Method and system for measuring depletion gas pressure in the early evaluation stage of coalbed methane development

Abstract

The present invention relates to the field of coalbed methane development, and in particular to a method and system for measuring the exhaustion gas production pressure in the early evaluation stage of coalbed methane development; the method comprises the following steps: conducting a simulation experiment, and simultaneously monitoring the internal pressure P1 of the coal sample and the gas flow rate at the gas outlet; fitting to obtain a prediction model of gas production per ton of coal and reservoir pressure; fitting to obtain a mathematical equation for predicting reservoir pressure distribution in the coal seam; and substituting the critical desorption pressure of the coal sample into the gas production influence radius r of coalbed methane exhaustion; calculating the economic limit gas production per ton of coal according to the gas production influence radius of coalbed methane exhaustion and the abandoned coalbed methane production; and obtaining the reservoir pressure corresponding to the economic limit gas production per ton of coal according to the prediction model of gas production per ton of coal and reservoir pressure, as the exhaustion gas production pressure of the target coal seam. The present invention is based on accurate data of the mined coal seam, and can effectively determine the exhaustion gas production pressure in the early evaluation stage of coalbed methane development.

Description

Technical Field

[0001] The invention relates to the field of coalbed methane development, and in particular to a method and system for measuring exhaustion gas production pressure in the early evaluation stage of coalbed methane development. Background Art

[0002] The exhaustion gas production pressure of a coalbed methane reservoir refers to the limit pressure of a horizontal well that has industrial mining value. It is an important parameter for calculating the recovery rate or recoverable reserves of a coalbed methane reservoir. It is also an important indicator and basis for the development design of a coalbed methane field and the demonstration design of ground engineering. The lower the exhaustion gas production pressure, the higher the final recovery rate of the gas reservoir. At present, the relationship between exhaustion gas production pressure and gas production is often obtained with the help of actual coalbed methane well production data. However, in the early development and evaluation stage of a coalbed methane field, there is no actual coalbed methane production data. In order to solve the above problems, the existing technology often uses the analogy method to obtain the relationship between gas production pressure and gas production, but because it is not very targeted, it often leads to large errors, which is not conducive to the decision-making of coalbed methane development.

[0003] Therefore, it is necessary to propose new measurement devices and methods to determine the depletion gas production pressure in the early evaluation stage of coalbed methane development and provide a more accurate basis for coalbed methane development decision-making. Summary of the invention

[0004] The purpose of the present invention is to address the problem in the prior art that it is difficult to accurately measure the depletion gas pressure in the early stage, and to provide a depletion gas pressure measurement method and system with high accuracy in the early evaluation stage of coalbed methane development, which can provide a reference for coalbed methane development decisions.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for measuring the exhaustion gas production pressure in the early evaluation stage of coalbed methane development, which is implemented by a exhaustion gas production simulation device, wherein the exhaustion gas production simulation device comprises a sample holder, the sample holder is used to place the coalbed methane coal sample, one end of the sample holder is provided with an air inlet, and the other end is provided with an air outlet, the air inlet, the middle of the sample holder and the air outlet are respectively provided with pressure detectors, which are respectively used to detect the air inlet pressure P0, the internal pressure P1 of the coal sample and the air outlet pressure P2; the air inlet is provided with a constant pressure and constant speed pump, which is used to continuously inject methane into the coal sample; the air outlet is provided with a pressure relief valve and a flow meter; and the following steps are included:

[0006] Step S1: dry the coal sample of the target coal seam and place it in the sample holder, fill it with methane using the constant pressure and constant speed pump until the internal pressure of the coal sample is equal to the actual reservoir pressure of the coal seam, then control the pressure relief valve to make the pressure drop process at the gas outlet consistent with the bottom hole pressure drop process during the actual coalbed methane well development process, and monitor the internal pressure P1 of the coal sample and the gas flow rate at the gas outlet at the same time;

