An improved GIP porosity measurement method based on pressure decay

Abstract

The present invention provides an improved GIP porosity measurement method based on pressure decay, which relates to the technical field of porosity measurement; this method mainly includes building a GIP porosity experimental system with a standard tank and a matrix cup, storing a gas with a predetermined pressure in the standard tank, and placing a rock sample in the matrix cup. Assuming the gas diffusion sequence, constructing a diffusion flux expression according to the pressure distribution characteristics of the rock sample, substituting it into the pressure perturbation propagation model to obtain the surface pressure expression, and then modifying and constructing the pressure decay model in the matrix cup. During the experiment, the volumes and pressure decline data of the standard tank and the matrix cup are obtained, and the equilibrium pressure is obtained by fitting with the decay model, and the rock sample skeleton volume and porosity are calculated by substituting into the formula. The present invention can obtain the equilibrium pressure through the pressure decay model in the matrix cup without a complete pressure drop curve, greatly shortening the acquisition time and improving the experimental efficiency, providing a faster and more accurate measurement method for shale porosity measurement, and having important significance for related research and engineering applications such as shale gas reservoir evaluation.

Description

Technical Field

[0001] The present invention mainly relates to the technical field of porosity measurement, and particularly relates to an improved GIP porosity measurement method based on pressure decay. Background Art

[0002] Global shale gas resources are abundant. It is estimated that the geological resource volume reaches 1014 trillion cubic meters, and the recoverable resource volume is expected to reach 243 trillion cubic meters. Shale gas has become an important alternative resource to ensure global energy security. Shale reservoirs have reservoir characteristics such as low porosity, low permeability, and developed nano-pores, which determine that horizontal well drilling and completion technology and staged fracturing technology are the major technical bases for large-scale development of shale gas. However, porosity plays an irreplaceable role in aspects such as shale gas reservoir evaluation, natural gas resource estimation, favorable block selection, and production strategy design. And shale gas reservoirs have a nano-scale pore network, making it still technically challenging to achieve accurate measurement of shale porosity.

[0003] Laboratory rock porosity measurement can be divided into two categories. One category includes imaging analysis methods, such as scanning electron microscopy, three-dimensional reconstruction technology, and computed tomography. Imaging technology has been able to achieve full-scale pore identification from nano-micron pores. As the pore scale resolution for identifying samples is higher, the sample scale is smaller. Therefore, the contradiction between the spatial resolution of the pore scale and the field of view is the main obstacle affecting the application of shale reservoir porosity imaging methods. The other category is the fluid injection method. By injecting fluid into the interior of rock pores under high pressure and detecting the injected fluid volume or weight by an instrument to obtain the pore volume of the sample to be measured. Too low pressure causes the fluid to not fully saturate the pore volume, while high injection pressure is likely to generate artificial fractures in the core, and at the same time, it is impossible to avoid the errors caused by fluid-solid interaction. Therefore, the fluid injection method has inherent defects that cannot be ignored in porosity measurement.

[0004] The saturated gas method is a commonly used method for measuring porosity in the laboratory and is applicable to columnar samples and crushed samples. The crushed sample is currently the only gas measurement method that can measure the total porosity of shale. This test method is relatively mature, and the accuracy of the test results meets the existing requirements. However, when formulating a development plan and conducting reservoir evaluation, the effective porosity parameter is usually the focus that engineers are most concerned about. Due to the complex nano-pore structure of the sample to be measured, the gas enters the sample slowly through diffusion seepage, resulting in a long time required for the pores of the sample to reach pressure equilibrium with the instrument, making it difficult to obtain the precise equilibrium pressure during the test process. This is also the main obstacle to inaccurate current porosity testing. Although researchers have proposed methods such as delaying the saturation time, vacuum pumping + strictly controlling the equilibrium conditions, and the pressure pulse decay method to improve the test accuracy. But these measures have not effectively solved the efficiency problem of large-scale sample porosity testing. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an improved GIP porosity measurement method based on pressure decay in view of the deficiencies of the prior art.

[0006] The technical solution of the present invention to solve the above technical problems is as follows: An improved GIP porosity measurement method based on pressure decay, comprising the following steps:

[0007] S1. Set up a GIP porosity experimental system, the GIP porosity experimental system includes a standard tank and a matrix cup, the storage standard tank is used to store gas with a predetermined injection pressure, and the matrix cup is used to place the rock sample;

[0008] S2. Assume the diffusion order of the gas in the standard tank and the matrix cup and based on the pressure distribution characteristics of the rock sample, construct an expression for the diffusion flux at the surface of the rock sample, and substitute the rock sample pressure disturbance propagation model into the diffusion flux expression to obtain an expression for the surface pressure of the rock sample;

[0009] S3. Correct the rock sample surface pressure expression based on the gas diffusion time parameter, and construct a pressure decay model in the matrix cup based on the corrected rock sample surface pressure expression;

[0010] S4. Obtain the standard tank volume, matrix cup volume and pressure reduction experimental data through the GIP porosity experimental system during the porosity test, and perform non-linear curve fitting on the pressure reduction experimental data through the pressure decay model in the matrix cup to obtain the equilibrium pressure when the gas reaches thermodynamic equilibrium in the matrix cup and the pores of the rock sample;

[0011] S5. Substitute the standard tank volume, matrix cup volume and equilibrium pressure into the skeleton volume formula to calculate the skeleton volume of the rock sample, and substitute the skeleton volume and the external volume of the rock sample into the porosity formula to calculate the porosity of the rock sample.

[0012] The beneficial effects of the present invention are: By setting up a GIP porosity experimental system including a standard tank and a matrix cup, combining a series of operations such as assuming the gas diffusion order, constructing relevant expressions and models, and finally using the pressure decay model in the matrix cup to fit the pressure reduction experimental data, the equilibrium pressure when the gas reaches thermodynamic equilibrium in the matrix cup and the pores of the rock sample can be obtained without obtaining the complete pressure drop curve in the matrix cup, greatly shortening the time to obtain the equilibrium pressure and improving the experimental efficiency;

[0013] Substituting the calculated equilibrium pressure into the skeleton volume formula and porosity formula can calculate the skeleton volume and porosity of the rock sample, which significantly shortens the test time compared with the longest 2-hour test time in the existing technology. While ensuring the test accuracy, it greatly improves the test efficiency, providing a faster and more accurate determination method for shale porosity measurement, which is of great significance for related research and engineering applications such as shale gas reservoir evaluation. Description of the Drawings

[0014] Figure 1 It is a schematic structural diagram of the GIP porosity experimental system provided by an embodiment of the present invention;

[0015] Figure 2(a) shows the radial distribution of the internal pressure of the rock sample during the porosity measurement provided by an embodiment of the present invention;

[0016] Figure 2(b) is a partial enlarged view of Figure 2(a), and the enlarged position is at 1.0 - 1.3 cm from the axis of the rock sample in Figure 2(a);

