A drainage technology for inhibiting fluctuation of bottom hole flowing pressure of in-situ coalbed gas well

Abstract

The application discloses a kind of inhibiting in situ coalbed gas well bottom hole flowing pressure fluctuation drainage mining process, belong to coalbed gas development technical field.In situ coalbed gas well drainage mining process is divided into test pumping, drainage pressure reduction, pressure build-up drainage, production control drainage, pressure reduction production, pressure control stable production, gas production attenuation 7 stages.Test pumping stage, determine coalbed liquid supply capacity;Drainage pressure reduction stage, gradually reduce bottom hole flowing pressure by drainage, so that coalbed starts to desorb;Pressure build-up drainage stage, slowly reduce bottom hole flowing pressure, so that casing pressure gradually rises to the set upper limit value;Production control drainage stage, maintain low daily gas production, continuously drain and reduce pressure and evaluate bottom hole flowing pressure fluctuation;Pressure reduction production stage, under high casing pressure, relatively flowing pressure daily drop, drain and produce gas, stepwise increase daily gas production;Pressure control stable production stage, gradually reduce casing pressure and stabilize bottom hole flowing pressure daily drop, so that gas and water are stably produced;Gas production attenuation stage, further reduce casing pressure and bottom hole flowing pressure, maintain daily gas production higher than the lower limit of industrial gas flow.

Description

Technical Field

[0001] This invention relates to a drainage process for suppressing bottom-hole flowing pressure fluctuations in in-situ coalbed methane wells, particularly a drainage process applicable to vertical and directional wells in low-permeability coal reservoirs in high-stress coal-bearing development zones. By optimizing the drainage operation system, this process avoids damage to coal reservoir permeability and improves the high-yield and long-term stable production of in-situ coalbed methane wells, belonging to the field of coalbed methane development technology. Background Technology

[0002] Compared to coalbed methane exploration and development in depressurization zones and goaf areas, in-situ coalbed methane resources are larger, of higher quality, and have more mature exploration technologies. Therefore, vigorously promoting in-situ coalbed methane exploration and development can significantly increase my country's coalbed methane production and make it an important supplement to conventional natural gas supply. The mechanism of in-situ coalbed methane development is to continuously reduce coal reservoir pressure by continuously drawing water to achieve coalbed methane desorption and production. To avoid damage to the coal reservoir during drainage, in-situ coalbed methane well drainage usually needs to follow the principles of continuity, slowness, and stability. However, during the drainage process of in-situ coalbed methane wells, fluctuations in bottom hole flowing pressure are commonly observed, and the type, amplitude, and frequency of bottom hole flowing pressure fluctuations vary significantly among different wells and at different drainage stages, leading to a serious risk of coal reservoir damage during in-situ coalbed methane well drainage.

[0003] Bottom-hole flowing pressure fluctuations occur after gas production in in-situ coalbed methane wells, and are therefore closely related to the gas-water two-phase flow in the surrounding coal reservoir. Since bottom-hole flowing pressure fluctuations in in-situ coalbed methane wells are simultaneously influenced by coalbed methane geological factors and fracturing-drainage engineering factors, their fluctuation types, amplitudes, and frequencies vary significantly. The Upper Permian coal seam group development area in southwestern my country, represented by the Liupanshui coalfield in Guizhou Province, is rich in coalbed methane resources with enormous development potential. Currently, hydraulic fracturing is the primary method used for coal reservoir stimulation in this area. Due to high inland stress, low reservoir permeability, and high gas saturation in this region, significant bottom-hole flowing pressure fluctuations occur during the drainage process of in-situ coalbed methane wells. This not only damages the permeability of the coal reservoir but also poses significant challenges to the drainage management of coalbed methane wells, hindering high and stable production. In order to comprehensively promote the development of in-situ coalbed methane resources in the Upper Permian in Southwest my country, it is necessary to solve problems such as bottom hole pressure fluctuations, velocity and pressure sensitivity damage of coal reservoirs, and difficulty in controlling drainage parameters from the aspects of drainage and production technology, so as to improve the gas production effect of in-situ coalbed methane wells and the economic benefits of coalbed methane development. Summary of the Invention

[0004] Technical Problem: The purpose of this invention is to overcome the problems existing in the prior art and provide a drainage process that suppresses bottom-hole flowing pressure fluctuations in in-situ coalbed methane wells. This addresses the problem that traditional drainage methods in low-permeability coal seams in high-stress areas can easily lead to sensitive damage to in-situ coalbed methane well reservoirs, resulting in low gas production and short stable production time.

