

[0005] Porosity testing methods all have numerous limitations, and a single method can hardly accurately characterize the actual reservoir space in shale. Helium porosity measurement is an effective porosity testing method widely used in domestic laboratories. Key parameters of this method are the quantitative evaluation of apparent volume and skeletal volume, and are also affected by temperature, pressure gauge accuracy, and stability. Summary of the Invention

[0006] The embodiments of the present invention provide a method and application for measuring the effective porosity of shale that takes into account both lamellar fractures and matrix pores, which can achieve the separate quantification of the matrix porosity and lamellar fracture porosity of shale samples, providing a basis for resource evaluation and sweet spot prediction.

[0007] In a first aspect, an embodiment of the present invention provides a method for determining effective porosity of shale taking into account both lamellae fractures and matrix pores, comprising performing the following steps on at least one shale sample:

[0008] Performing a helium effective porosity measurement experiment on the rock sample until a first set time is reached, and obtaining a plurality of experimental data including data pairs of experimental time and effective porosity;

[0009] Based on the mathematical model system of effective porosity and experimental time, a first fitting relationship between the effective porosity of the rock sample and the experimental time is obtained by fitting the experimental data. According to the rock sample selection direction and the first fitting relationship, the effective porosity, matrix porosity and lamellae porosity of the rock sample are respectively determined.

[0010] In a second aspect, an embodiment of the present invention provides a method for evaluating the reservoir space of a shale reservoir, comprising using the above-described method to respectively determine the effective porosity, matrix porosity, and lamellar fracture porosity of a rock sample in a shale reservoir;

[0011] The reservoir space of shale reservoir is evaluated based on the determined effective porosity, matrix porosity and lamellae porosity.

[0012] The beneficial effects of the above technical solutions provided by the embodiments of the present invention include at least:

[0013] (1) Current porosity measurements usually only obtain the value of effective porosity or total porosity, and are unable to quantitatively distinguish the effective porosity contributed by the lamellar fracture storage space and the matrix storage space. The embodiment of the present invention provides a method for determining the effective porosity of shale that takes both lamellar fractures and matrix pores into consideration. By using a helium measurement method, a porosity measurement experiment is conducted for a sufficient time, so that helium molecules can fully penetrate into the lamellar fractures and matrix pores of the rock sample, and the obtained effective porosity has a higher accuracy. In addition, the effective porosity, matrix porosity, and lamellar fracture porosity of the rock sample are determined according to the rock sample selection direction and the fitting relationship between porosity and time, thereby achieving the quantitative characterization of matrix porosity and lamellar fracture porosity, and providing a basis for shale reservoir resource evaluation and sweet spot prediction.

[0014] (2) There are differences in the reasonable experimental time for helium porosity determination of different types of shales. Reasonable time determines more accurate porosity values ​​and controls experimental costs, time, and labor costs. The embodiment of the present invention provides a method for determining the effective porosity of shale that takes into account both lamellae and matrix pores, and establishes a model relationship between effective porosity and experimental time; a mathematical model is established to determine the required optimal experimental time, obtain experimental data, and use the mathematical model to fit the experimental data to obtain a first fitting relationship between the effective porosity of the rock sample and the experimental time. The effective porosity, matrix porosity, and lamellae porosity of the rock sample are determined based on the first fitting relationship. This not only ensures the accuracy of various effective porosity values, but also saves experimental costs, time, and labor costs.

[0015] Other features and advantages of the present invention will be described in the following description, and in part will become apparent from the description, or will be understood by practicing the present invention. The purposes and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description, claims, and drawings.

[0016] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0018] Figure 1 This is a flow chart of the method for measuring effective porosity of shale in Example 1 of the present invention;

[0019] Figure 2 This is a schematic structural diagram of the parallel bedding plug sample B25-1 in Example 2 of the present invention;

[0020] Figure 3 This is a schematic structural diagram of the vertical bedding plug sample B25-2 in Example 2 of the present invention;

[0021] Figure 4 This is a graph showing the relationship between the helium effective porosity and the measurement time of the B25-1 sample in Example 2 of the present invention;

[0022] Figure 5 This is a graph showing the relationship between the helium effective porosity and the measurement time of the B25-2 sample in Example 2 of the present invention;

[0023] Figure 6 This is a prediction diagram of the helium effective porosity of the B25-1 sample in Example 2 of the present invention;

[0024] Figure 7 This is a distribution diagram of the predicted range of helium effective porosity of the B25-2 sample in Example 2 of the present invention. DETAILED DESCRIPTION

[0025] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0026] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0027] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the invention belongs. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In the event of any conflict with any incorporated document, the content of this specification shall prevail.

