Overburdened gas porosity measurement apparatus and method

Abstract

The present application belongs to the field of oil and gas exploration, and particularly relates to a device and method for determining gas-measured porosity under overburden pressure. The device for determining gas-measured porosity under overburden pressure comprises: a core holder for accommodating a rock sample, a pressure boosting device connected to the core holder to apply overburden pressure to the rock sample, a space-adjustable gas chamber with an internal space volume capable of being adjusted, the space-adjustable gas chamber being connected to the core holder, and a gas measurement system connected to the core holder and the space-adjustable gas chamber to pass in gas with a preset pressure, thereby measuring the porosity of the rock sample. In the present application, during the calibration of the pore volume, the continuous change of the internal space of the space-adjustable gas chamber is used to obtain a pore volume-pressure standard curve with multiple measuring points through a single gas expansion process, thereby avoiding the purpose of calibrating the pore volume by repeatedly loading and unloading the sample in the core holder, and greatly simplifying the steps of calibrating the pore volume.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas exploration, and particularly relates to a device and method for measuring gas porosity under overburden pressure. Background Technology

[0002] Under burial conditions, rocks are subjected to overlying strata pressure, and the compression of the rock skeleton leads to a decrease in porosity. To accurately estimate oil and gas reserves under burial conditions, it is necessary to conduct overlying porosity tests on the rocks under experimental conditions to restore their reservoir capacity under the original strata conditions.

[0003] Chinese patent CN102353625A discloses a method for measuring overburden porosity using water, primarily relying on pore water pressure under different overburden pressures to calculate porosity changes. Compared to water, gas-based porosity measurement utilizes low-pressure gas diffusion, offering a faster advantage. GB / T 29172 "Core Analysis Methods" and SY / T 6385 2016 "Methods for Determining Porosity and Permeability of Rocks under Overburden Pressure" specify the procedure for gas-based porosity measurement under overburden pressure. The main principle is that after the rock core is loaded into a core holder, a pressurization device is used to achieve the specified overburden pressure conditions. Then, using the gas expansion method and Boyle's law, the pore volume of the rock under different overburden pressures is obtained based on the pressure changes before and after expansion, thus yielding the corresponding overburden porosity. Although the overburden porosimeter is currently a relatively mature piece of equipment in laboratories, a cumbersome issue remains in its use: calibrating a set of pore volumes in the core holder to obtain a standard curve of pore volume versus pressure values, which is crucial for obtaining the pore volume of unknown samples.

[0004] Overburden porosity can test cylindrical or cubic samples of different diameters. Existing overburden porosity meters require the preparation of a set of stainless steel cylindrical or cubic standard samples with a fixed volume. Generally, at least 5 standard sample points are needed, and each standard sample needs to be measured in a core holder. This requires cumbersome steps such as sample loading, overburden loading, and testing. The sample loading process is even more troublesome for standard samples or cubic samples with larger diameters.

[0005] To optimize the overburden porosimeter, consider calibrating the pore volume in components outside the core holder. This avoids the cumbersome process of repeatedly loading and unloading samples in the core holder for pore volume calibration. This is especially true for overburden porosimeters for full-diameter or cubic samples; calibrating the pore volume outside the core holder can significantly simplify the calibration process. Summary of the Invention

[0006] This invention provides a device and method for measuring gas porosity under pressure, which is used to solve at least one of the above-mentioned technical problems.

[0007] A first aspect of the present invention provides an apparatus for measuring gas porosity under pressure, comprising:

[0008] Core holders are used to hold rock samples.

[0009] A pressurizing device, which is connected to the core holder, to apply overburden pressure to the rock sample;

[0010] An adjustable air chamber, the volume of which can be adjusted, is connected to the core holder; and

[0011] A gas measurement system is connected to the core holder and the adjustable gas chamber to introduce gas at a preset pressure, thereby measuring the porosity of the rock sample.

[0012] In one embodiment, the adjustable air chamber includes a cylinder and an adjustment assembly.

[0013] The first end of the adjustment component is disposed inside the cylinder, and the first end of the adjustment component is slidably and sealed to the inner wall of the cylinder. The first end of the adjustment component can reciprocate along the axial direction of the cylinder to adjust the volume of the internal space of the adjustable air chamber.