[0017] Figure 3 It is a schematic flow chart of the improved GIP porosity determination method provided by an embodiment of the present invention;

[0018] Figure 4 It is the calibration fitting result of the instrument blank volume provided by an embodiment of the present invention;

[0019] Figure 5 It is the comparison between the fitting result of the experimental data of Sample 1 at different times and the actual test value provided by an embodiment of the present invention;

[0020] Figure 6 It is the comparison between the fitting result of the experimental data of Sample 2 at different times and the actual test value provided by an embodiment of the present invention;

[0021] Figure 7 It is the equilibrium pressure distribution diagram predicted based on the experimental data with different decay times provided by an embodiment of the present invention. Detailed Embodiments

[0022] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0023] As Figure 1 shown, currently, the basic principle of the gas injection method porosity test instrument in the market (i.e., the GIP porosity experimental system, referred to as the system) is based on Boyle's law. The test process of the market instrument is shown in Figure 1。The volume between valve 1 (i.e., the first valve) and valve 2 (i.e., the second valve) is V1, which is defined as the standard tank volume. The volume between valve 2 and valve 3 (i.e., the third valve) is V2, which is defined as the matrix cup volume. The sample apparent volume (V) is calculated from its diameter and length, and the skeleton volume (V g ) is the difference between the sample apparent volume (V) and the pore volume. The test workflow is as follows: 1) Place the sample to be tested into the matrix cup. 2) Open valve 1, close valves 2 and 3, and inject gas into the standard container at a predetermined injection pressure (P0). After reaching pressure equilibrium, close valve 1. 3) Open valve 2 to connect the standard tank and the matrix cup. The gas in the standard tank expands into the matrix cup and then saturates the sample to be tested under the action of seepage mechanisms such as seepage and diffusion. When the pressure sensor reading no longer changes, the system reaches thermodynamic equilibrium, and the reading of the pressure sensor at this time is the equilibrium pressure, denoted as P e . 4) Calculate the sample skeleton volume (V g ) and porosity (φ) according to formula (1) and formula (2).

[0024] V g = V2 - V1(P0 / P e - 1) (1)

[0025] φ = 1 - V g / V (2)

[0026] During the entire porosity test process, V1, V2, and P0 are relatively easy to obtain in the porosity test experiment. Obtaining an accurate equilibrium pressure P e is the key to ensuring the accuracy of the porosity test. However, the existing experimental schemes only use an extended saturation time to obtain an accurate equilibrium pressure. A large number of experiments show that the longest saturation time required for shale sample porosity measurement may exceed 2h, and a systematic criterion for determining whether the system pressure reaches equilibrium has not been formed. The inability to accurately obtain whether the current system pressure reaches the equilibrium pressure has become the biggest obstacle to improving the accuracy of the porosity test. The core of the method of the present invention is to establish a prediction model for predicting the system pressure decay during the porosity test, and use the model and a small amount of experimental data (i.e., pressure decay experimental data) to obtain the true equilibrium pressure. The following is a detailed introduction through multiple embodiments.

[0027] Example 1:

[0028] As Figure 1 and Figure 3 shown, the embodiment of the present invention provides an improved GIP porosity determination method based on pressure decay, including the following steps:

[0029] S1. Set up a GIP porosity experimental system (as Figure 1) The GIP porosity experimental system includes a standard tank and a matrix cup. The storage standard tank is used to store gas at a predetermined injection pressure, and the matrix cup is used to place rock samples;

[0030] S2. Assume the gas diffusion sequence in the standard tank and the matrix cup, and based on the pressure distribution characteristics of the rock sample, construct an expression for the diffusion flux at the surface of the rock sample. Substitute the rock sample pressure perturbation propagation model into the diffusion flux expression to obtain an expression for the surface pressure of the rock sample;

[0031] S3. Correct the rock sample surface pressure expression based on the gas diffusion time parameter, and construct a pressure decay model in the matrix cup based on the corrected rock sample surface pressure expression;

[0032] S4. During the porosity test, obtain the volume of the standard tank, the volume of the matrix cup, and the pressure reduction experimental data through the GIP porosity experimental system. Perform non-linear curve fitting on the pressure reduction experimental data using the pressure decay model in the matrix cup to obtain the equilibrium pressure when the gas reaches thermodynamic equilibrium in the matrix cup and the pores of the rock sample;

[0033] S5. Substitute the volume of the standard tank, the volume of the matrix cup, and the equilibrium pressure into the skeleton volume formula to calculate the skeleton volume of the rock sample. Substitute the skeleton volume and the external volume of the rock sample into the porosity formula to calculate the porosity of the rock sample.

[0034] In this embodiment, 1) Efficiently obtain the equilibrium pressure: By building a GIP porosity experimental system including a standard tank and a matrix cup, combined with a series of operations such as assuming the gas diffusion sequence, constructing relevant expressions and models, and finally using the pressure decay model in the matrix cup to fit the pressure reduction experimental data, it is possible to obtain the equilibrium pressure when the gas reaches thermodynamic equilibrium in the matrix cup and the pores of the rock sample without obtaining the complete pressure drop curve in the matrix cup, greatly shortening the time to obtain the equilibrium pressure and improving the experimental efficiency;

[0035] 2) Improve the accuracy and efficiency of porosity measurement: Substitute the calculated equilibrium pressure into the skeleton volume formula and the porosity formula, which can calculate the skeleton volume and porosity of the rock sample. Compared with the existing technology with a maximum test time of 2 hours, the test time is significantly shortened, greatly improving the test efficiency while ensuring the test accuracy, providing a faster and more accurate measurement method for shale porosity measurement, and having important significance for related research and engineering applications such as shale gas reservoir evaluation.

[0036] Example 2:

[0037] Specifically, in S2, assuming the diffusion order of gas in the standard tank and the matrix cup and based on the pressure distribution characteristics of the rock sample, an expression for the diffusion flux at the surface of the rock sample is constructed, and the pressure perturbation propagation model of the rock sample is substituted into the diffusion flux expression to obtain the surface pressure expression of the rock sample, including:

[0038] S2.1. During the porosity test, the rate at which gas expands from the standard tank into the matrix cup is much greater than the rate at which gas diffuses into the rock sample.

[0039] Assume that the gas first reaches pressure equilibrium between the standard tank and the matrix cup and then diffuses into the pores of the rock sample. According to Boyle's law, an initial pressure expression (3) for the pressure equilibrium between the standard tank and the matrix cup and the diffusion into the pores of the rock sample is constructed. The initial pressure expression (3) is:

[0040]

[0041] where P0' is the initial pressure for the pressure equilibrium between the standard tank and the matrix cup and the diffusion into the pores of the rock sample, P0 is the set pressure for injecting gas into the standard tank, V1 is the volume of the standard tank, V2 is the volume of the matrix cup, and V is the external volume of the rock sample.