[0028] In the description of the present invention, it should be noted that the terms “include”, “including”, “have”, “contain”, etc. are all open terms, which mean including but not limited to.

[0029] Helium effective porosity measurement is a widely used effective porosity test method in domestic laboratories. The inventors discovered that the accuracy of shale effective porosity determined by existing helium effective porosity measurement techniques needs to be improved. Furthermore, existing techniques can only measure the effective porosity of shale, failing to distinguish between matrix pores and lamellar fracture pores. Further research and development led the inventors to the present invention. The present invention provides a method and application for measuring shale effective porosity that considers both lamellar fractures and matrix pores, enabling the separate quantification of matrix porosity and lamellar fracture porosity in shale samples.

[0030] Example 1

[0031] A first embodiment of the present invention provides a method for measuring the effective porosity of shale taking both lamellae fractures and matrix pores into consideration.

[0032] The national standard stipulates that the porosity value measured at 25 minutes in the helium effective porosity measurement experiment shall be used as the effective porosity value of the shale. However, the inventors found in a large number of experiments that as the experimental time gradually increases, gas molecules are continuously injected into the shale pores, that is, the porosity value measured in the experiment gradually increases with the increase of the experimental time, but the increase in magnitude gradually decreases with the extension of the experimental time. Based on the considerations of experimental cost, time and manpower cost, it is impossible to increase the time of each helium porosity measurement experiment to a sufficiently long time in actual work. Therefore, in this embodiment, helium porosity measurement experiments are first carried out on multiple shale samples until the second set time is reached, and a general relationship (mathematical model) between porosity and experimental time is established based on the experimental data. Based on the established mathematical model of porosity and experimental time, in the subsequent helium porosity measurement experiment, the experimental time does not need to reach the second set time. According to empirical statistics, the experimental time reaches the first set time. Based on the experimental data combined with the pre-established mathematical model of porosity and experimental time, the first fitting relationship between the porosity of the rock sample and the experimental time can meet the experimental accuracy requirements.

[0033] Specifically, the second set time is usually set to be greater than 50 hours; the first set time is set according to sample differences and is usually set to 15 to 25 hours.

[0034] The above mathematical model is pre-established in the following way:

[0035] Helium effective porosity measurement experiments are performed on multiple shale samples until a second set time is reached, obtaining multiple experimental data including data pairs of experimental time and effective porosity. The experimental data are fitted to obtain a second fitting relationship between effective porosity and experimental time; and a mathematical model of effective porosity and experimental time is established based on the second fitting relationship for each of the multiple shale samples.

[0036] Specifically, the mathematical model of effective porosity and experimental time is:

[0037] Porosity=a×t b +c

[0038] Where Porosity is the effective porosity, t is the experimental time, and a, b, and c are constants to be determined.

[0039] Reference Figure 1 As shown, the method for determining the effective porosity of shale taking into account both lamellae fractures and matrix pores provided in the first embodiment of the present invention includes performing the following steps on at least one shale sample:

[0040] Step S11: performing a helium effective porosity measurement experiment on the rock sample until a first set time is reached, and obtaining a plurality of experimental data including data pairs of experimental time and effective porosity.

[0041] During the helium effective porosity measurement experiment, the effective porosity is determined in real time through the apparent volume of the rock sample and the real-time determined skeleton volume. Therefore, the key to improving the accuracy of effective porosity experimental data is to accurately obtain the apparent volume and skeleton volume.

[0042] In this example, a measurement method was used to determine the apparent volume of a rock sample. The sample was a plunger sample (i.e., a cylindrical rock sample) and wire-cut. The apparent volume was measured using an electronic vernier caliper. The outer diameter and length of the rock sample were measured with the caliper, and the calculated volume was used as the apparent volume. This measurement method avoids the effects of clay mineral swelling due to water absorption during the liquid measurement process, reducing measurement errors. Furthermore, wire cutting prevents cracks caused by manual handling and ensures the integrity of the rock sample. Furthermore, wire cutting results in a smoother surface, which improves the accuracy of the apparent volume determined by the measurement method.