[0007] Step S2: obtaining multiple sets of coal sample internal pressure P1 and corresponding outlet gas flow data, calculating the gas production per ton of coal according to the outlet gas flow, and fitting a relationship equation between gas production per ton of coal and internal pressure of coal sample based on the gas production per ton of coal and the corresponding coal sample internal pressure P1 data as a prediction model of gas production per ton of coal and reservoir pressure;

[0008] Step S3: According to the pressure values ​​measured at each pressure measurement point at the end of the simulated experiment gas production, and the distance between each pressure measurement point and the gas outlet, a mathematical equation for predicting the reservoir pressure distribution in the coal seam is obtained by fitting with a logarithmic equation; and the critical desorption pressure of the coal sample is substituted into the mathematical equation for predicting the reservoir pressure distribution in the coal seam, and the distance value corresponding to the critical desorption pressure of the coal sample is calculated as the influence radius r of coalbed methane depletion gas production; the logarithmic equation used for fitting is:

[0009] P = D·ln(d)+E;

[0010] Among them, P represents the pressure value measured at the pressure measurement point at the end of the simulation experiment, d represents the distance between the pressure measurement point and the gas outlet, and D and E are the corresponding fitting constants;

[0011] Step S4: Calculate the economic limit gas production per ton of coal according to the gas production impact radius r of coalbed methane depletion and the abandoned coalbed methane production;

[0012] Step S5, substitute the economic limit gas production per ton of coal obtained in step S4 as the gas production per ton of coal into the gas production per ton of coal-reservoir pressure prediction model obtained in step S2 to obtain the reservoir pressure corresponding to the economic limit gas production per ton of coal, which is the coal seam depletion gas production pressure of the target coal seam.

[0013] The step S1 specifically includes the following steps:

[0014] Step S11: placing the dried coal sample in a sample holder of a depletion gas production simulation device and continuously evacuating the sample;

[0015] Step S12: setting the confining pressure of the coal sample in the sample holder to the actual vertical ground stress of the target coal seam, setting the axial pressure to the average horizontal ground stress of the coal seam, and setting the temperature to the real ground temperature of the coal seam through the confining pressure control system, the axial pressure control system and the temperature control system;

[0016] Step S13: filling the coal sample in the sample holder with methane through a constant pressure and constant speed pump, and monitoring the internal pressure P1 of the coal sample in real time until the internal pressure P1 of the coal sample is equal to the actual reservoir pressure P3 of the coal seam;

[0017] Step S14: Control the opening of the pressure relief valve to control the outlet pressure P2, so that the decreasing process of the outlet pressure P2 is consistent with the decreasing process of the bottom hole flow pressure during the actual coalbed methane well development process; during the decreasing process of the outlet pressure P2, monitor the internal pressure P1 of the coal sample and the outlet gas flow in real time.

[0018] The specific method of step S2 is:

[0019] Step S21: obtaining multiple groups of coal sample internal pressure and corresponding outlet gas flow data;

[0020] Step S22: Calculate the gas production per ton of coal according to the gas flow rate at the gas outlet. The calculation formula is:

[0021] T = L / M0;

[0022] Among them, T represents the gas production per ton of coal, L represents the gas flow rate at the gas outlet, and M0 represents the mass of the coal sample;

[0023] Step S23: Combine multiple groups of coal sample internal pressure P1 data corresponding to the gas production per ton of coal, perform data fitting to obtain the relationship equation between the gas production per ton of coal and the internal pressure of the coal sample of the target coal seam, and use the fitted relationship equation between the gas production per ton of coal and the internal pressure of the coal sample as the prediction model of the gas production per ton of coal and the reservoir pressure of the target coal seam.

[0024] In step S2, a quadratic equation is used to fit the data to obtain the relationship equation between gas production per ton of coal and internal pressure of the coal sample. The fitting equation is:

[0025] T=AP1 2 +BP1+C0;

[0026] Among them, T represents the gas production per ton of coal, A, B, C0 represent the three fitting parameters of the quadratic equation, and P1 represents the internal pressure of the coal sample;

[0027] In step S5, the calculation formula for the depleted gas production pressure of the target coal seam is:

[0028] ;

[0029] in, It represents the depletion gas production pressure of the target coal seam, and Q represents the economic limit gas production per ton of coal.

