A method and system for determining the spacing between fracturing stages in a coal reservoir

Abstract

The invention discloses a method and a system for determining the spacing of coal reservoir fracturing sections, and relates to the technical field of fracturing. The method comprises the following steps: placing a cubic reservoir sample in a vacuum tank, filling the vacuum tank with methane gas until methane is fully adsorbed, then injecting water into the vacuum tank to replace the methane gas, pressurizing the water in the vacuum tank to a pump stop pressure during fracturing, and finally releasing the pressure in the vacuum tank and desorbing until the desorbed gas no longer increases, thereby obtaining the desorption equilibrium time of the current reservoir sample; repeating the above steps to obtain the desorption equilibrium time of cubic reservoir samples with different side lengths, and obtaining a relationship between the desorption equilibrium time and the sample side length; in the invention, the relationship between the desorption equilibrium time and the sample side length is obtained, and the fracturing section spacing corresponding to the planned development period is back-calculated by using the relationship, thereby effectively improving the optimization effect of the fracturing section spacing.

Description

Technical Field

[0001] The invention relates to the technical field of fracturing, and in particular to a method for determining the spacing of fracturing sections in a coal reservoir and a determination system thereof. Background Art

[0002] In coal mining, the rational application of fracturing technology is one of the key factors to improve mining efficiency, and the spacing between fracturing sections is an important parameter for the implementation of fracturing technology.

[0003] The currently used fracturing stage spacing determination technology mainly optimizes the fracturing stage spacing based on geological data and fracture simulation results. It is necessary to obtain basic formation data such as lithology, thickness and fracture development through geological exploration, and then use acoustic logging and imaging logging to evaluate the physical and mechanical properties of the formation, and use microseismic monitoring technology to track the expansion of fractures to evaluate the morphology and distribution of fractures. After collecting and evaluating relevant data, numerical simulation software is used to simulate the fracture expansion process to determine and optimize the theoretically optimal stage spacing. However, the fracturing stage spacing determination technology based on geological data and fracture simulation ignores the desorption characteristics and long-term development needs of coalbed methane wells, resulting in a mismatch between the fracturing stage spacing design and the actual desorption efficiency, which in turn causes the problem of poor optimization of the fracturing stage spacing, affecting economic benefits and sustainable production.

[0004] Therefore, people are in urgent need of a method and system for determining the spacing of coal reservoir fracturing stages with good optimization effect of fracturing stage spacing. Summary of the invention

[0005] The purpose of the present invention is to provide a method and a system for determining the spacing between fracturing stages in a coal reservoir, so as to solve the problems existing in the above-mentioned prior art. The relationship between the desorption equilibrium time and the side length of the sample is obtained by means of simulation experiments, so as to determine the relationship between the planned development years and the spacing between fracturing stages, so as to improve the optimization effect of the spacing between fracturing stages.

[0006] To achieve the above object, the present invention provides the following solution: The present invention provides a method for determining the spacing of coal reservoir fracturing stages, comprising the following steps:

[0007] S1: Obtain multiple cubic reservoir samples with different side lengths;

[0008] S2: placing a single reservoir sample in a vacuum tank;

[0009] S3: Fill the vacuum tank with methane gas at the actual reservoir pressure until methane is fully adsorbed in the reservoir sample;

[0010] S4: inject water into the vacuum tank to replace the methane gas, and keep the pressure inside the vacuum tank not less than the actual reservoir pressure;

[0011] S5: pressurizing the water in the vacuum tank to the pump stop pressure during the fracturing process;

[0012] S6: reduce the water pressure in the vacuum tank to atmospheric pressure, and keep the water pressure in the vacuum tank the same as the atmospheric pressure until the desorbed gas no longer increases;

[0013] S7: Obtain the desorption equilibrium time of the current reservoir sample;

[0014] S8: repeating steps S2-S7 to obtain the desorption equilibrium time of multiple reservoir samples;

[0015] S9: The linear regression method is used to obtain the relationship between the desorption equilibrium time and the sample side length, and the side length of the reservoir sample corresponding to the planned development period is calculated in reverse. The side length of the reservoir sample is the fracturing stage spacing.

[0016] Preferably, the reservoir sample is subjected to vacuum drying treatment before being placed in the vacuum tank.

[0017] Preferably, the temperature inside the vacuum tank is controlled to the actual temperature of the reservoir.