[0014] In one embodiment, the adjustment assembly includes a micrometer head, the micrometer head comprising a micrometer screw capable of extending and retracting along the axial direction of the cylinder.

[0015] The volume of the internal space of the adjustable air chamber is adjusted by regulating the axial extension and retraction of the micrometer screw.

[0016] In one embodiment, the adjustment assembly further includes an extension rod.

[0017] The extension rod is disposed at the end of the micrometer screw of the micrometer head, and the extension rod is slidably and sealed to the inside of the cylinder. The diameter of the extension rod is smaller than the diameter of the micrometer screw.

[0018] In one embodiment, a sealing ring is provided at the first end of the adjusting component, and the first end of the adjusting component is slidably and sealed to the inner wall of the cylinder through the sealing ring.

[0019] In one embodiment, the adjustable air chamber is connected to the core holder via a first pipeline and a second pipeline, respectively.

[0020] One end of the first pipeline is connected to the adjustable air chamber, and the other end of the first pipeline is connected to the first side of the core holder.

[0021] One end of the second pipeline is connected to the adjustable air chamber, and the other end of the second pipeline is connected to the second side of the core holder opposite to its first side.

[0022] The gas measurement system is connected to the first pipeline and / or the second pipeline.

[0023] In one embodiment, the gas measurement system includes a gas source, a gas injection line, and a pressure sensor.

[0024] The outlet end of the gas source is equipped with a pressure reducing valve and is connected to the core holder and the adjustable gas chamber through the gas injection pipeline. A first valve and a second valve are sequentially arranged on the gas injection pipeline, with the first valve being closer to the pressure reducing valve than the second valve.

[0025] The pressure sensor is connected to the pipeline between the first valve and the second valve.

[0026] In one embodiment, the gas detection system further includes a venting line.

[0027] The venting pipeline is connected between the first valve and the second valve, and a micro-leakage valve and a shut-off valve are sequentially installed on the venting pipeline along the flow direction of the gas inside it.

[0028] A second aspect of the present invention provides a method for measuring porosity using the above-described gas porosity measuring device under pressure, comprising a pore volume-pressure calibration step:

[0029] Step 11: Fix the standard sample in the core holder and apply the first fixing pressure to the standard sample through the pressurization device;

[0030] Step 12: Diffusion of the gas at the first preset pressure in the gas measurement system into the core holder and the adjustable gas chamber;

[0031] Step 13: Gradually increase the volume of the internal space of the adjustable air chamber from zero, and record the gas pressure value after each increase in the volume of the internal space of the adjustable air chamber, until the preset number of times is reached, thereby obtaining the pore volume-pressure standard curve.

[0032] In one embodiment, the method further includes a step of measuring the porosity of the sample to be tested:

[0033] Step 21: Adjust the volume of the internal space of the adjustable air chamber to zero;

[0034] Step 22: Fix the sample to be tested in the core holder and apply a second fixing pressure to the sample to be tested through the pressurization device;

[0035] Step 23: Diffusion of the gas at the second preset pressure in the gas measurement system into the core holder and the adjustable gas chamber;

[0036] Step 24: After the gas diffuses into the pores of the sample to be tested to equilibrium, record the equilibrium pressure, and obtain the pore volume of the sample to be tested based on the equilibrium pressure and the pore volume-pressure standard curve obtained in step 13.

[0037] The second preset pressure is equal to the first preset pressure.

[0038] Compared with the prior art, the advantages of the present invention are as follows:

[0039] This invention designs a spatially adjustable gas chamber connected in parallel with a core holder. The internal space of the chamber can be adjusted. During pore volume calibration, the continuous change of the internal space allows for the generation of a pore volume-pressure standard curve with multiple measuring points through a single gas expansion process. This avoids the need for multiple loading and unloading of samples in the core holder to achieve pore volume calibration, and greatly simplifies the pore volume calibration process. Attached Figure Description

[0040] The invention will now be described in more detail with reference to embodiments and the accompanying drawings.