[0030] In step S3, the calculation formula of the radius r of gas production impact of coalbed methane depletion is:

[0031] r= ;

[0032] Wherein, P' represents the critical desorption pressure of the coal sample.

[0033] In step S4, the calculation formula of the economic limit gas production per ton of coal is:

[0034] Q= q ga / πr 2 hρ;

[0035] Among them, Q represents the economic limit gas production per ton of coal, q ga represents the abandoned coalbed methane production of coalbed methane horizontal wells, h represents the average thickness of coalbed methane; the abandoned coalbed methane production of coalbed methane horizontal wells q ga The calculation formula is:

[0036] q ga = (C + W) / (Jη);

[0037] Where C is the operating cost; W is the tax and surcharge; J is the coalbed methane selling price; η is the commodity rate.

[0038] In step S1, the temperature, confining pressure and axial pressure in the sample holder are set according to the formation temperature and the characteristics of the in-situ stress field.

[0039] In addition, the present invention also provides a depleted gas production pressure measurement system in the early evaluation stage of coalbed methane development, which is used to implement the depleted gas production pressure measurement method in the early evaluation stage of coalbed methane development, including a depleted gas production simulation device and a control module; the control module is connected to the pressure detector, constant pressure and constant speed pump, pressure relief valve, and flow meter, and is used to control the working status of the constant pressure and constant speed pump and pressure relief valve, and is also used to collect pressure data and outlet gas flow data, and is used to execute the steps S2~S5.

[0040] The depletion gas production pressure measurement system for the early evaluation stage of coalbed methane development also includes a confining pressure control system, an axial pressure control system, and a temperature control system. The confining pressure control system, the axial pressure control system, and the temperature control system are respectively used to control the confining pressure, axial pressure, and temperature of the coalbed methane sample in the sample holder; the control module is connected to the confining pressure control system, the axial pressure control system, and the temperature control system.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] The present invention provides a method and system for measuring the exhaustion gas production pressure in the early evaluation stage of coalbed methane development. The method simulates the coalbed methane production through a exhaustion gas production simulation device, and obtains the internal pressure P1 of the coal sample and the corresponding gas flow data of the gas outlet to fit the gas production per ton of coal-reservoir pressure prediction model; then, the coalbed methane depletion gas production influence radius is calculated through the fitted reservoir pressure distribution prediction mathematical equation, and the economic limit gas production per ton of coal is calculated in combination with the abandoned coalbed methane production; finally, the exhaustion gas production pressure of the coal seam is calculated through the economic limit gas production per ton of coal and the gas production per ton of coal-reservoir pressure prediction model; the method can effectively determine the exhaustion gas production pressure in the early evaluation stage of coalbed methane development through the accurate data of the mined coal seam, has strong pertinence and high accuracy, can provide strong technical support for the decision-making of coalbed methane development, has great decision-making reference value, and can be widely used in coalbed methane development decision-making work. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 A schematic diagram of the structure of a depletion gas production pressure measuring device for the early evaluation stage of coalbed methane development provided by an embodiment of the present invention; wherein 1 is a sample holder, 2 is an air inlet, 3 is an air outlet, 4 is a pressure detector, and 5 is a pressure relief valve;

[0044] Figure 2 A curve diagram of reservoir pressure-gas production per ton of coal obtained in an embodiment of the present invention;

[0045] Figure 3 This is a curve diagram of reservoir pressure distribution. DETAILED DESCRIPTION