[0018] Preferably, before filling the vacuum tank with methane gas, the vacuum tank is tested for air tightness.

[0019] Preferably, after the air tightness test is completed, the gas used to test the air tightness is released, and after the release is completed, the vacuum tank body is evacuated.

[0020] Preferably, in step S6, the water pressure in the vacuum tank is gradually reduced to atmospheric pressure.

[0021] The present invention also provides a system for determining the spacing of coal reservoir fracturing sections used in the above-mentioned method for determining the spacing of coal reservoir fracturing sections, comprising a vacuum tank body, a methane input unit for filling with methane gas, a vacuum pump, a water supply unit for injecting water, a back pressure valve, and a gas flow detection unit for detecting changes in gas flow, wherein the methane input unit is connected to the vacuum tank body through a first valve, the water supply unit is connected to the vacuum tank body through a second valve, the vacuum pump is connected to the vacuum tank body through a third valve, the liquid inlet end of the back pressure valve is connected to the vacuum tank body through a fourth valve, the liquid outlet end is connected to the gas flow detection unit, and the pressure control end of the back pressure valve is connected to the back pressure control pump.

[0022] Preferably, the pipeline between the back pressure valve and the fourth valve is connected to one end of a branch pipeline, a fifth valve is arranged on the branch pipeline, and the other end of the branch pipeline is connected to the gas flow detection unit.

[0023] Preferably, a pressure sensor is provided on the pipeline between the vacuum tank body and the fourth valve.

[0024] Preferably, the coal reservoir fracturing section spacing determination system further comprises a gas supply unit for providing gas for detecting air tightness, and the gas supply unit is connected to the vacuum tank body.

[0025] Compared with the prior art, the present invention mainly achieves the following technical effects:

[0026] By obtaining the desorption equilibrium time of reservoir samples with different side lengths, the relationship between the desorption equilibrium time and the sample side length is obtained. This relationship represents the relationship between the planned development years and the fracturing stage spacing. The optimal fracturing stage spacing corresponding to the planned development years can be determined by back calculation, thereby improving the optimization effect of the final fracturing stage spacing.

[0027] Compared with the prior art, other solutions of the present invention have achieved the following technical effects:

[0028] Vacuum drying the obtained reservoir samples can not only more effectively remove moisture from the reservoir samples, helping to maintain the chemical and physical properties of the samples, but also avoid oxidation reactions, maintain structural stability, and thus improve the accuracy of subsequent experiments.

[0029] The airtightness test can ensure that the vacuum tank has no leaks, and vacuuming can be performed after the airtightness test is completed to further improve the accuracy of subsequent experiments.

[0030] By slowly reducing the water pressure, the simulation realism is improved, which is beneficial to improving the accuracy of the experiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0032] Figure 1 A system diagram of a system for determining the spacing between coal reservoir fracturing stages according to the present invention;

[0033] Among them, 1. vacuum tank; 2. vacuum pump; 3. back pressure valve; 4. back pressure control pump; 5. constant pressure and constant speed pump; 6. gas-water separator; 7. gas flow meter; 8. methane bottle; 9. test gas bottle; 10. pressure gauge; 11. pressure sensor; 12. first valve; 13. second valve; 14. third valve; 15. fourth valve; 16. fifth valve; 17. first pressure regulating valve; 18. second pressure regulating valve. DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] The purpose of the present invention is to provide a method and a system for determining the spacing between fracturing stages in a coal reservoir, so as to solve the problems existing in the prior art. The relationship between the desorption equilibrium time and the side length of the sample is obtained by means of simulation experiments, so as to determine the relationship between the planned development years and the spacing between fracturing stages, so as to improve the optimization effect of the spacing between fracturing stages.

[0036] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0037] Please refer to Figure 1 As shown, the present invention provides a method for determining the spacing of coal reservoir fracturing sections, comprising the following steps: S1: performing linear cutting on the collected reservoir samples to obtain a plurality of cubic reservoir samples with different side lengths, and the samples can be numbered in this step;

[0038] S2: placing a single reservoir sample in the vacuum tank 1. After the single reservoir sample is placed in the vacuum tank 1, the vacuum tank 1 needs to be kept in a vacuum state. The vacuum degree in the vacuum tank 1 can be controlled by a vacuum pump 2;