[0041] Figure 1 This is a schematic diagram of the structure of a gas porosity measuring device under surcharge in one embodiment of the present invention;

[0042] Figure 2 This is a schematic diagram of the structure of the space-adjustable air chamber in one embodiment of the present invention;

[0043] Figure 3 This is a schematic diagram of the structure of the space-adjustable air chamber in another embodiment of the present invention;

[0044] Figure 4 This is a flowchart of a method for measuring gas porosity under surcharge in one embodiment of the present invention.

[0045] Figure label:

[0046] 1. Core holder; 2. Pressurization device; 3. Adjustable gas chamber; 4. Gas measurement system;

[0047] 31. Cylinder body; 32. Micrometer head; 33. Extension rod; 34. Sealing ring;

[0048] 35. Fixed bracket; 321. Micrometer screw;

[0049] 41. Gas source; 42. Pressure sensor; 43. Pressure reducing valve; 44. Gas injection line;

[0050] 45. First valve; 46. Second valve; 47. Vent line; 48. Leakage valve;

[0051] 49. Stop valve. Detailed Implementation

[0052] The invention will now be further described with reference to the accompanying drawings.

[0053] like Figure 1-3 As shown, according to a first aspect of the present invention, the gas-based porosity measuring device under overburden pressure includes: a core holder 1, a pressurizing device 2, a space-adjustable gas chamber 3, and a gas measuring system 4. The core holder 1 is used to hold a rock sample; the pressurizing device 2 is connected to the core holder 1 to apply overburden pressure to the rock sample; the volume of the internal space of the space-adjustable gas chamber 3 is adjustable, and the space-adjustable gas chamber 3 is connected to the core holder 1; the gas measuring system 4 is connected to the core holder 1 and the space-adjustable gas chamber 3 to introduce gas at a preset pressure into both, thereby measuring the porosity of the rock sample.

[0054] In this invention, a set of adjustable gas chamber 3 connected in parallel with core holder 1 is designed. Its internal space can be adjusted. During pore volume calibration, the continuous change of its internal space is used to obtain a pore volume-pressure standard curve with multiple measuring points through a single gas expansion process. This avoids the need for multiple loading and unloading of samples in core holder 1 to achieve the purpose of pore volume calibration, and greatly simplifies the steps of pore volume calibration.

[0055] Example 1

[0056] In this embodiment, the adjustable air chamber 3 includes a cylinder 31 and an adjustment component. The first end of the adjustment component is disposed inside the cylinder 31 and is slidably and sealingly connected to the inner wall of the cylinder 31. The first end of the adjustment component can reciprocate along the axial direction of the cylinder 31 to adjust the volume of the internal space of the adjustable air chamber 3.

[0057] Specifically, a sealing ring 34 is provided at the first end of the adjusting component, and the first end of the adjusting component is slidably and sealed to the inner wall of the cylinder 31 through the sealing ring 34.

[0058] The adjustment assembly includes a micrometer head 32, which includes a micrometer screw 321 capable of extending and retracting along the axial direction of the cylinder 31. The volume of the internal space of the adjustable air chamber 3 is adjusted by regulating the axial extension and retraction of the micrometer screw 321 of the micrometer head.

[0059] The micrometer head 32 is a measuring instrument that uses the principle of a screw pair to read the axial movement of its micrometer screw 321 and has a mounting position. Generally, the micrometer head 32 consists of a fixed part mounting sleeve, a bushing, and a movable part micrometer screw 321, a micrometer cylinder, and a fine adjustment knob. In addition, the micrometer head 32 generally has two-stage threads for coarse and fine adjustment.

[0060] The micrometer screw 321 of the micrometer head 32 can achieve a movement accuracy of 0.01mm.

[0061] In this embodiment, a micrometer head 32 is used as an adjustment component for the internal space of the adjustable air chamber 3. By continuously adjusting the axial movement of its micrometer screw 321, the internal space of the cylinder 31 can be continuously varied. The change in the internal space of the adjustable air chamber 3 can be calculated by the moving distance of the micrometer screw 321 of the micrometer head 32 and the inner diameter of the cylinder 31, that is, the moving distance of the micrometer screw 321 corresponds to the corresponding pore volume.