[0046] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0047] Embodiment 1

[0048] Embodiment 1 of the present invention provides a method for measuring the exhaustion gas production pressure in the early evaluation stage of coalbed methane development, which is implemented based on an exhaustion gas production simulation device, such as Figure 1As shown, the exhaustion gas production simulation device includes a sample holder 1, the sample holder 1 is used to place the coal sample of coalbed methane, one end of the sample holder 1 is provided with an air inlet 2, and the other end is provided with an air outlet 3, the air inlet 2, the middle of the sample holder 1 and the air outlet 3 are respectively provided with pressure detectors 4, which are respectively used to detect the air inlet pressure P0, the internal pressure P1 of the coal sample and the air outlet pressure P2; the air inlet 2 is provided with a constant pressure and constant speed pump for continuously injecting methane into the coal sample, and the air outlet 3 is provided with a pressure relief valve 5 and a flow meter; the measuring method includes the following steps:

[0049] Step S1: A simulation experiment is carried out using the measuring device: the coal sample of the target coal seam is dried and placed in the sample holder 1, and methane is filled into the sample using the constant pressure and constant speed pump until the internal pressure of the coal sample is equal to the actual reservoir pressure of the coal seam, and then the pressure relief valve is controlled to make the pressure drop process at the gas outlet consistent with the bottom hole pressure drop process during the actual coalbed methane well development process, while monitoring the internal pressure P1 of the coal sample and the gas flow rate at the gas outlet.

[0050] The step S1 specifically includes the following steps:

[0051] Step S11: placing the dried coal sample in the sample holder 1 of the coalbed methane depletion gas production simulation device, and continuously evacuating the sample holder 1 at the gas outlet 3 through a vacuum pump;

[0052] Step S12: The confining pressure of the coal sample in the sample holder 1 is set to the actual vertical ground stress of the target coal seam, the axial pressure is set to the horizontal ground stress of the coal seam, and the temperature is set to the real ground temperature of the coal seam through the confining pressure control system, the axial pressure control system and the temperature control system;

[0053] Step S13: methane is charged into the coal sample in the sample holder 1 through the constant pressure and constant speed pump of the air inlet 2, and the internal pressure P1 of the coal sample is monitored in real time until the internal pressure P1 of the coal sample is equal to the actual reservoir pressure P3 of the coal seam;

[0054] Step S14: Control the opening of the pressure relief valve to control the outlet pressure P2, so that the decreasing process of the outlet pressure P2 is consistent with the decreasing process of the bottom hole flow pressure during the actual coalbed methane well development process; during the decreasing process of the outlet pressure P2, monitor the internal pressure P1 of the coal sample and the outlet gas flow L in real time.

[0055] Among them, the process of bottom hole pressure drop during actual coalbed methane well development is empirical data.

[0056] Step S2: Obtain multiple groups of coal sample internal pressure and corresponding outlet gas flow data, and fit to obtain a gas production per ton of coal-reservoir pressure prediction model.

[0057] Specifically, the specific method of step S2 is:

[0058] Step S21: obtaining multiple groups of coal sample internal pressure and corresponding outlet gas flow data;

[0059] Step S22: Calculate the gas production per ton of coal according to the gas flow rate at the gas outlet. The calculation formula is:

[0060] T = L / M0; (1)

[0061] Among them, T represents the gas production per ton of coal, L represents the gas flow rate at the gas outlet, and M0 represents the mass of the coal sample;

[0062] Step S23: Combine multiple groups of gas production per ton of coal and coal sample internal pressure P1 data corresponding to gas flow rates at different gas outlets, perform data fitting to obtain a relationship equation between gas production per ton of coal and internal pressure of coal sample for the target coal seam, and use the fitted relationship equation between gas production per ton of coal and internal pressure of coal sample as a prediction model for gas production per ton of coal and reservoir pressure. Specifically, in this embodiment, the quadratic equation T=AP1 is used. 2 +BP1+C0 are used to fit the data to obtain the relationship equation between the gas production per ton of coal in the coal seam and the internal pressure of the coal sample, where A, B, and C0 represent the three fitting parameters of the quadratic equation.