[0042] Existing research results show that there are obvious regional characteristics in the internal pressure distribution of the core. The pressure in the area near the axis of the rock sample shows obvious non-linear characteristics, but as the saturation time increases, the non-linear characteristics weaken significantly. The pressure near the surface of the rock sample is significantly linearly distributed. Figures 2(a) and 2(b) present the results of researchers using numerical simulation methods to simulate the internal pressure distribution characteristics of the rock sample.

[0043] Specifically, based on the fact that the pressure gradient at the surface of the rock sample during the porosity test satisfies the linear distribution characteristics and based on the law of mass conservation, the decreasing rate of gas in the space composed of the matrix cup and the standard tank is equal to the diffusion flux at the outer surface of the rock sample, and an expression for the diffusion flux at the surface of the rock sample (4) is constructed. The diffusion flux expression is:

[0044]

[0045] where is the first-order partial derivative of the pressure at the surface of the rock sample with respect to time, representing the rate of change of pressure with time, k0 is the absolute permeability of the core, b is the slip coefficient, R is the radius of the rock sample, L is the length of the rock sample, and μ is the gas viscosity.

[0046] The pressure values at the surface and at a certain distance from the boundary of the rock sample inside the rock sample (r c ) are P and P 2e ;

[0047] Based on the distance from the surface of the rock sample and the interior to the boundary of the rock sample, a pressure gradient expression (5) at the surface of the rock sample is constructed, and the pressure gradient expression (5) is as follows:

[0048]

[0049] where is the first-order partial derivative of the pressure at the surface of the rock sample with respect to space, representing the rate of change of pressure with radius in the radial direction, and P 2e is the pressure at any arbitrary point inside the rock sample, and r c is the spatial position corresponding to the pressure P 2e ;

[0050] S2.4. The space composed of the standard tank and the matrix cup is a closed system that follows the principle of mass conservation, and the average pressure inside the rock sample is the easiest to obtain;

[0051] Based on the fact that the space composed of the standard tank and the matrix cup is a closed system that follows the principle of mass conservation, an average pressure expression (6) inside the pores of the rock sample is constructed, and the average pressure expression (6) is as follows:

[0052] V’P0’ + V P P 20 = V’P + V P P 2e ,

[0053] where V' is the unoccupied volume after the rock sample is placed in the system, with a size of V1 + V2 - V, and V P is the pore volume of the rock sample, and P 20 is the initial pressure inside the core, generally the local atmospheric pressure;

[0054] Since the value of V P is much smaller than V' in the equation, and the atmospheric pressure P 20 is much smaller than the test pressure values P and P 2e , V P and P 20 are ignored, and the average pressure expression is rewritten as the optimized average pressure expression (7):

[0055] P 2e = V'(P0' - P) / V p ;

[0056] S2.5. During the porosity test of laboratory columnar samples, the average pressure inside the rock sample propagates from the surface of the rock sample inward, and the pressure at the center of the rock sample axis increases with time. During the production process of a vertical well gas reservoir, over time, the wellbore pressure gradually decreases, and the outer boundary of the pressure drop funnel gradually spreads outward, but the pressure at the outer boundary of the pressure drop funnel remains constant. By comparing the law of internal pressure propagation in the rock sample during porosity testing with the law of internal pressure propagation in the reservoir of a vertical well gas reservoir, the internal and external boundary conditions of the two processes are consistent, the geometric shapes are similar, and the former is similar to the reverse process of the latter. Therefore, the pressure boundary expansion law of a vertical well gas reservoir is used to evaluate the spatial variation of the average pressure inside the sample during the porosity test.

[0057] In shale reservoirs, bedding and microfractures are the main conduits for natural gas flow. A pressure disturbance propagation model for multi-scale volume fracturing of pore-fracture reservoirs is used to characterize the pressure propagation law in shale samples. It is assumed that the shale gas reservoir consists of two concentric regions - the matrix system and the fracture system. The wellbore radius is defined as r w , and the fracture network region forms a circular ring radial seepage zone with a radius difference of r m -r w centered on the geometric center of the wellbore; the matrix region forms a circular ring radial seepage zone with a radius difference of r e -r m centered on the wellbore center. The production time (t) of a vertical well gas reservoir is proportional to the cube of the radius of the pressure drop funnel (r t ), and the proportionality coefficient is ξ. The pressure disturbance propagation model (8) can be expressed as:

[0058] t = (r t / ξ) 3 ,

[0059] In the pressure disturbance propagation model (8), r t is the outer boundary of the pressure drop funnel of the vertical well gas reservoir. Different from the pressure propagation from the wellbore to the outer boundary in a vertical well gas reservoir, during porosity testing, the pressure propagates from the surface of the rock sample to the inside of the core, and the propagation distance changes from r t to R - r c ;

[0060] Based on the pressure disturbance propagation model (8) of multi-scale volume fracturing of pore-fracture reservoirs in the pressure boundary expansion law of a vertical well gas reservoir and the pressure propagation law of the rock sample, a pressure disturbance propagation model of the rock sample is obtained. The pressure disturbance propagation model (9) of the rock sample is:

[0061] t = (R - r c ) 3 / ξ 3 ,

[0062] where t is the time of pressure disturbance propagation, and R - r cis the distance difference for the pressure to propagate from the surface of the rock sample to the interior of the core, and ξ is the proportionality coefficient.

[0063] S2.6. In the diffusion flux expression (4), the changes in k(P) and P are small compared to the change in (V'+V P )P - V'P0'. Therefore, k(P) and P in the diffusion flux expression (4) are approximately treated as constants k(P0') and P0'.

[0064] Substitute the initial pressure expression (3), the pressure gradient expression (5), the average pressure expression (7), the rock sample pressure perturbation propagation model (9), and the pressure attenuation coefficient in the matrix cup into the diffusion flux expression (4) to obtain the rock sample surface pressure expression. The rock sample surface pressure expression (10) is:

[0065]

[0066] where ξ1 is the pressure attenuation coefficient in the matrix cup. represents the equilibrium pressure at which the gas reaches complete equilibrium in the matrix cup and the pores of the rock sample (i.e., P e ), represents the pressure drop in the matrix cup due to the gas diffusing into the pores of the rock sample.

[0067] In this embodiment, 1) accurately construct the initial pressure expression: By assuming the gas diffusion order and constructing the initial pressure expression for the pressure equilibrium between the standard tank and the matrix cup and the diffusion into the pores of the rock sample based on Boyle's law, it provides the key starting parameters for subsequent calculations. This expression comprehensively considers factors such as the volume of the standard tank, the volume of the matrix cup, the external volume of the rock sample, and the set pressure of gas injection, making the determination of the initial pressure more scientific and reasonable, and helping to improve the accuracy of porosity measurement.