[0062] The diameter of the cylinder 31 can be designed according to the range of pore volume variation required by the device, and the displacement range of the micrometer head 32 can also be selected according to the range of pore volume variation.

[0063] like Figure 2 As shown, in one embodiment, the end of the micrometer screw 321 of the micrometer tip is slidably and sealingly connected to the inner wall of the cylinder 31. Specifically, a sealing ring 34 is provided at the end of the micrometer screw 321 of the micrometer tip.

[0064] like Figure 3 As shown, in another embodiment, the adjustment assembly includes a micrometer head 32 and an extension rod 33. The extension rod 33 is disposed at the end of the micrometer screw 321 of the micrometer head 32. The extension rod 33 is slidably and sealed to the inside of the cylinder 31, and the diameter of the extension rod 33 is smaller than the diameter of the micrometer screw 321. Specifically, a sealing ring 34 is provided at the end of the extension rod 33.

[0065] Thus, if the diameter of the designed cylinder 31 is smaller than the diameter of the micrometer screw, a thinner extension rod 33 can be fitted at the front end of the micrometer screw to achieve a smaller adjustable pore volume.

[0066] Preferably, the adjustable air chamber 3 further includes a fixed support 35, and the cylinder 31 is mounted on the fixed support 35.

[0067] Example 2

[0068] This embodiment describes the differences from the above embodiments, while the similarities will not be repeated.

[0069] In this embodiment, the gas measurement system 4 includes a gas source 41, a gas injection line 44, and a pressure sensor 42. The outlet end of the gas source 41 is equipped with a pressure reducing valve 43 and is connected to the core holder 1 and the adjustable gas chamber 3 via the gas injection line 44. A first valve 45 and a second valve 46 are sequentially installed on the gas injection line 44, with the first valve 45 positioned closer to the pressure reducing valve 43 than the second valve 46. The pressure sensor 42 is connected to the pipeline between the first valve 45 and the second valve 46.

[0070] The gas source 41 provides the gas for measurement, and its outlet is equipped with a pressure reducing valve 43 to reduce the pressure of the outflowing gas. The gas source 41 is connected to the core holder 1 and the adjustable gas chamber 3 via an injection pipeline 44. A first valve 45 and a second valve 46 are spaced apart on the injection pipeline 44 to control the gas flow and injection volume. By closing the second valve 46 and opening the first valve 45 for a period of time before closing it again, a certain pressure of gas can be stored in the pipeline between the two valves. Then, by opening the second valve 46, the gas expands and enters the rock sample in the core holder 1 and the adjustable gas chamber 3, thereby controlling the injection volume.

[0071] Preferably, the gas measurement system 4 further includes a venting line 47. The venting line 47 is connected to the pipeline between the first valve 45 and the second valve 46, and a micro-leakage valve 48 and a shut-off valve 49 are sequentially arranged on the venting line 47 along the flow direction of the gas inside. In this way, when the gas pressure in the pipeline between the first valve 45 and the second valve 46 is greater than the required or set pressure, the micro-leakage valve 48 and the shut-off valve 49 on the venting line 47 are opened, thereby allowing the gas pressure in the pipeline between the first valve 45 and the second valve 46 to gradually decrease. When the pressure decreases to the required or set pressure, the shut-off valve 49 is closed, and the gas pressure can be stabilized at a fixed required pressure value.

[0072] More preferably, the adjustable gas chamber 3 is connected to the core holder 1 via a first pipeline and a second pipeline. One end of the first pipeline is connected to the adjustable gas chamber 3, and the other end is connected to a first side of the core holder 1. One end of the second pipeline is connected to the adjustable gas chamber 3, and the other end is connected to a second side of the core holder 1 opposite to its first side. The gas injection pipeline 44 of the gas measurement system 4 is connected to the first pipeline and / or the second pipeline. In this way, the core holder 1 and the adjustable gas chamber 3 are connected by two pipelines, and the gas entering through the injection pipeline is introduced into both ends (e.g., the upper and lower ends) of the rock sample, allowing the gas to diffuse more quickly in the sample pores, which helps to shorten the testing time.