[0063] like Figure 2 As shown in the figure, the fitting curves between the total gas production, free gas volume, adsorbed gas volume and the internal pressure of the coal sample are shown. The relationship equations obtained by fitting the above quadratic equation are:

[0064] T1=0.0831P1 2 -1.5434P1+7.2919;R 2 =0.996;

[0065] T2=-0.021P1 2 +0.019P1+0.0720;R 2 =0.788;

[0066] T3=0.0852P1 2 -1.5626P1+7.2200;R 2 =0.996;

[0067] In the formula, T1 represents the total gas production, T2 represents the free gas volume, and T3 represents the adsorbed gas volume. The relationship equation corresponding to the above total gas production is the gas production per ton of coal-reservoir pressure prediction model.

[0068] Step S3: According to the pressure values ​​measured at each pressure measurement point at the end of the simulated experiment gas production, as well as the distance between each pressure measurement point and the outlet pressure measurement point, a mathematical equation for predicting the reservoir pressure distribution in the coal seam is fitted; and the critical desorption pressure of the coal sample is substituted into the equation, and the distance value corresponding to the critical desorption pressure of the coal sample is calculated as the influence radius r of coalbed methane depletion gas production.

[0069] Specifically, in step S3, a logarithmic equation is used to fit the mathematical equation for predicting reservoir pressure distribution in the coal seam. The critical desorption pressure is measured by an isothermal adsorption test.

[0070] Specifically, in this embodiment, the logarithmic equation used for fitting is:

[0071] P = D·ln(d)+E; (2)

[0072] Among them, P represents the pressure value measured at the pressure measurement point at the end of the simulated experiment gas production, d represents the distance between the pressure measurement point and the gas outlet, and D and E are the corresponding fitting constants. Taking the time corresponding to 8500s as the end of gas production, the mathematical equation for predicting reservoir pressure distribution is: P=0.2951·ln(d)+1.9658. According to formula (2), the calculation formula for the influence radius r of coalbed methane depletion gas production is:

[0073] r= ; (3)

[0074] Wherein, e represents the natural constant, and P' represents the critical desorption pressure of the coal sample.

[0075] like Figure 3 As shown, the test data of different pressure measurement points (inlet pressure P0, coal sample internal pressure P1, outlet pressure P2) at different times during the above-mentioned simulated experimental gas production process are recorded, which can simulate the pressure drop law during the coalbed methane depletion production process; wherein, a rectangular coordinate system is established with the outlet 3 as the coordinate origin, the horizontal axis coordinate 0 cm corresponds to the position of the outlet 3, the horizontal axis coordinate -10 cm corresponds to the position of the inlet 2, and the length of the simulation device is 10 cm; specifically, in this embodiment, according to the values ​​of each pressure measurement point at the end of the simulated experiment gas production (8500s) and the position of each pressure measurement point, the mathematical equation for predicting the reservoir pressure distribution in the coal seam is fitted, which conforms to the logarithmic distribution relationship.

[0076] Then, the critical desorption pressure of the coal sample is substituted into the mathematical equation for predicting the reservoir pressure distribution, and the distance value corresponding to the critical desorption pressure of the coal sample is calculated as the radius r of influence of coalbed methane depletion and gas production; wherein, coalbed methane is mainly adsorbed on the pore wall of the coal seam in the form of adsorbed gas, and the reservoir pressure needs to be reduced to the critical desorption pressure so that the coalbed methane is desorbed from the pore wall and then produced from the coalbed methane well; therefore, at the end of coalbed methane exploitation, the position where the reservoir pressure is the critical desorption pressure is the farthest radius that can be affected by coalbed methane depletion and gas production during the entire coalbed methane exploitation process.

[0077] Furthermore, the coal seam mass M within the reservoir pressure influence range is calculated based on the average coal seam gas thickness h, gas production influence radius r and coal seam density ρ. The calculation formula is:

[0078] M=πr 2 hρ. (4)

[0079] Step S4: Calculate the economic limit gas production per ton of coal according to the gas production impact radius r of coalbed methane depletion and the abandoned coalbed methane production.