[0068] 2) reasonably construct the diffusion flux expression: Based on the linear distribution characteristics of the pressure gradient on the surface of the rock sample and the law of mass conservation, construct the diffusion flux expression at the surface of the rock sample, which can be simplified into a one-dimensional differential equation of the pressure in the matrix cup over time. This helps to deeply understand the diffusion process of gas between the matrix cup and the pores of the rock sample, provides a theoretical basis for accurately simulating and predicting pressure changes, and thus enables a better grasp of the physical phenomena during the porosity test and improves the test accuracy.

[0069] 3) precisely construct the pressure gradient expression: Consider the pressure values on the surface of the rock sample and at a certain distance from the rock sample boundary inside to construct the pressure gradient expression, which clarifies the distribution and change relationship of the pressure in the rock sample. It is crucial for studying the pressure propagation law inside the rock sample, can provide a basis for accurately calculating parameters such as the average pressure in the pores of the rock sample subsequently, and further improve the accuracy of porosity measurement.

[0070] 4) Optimize the average pressure expression: Construct and optimize the average pressure expression within the pores of the rock sample based on the characteristics of the space composed of the standard tank and the matrix cup. Simplify the expression by reasonably neglecting small quantities, making the calculation of the average pressure more convenient and accurate, reducing unnecessary computational complexity, while ensuring the reliability of the calculation results, which helps improve the efficiency and accuracy of porosity determination.

[0071] 5) Effectively introduce the pressure perturbation propagation model: Compare the porosity test with the pressure propagation law in a vertical well gas reservoir, and introduce an appropriate pressure perturbation propagation model to obtain the pressure perturbation propagation model of the rock sample. This model can more accurately describe the pressure propagation law inside the rock sample, better conform to the actual situation, provide an important basis for accurately calculating the rock sample surface pressure expression, and is conducive to improving the accuracy of porosity determination.

[0072] Example 3:

[0073] Specifically, in S3, the second term in the rock sample surface pressure expression (10) (i.e., ) represents the pressure drop generated in the matrix cup due to the gas diffusing into the pores of the rock sample. In the pressure decay model of the matrix cup established in the above example, it is assumed that the gas in the standard tank first expands into the matrix cup, and then the gas in the matrix cup diffuses into the rock sample. In actual experiments, the process of the gas in the standard tank entering the matrix cup and the pores of the rock sample occurs simultaneously. Therefore, the pressure drop of the gas diffusing into the rock sample is less than the theoretical value. Introduce the coefficient η to modify the second term in the rock sample surface pressure expression (10);

[0074] Based on the gas diffusion time parameter, correct the rock sample surface pressure expression, and construct a pressure decay model in the matrix cup based on the corrected rock sample surface pressure expression, including:

[0075] Based on the fact that the actual pressure drop of the gas diffusing into the rock sample is less than the theoretical pressure drop, substitute the ratio η of the actual pressure drop in the matrix cup to its theoretical pressure drop into the rock sample surface pressure expression to obtain the corrected rock sample surface pressure expression. The corrected rock sample surface pressure expression (12) is:

[0076]

[0077] where η < 1;

[0078] Since the start time of the gas diffusion from the matrix cup into the pores of the rock sample cannot be determined, let the recorded decay time be t r , and the difference between the recorded time and the actual time is t0. Substitute t rSum with t0 to obtain the gas diffusion time parameter, substitute the gas diffusion time parameter into the corrected expression of the surface pressure of the rock sample, and obtain the pressure decay model in the matrix cup. The pressure decay model (16) in the matrix cup is as follows:

[0079]

[0080] where P e is the equilibrium pressure when the gas reaches complete equilibrium in the matrix cup and the pores of the rock sample, ξ2 is the difference between the initial pressure and the equilibrium pressure when the gas diffuses from the matrix cup to the pores of the rock sample,

[0081] In this embodiment, 1) Modify the surface pressure expression: Considering the difference between the gas diffusion pressure drop in the actual experiment and the theory, introduce a coefficient to modify the surface pressure expression of the rock sample, making the expression more in line with the actual situation, reducing the deviation between theory and practice, improving the simulation accuracy of the model for the experimental process, and thus helping to improve the accuracy of porosity measurement.

[0082] 2) Construct the pressure decay model in the matrix cup: Based on the corrected surface pressure expression, consider the gas diffusion time parameter to construct the pressure decay model in the matrix cup. This model can more accurately reflect the change law of the pressure in the matrix cup with time. When fitting with experimental data, key parameters such as the equilibrium pressure can be obtained more accurately and quickly, improving the accuracy and efficiency of porosity measurement.

[0083] Example 4:

[0084] The GIP porosity experimental system further includes a gas source bottle, a first valve, a second valve, a third valve and a pressure sensor;

[0085] One end of the first valve is connected to the gas source bottle, and the other end is connected to the standard tank, which is used to control the injection of gas into the standard tank; one end of the second valve is connected to the standard tank, and the other end is connected to the matrix cup, which is used to control the connection between the standard tank and the matrix cup; one end of the third valve is connected to the matrix cup, and the other end is connected to the outside, which is used to evacuate the gas in the GIP porosity experimental system; the pressure sensor is connected between the first valve and the second valve, which is used to detect the pressure change in the GIP porosity experimental system.

[0086] In this embodiment, it is clarified that in addition to the standard tank and the matrix cup, the GIP porosity experimental system further includes components such as a gas source bottle, valves, and pressure sensors, and the functions of each component are clearly defined. The gas source bottle provides the gas source, the valves control the gas flow direction and the on / off of the system, and the pressure sensors detect the pressure changes. Such a complete system composition ensures that the experimental system can operate stably and accurately, providing a reliable hardware foundation for porosity measurement and helping to improve the accuracy of experimental data and the reliability of measurement results.

[0087] Example 5:

[0088] In S3, during the porosity test by the GIP porosity experimental system, the volume of the standard tank, the volume of the matrix cup, and the pressure decay experimental data are obtained. The pressure decay experimental data is subjected to non-linear curve fitting through the pressure decay model in the matrix cup to obtain the equilibrium pressure when the gas reaches thermodynamic equilibrium in the matrix cup and the pores of the rock sample, including:

[0089] S3.1. Prepare the rock sample;

[0090] S3.2. Conduct an airtightness test on the GIP porosity experimental system;

[0091] S3.3. Use metal blocks with multiple different volumes to perform blank volume calibration operations under different states of the GIP porosity experimental system, and record the pressure test data corresponding to each metal block in each state. Based on the pressure test data, the volume of the standard tank and the volume of the matrix cup are calculated;

[0092] S3.4. Place the rock sample in the matrix cup, use the GIP porosity experimental system to perform porosity test on the rock sample, and use the pressure sensor to detect and record the pressure decay experimental data of the pressure changing with time;

[0093] S3.5. Import the pressure decay model in the matrix cup as a fitting function into the origin tool, and perform non-linear curve fitting on the pressure decay experimental data through the pressure decay model in the matrix cup in the origin tool to obtain the equilibrium pressure P e of the gas reaching thermodynamic equilibrium in the matrix cup and the pores of the rock sample, the pressure decay coefficient ξ1 in the matrix cup, the difference ξ2 between the initial pressure and the equilibrium pressure when the gas diffuses from the matrix cup to the pores of the rock sample, and the difference t0 between the recorded time and the actual time.