[0073] Example 3

[0074] This embodiment describes the differences from the above embodiments, while the similarities will not be repeated.

[0075] The core holder 1 can be a core holder 1 that meets different sample sizes. The rock sample shape can be a cylindrical plug or a cubic block. Then the core holder 1 is a corresponding cylindrical core holder or a triaxial core holder.

[0076] If the rock sample used is a cylindrical plug sample, the sample diameter corresponding to core holder 1 is generally 25mm, 38mm, 50mm or 100mm. If the rock sample used is a cubic block sample, the sample size corresponding to core holder 1 is generally 25×25mm, 38×38mm or 50×50mm.

[0077] Meanwhile, the core holder 1 can be a core holder with circumferential loading and overburden, or a core holder 1 with both circumferential and axial loading and overburden.

[0078] The pressurization device 2 uses a liquid pressurization pump, which can be a manual pump or an automatic pump, and is mainly used for loading the overburden in the core holder 1.

[0079] like Figure 4 As shown, according to a second aspect of the present invention, the method for measuring porosity using the above-described pressure-controlled gas porosity measuring device includes a step of pore volume-pressure calibration and a step of measuring the porosity of the sample to be tested.

[0080] The steps involved in pore volume-pressure calibration include:

[0081] Step 11: Fix the standard sample in the core holder 1, and apply the first fixing pressure to the standard sample through the pressurization device 2.

[0082] Step 12: Diffusion of the gas at the first preset pressure in the gas measurement system 4 into the core holder 1 and the space adjustable gas chamber 3.

[0083] Step 13: Gradually increase the volume of the internal space of the adjustable air chamber 3 from zero, and record the gas pressure value after each increase in the volume of the internal space of the adjustable air chamber 3, until the preset number of times is reached, thereby obtaining the pore volume-pressure standard curve.

[0084] Specifically, in step 13, the first end of the adjustment component is first adjusted to the front end of the cylinder 31, so that the volume of the internal space of the adjustable air chamber 3 is zero. Then, the first end of the adjustment component is moved a certain distance along the axial direction until the preset number of times is reached, so that the volume of the internal space of the adjustable air chamber 3 increases gradually.

[0085] The preset number of times is at least 5.

[0086] In the pore volume-pressure calibration step, the adjustable gas chamber 3 is connected in parallel with the core holder 1. By utilizing the continuous change of its internal space, a pore volume-pressure standard curve with multiple measuring points can be obtained through a single gas expansion process. This avoids the need for multiple loading and unloading of samples in the core holder 1 to achieve the purpose of pore volume calibration, and greatly simplifies the pore volume calibration step.

[0087] The steps for determining the porosity of the sample to be tested include:

[0088] Step 21: Adjust the volume of the internal space of the adjustable air chamber 3 to zero.

[0089] Specifically, the temperature measuring screw of the micrometer head 32 is adjusted to the front end of the cylinder 31.

[0090] Step 22: Fix the sample to be tested in the core holder 1, and apply a second fixing pressure to the sample to be tested through the pressurizing device 2.

[0091] Step 23: Diffusion of the gas at the second preset pressure in the gas measurement system 4 into the core holder 1 and the space adjustable gas chamber 3.

[0092] Step 24: After the gas diffuses into the pores of the sample to be tested to equilibrium, record the equilibrium pressure. Obtain the pore volume of the sample to be tested based on the equilibrium pressure and the pore volume-pressure standard curve obtained in Step 13.

[0093] The second preset pressure is equal to the first preset pressure.

[0094] Step 24 is followed by step 25, which involves calculating porosity: obtaining the total volume of the sample by measurement, and then calculating the porosity value under overburden pressure according to porosity = pore volume / total volume.

[0095] In the step of measuring the porosity of the sample to be tested, by adjusting the volume of the internal space of the adjustable air chamber 3 to zero, the corresponding porosity is zero.

[0096] It should be noted that the aforementioned standard sample refers to a solid sample of a standard shape. In specific implementations, the standard sample can be a solid metal block, such as a steel core. The porosity and permeability of a solid metal block will not change due to loading and unloading stress, making the test results more accurate. However, this invention can also use real rock samples with known pore volumes or cores made of other alloys for testing; the specific choice depends on the required precision, etc., of the user.