[0080] Specifically, in step S4, the calculation formula for the economic limit gas production per ton of coal is:

[0081] Q= q ga / M=q ga / πr 2 hρ; (5)

[0082] Among them, Q represents the economic limit gas production per ton of coal, q ga represents the abandoned coalbed methane production of coalbed methane horizontal wells, M represents the coal seam quality within the influence range of reservoir pressure, h represents the average thickness of coalbed methane, and ρ represents the coal seam density.

[0083] As the production of coalbed methane horizontal wells decreases after entering the decline period, until the production cash inflow is equal to the cash outflow, the exhaustion time of coalbed methane horizontal well production occurs. At this time, the single well production of coalbed methane horizontal wells is the abandoned coalbed methane production; therefore, the present invention calculates the economic limit gas production of coalbed methane production based on the operating costs, taxes, surcharges, coalbed methane sales prices and commodity rates of the coalbed methane engineering company (the daily gas production of the coalbed methane well is sold and the income is lower than the sum of the operating costs, taxes, surcharges, etc., and continuing the mining of the coalbed well will continue to lose money at this time), the abandoned coalbed methane production of the coalbed methane horizontal well q ga The calculation formula is:

[0084] = (C + W) / (Jη); (6)

[0085] In the formula, q ga is the abandoned coalbed methane production, m 3 / d; C is the operating cost, RMB / d; W is the tax and surcharge, RMB / d; J is the coal-bed methane price, yuan / m 3 ; η is the commodity rate, %.

[0086] Step S5: Determine the depletion gas production pressure of the target coal seam based on the economic limit gas production per ton of coal obtained in step S4 and in combination with the gas production per ton of coal-reservoir pressure prediction model.

[0087] Specifically, the economic limit gas production per ton of coal Q obtained in step S4 is substituted as T1 into the relationship equation between gas production per ton of coal and internal pressure of coal sample obtained in step S2, and the internal pressure of the coal sample corresponding to the economic limit gas production per ton of coal is calculated, which is the depletion gas production pressure of the target coal seam. In this embodiment, the smaller solution of the equation is used as the depletion gas production pressure of the coal seam, and its calculation formula is:

[0088] ; (7)

[0089] in, Indicates the depletion gas production pressure of the target coal seam.

[0090] In summary, the present invention simulates coalbed methane production by adopting a depletion gas production simulation device, and obtains a gas production per ton of coal-reservoir pressure prediction model by fitting the acquired internal pressure P1 of the coal sample with the corresponding gas flow data of the gas outlet; then the coalbed methane depletion gas production influence radius r is calculated by the fitted reservoir pressure distribution prediction mathematical equation, and the abandoned coalbed methane production of the coalbed methane horizontal well is obtained by the production cash inflow status, and the economic limit gas production per ton of coal is calculated; finally, the coal seam depletion gas production pressure is calculated by the economic limit gas production per ton of coal and the gas production per ton of coal-reservoir pressure prediction model; it can effectively determine the depletion gas production pressure in the early evaluation stage of coalbed methane development, has strong pertinence and high accuracy, can provide strong technical parameter support for coalbed methane development decision-making, has great decision-making reference value, and can be applied to coalbed methane development decision-making work.

[0091] Embodiment 2

[0092] Embodiment 2 of the present invention provides a depletion gas production pressure measuring device for the early evaluation stage of coalbed methane development, including a depletion gas production simulation device and a control module; specifically, the depletion gas production simulation device includes a sample holder 1, the sample holder 1 is used to place a coalbed methane sample, one end of the sample holder 1 is provided with an air inlet 2, and the other end is provided with an air outlet 3, the air inlet 2, the middle part of the sample holder 1 and the air outlet 3 are respectively provided with pressure detectors 4, which are respectively used to detect the air inlet pressure P0, the internal pressure P1 of the coal sample and the air outlet pressure P2; the air inlet is provided with a constant pressure and constant speed pump, which is used to continuously inject methane into the coal sample, and the air outlet is provided with a pressure relief valve and a flow meter, the pressure relief valve is used to control the pressure value at the air outlet of the sample holder 1, and the flow meter is used to detect the gas flow output from the air outlet.