[0043] Equilibration time, gas injection direction, injection pressure, ambient temperature and pressure gauge sensitivity all affect the accurate acquisition of skeleton volume.

[0044] In this embodiment, a high-precision pressure sensor is used to measure pressure. Under the conditions of constant ambient temperature and injection pressure, a helium effective porosity measurement experiment is performed on the rock sample. The instrument is kept at a constant temperature to avoid external interference, thereby improving the measurement accuracy.

[0045] Regarding the optimization of the gas injection direction, rock samples are first cut based on the relationship between the shale and the fractures. At least one shale sample is a first sample with an axial direction parallel to the fracture direction; or at least one shale sample is a second sample with an axial direction perpendicular to the fracture direction; or at least one shale sample is a first sample with an axial direction parallel to the fracture direction and a second sample with an axial direction perpendicular to the fracture direction. The first and second samples are taken from the same core at the same depth (so that the first and second samples can represent samples at the same location). The rock sample is a plunger sample, and the injection direction is perpendicular to the axis of the rock sample. Therefore, the injection direction is perpendicular to the fracture direction of the first rock sample and parallel to the fracture direction of the second rock sample.

[0046] The axial direction here refers to the direction perpendicular to the cross section of the rock sample.

[0047] Step S12: Based on the mathematical model of shale effective porosity and experimental time, a first fitting relationship between the effective porosity of the rock sample and the experimental time is obtained by fitting the experimental data. The effective porosity, matrix porosity, and lamellae porosity of the rock sample are determined according to the rock sample selection direction and the first fitting relationship.

[0048] The rock sample selection direction here refers to the relationship between the axial direction of the rock sample obtained by cutting and the direction of the rock sample lamellae. Taking the first rock sample and the second rock sample as an example, the injection direction is perpendicular to the lamellae direction of the first rock sample and parallel to the lamellae direction of the second rock sample. Therefore, for the first rock sample, the gas molecules are first injected into the matrix pores of the rock sample and then into the lamellae of the rock sample; for the second rock sample, the fluid is first injected into the lamellae of the rock sample and then into the matrix pores of the rock sample.

[0049] Therefore, according to the first fitting relationship, the first porosity when the effective porosity change gradient begins to be less than the first gradient threshold and the second porosity when the porosity change gradient begins to be less than the second gradient threshold can be determined respectively, and the first porosity is used as the effective porosity of the rock sample; if the shale rock sample is the first rock sample, the second porosity is determined as the matrix porosity of the rock sample, and the difference between the effective porosity and the matrix porosity is determined as the lamellar fracture porosity; if the shale rock sample is the second rock sample, the second porosity is determined as the lamellar fracture porosity of the rock sample, and the difference between the effective porosity and the lamellar fracture porosity is determined as the matrix porosity.

[0050] The porosity gradient here is defined as the ratio of the difference between the porosity at the current moment and the porosity at the moment before the current moment to the difference between the two corresponding moments. The first gradient threshold is lower than the second gradient threshold. This means that the first porosity is the porosity at which porosity essentially remains constant over time, while the second porosity is the porosity at the inflection point of the relationship curve corresponding to the second fitting relationship.

[0051] Current porosity measurements typically only yield effective porosity or total porosity values, and are unable to quantitatively distinguish the effective porosity contributed by lamellae reservoir space and matrix reservoir space. A first embodiment of the present invention provides a method for determining shale effective porosity that takes both lamellae and matrix pores into account. By using a helium measurement method, effective porosity measurement experiments are conducted for a sufficient time to allow helium molecules to fully penetrate the lamellae and matrix pores of the rock sample, resulting in a more accurate effective porosity. Furthermore, the effective porosity, matrix porosity, and lamellae porosity of the rock sample are determined based on the rock sample selection direction and the fitting relationship between porosity and time, thereby achieving separate quantitative characterization of matrix porosity and lamellae porosity.