[0005] Technical solution: In order to achieve the above objectives, the present invention provides a drainage and production process for suppressing bottom pressure fluctuations in in-situ coalbed methane wells, comprising seven stages: trial pumping stage, fluid drainage and pressure reduction stage, pressure stagnation and fluid drainage stage, production control and fluid drainage stage, pressure reduction and production increase stage, pressure control and production stabilization stage, and gas production decline stage. Step (a): In the aforementioned trial pumping stage, the on-site drainage personnel begin the process from the installation and start-up of the drainage equipment at the in-situ coalbed methane well to the stable descent of the annular fluid level in the casing of the in-situ coalbed methane well. The pumping frequency of the pumping unit is gradually adjusted from low to high until the end of this stage. By continuously monitoring and analyzing the bottom-hole flowing pressure and the annular fluid level of the in-situ coalbed methane well, the coal seam fluid supply capacity is comprehensively evaluated, the daily water production variation range of the in-situ coalbed methane well during the drainage process is predicted, and the amplitude and type of bottom-hole flowing pressure fluctuations in the gas-water two-phase flow stage are inferred. This provides a basis for determining the drainage parameters, including the daily water production, the daily decrease in bottom-hole flowing pressure, and the rate of descent of the annular fluid level in the casing, in the later stage of the trial pumping stage. Step (b): The aforementioned fluid drainage and pressure reduction stage begins with a stable and slow descent of the annular fluid level in the casing and ends when the coal reservoir in the in-situ coalbed methane well reaches critical desorption and the annular pressure in the casing just begins to appear. From the end of the trial pumping stage to the end of this stage, the on-site drainage personnel will drain water according to the daily water production determined in the trial pumping stage and maintain a relatively stable daily water production. During the drainage process, they will closely monitor the changes in bottom hole flowing pressure and further infer the change law of bottom hole flowing pressure in the in-situ coalbed methane well during the gas-water two-phase flow stage. They will maintain the daily decrease in bottom hole flowing pressure within 0.025 to 0.035 MPa / d, increase the fracturing fluid return rate before the annular pressure in the casing of the in-situ coalbed methane well appears, and increase the expansion range of the coal reservoir pressure drop funnel. Step (c): The pressure-holding and fluid-draining stage begins after the annular pressure of the in-situ coalbed methane well becomes apparent, and continues until the annular pressure rises to the upper limit of the annular pressure. On-site production personnel control the daily decrease in bottom hole flowing pressure by reducing the pumping unit strokes, so that the annular pressure gradually increases and then continues to hold pressure. During the pressure-holding and fluid-draining process, the bottom hole flowing pressure fluctuation is closely observed, and the type and characteristics of the bottom hole flowing pressure fluctuation are compared and determined to predict the bottom hole flowing pressure fluctuation characteristics of the in-situ coalbed methane well in the subsequent production control and fluid-draining stage and the pressure reduction and production increase stage. Step (d): The controlled production and drainage stage begins when the annular pressure of the in-situ coalbed methane well rises to the upper limit of the control range, and ends when the daily decrease in bottom hole flowing pressure, daily gas production, and annular pressure of the in-situ coalbed methane well become relatively stable. The in-situ coalbed methane well maintains relatively stable annular pressure and daily decrease in bottom hole flowing pressure until the daily decrease in bottom hole flowing pressure stabilizes. At this point, the on-site drainage personnel open the gas pipeline valves, gradually increasing the daily gas production of the in-situ coalbed methane well from 0 to 100 m³ / h. 3 / d, then gradually increased to 200 m 3 / d, during this process, the bottom pressure fluctuation begins to appear and intensifies. The bottom pressure fluctuation of the in-situ coalbed methane well should be closely observed to determine the type and characteristics of the bottom pressure fluctuation. The evolution characteristics of the bottom pressure fluctuation of the in-situ coalbed methane well during the pressure reduction and production increase stage should be predicted, and this should be used as the basis for the daily decrease of bottom pressure and the adjustment of casing annulus pressure during the production control and liquid discharge stage. Step (e): The pressure reduction and production enhancement stage begins when the daily decrease in bottom hole flowing pressure, daily gas production, and casing annular pressure of the in-situ coalbed methane well are relatively stable, and ends when the daily gas production of the in-situ coalbed methane well reaches a relatively high stable state under the condition of stable casing annular pressure. The duration of the pressure reduction and production enhancement stage is controlled by factors such as the daily decrease in bottom hole flowing pressure during drainage and the difference in casing annular liquid level corresponding to desorption of multiple coal reservoirs. The fluctuation of bottom hole flowing pressure of coalbed methane well gradually increases. The daily decrease in bottom hole flowing pressure and casing annular pressure at the beginning of the pressure reduction and production enhancement stage should be determined according to the type and characteristics of bottom hole flowing pressure fluctuation in the later stage of production control and liquid drainage. During the process of step-wise reduction of bottom hole flowing pressure to enhance the daily gas production of in-situ coalbed methane well, the changes in the type and characteristics of bottom hole flowing pressure fluctuation should be closely observed, and this should be used as the basis for adjusting the daily decrease in bottom hole flowing pressure, casing annular pressure, and daily gas production enhancement rate during the pressure reduction and production enhancement stage. Step (f): The pressure control and stable production stage begins with the desorption and gas supply from multiple coal reservoirs in the in-situ coalbed methane well and stabilizes gas production, and ends when the daily gas production of the in-situ coalbed methane well stabilizes at a high level. The duration of the pressure control and stable production stage is controlled by factors such as the gas supply volume, the well-controlled resources of the in-situ coalbed methane well, and the continuity of the drainage process. The gas-water production rate of the in-situ coalbed methane well and the gas-water two-phase flow state in the coal reservoir have stabilized, and the bottom hole pressure fluctuations have gradually weakened. On-site drainage personnel reduce the coal reservoir pressure by gradually reducing the daily decrease in bottom hole pressure and the annulus pressure of the casing, thereby releasing the coal reservoir's desorption and gas supply capacity and maintaining the long-term stable production of the in-situ coalbed methane well. Step (g): The gas production decline stage begins when the daily gas production of the in-situ coalbed methane well starts to decline and ends when the daily gas production drops below the lower limit of the industrial gas flow rate. On-site drainage personnel need to further increase the daily reduction of bottom hole pressure, reduce casing annular pressure and bottom hole pressure, slow down the rate of decline of daily gas production of the in-situ coalbed methane well, and make the daily gas production higher than the lower limit of the industrial gas flow rate of the regional in-situ coalbed methane well.