[0094] It should be understood that in S3.4, "using the pressure sensor to detect and record the pressure decay experimental data of the pressure changing with time", in the actual application process, according to the fitting results later, it is only necessary to record the experimental data corresponding to the law of the pressure changing with time within 10 min - 20 min.

[0095] In this embodiment, the complete process of obtaining the volume of the standard tank, the volume of the matrix cup, and the experimental data of pressure reduction through the experimental system, and fitting the equilibrium pressure by using the pressure decay model in the matrix cup is elaborated in detail. Only the experimental data within a relatively short time period needs to be recorded, and there is no need to obtain the complete pressure drop curve in the matrix cup, so that the equilibrium pressure when the matrix cup and the sample pores reach complete pressure equilibrium can be fitted. The test time is significantly lower than the longest 2-hour test time in the prior art, and great improvements have been made in terms of test accuracy and time efficiency.

[0096] Example 6:

[0097] In S3.2, the airtightness of the GIP porosity experimental system is detected, including:

[0098] Close the third valve, open the first valve and the second valve in sequence. The gas in the gas source bottle expands and enters the GIP porosity experimental system. Observe the value of the pressure sensor. When the pressure of the pressure sensor reaches the set pressure, close the first valve;

[0099] Wait for the GIP porosity experimental system to be constant for the set time. If the reading of the system pressure sensor does not change, the airtightness test is passed. If the pressure of the GIP porosity experimental system decreases, reinforce all the joints of the GIP porosity experimental system and conduct the airtightness test again until the GIP porosity experimental system passes the airtightness test.

[0100] In this embodiment, through specific airtightness detection steps, including operations such as closing valves, inflating, observing pressure changes, and judging after a constant time, the airtightness of the system can be effectively detected, potential gas leakage problems can be timely discovered and solved, the tightness of the experimental system during the test is ensured, experimental errors caused by airtightness problems are avoided, the accuracy of experimental data is improved, and thus the accuracy of porosity measurement is helped to be improved.

[0101] Example 7:

[0102] In S3.3, blank volume calibration operations are carried out on the GIP porosity experimental system using metal blocks with different volumes respectively, and the pressure test data corresponding to each metal block in each state are recorded, including:

[0103] Prepare metal blocks with different volumes, number the metal blocks, and put the metal block numbered 1 into the matrix cup with a volume of V i , close the first valve, the second valve and the third valve. First, open the first valve to make the gas in the gas source bottle expand and enter the standard tank. When the pressure sensor reaches 1.5 MPa, close the first valve and wait for the reading of the pressure sensor to be stable. Record the pressure test data as P 11, this state is set as the first state; open the second valve, wait again for the reading of the pressure sensor to stabilize, and record the reading as P 21 , this state is set as the second state; open the second valve and the third valve, evacuate the gas in the GIP porosity experiment system, put the metal block numbered 2 into the matrix cup, repeat the above operations, and record the corresponding pressure test data of the GIP porosity experiment system for all metal blocks in the first state and the second state as P 1i and P 2i , where i is the number corresponding to the metal block.

[0104] In this embodiment, the operation process of blank volume calibration using multiple metal blocks with different volumes is described in detail, including steps such as numbering, placing metal blocks, controlling valves, and recording pressure data in different states. This precise operation method can obtain multiple sets of accurate pressure test data, providing a rich and reliable data basis for calculating the standard tank volume and matrix cup volume based on Boyle's law later, helping to improve the accuracy of volume calculation, and thus ensuring the accuracy of porosity determination.

[0105] Example 8:

[0106] Specifically, in S3.3, calculating the standard tank volume and matrix cup volume based on the pressure test data includes:

[0107] Based on Boyle's law, substituting the metal block volume V i , the standard tank volume V1, the matrix cup volume V2, and the pressure test data P 1i and P 2i into the relational expression:

[0108]

[0109] Calculate to obtain the standard tank volume V1 and the matrix cup volume V2.

[0110] In this embodiment, based on Boyle's law, the standard tank volume and matrix cup volume are calculated using the metal block volume and pressure test data. This calculation method has a scientific basis and can accurately determine the volume parameters of key components in the experimental system. These accurate volume parameters play an important role in the subsequent porosity calculation process, directly affecting the accuracy of the porosity calculation result, and ensuring the accuracy and reliability of the entire porosity determination process.

[0111] Example 9:

[0112] In S5, substituting the standard tank volume, matrix cup volume, and equilibrium pressure into the skeleton volume formula to calculate the skeleton volume of the rock sample, and substituting the skeleton volume of the rock sample and the external volume of the rock sample into the porosity formula to calculate the porosity of the rock sample, including:

[0113] To ensure that the blank volume in the matrix cup is fully filled, metal pads are added to the matrix cup. The matrix volume, the standard tank volume, the equilibrium pressure, and the metal pad volume are substituted into the skeleton volume formula to calculate the skeleton volume of the rock sample. The skeleton volume formula is as follows:

[0114] V g = V2 - V i - V1(P0 / P e - 1),

[0115] where V g is the skeleton volume of the rock sample;

[0116] The skeleton volume of the rock sample and the external volume of the rock sample are substituted into the porosity formula to calculate the porosity of the rock sample. The porosity formula is as follows:

[0117] φ = 1 - V g / V.

[0118] In this embodiment, considering the addition of metal pads to the matrix cup to ensure the filling of the blank volume, relevant parameters are substituted into the modified skeleton volume formula to calculate the skeleton volume of the rock sample, and then further substituted into the porosity formula to calculate the porosity. This calculation method fully considers the actual experimental situation, making the calculation of the skeleton volume and porosity more accurate, more in line with the actual situation, improving the accuracy of the porosity measurement result, and through comparison with the true porosity, the relative error is small, verifying the reliability of this calculation method.

[0119] The application of the improved GIP porosity test method is introduced below through specific parameter examples.

[0120] 1. Prepare the rock sample. Use a vernier caliper to measure the diameter and length of the sample, and then place the sample to be measured in an incubator and dry it for 24 hours. After completion, take it out of the incubator and put it into a dry container to wait for the sample to cool naturally. In the equilibrium pressure prediction model, gas only diffuses into the pores of the rock sample from the side of the rock sample. Therefore, epoxy resin is used to seal the two end faces of the rock sample to prevent gas from diffusing into the pores of the rock sample from both ends during the porosity measurement. The volume of the epoxy resin used for sealing is much smaller than the skeleton volume of the sample. Therefore, the influence of the applied epoxy resin on the test result can be ignored. To more reliably verify the reliability of this method, two rock samples (i.e., Sample 1 and Sample 2) are used for the experiment. The rock samples used in this specific parameter example are shown in Table 1. Table 1 is the original data of the sample specifications.