[0052] Helium porosity measurement experiments require a sufficiently long experiment time to obtain more accurate porosity values, which requires higher experimental costs, time, and labor costs. The shale porosity measurement method provided in Example 1 of the present invention, which takes into account both lamellar fractures and matrix pores, pre-establishes a mathematical model of porosity and experimental time. Experimental data is obtained using an experimental time much shorter than that required to establish the mathematical model. The mathematical model is then used to fit the experimental data to obtain a first fitting relationship between the rock sample's porosity and experimental time. The effective porosity, matrix porosity, and lamellar fracture porosity of the rock sample are then determined based on this first fitting relationship. This ensures the accuracy of various porosity values ​​while saving experimental costs, time, and labor costs.

[0053] Example 2

[0054] The second embodiment of the present invention provides a specific application embodiment of a shale porosity measurement method that takes both lamellae fractures and matrix pores into consideration.

[0055] In this example, the samples used in the experiment were taken from the xx section of the xx basin. Two pure shale plugs were obtained from the same core at the same depth after wire cutting: parallel-bedding plug sample B25-1 and perpendicular-bedding plug sample B25-2. The bedding fractures in B25-1 were parallel to the sample axis, while those in B25-2 were perpendicular to the sample axis. Qualitative observations were made using thin sections and scanning electron microscopy, followed by helium effective porosity measurement and effective evaluation using numerical simulation.

[0056] The samples were prepared and observed by thin sections, scanning electron microscopy, and nano-CT on the cylindrical surface of the plunger. The samples showed felsic laminae, clay laminae, and shell laminae, with the mineral composition mainly composed of clay minerals, quartz, and carbonate minerals. Irregular lamellae parallel to the bedding were observed in the CT model. Figure 2 It can be seen that the parallel-bedding plug sample B25-1 has two relatively straight lamellae, which are perpendicular to the cylindrical surface of the plug; Figure 3 It can be seen that the vertical bedding plug sample B25-2 has two interlaced lamellae and other smaller horizontal cracks, all parallel to the cylindrical surface of the plug.

[0057] The effective porosity of samples B25-1 and B25-2 was measured using helium porosimetry, with an initial injection pressure of 100 psi and a test environment (temperature) controlled at 24±1°C. The apparent volume of B25-1 measured 2.506 cm in diameter, 1.954 cm in length, and a mass of 23.44 g; the apparent volume of B25-2 measured 2.494 cm in diameter, 1.143 cm in length, and a mass of 13.49 g. Long-term effective porosity measurements of the samples revealed that the effective porosity of B25-1, a parallel-bedding plug, was 5.5% at 25 minutes, as specified by the national standard, and 8.64% at 53 hours. The effective porosity showed a uniform upward trend throughout the POR-T period, with no apparent inflection point. The effective porosity of B25-2, a perpendicular-bedding plug, was 6.68% at 25 minutes, as specified by the national standard, and 8.38% at 53 hours. Throughout the POR-T period, the effective porosity showed a gradual upward trend after a clear inflection point.

[0058] By means of numerical experiments, the physical experimental results are fitted to predict the effective porosity value in the subsequent time. The effective porosity, pressure, and temperature variation over time within 66 hours are obtained from the parallel bedding sample B25-1 experiment. The power function approximation method is used to perform curve fitting, such as Figure 4 As shown, Porosity = -28.45t -0.03272 +27.9, the approximation errors of the fitting curve are SSE = 1.17, R-square = 0.9899, ​​and RMSE = 0.1855. From the above error analysis results, it can be seen that R-square is close to 1, indicating that the fitting curve is almost consistent with the scattered points. The RMSE shows that the absolute value of the estimated error of effective porosity is very low. Therefore, this function can be used as an approximation function curve for the change of effective porosity over time. The effective porosity, pressure, and temperature over time values ​​were obtained in the vertical bedding sample B25-2 experiment within 53 hours. The power function approximation method was used to perform curve fitting, as shown in the following example: Figure 5 As shown, Porosity = -10.09t -0.2261 +8.349, the approximation errors of the fitting curve are SSE = 5.41, R-square = 0.9117, and RMSE = 0.3547. From the above error analysis results, it can be seen that R-square is close to 1, indicating that the fitting curve is almost consistent with the scatter points. The RMSE shows that the absolute value of the estimated error of the effective porosity is very low. Therefore, this function can be used as an approximation function curve of the change of effective porosity with time.