[0039] S3: methane gas is charged into the vacuum tank 1 at the actual reservoir pressure, the methane gas charging state is maintained, and the pressure in the vacuum tank 1 is stabilized at the actual reservoir pressure for more than 168 hours, until methane is fully adsorbed in the reservoir sample, and the pressure in the vacuum tank 1 can be monitored in real time by the pressure sensor 11;

[0040] S4: inject water into the vacuum tank body 1 to replace the methane gas. In this step, the pressure in the vacuum tank body 1 needs to be controlled to be the actual pressure of the reservoir. The control method can be a back pressure valve 3 or a cartridge valve. The control end pressure of the back pressure valve 3 and the cartridge valve is set to the actual pressure of the reservoir. The liquid inlet ends of the two valves are connected to the top of the vacuum tank body 1. After observing that water comes out of the liquid outlet ends of the back pressure valve 3 and the cartridge valve, it means that the replacement step is completed. At the same time, the water injection amount can also be recorded, and the volume of the space in the vacuum tank body 1 can be compared with the volume of the water injection amount to assist in determining that the replacement step is completed;

[0041] S5: pressurizing the water in the vacuum tank 1 to the pump stop pressure during the fracturing process by operating the pump body. In this step, the discharge path connecting the vacuum tank 1 and the back pressure valve 3 or the cartridge valve is closed;

[0042] S6: reducing the water pressure in the vacuum tank 1 to the atmospheric pressure, and maintaining the water pressure in the vacuum tank 1 to be the same as the atmospheric pressure. During this process, the methane adsorbed in the reservoir sample begins to desorb until the desorbed gas no longer increases;

[0043] S7: Obtain the desorption equilibrium time of the current reservoir sample;

[0044] S8: repeating steps S2-S7 to obtain desorption equilibrium time of multiple reservoir samples with different side lengths;

[0045] S9: The linear regression method is used to obtain the relationship between the desorption equilibrium time and the sample side length. Based on this relationship, the side length of the reservoir sample is calculated using the planned development years. The side length of the reservoir sample is the fracturing stage spacing.

[0046] This method obtains the desorption equilibrium time of reservoir samples with different side lengths to obtain the relationship between the desorption equilibrium time and the sample side length. This relationship represents the relationship between the planned development years and the fracturing stage spacing. The optimal fracturing stage spacing corresponding to the planned development years can be determined by back calculation, thereby improving the optimization effect of the final fracturing stage spacing.

[0047] Before the reservoir sample is placed in the vacuum tank 1, it is subjected to vacuum drying. Taking the coal sample as an example, the drying temperature does not exceed 120°C, which can not only more effectively remove moisture from the coal sample and help maintain the chemical and physical properties of the sample, but also avoid oxidation reactions and maintain structural stability, thereby improving the accuracy of subsequent experiments.

[0048] The temperature inside the vacuum tank 1 is controlled to the actual temperature of the reservoir to improve the realism of the simulation, and the actual temperature of the reservoir can be obtained by field temperature measurement.

[0049] Before filling the vacuum tank body 1 with methane gas, the vacuum tank body 1 is tested for air tightness. The gas used for the air tightness test is preferably helium. When the helium pressure in the vacuum tank body 1 reaches the pump stop pressure during the fracturing process, the filling of helium is stopped. If the pressure change in the vacuum tank body 1 is less than 1MPa within 10 hours, it indicates that the air tightness of the test device is good, otherwise it indicates that the air tightness is poor. It is necessary to determine the leakage point and repair it until the device is leaking. The accuracy of the experiment can be effectively improved through air tightness testing.

[0050] After the air tightness test is completed, it is necessary to open the exhaust path of the vacuum tank body 1 to release the gas used to test the air tightness, and after the release is completed, close the exhaust path and evacuate the vacuum tank body 1 to improve the experimental accuracy.

[0051] In order to improve the realism of the simulation, in step S6, the water pressure in the vacuum tank 1 is gradually reduced to atmospheric pressure to improve the experimental accuracy.