[0121] Table 1

[0122] Rock sample number Length / mm Diameter / mm Sample 1 52.05 25.13 Sample 2 60.66 25.11

[0123] 2. Instrument Calibration. The flow chart of the instruments used in the experiment is shown in Figure 1 , and the airtightness detection and blank volume calibration of the instruments are required for each test. 1) Airtightness detection. Close valve 3 (i.e., the third valve), and successively open valve 1 (i.e., the first valve) and 2 (i.e., the second valve). The gas in the gas cylinder expands and enters the system. Observe the value of the pressure sensor. When the pressure of the pressure sensor reaches 3 MPa, close valve 1. Wait for the system to be stable for 1 hour. If the reading of the system pressure sensor does not change, the airtightness detection is passed; if the system pressure decreases, reinforce all the connections of the instrument and conduct the airtightness detection again until the instrument passes the airtightness detection. 2) As Figure 4 shown, blank volume calibration. Place the metal cylinder block into the matrix cup, with a volume of V i . Close valves 1, 2, and 3. First, open valve 1 to make the gas in the gas source expand and enter the standard tank. When the pressure sensor reaches 1.5 MPa, close valve 1 and wait for the reading of the pressure sensor to be stable. Record the reading as P 11 . This state is state 1 (i.e., the first state). Then open valve 2 and wait for the reading of the pressure sensor to be stable again. Record the reading as P 21 . This state is state 2 (i.e., the second state). Open valves 2 and 3 to evacuate the gas in the system. Replace the volume of the metal block in the matrix cup and repeat the above operations. A total of 4 groups of metal blocks with different volumes are measured for the system pressure in two states, and the readings are respectively recorded as P 1i , P 2i , where i is 2, 3, 4 respectively. Table 2 shows the pressure values of metal blocks with different volumes in state 1 and state 2. The pressures of the two states measured for different metal blocks are shown in Table 2. According to Boyle's law, the volume of the metal block, the volume of the standard tank, the volume of the matrix cup, and the pressure value satisfy the relational expression (17):

[0124]

[0125] By fitting according to the data in Table 2 and the relational expression (17), the volume of the standard tank V1 = 23.517 cm 3 , and the volume of the matrix cup V2 = 38.578 cm 3 .

[0126] Table 2

[0127] <![CDATA[Volume V of the metal block i / cm 3 > <![CDATA[P 1i / MPa]]> <![CDATA[P 2i / MPa]]> <![CDATA[P 1i / P 2i > 12.282 1.489 0.703 2.118 18.427 1.490 0.803 1.856 23.041 1.483 0.890 1.666 24.603 1.482 0.932 1.590

[0128] 3. Measurement of the original data of the pressure decreasing law in the matrix cup. Take the sample to be tested out of the drying container and place it into the matrix cup. When placing the sample, if the external volume of the rock sample is not enough to completely fill the blank volume in the matrix cup, add a metal spacer block with a known external volume V i, and close valves 1, 2, and 3. First, open valve 1, and the gas from the air source expands into the standard tank. When the reading of the pressure sensor reaches 200 psi, close valve 1 and wait for the system pressure to stabilize. Record the reading of the pressure sensor as P0. Next, open valve 2 and simultaneously open the pressure sensor recording software to record the change of the sensor pressure over time. In this embodiment, in order to verify whether the equilibrium pressure predicted according to the established equilibrium pressure prediction model is consistent with the actual value, the pressure decay curve is completely recorded. The criterion for determining pressure stability is that the pressure does not change within 10 minutes. In the actual application process, according to the fitting results described later, only the change law of the pressure over time within 10 min - 20 min needs to be recorded. Finally, open valves 2 and 3 to evacuate the gas in the instrument system. Table 3 shows the original data of the pressure decrease in the matrix cup during the porosity test of two samples. The pressure in the matrix cup of Sample 1 reaches equilibrium when the time exceeds 86.55 min, and the pressure in the matrix cup of Sample 2 reaches equilibrium when the saturation time exceeds 39.90 min.

[0129] Table 3

[0130]

[0131]

[0132] 4. Obtaining the equilibrium pressure. Use the origin software tool to perform non-linear curve fitting on the obtained experimental data. The function used for fitting is Equation 16 (i.e., the pressure decay model in the matrix cup). When fitting, it is necessary to define the assignment method for P e , ξ2, ξ1, and t0 in Formula 16. To ensure that the fitting function satisfies convergence after assignment, the assignment method is as follows: The equilibrium pressure P e adopts the pressure value at the end of the experiment. For example, the assignments for Sample 1 and Sample 2 during fitting are 150.4 psi and 141.6 psi respectively; ξ2 refers to the difference between the initial pressure and the equilibrium pressure when the gas diffuses from the matrix cup to the sample, and it is assigned using the pressure difference at the beginning and end of the experiment. For example, the assignments for Sample 1 and Sample 2 during fitting are 2.4 psi (152.8 - 150.4) and 3.3 psi (144.9 - 141.6) respectively. The assignment values of ξ1 and t0 have little influence on the convergence during function fitting, and they are assigned 1 and 0 respectively. Figure 5 and Figure 6 are the fitting effect diagrams of the decreasing pressure experimental data and the model. Figure 7Table 4 presents the equilibrium pressure values predicted for two samples based on experimental data with different decreasing times. The difference in the equilibrium pressure simulated using the experimental data after 10 minutes is very small. Therefore, the method provided by the present invention only needs to measure the pressure decreasing data within 10 minutes during actual practice, and then the pressure at equilibrium during the porosity test can be predicted according to the model, greatly improving the measurement efficiency. Table 4 shows the equilibrium pressure values predicted based on experimental data with different decreasing times.

[0133] Table 4

[0134]

[0135] 5. Calculation of sample porosity. During the porosity test, in order to ensure that the blank volume in the matrix cup can be fully filled, metal pads are added. At this time, the formula for calculating the skeleton volume in formula (1) is changed to formula (18):

[0136] V g = V2 - V i - V1(P0 / P e - 1) (18)

[0137] The equilibrium pressure values obtained from the equilibrium pressure prediction model established by the present invention and the true equilibrium pressure are respectively used to calculate the porosity values of the two samples. The original data table used for the calculation is Table 5. The porosity is calculated using formula (18) and formula (2). The calculation results are shown in Table 6. As can be seen from Table 5, the relative error between the porosity values obtained by the method established in this patent and the true porosity is relatively small, and for both samples, it is not more than 0.2%. It can be seen that the porosity test method established by the present invention has good reliability. At the same time, as described above, the time required for the two samples to reach the equilibrium pressure is 86.55 minutes and 39.90 minutes respectively, while the time required by the present invention is 10 minutes, significantly improving the test efficiency. Table 5 shows the original data required for calculating the porosity of the two samples. Table 6 shows the porosity results calculated from the predicted equilibrium pressure value of the new model and the true equilibrium pressure.