[0093] The control module is connected to the pressure detector 4, the constant pressure and constant speed pump, the pressure relief valve, and the flow meter, and is used to control the working status of the constant pressure and constant speed pump and the pressure relief valve, and is also used to collect pressure data and gas flow data at the outlet, and to execute steps S2 to S5 described in Example 1, and then calculate the depletion gas production pressure value of the target coal seam.

[0094] Furthermore, a depletion gas production pressure measuring device in the early evaluation stage of coalbed methane development in this embodiment also includes a confining pressure control system, an axial pressure control system, and a temperature control system. The confining pressure control system, the axial pressure control system, and the temperature control system are used to control the confining pressure, axial pressure, and temperature of the coal sample in the sample holder 1, respectively; the control module is connected to the confining pressure control system, the axial pressure control system, and the temperature control system.

[0095] In this embodiment, by setting the control module, the automatic measurement and calculation of the exhaustion gas production pressure value of the target coal seam can be achieved, thereby improving the measurement efficiency.

[0096] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for measuring the exhaustion gas pressure in the early evaluation stage of coalbed methane development, characterized in that: The exhaustion gas production simulation device is used for implementation. The exhaustion gas production simulation device includes a sample holder, the sample holder is used to place the coal sample of coalbed methane, one end of the sample holder is provided with an air inlet, and the other end is provided with an air outlet. The air inlet, the middle of the sample holder and the air outlet are respectively provided with pressure detectors, which are respectively used to detect the air inlet pressure P0, the internal pressure P1 of the coal sample and the air outlet pressure P2; the air inlet is provided with a constant pressure and constant speed pump, which is used to continuously inject methane into the coal sample; The gas outlet is provided with a pressure relief valve and a flow meter; the process comprises the following steps: Step S1: dry the coal sample of the target coal seam and place it in the sample holder, fill it with methane using the constant pressure and constant speed pump until the internal pressure of the coal sample is equal to the actual reservoir pressure of the coal seam, then control the pressure relief valve to make the pressure drop process at the gas outlet consistent with the bottom hole pressure drop process during the actual coalbed methane well development process, and monitor the internal pressure P1 of the coal sample and the gas flow rate at the gas outlet at the same time; Step S2: Obtain multiple groups of coal sample internal pressure P1 and corresponding outlet gas flow data, calculate the gas production per ton of coal according to the outlet gas flow, and perform data fitting based on the gas production per ton of coal and the corresponding coal sample internal pressure P1 data to obtain the relationship equation between gas production per ton of coal and internal pressure of coal sample as a prediction model of gas production per ton of coal and reservoir pressure; the fitting equation is: T=AP1 2 +BP1+C0; Among them, T represents the gas production per ton of coal, A, B, C0 represent the three fitting parameters of the quadratic equation, and P1 represents the internal pressure of the coal sample; Step S3: According to the pressure values ​​measured at each pressure measurement point at the end of the simulated experiment gas production, and the distance between each pressure measurement point and the outlet pressure measurement point, a mathematical equation for predicting reservoir pressure distribution in the coal seam is fitted, and the fitting equation is: P=D·ln(d)+E; P represents the pressure value measured at the pressure measurement point at the end of the simulated experiment gas production, d represents the distance between the pressure measurement point and the outlet, and D and E are corresponding fitting constants; calculate the radius r of gas production influence of coalbed methane depletion; the calculation formula is: r= ； Where, e represents the natural constant, P' represents the critical desorption pressure of the coal sample; Step S4: Calculate the economic limit gas production per ton of coal according to the gas production impact radius r of coalbed methane depletion and the abandoned coalbed methane production; the calculation formula is: Q= q ga / πr 2 hρ; Among them, Q represents the economic limit gas production per ton of coal, q ga represents the abandoned coalbed methane production of coalbed methane horizontal wells, h represents the average thickness of coalbed methane, and ρ represents the coal seam density; Step S5, calculate the coal seam exhaustion gas production pressure of the target coal seam, the calculation formula is: ； in, It represents the depletion gas production pressure of the target coal seam, and Q represents the economic limit gas production per ton of coal.