[0052] The present invention also provides a determination system used in the above-mentioned method for determining the spacing of coal reservoir fracturing sections, comprising a vacuum tank body 1, a methane input unit for filling with methane gas, a vacuum pump 2, a water supply unit for injecting water, a back pressure valve 3 and a gas flow detection unit for detecting changes in gas flow, wherein the methane input unit is connected to the vacuum tank body 1 through a first valve 12, the water supply unit is connected to the vacuum tank body 1 through a second valve 13, the vacuum pump 2 is connected to the vacuum tank body 1 through a third valve 14, the liquid inlet end of the back pressure valve 3 is connected to the vacuum tank body 1 through a fourth valve 15, and the liquid outlet end is connected to the gas flow detection unit, and the pressure control of the back pressure valve 3 The end is connected with the back pressure control pump 4, and the back pressure control pump 4 pumps the pressure liquid to the pressure control end of the back pressure valve 3, wherein the methane input unit comprises a methane bottle 8 containing methane with a certain pressure and a first pressure regulating valve 17, and the methane bottle 8 is connected with the vacuum tank body 1 through the first pressure regulating valve 17. Of course, the methane bottle 8 can also be replaced by the tank body, and the first pressure regulating valve 17 adjusts the output pressure of the methane bottle 8; the water supply unit is a water delivery pump, and preferably a constant pressure and constant speed pump 5 is used. The constant pressure can stably maintain the water pressure in the vacuum tank body 1, and the constant speed is convenient for calculating the water injection amount through time; the gas flow detection unit is a gas flow meter 7, and preferably, a gas-water separator 6 is arranged between the gas flow meter 7 and the vacuum tank body 1 to separate the water in the gas.

[0053] The back-pressure valve 3 and the back-pressure control pump 4 can be replaced by a cartridge valve and a corresponding hydraulic system.

[0054] The pipeline between the back pressure valve 3 and the fourth valve 15 is connected to one end of a branch pipeline, a fifth valve 16 is arranged on the branch pipeline, and the other end of the branch pipeline is connected to the gas flow detection unit. Through the arrangement of the branch pipeline, a direct pressure relief channel is provided.

[0055] A pressure sensor 11 is provided on the pipeline between the vacuum tank body 1 and the fourth valve 15 for monitoring the current pressure in the vacuum tank body 1 .

[0056] The coal reservoir fracturing section spacing determination system also includes a gas supply unit for providing gas for detecting air tightness. Preferably, the gas for detecting air tightness is helium. The gas supply unit is connected to the vacuum tank body 1. The gas supply unit includes a detection gas cylinder 9 and a second pressure regulating valve 18. The detection gas cylinder 9 is connected to the vacuum tank body 1 through the second pressure regulating valve 18. The second pressure regulating valve 18 is used to adjust the output pressure of the detection gas cylinder 9.

[0057] In order to facilitate monitoring of the methane cylinder 8 and detecting the output pressure of the gas cylinder 9, a pressure gauge 10 can be installed on the pipeline between the first pressure regulating valve 17 and the vacuum tank body 1 and on the pipeline between the second pressure regulating valve 18 and the vacuum tank body 1; in order to save the layout of pipelines, the first pressure regulating valve 17 and the second pressure regulating valve 18 are connected to the vacuum tank body 1 through a common pipeline, and the pressure gauge 10 and the first valve 12 are both arranged on the common pipeline.

[0058] A heater can be set inside or outside the vacuum tank body 1 to heat the inner cavity of the vacuum tank body 1 to the actual temperature of the reservoir, and a thermometer can be set to detect whether the temperature inside the vacuum tank body 1 reaches the actual temperature of the reservoir. When the heater is set inside, it needs to be waterproofed. When the heater is set outside, the outer shell of the vacuum tank body 1 needs to use a heat-conductive material.

[0059] When the system is in use and in the step of charging methane, the first pressure regulating valve 17 and the first valve 12 are opened, and the other valves are in a closed state until the methane is charged; in the step of injecting water to replace methane, the constant pressure and constant speed pump 5 is started at a constant speed, the second valve 13 and the fourth valve 15 are opened, and the other valves are in a closed state until the water injection replacement is completed; in the step of adjusting the water pressure in the vacuum tank body 1 to the pump stop pressure during the fracturing process, the second valve 13 is opened first, and the other valves are in a closed state, and the constant pressure and constant speed pump 5 is started at a constant pressure, and after reaching the pump stop pressure during the fracturing process, the second valve 13 is closed; in the step of releasing the water pressure in the vacuum tank body 1 to atmospheric pressure, the fourth valve 15 is initially at In the open state, the other valves are in the closed state, and when the pumping pressure of the back pressure control pump 4 reaches the atmospheric pressure, the fifth valve 16 is opened; in the step of air tightness detection, the second pressure regulating valve 18 and the first valve 12 are opened to fill the test gas, and the other valves are in the closed state. When the pressure in the vacuum tank body 1 reaches the pump stop pressure during the fracturing process, the second pressure regulating valve 18 and the first valve 12 are closed, and at the same time, the control end pressure of the back pressure valve 3 can be set to the pump stop pressure during the fracturing process, and the fourth valve 15 is kept open throughout the air tightness detection step, the air tightness of the fourth valve 15 can be additionally detected, and when helium is filled, it can be ensured that the filling pressure is at most the pump stop pressure during the fracturing process.