[0059] The power function obtained is used to predict the results of the parallel bedding sample B25-1 for the next 18 days, and the results are Figure 6The effective porosity prediction curve is shown in the figure. As can be seen from the figure, the small stars represent the known experimental effective porosity data, and the dotted line represents the predicted curve. It can be seen that the effective porosity increases with time. It is expected that the effective porosity will be close to 10% when the experiment is carried out for 18 days, which is exactly 9.9782%. For the vertical bedding sample B25-2, see Figure 5 As shown, Figure 5 The scattered points in the distribution are the measured values ​​of physical experiments, and the curve is the effective porosity predicted by numerical fitting. Porosity = a × t b +c. Combine the three parameters a, b, and c within the adjustable range and draw a graph as shown below. Figure 7 The effective porosity curve range is the range of effective porosity (it can be seen that the experimental values ​​are all within the predicted range). It is estimated that the maximum effective porosity will be close to 9% after 18 days of the experiment, which is 8.1505% to be exact.

[0060] The overall performance is that the helium effective porosity curve of the parallel bedding sample B25-1 shows an early inflection point with no obvious, and an overall uniform upward trend. The effective porosity value at 25 minutes is 5.5%, the effective porosity value at 53 hours is 8.64%, and the effective porosity is close to 10% at 18 days. The helium effective porosity curve of the vertical bedding sample B25-2 shows an early and obvious inflection point, and an overall slow upward trend in the later period. The effective porosity value at 25 minutes is 6.68%, the effective porosity value at 53 hours is 8.38%, and the effective porosity is close to 9% at 18 days.

[0061] Experimental results analysis of Example 2 of the present invention:

[0062] Pure shale has well-developed lamellae fractures, and densely developed lamellae fractures are not only seepage channels for oil and gas resources, but also important storage spaces. For tight oil and gas reservoirs with generally poor physical properties, the effective porosity is low, and the relative error of porosity measurement is greater, so accurate porosity measurement is particularly important. The current helium porosity measurement standard requires a measurement equilibrium time of 25 minutes, and the experimental result obtained is the total effective porosity value of the sample. In the practice of this method, the effective porosity values ​​obtained in the 25-minute test were 5.5% and 6.68%, respectively, corresponding to the measured values ​​of 8.64% and 8.38% at 66 hours and 53 hours, with errors of 59% and 25.4%, and corresponding to the predicted maximum values ​​of 10% and 9% at 18 days, with errors of 45% and 25.7%. Furthermore, for samples with parallel bedding, the maximum helium contact surface parallel to the bedding plane during testing. Therefore, the contributions of pores and fractures remained largely consistent during this process, leading to a less pronounced inflection point in the POR-T curve and a uniform increase in effective porosity. For samples with perpendicular bedding, the maximum helium contact surface perpendicular to the bedding plane during testing. Therefore, the early helium effective porosity determination was dominated by well-connected lamellae fractures, followed by smaller pores. This resulted in an early and pronounced inflection point in the POR-T curve, followed by a slow increase in effective porosity. This further optimized the key parameters for helium effective porosity determination—equilibrium time and gas injection direction—and effectively characterized lamellae fractures.

[0063] Based on the inventive concept of the present invention, an embodiment of the present invention also provides a method for evaluating the storage space of a shale reservoir, comprising using the method described above to determine the effective porosity, matrix porosity, and lamellae porosity of a rock sample in a shale reservoir; and performing a storage space evaluation on the shale reservoir based on the determined effective porosity, matrix porosity, and lamellae porosity.

[0064] It should be understood that the specific order or hierarchy of steps in the disclosed processes is an example of an exemplary method. Based on design preferences, it should be understood that the specific order or hierarchy of steps in the process can be rearranged without departing from the scope of the present disclosure. The accompanying method claims present elements of the various steps in an exemplary order and are not intended to be limited to the specific order or hierarchy described.

[0065] In the foregoing detailed description, various features are grouped together in a single embodiment to simplify the disclosure. This method of disclosure should not be interpreted as reflecting an intention that embodiments of the claimed subject matter require more features than are expressly recited in each claim. On the contrary, as reflected in the appended claims, the invention comprises less than all the features of any individual disclosed embodiment. The appended claims are therefore hereby expressly incorporated into the detailed description, with each claim standing on its own as a separate preferred embodiment of the invention.