2. The method for measuring the exhaustion gas pressure in the early evaluation stage of coalbed methane development according to claim 1 is characterized in that: The step S1 specifically includes the following steps: Step S11: placing the dried coal sample in a sample holder of a depletion gas production simulation device and continuously evacuating the sample; Step S12: setting the confining pressure of the coal sample in the sample holder to the actual vertical ground stress of the target coal seam, setting the axial pressure to the average horizontal ground stress of the coal seam, and setting the temperature to the real ground temperature of the coal seam through the confining pressure control system, the axial pressure control system and the temperature control system; Step S13: filling the coal sample in the sample holder with methane through a constant pressure and constant speed pump, and monitoring the internal pressure P1 of the coal sample in real time until the internal pressure P1 of the coal sample is equal to the actual reservoir pressure P3 of the coal seam; Step S14: Control the opening of the pressure relief valve to control the outlet pressure P2, so that the decreasing process of the outlet pressure P2 is consistent with the decreasing process of the bottom hole flow pressure during the actual coalbed methane well development process; during the decreasing process of the outlet pressure P2, monitor the internal pressure P1 of the coal sample and the outlet gas flow in real time.

3. The method for measuring the exhaustion gas pressure in the early evaluation stage of coalbed methane development according to claim 1 is characterized in that: The specific method of step S2 is: Step S21: obtaining multiple groups of coal sample internal pressure and corresponding outlet gas flow data; Step S22: Calculate the gas production per ton of coal according to the gas flow rate at the gas outlet. The calculation formula is: T = L / M0; Among them, T represents the gas production per ton of coal, L represents the gas flow rate at the gas outlet, and M0 represents the mass of the coal sample; Step S23: Combine multiple groups of coal sample internal pressure P1 data corresponding to the gas production per ton of coal, perform data fitting to obtain the relationship equation between the gas production per ton of coal and the internal pressure of the coal sample of the target coal seam, and use the fitted relationship equation between the gas production per ton of coal and the internal pressure of the coal sample as the prediction model of the gas production per ton of coal and the reservoir pressure of the target coal seam.

4. The method for measuring the exhaustion gas pressure in the early evaluation stage of coalbed methane development according to claim 1 is characterized in that: Abandoned coalbed methane production from horizontal coalbed methane wells q ga The calculation formula is: q ga =(C+W) / (Jη); Where, C is the operating cost; W is the tax and surcharge; J is the coalbed methane selling price; η is the commodity rate.

5. The method for measuring the exhaustion gas pressure in the early evaluation stage of coalbed methane development according to claim 1 is characterized in that: In step S1, the temperature, confining pressure and axial pressure in the sample holder are set according to the formation temperature and the characteristics of the in-situ stress field.

6. A system for measuring depletion gas pressure in the early evaluation stage of coalbed methane development, used for implementing a method for measuring depletion gas pressure in the early evaluation stage of coalbed methane development as claimed in any one of claims 1 to 5, characterized in that: It includes a depletion gas production simulation device and a control module; the control module is connected to the pressure detector, the constant pressure and constant speed pump, the pressure relief valve, and the flow meter, and is used to control the working status of the constant pressure and constant speed pump and the pressure relief valve, and is also used to collect pressure data and outlet gas flow data, and is used to execute steps S2~S5.

7. The exhaustion gas pressure measurement system for the early evaluation stage of coalbed methane development according to claim 6 is characterized in that: It also includes a confining pressure control system, an axial pressure control system, and a temperature control system, wherein the confining pressure control system, the axial pressure control system, and the temperature control system are used to control the confining pressure, the axial pressure, and the temperature of the coal sample in the sample holder (1), respectively; and the control module is connected to the confining pressure control system, the axial pressure control system, and the temperature control system.

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