[0138] Table 5

[0139]

[0140] Table 6

[0141]

[0142] The advantages of the present invention are:

[0143] 1) Efficiently obtain the equilibrium pressure: By building a GIP porosity experimental system including a standard tank and a matrix cup, and through a series of operations such as combining the assumed gas diffusion sequence, constructing relevant expressions and models, etc., finally, the pressure decay model in the matrix cup is used to fit the pressure decline experimental data. Without obtaining the complete pressure drop curve in the matrix cup, the equilibrium pressure when the gas reaches thermodynamic equilibrium in the matrix cup and the pore of the rock sample can be obtained, greatly shortening the time to obtain the equilibrium pressure and improving the experimental efficiency.

[0144] 2) Improve the accuracy and efficiency of porosity measurement: Substituting the calculated equilibrium pressure into the skeleton volume formula and the porosity formula can calculate the skeleton volume and porosity of the rock sample. The accuracy error of this method for measuring the shale porosity is controlled within 5%, and the measurement time does not exceed 20 minutes, which is significantly shorter than the longest measurement time of 2 hours in the existing technology. While ensuring the measurement accuracy, it greatly improves the measurement efficiency, providing a faster and more accurate determination method for shale porosity measurement, which is of great significance for related research and engineering applications such as shale gas reservoir evaluation.

[0145] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the term "including", "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device.

[0146] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An improved GIP porosity determination method based on pressure decay, characterized in that: The steps include: S1. Building a GIP porosity experimental system, wherein the GIP porosity experimental system comprises a standard tank and a matrix cup, wherein the standard tank is used to store gas at a predetermined injection pressure, and the matrix cup is used to place a rock sample; S2. Assuming the diffusion order of gas in the standard tank and the matrix cup and based on the pressure distribution characteristics of the rock sample, a diffusion flux expression at the surface of the rock sample is constructed, and the rock sample pressure disturbance propagation model is substituted into the diffusion flux expression to obtain the surface pressure expression of the rock sample; S3, correcting the rock sample surface pressure expression based on the gas diffusion time parameter, and constructing a pressure decay model in the matrix cup based on the corrected rock sample surface pressure expression; S4. Obtaining the standard tank volume, matrix cup volume and pressure decline experimental data during the porosity test by using the GIP porosity experimental system, performing nonlinear curve fitting on the pressure decline experimental data by using the pressure decay model in the matrix cup, and obtaining the equilibrium pressure when the gas reaches thermodynamic equilibrium in the matrix cup and the pores of the rock sample; S5. Substitute the standard tank volume, matrix cup volume and equilibrium pressure into the skeleton volume formula to calculate the skeleton volume of the rock sample, and substitute the skeleton volume of the rock sample and the apparent volume of the rock sample into the porosity formula to calculate the porosity of the rock sample.

2. The improved GIP porosity determination method according to claim 1, characterized in that: In S2, assuming the diffusion order of gas in the standard tank and the matrix cup and based on the pressure distribution characteristics of the rock sample, a diffusion flux expression at the surface of the rock sample is constructed, and the rock sample pressure disturbance propagation model is substituted into the diffusion flux expression to obtain the rock sample surface pressure expression, including: S2.

1. Assuming that the gas first reaches pressure equilibrium between the standard tank and the matrix cup and then diffuses into the pores of the rock sample, the initial pressure expression for reaching pressure equilibrium between the standard tank and the matrix cup and diffusing into the pores of the rock sample is constructed according to Boyle's law. The initial pressure expression is: Among them, P0' is the initial pressure at which the pressure between the standard tank and the matrix cup reaches equilibrium and diffuses into the pores of the rock sample, P0 is the set pressure for injecting gas into the standard tank, V1 is the volume of the standard tank, V2 is the volume of the matrix cup, and V is the apparent volume of the rock sample; S2.

2. Based on the fact that the pressure gradient on the surface of the rock sample during the porosity test satisfies the linear distribution characteristics, and based on the law of mass conservation, the decreasing rate of the gas in the space composed of the matrix cup and the standard tank is equal to the diffusion flux on the outer surface of the rock sample, the diffusion flux expression on the surface of the rock sample is constructed, and the diffusion flux expression is: in, is the first-order partial differential of pressure on the surface of the rock sample with respect to time, indicating the rate of change of pressure with time, k0 is the absolute permeability of the core, b is the slip coefficient, R is the radius of the rock sample, L is the length of the rock sample, and μ is the gas viscosity; S2.

3. Based on the distance between the surface and the interior of the rock sample and the boundary of the rock sample, a pressure gradient expression at the surface of the rock sample is constructed. The pressure gradient expression is: in, is the first-order partial differential of the pressure on the surface of the rock sample with respect to space, indicating the rate of change of pressure with radius in the radial direction, P 2e is the pressure at any point inside the rock sample, r c is the pressure P 2e The corresponding spatial position; S2.

4. Based on the space composed of the standard tank and the matrix cup being a closed system following the principle of mass conservation, an expression for the average pressure in the pores of the rock sample is constructed. The expression for the average pressure is: V′P0′+V P P 20 =V′P+V P P 2e , Where V' is the unoccupied volume after the rock sample is placed in the system, which is V1+V2-V, V P is the pore volume of the rock sample, P 20 is the initial pressure inside the core, P0' is the initial pressure at which the pressure between the standard tank and the matrix cup reaches pressure equilibrium and diffuses into the pores of the rock sample; Since V P The value is much smaller than V' in the equation, and the atmospheric pressure P 20 Much smaller than the test pressure values ​​P and P 2e , V P and P 20 Ignore, and rewrite the average pressure expression into the optimized average pressure expression: P 2e =V'(P0'-P) / V p ; S2.

5. Based on the pressure disturbance propagation model of multi-scale volume fracturing of pore-fracture reservoirs in the vertical well gas reservoir pressure boundary expansion law and the pressure propagation law of rock samples, a rock sample pressure disturbance propagation model is obtained. The rock sample pressure disturbance propagation model is: t=(Rr c ) 3 / x 3 , Where t is the propagation time of the pressure disturbance, Rr c is the distance difference of pressure propagation from the surface of the rock sample to the inside of the core, ξ is the proportionality coefficient; S2.

6. Substitute the initial pressure expression, pressure gradient expression, average pressure expression, rock sample pressure disturbance propagation model, and pressure attenuation coefficient in the matrix cup into the diffusion flux expression to obtain the rock sample surface pressure expression, which is: Where ξ1 is the pressure attenuation coefficient in the matrix cup, represents the pressure drop in the matrix cup due to the diffusion of gas into the pores of the rock sample, It indicates the equilibrium pressure at which the gas reaches complete equilibrium in the matrix cup and the pores of the rock sample.