[0066] The above description includes examples of one or more embodiments. Of course, it is impossible to describe all possible combinations of components or methods for the purpose of describing the above embodiments, but it will be appreciated by those skilled in the art that the various embodiments may be further combined and arranged. Therefore, the embodiments described herein are intended to encompass all such changes, modifications and variations that fall within the scope of protection of the appended claims. In addition, with respect to the term "comprising" used in the specification or claims, the word is encompassed in a manner similar to the term "including," as explained in terms of "including," used as a transitional word in the claims. In addition, any term "or" used in the specification of the claims is intended to mean "non-exclusive or." The terms "first" and "second" are used for descriptive purposes only and are not to be understood as indicating or implying relative importance.

Claims

1. A method for determining effective porosity of shale taking into account both lamellar fractures and matrix pores, characterized in that: The method comprises performing the following steps on at least one shale rock sample: Performing a helium porosity measurement experiment on the rock sample until a first set time is reached, and obtaining a plurality of experimental data including data pairs of experimental time and effective porosity; Based on the mathematical model of shale effective porosity and experimental time, a first fitting relationship between the effective porosity of the rock sample and the experimental time is obtained by fitting the experimental data. The effective porosity, matrix porosity and lamellae porosity of the rock sample are respectively determined according to the rock sample selection direction and the first fitting relationship.

2. The method according to claim 1, wherein The at least one shale rock sample is a first rock sample whose axis is parallel to the lamellae fracture direction; or The at least one shale rock sample is a second rock sample whose axis is perpendicular to the lamellae fracture direction; or The at least one shale rock sample comprises a first rock sample whose axis is parallel to the direction of the lamina fractures and a second rock sample whose axis is perpendicular to the direction of the lamina fractures. The first rock sample and the second rock sample are taken from the same depth of the same core.

3. The method according to claim 2, wherein Determining the effective porosity, matrix porosity, and lamellar fracture porosity of the rock sample according to the rock sample selection direction and the first fitting relationship, including: determining, based on the first fitting relationship, a first porosity at which the effective porosity change gradient begins to be less than a first gradient threshold and a second porosity at which the porosity change gradient begins to be less than a second gradient threshold, respectively; using the first porosity as the effective porosity of the rock sample, and the first gradient threshold being less than the second gradient threshold; If the shale rock sample is the first rock sample, the second porosity is determined as the matrix porosity of the rock sample, and the difference between the effective porosity and the matrix porosity is determined as the lamellar fracture porosity; If the shale rock sample is the second rock sample, the second porosity is determined as the lamellar fracture porosity of the rock sample, and the difference between the effective porosity and the lamellar fracture porosity is determined as the matrix porosity.

4. The method according to claim 1, wherein The helium effective porosity measurement experiment on the rock sample includes: Under the conditions of constant ambient temperature and injection pressure, helium effective porosity measurement experiments were carried out on rock samples.

5. The method according to claim 1, wherein The mathematical model is established in the following way: Performing helium effective porosity measurement experiments on multiple shale samples until a second set time is reached, obtaining multiple experimental data including data pairs of experimental time and effective porosity, and fitting the experimental data to obtain a second fitting relationship between effective porosity and experimental time; A mathematical model of shale effective porosity and experimental time is established through the second fitting relationship of each of the multiple shale samples.

6. The method according to claim 5, wherein The second set time is greater than 50 hours.

7. The method according to claim 5, wherein The first set time is 15 to 25 hours.

8. The method according to claim 5, wherein The mathematical model is: Porosity=a×t b +c Where Porosity is the effective porosity, t is the experimental time, and a, b, and c are constants to be determined.

9. The method according to claim 1, wherein The shale rock sample is a plunger sample obtained by wire cutting method.

10. The method according to claim 1, wherein The helium effective porosity measurement experiment is performed on the rock sample until a first set time is reached and experimental data including multiple experimental times and effective porosity data is obtained, further comprising: The apparent volume of the rock sample is determined by measurement.

11. A method for evaluating the reservoir space of a shale reservoir, characterized in that: include: Using the method according to any one of claims 1 to 10, respectively determining the effective porosity, matrix porosity and lamellar fracture porosity of a rock sample in a shale reservoir; The reservoir space of shale reservoir is evaluated based on the determined effective porosity, matrix porosity and lamellae porosity.

Images