[0060] Adaptive changes made according to actual needs are all within the protection scope of the present invention.

[0061] It should be noted that it is obvious to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, the embodiments should be regarded as exemplary and non-restrictive from any point of view, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention. Any reference numerals in the claims should not be regarded as limiting the claims involved.

[0062] The present invention uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the method and core ideas of the present invention. At the same time, for those skilled in the art, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for determining the spacing between coal reservoir fracturing stages, characterized in that: The following steps are involved: S1: Obtain multiple cubic reservoir samples with different side lengths; S2: placing a single reservoir sample in a vacuum tank; S3: Fill the vacuum tank with methane gas at the actual reservoir pressure until methane is fully adsorbed in the reservoir sample; S4: inject water into the vacuum tank to replace the methane gas, and keep the pressure inside the vacuum tank not less than the actual reservoir pressure; S5: pressurizing the water in the vacuum tank to the pump stop pressure during the fracturing process; S6: reduce the water pressure in the vacuum tank to atmospheric pressure, and keep the water pressure in the vacuum tank the same as the atmospheric pressure until the desorbed gas no longer increases; S7: Obtain the desorption equilibrium time of the current reservoir sample; S8: repeating steps S2-S7 to obtain the desorption equilibrium time of multiple reservoir samples; S9: The linear regression method is used to obtain the relationship between the desorption equilibrium time and the sample side length, and the side length of the reservoir sample corresponding to the planned development period is calculated in reverse. The side length of the reservoir sample is the fracturing stage spacing.

2. The method for determining the spacing between coal reservoir fracturing stages according to claim 1, characterized in that: Before the reservoir samples are placed in the vacuum tank, they are vacuum dried.

3. The method for determining the spacing between coal reservoir fracturing stages according to claim 1, characterized in that: Control the temperature inside the vacuum tank to the actual temperature of the reservoir.

4. The method for determining the spacing between coal reservoir fracturing stages according to claim 1, characterized in that: Before filling the vacuum tank with methane gas, the vacuum tank is tested for air tightness.

5. The method for determining the spacing between coal reservoir fracturing stages according to claim 4, characterized in that: After the air tightness test is completed, the gas used to test the air tightness is released, and after the release is completed, the vacuum tank body is evacuated.

6. The method for determining the spacing between coal reservoir fracturing stages according to claim 1, characterized in that: In step S6, the water pressure in the vacuum tank is gradually reduced to atmospheric pressure.

7. A system for determining the spacing between coal reservoir fracturing stages used in the method for determining the spacing between coal reservoir fracturing stages as claimed in any one of claims 1 to 6, characterized in that: It includes a vacuum tank body, a methane input unit for filling methane gas, a vacuum pump, a water supply unit for injecting water, a back pressure valve and a gas flow detection unit for detecting changes in gas flow, wherein the methane input unit is connected to the vacuum tank body through a first valve, the water supply unit is connected to the vacuum tank body through a second valve, the vacuum pump is connected to the vacuum tank body through a third valve, the liquid inlet end of the back pressure valve is connected to the vacuum tank body through a fourth valve, the liquid outlet end is connected to the gas flow detection unit, and the pressure control end of the back pressure valve is connected to the back pressure control pump.

8. The system for determining the spacing between coal reservoir fracturing stages according to claim 7, characterized in that: The pipeline between the back pressure valve and the fourth valve is connected to one end of a branch pipeline, a fifth valve is arranged on the branch pipeline, and the other end of the branch pipeline is connected to the gas flow detection unit.

9. The system for determining the spacing between coal reservoir fracturing stages according to claim 7, characterized in that: A pressure sensor is arranged on the pipeline between the vacuum tank and the fourth valve.

10. The system for determining the spacing between coal reservoir fracturing stages according to claim 7, characterized in that: The coal reservoir fracturing section spacing determination system further comprises a gas supply unit for providing gas for detecting air tightness, and the gas supply unit is connected to the vacuum tank body.

Images