[0097] The cross-sectional shape and dimensions of the standard sample should be consistent with those of the sample to be tested, so that both the standard sample and the sample to be tested can be held firmly by the core holder 1. Specifically, if the core holder 1 is suitable for cylindrical samples, and the sample to be tested is a cylinder with a diameter of 25 mm, then the standard sample is also a cylinder with a diameter of 25 mm, and their heights can be the same or different. Similarly, if the core holder 1 is suitable for cubic samples, then the cross-sections of both the standard sample and the sample to be tested should be rectangular and the same size, while their heights can be the same or different.

[0098] Furthermore, the second fixed overburden pressure in step 22 is a series of overburden pressures simulating formation conditions (which can be set to 5 MPa to 100 MPa, or even higher pressures), used to obtain the pore volume in the sample under different overburden pressures. Since the overburden pressure has little effect on the deformation of the solid metal block, the first fixed overburden pressure in step 11 is simply to tighten the solid metal block, and a fixed pressure of 5 MPa is generally sufficient.

[0099] Example 4

[0100] The gas porosity measuring device under overburden pressure in this embodiment is mainly divided into four parts: core holder 1, overburden pressure boosting pump, adjustable gas chamber 3, and gas measuring system 4.

[0101] Among them, the core holder 1 is suitable for cylindrical plug rock samples, and its corresponding sample diameter is designed to be 25mm. The core holder 1 is a cylindrical core holder 1 with circumferential and axial loading and pressure.

[0102] The pressurization device 2 employs an automatically controllable liquid booster pump. The pressure sensor 42 of the gas measurement system 4 has a range of 1 MPa and an accuracy of ±0.1% FS.

[0103] The adjustable air chamber 3 has a cylindrical body 31 made of stainless steel with an inner diameter of 8mm and a wall thickness of 2mm. The micrometer screw 321 of the micrometer head 32 has an outer diameter of 8mm, which matches the inner diameter of the cylindrical body 31. An O-ring seal 34 is installed at the end of the micrometer screw 321. Its displacement distance is divided into five positions: 5mm, 10mm, 20mm, 30mm, and 50mm. Based on the inner diameter and displacement, the corresponding pore volume V is calculated to be 251.32mm². 3 502.65mm 3 1005.31mm 3 1507.96mm 3 2513.27mm 3 .

[0104] During pore volume-pressure calibration, a gas pressure of 1 MPa is first introduced into the gas measurement system 4. After equilibrium is reached, the second valve 46 is opened to allow the gas to diffuse into the core holder 1, the adjustable gas chamber 3, and the pipeline between them. Then, the micrometer screw 321 of the micrometer head 32 is sequentially adjusted backward from the front end to positions of 5 mm, 10 mm, 20 mm, 30 mm, and 50 mm. At the same time, the corresponding pressures P1, P2, P3, P4, and P5 are recorded at each position. Then, a linear relationship graph between pressure P and pore volume V is plotted, i.e., the pore volume-pressure standard curve.

[0105] After the pore volume-pressure calibration was completed, the porosity of the sample was measured. The total volume of the sample obtained by measurement was 12000 mm². 3 After being loaded into the core holder 1, the pressure was increased to 5 MPa. The micrometer screw 321 of the micrometer head 32 was positioned at the foremost position (even when the internal volume of the adjustable gas chamber 3 was zero). A gas pressure of 1 MPa was introduced into the gas measurement system 4. After equilibrium was reached, the second valve 46 was opened, allowing the gas to diffuse into the core holder 1, the adjustable gas chamber 3, and the pipeline between them. The pressure obtained after the gas diffused and reached equilibrium in the sample pores was Ps. The pore volume Vs of the sample was calculated to be 252.3 mm² using the pore volume-pressure standard curve. 3 .

[0106] Calculate the porosity of the sample under a 5 MPa overburden pressure.

[0107] In the description of this invention, it should be understood that the terms "front," "rear," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature.