3. The improved GIP porosity determination method according to claim 2, characterized in that: In S3, the rock sample surface pressure expression is corrected based on the gas diffusion time parameter, and a pressure decay model in the matrix cup is constructed based on the corrected rock sample surface pressure expression, including: Based on the fact that the actual pressure drop of gas diffusion into the rock sample is less than the theoretical pressure drop, the ratio η of the actual pressure drop in the matrix cup to its theoretical pressure drop is substituted into the rock sample surface pressure expression to obtain a corrected rock sample surface pressure expression, which is: Among them, η<1; Since it is impossible to determine the start time of the diffusion of gas in the matrix cup into the pores of the rock sample, the recorded decay time is assumed to be t r , the difference between the recorded time and the actual time is t0, and t r The gas diffusion time parameter is obtained by summing it with t0, and the gas diffusion time parameter is substituted into the modified rock sample surface pressure expression to obtain the pressure decay model in the matrix cup. The pressure decay model in the matrix cup is: Among them, P e is the equilibrium pressure when the gas reaches complete equilibrium in the matrix cup and the pores of the rock sample, ξ2 is the difference between the initial pressure and the equilibrium pressure when the gas diffuses from the matrix cup to the pores of the rock sample.

4. The improved GIP porosity determination method according to claim 3, characterized in that: The GIP porosity experimental system also includes a gas source bottle, a first valve, a second valve, a third valve and a pressure sensor; One end of the first valve is connected to the gas source bottle, and the other end is connected to the standard tank, which is used to control the injection of gas into the standard tank; one end of the second valve is connected to the standard tank, and the other end is connected to the matrix cup, which is used to control the connection between the standard tank and the matrix cup; one end of the third valve is connected to the matrix cup, and the other end is connected to the outside, which is used to evacuate the gas in the GIP porosity experimental system; the pressure sensor is connected between the first valve and the second valve, and is used to detect the pressure change in the GIP porosity experimental system.

5. The improved GIP porosity determination method according to claim 4, characterized in that: In S3, the standard tank volume, matrix cup volume and pressure decline experimental data are obtained during the porosity test by using the GIP porosity experimental system, and the pressure decline experimental data are subjected to nonlinear curve fitting by using the pressure decay model in the matrix cup to obtain the equilibrium pressure when the gas reaches thermodynamic equilibrium in the matrix cup and the pores of the rock sample, including: S3.

1. Prepare rock samples; S3.

2. Conduct air tightness test on the GIP porosity experimental system; S3.3, using a plurality of metal blocks of different volumes to perform blank volume calibration operations in different states of the GIP porosity experimental system, and recording the pressure test data corresponding to each metal block in each state, and calculating the standard tank volume and the matrix cup volume based on the pressure test data; S3.4, placing the rock sample in a matrix cup, using the GIP porosity test system to test the rock sample for porosity, and using a pressure sensor to detect and record the pressure decrease experimental data of the pressure change over time; S3.5, the pressure decay model in the matrix cup is imported into the origin tool as a fitting function, and the pressure decrease experimental data is subjected to nonlinear curve fitting by the pressure decay model in the matrix cup in the origin tool to obtain the equilibrium pressure P when the gas reaches thermodynamic equilibrium in the matrix cup and the pores of the rock sample. e , the pressure attenuation coefficient in the matrix cup ξ1, the difference between the initial pressure and the equilibrium pressure when the gas diffuses from the matrix cup to the pores of the rock sample ξ2, and the difference between the recorded time and the actual time t0.

6. The improved GIP porosity determination method according to claim 5, characterized in that: In S3.2, the air tightness test of the GIP porosity experimental system is performed, including: Close the third valve, open the first valve and the second valve in sequence, and the gas in the gas source bottle expands into the GIP porosity experimental system. Observe the value of the pressure sensor, and close the first valve when the pressure of the pressure sensor reaches the set pressure; Wait until the GIP porosity experiment system is constant for the set time. If the reading of the system pressure sensor does not change, the air tightness test is passed. If the pressure of the GIP porosity experiment system decreases, all connections of the GIP porosity experiment system are reinforced and the air tightness test is performed again until the GIP porosity experiment system passes the air tightness test.

7. The improved GIP porosity determination method according to claim 5, characterized in that: In S3.3, a plurality of metal blocks of different volumes are used to perform blank volume calibration operations in different states of the GIP porosity experimental system, and the pressure test data corresponding to each metal block in each state is recorded, including: Prepare a plurality of metal blocks of different volumes and number the metal blocks. Put the metal block numbered 1 into the matrix cup with a volume of V. i , close the first valve, the second valve and the third valve, open the first valve first, let the gas in the gas source bottle expand into the standard tank, when the pressure sensor reaches 1.5MPa, close the first valve, wait for the pressure sensor reading to stabilize, and record the pressure test data as P 11 , this state is set as the first state; open the second valve, wait for the pressure sensor reading to stabilize again, and record the reading as P 21 , this state is set as the second state; open the second valve and the third valve to exhaust the gas in the GIP porosity experimental system, put the metal block numbered 2 into the matrix cup, repeat the above operation, and record the pressure test data of the GIP porosity experimental system corresponding to all metal blocks in the first state and the second state as P 1i and P 2i , where i is the number corresponding to the metal block.

8. The improved GIP porosity determination method according to claim 7, characterized in that: In S3.3, the standard tank volume and the matrix cup volume are calculated based on the pressure test data, including: Based on Boyle's law, the volume of the metal block V i , standard tank volume V1, matrix cup volume V2 and pressure test data P 1i and P 2i Substituting into the relation: The standard tank volume V1 and matrix cup volume V2 are calculated.

9. The improved GIP porosity determination method according to claim 5, characterized in that: In S5, the standard tank volume, the matrix cup volume and the equilibrium pressure are substituted into the skeleton volume formula to calculate the skeleton volume of the rock sample, and the skeleton volume and the apparent volume of the rock sample are substituted into the porosity formula to calculate the porosity of the rock sample, including: In order to ensure that the blank volume in the matrix cup is fully filled, a metal block is added to the matrix cup, and the standard tank volume, matrix cup volume, equilibrium pressure and metal block volume are substituted into the skeleton volume formula to calculate the skeleton volume of the rock sample. The skeleton volume formula is: V g =V2-V i -V1(P0 / P e -1), Among them, V s is the skeleton volume of the rock sample, P e V is the equilibrium pressure when the gas reaches complete equilibrium in the matrix cup and the pores of the rock sample, i is the volume of the metal block; The porosity of the rock sample is calculated by substituting the skeleton volume and the apparent volume of the rock sample into the porosity formula. The porosity formula is: φ=1-V g / V。

Images