[0108] Although the invention has been described with reference to preferred embodiments, various modifications can be made and components can be replaced with equivalents without departing from the scope of the invention. In particular, the technical features mentioned in the various embodiments can be combined in any manner as long as there is no structural conflict. The invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A method for determining porosity using a pressure-controlled gas porosity measuring device, characterized in that, A device for measuring gas porosity under accretion pressure, comprising: Core holders are used to hold rock samples. A pressurizing device, which is connected to the core holder, to apply overburden pressure to the rock sample; An adjustable air chamber, the volume of which can be adjusted, is connected to the core holder; and A gas measurement system is connected to the core holder and the adjustable gas chamber to introduce gas at a preset pressure to measure the porosity of the rock sample. The method includes the steps of pore volume-pressure calibration: Step 11: Fix the standard sample in the core holder and apply the first fixing pressure to the standard sample through the pressurization device; Step 12: Diffusion of the gas at the first preset pressure in the gas measurement system into the core holder and the adjustable gas chamber; Step 13: Gradually increase the volume of the internal space of the adjustable air chamber from zero, and record the gas pressure value after each increase in the volume of the internal space of the adjustable air chamber, until the preset number of times is reached, thereby obtaining the pore volume-pressure standard curve.

2. The method according to claim 1, characterized in that, The adjustable air chamber includes a cylinder and an adjustment assembly. The first end of the adjustment component is disposed inside the cylinder, and the first end of the adjustment component is slidably and sealed to the inner wall of the cylinder. The first end of the adjustment component can reciprocate along the axial direction of the cylinder to adjust the volume of the internal space of the adjustable air chamber.

3. The method according to claim 2, characterized in that, The adjustment assembly includes a micrometer head, which includes a micrometer screw capable of extending and retracting along the axial direction of the cylinder. The volume of the internal space of the adjustable air chamber is adjusted by regulating the axial extension and retraction of the micrometer screw.

4. The method according to claim 3, characterized in that, The adjustment assembly also includes an extension rod. The extension rod is disposed at the end of the micrometer screw, and the extension rod is slidably and sealed to the inner wall of the cylinder. The diameter of the extension rod is smaller than the diameter of the micrometer screw.

5. The method according to any one of claims 2-4, characterized in that, The first end of the adjusting component is provided with a sealing ring, and the first end of the adjusting component is slidably and sealed to the inner wall of the cylinder through the sealing ring.

6. The method according to any one of claims 1-4, characterized in that, The adjustable air chamber is connected to the core holder via a first pipeline and a second pipeline, respectively. One end of the first pipeline is connected to the adjustable air chamber, and the other end of the first pipeline is connected to the first side of the core holder. One end of the second pipeline is connected to the adjustable air chamber, and the other end of the second pipeline is connected to the second side of the core holder opposite to its first side. The gas measurement system is connected to the first pipeline and / or the second pipeline.

7. The method according to any one of claims 1-4, characterized in that, The gas measurement system includes a gas source, a gas injection line, and a pressure sensor. The outlet end of the gas source is equipped with a pressure reducing valve and is connected to the core holder and the adjustable gas chamber through the gas injection pipeline. A first valve and a second valve are sequentially arranged on the gas injection pipeline, with the first valve being closer to the pressure reducing valve than the second valve. The pressure sensor is connected to the pipeline between the first valve and the second valve.

8. The method according to claim 7, characterized in that, The gas detection system also includes a venting pipeline. The venting pipeline is connected between the first valve and the second valve, and a micro-leakage valve and a shut-off valve are sequentially installed on the venting pipeline along the flow direction of the gas inside it.

9. The method according to claim 1, characterized in that, It also includes the step of measuring the porosity of the sample to be tested: Step 21: Adjust the volume of the internal space of the adjustable air chamber to zero; Step 22: Fix the sample to be tested in the core holder and apply a second fixing pressure to the sample to be tested through the pressurization device; Step 23: Diffusion of the gas at the second preset pressure in the gas measurement system into the core holder and the adjustable gas chamber; Step 24: After the gas diffuses into the pores of the sample to be tested to equilibrium, record the equilibrium pressure, and obtain the pore volume of the sample to be tested based on the equilibrium pressure and the pore volume-pressure standard curve obtained in step 13. The second preset pressure is equal to the first preset pressure.

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