[0006] In step (a), the trial pumping stage lasts for 3 to 5 days from the installation and start-up of the in-situ coalbed methane well drainage equipment to the stable drop of the annulus liquid level in the casing of the in-situ coalbed methane well. By controlling the pumping frequency of the pumping unit during the trial pumping phase and adjusting the bottom hole flowing pressure and annular fluid level parameters of the in-situ coalbed methane wells by on-site production personnel, and considering that the coal reservoir contains saturated single-phase water flow during the trial pumping phase and there is no bottom hole flowing pressure fluctuation in the in-situ coalbed methane wells, the coal seam fluid supply capacity is comprehensively evaluated by predicting the bottom hole flowing pressure fluctuation during subsequent production based on the differences in the production capacity of the in-situ coalbed methane wells. When the coal seam liquid supply capacity is greater than 5 m³ during the trial pumping phase 3 / d, with the same daily decrease in bottom pressure, the daily water production is 20-30%. In the subsequent drainage process of in-situ coalbed methane wells, the bottom pressure fluctuation is usually weak, which is manifested as the bottom pressure fluctuation amplitude being less than 15KPa and the fluctuation frequency being 100 times / d. When the coal seam liquid supply capacity is less than 4 m³ 3 / d, with the same daily decrease in bottom hole pressure, the daily water production is as low as 20-30%. In the subsequent drainage process of in-situ coalbed methane wells, the bottom hole pressure fluctuation is usually strong, which is manifested as the bottom hole pressure fluctuation amplitude is greater than 50KPa and the fluctuation frequency is less than 40 times / d.

[0007] In step (b), the liquid drainage and pressure reduction stage begins from the stable and slow decline of the annular liquid level in the casing and ends when the annular pressure in the casing just begins to appear at the critical desorption of the coal reservoir in the in-situ coalbed methane well. The duration of the liquid drainage and pressure reduction stage is controlled by the gas saturation of the coal reservoir in the in-situ coalbed methane well, the critical desorption pressure, and the daily decrease in bottom hole flowing pressure during the drainage process of the in-situ coalbed methane well. For coal reservoirs with high gas saturation, the duration of the liquid drainage and depressurization stage is 10 to 30 days. For coal reservoirs with low gas saturation, the liquid drainage and depressurization stage lasts for 80–140 days. During the depressurization and drainage stage, the coal reservoir contains saturated single-phase water flow. Therefore, there is no bottom pressure fluctuation in the in-situ coalbed methane well. Based on the difference in production capacity during the depressurization and drainage stage of the in-situ coalbed methane well, the bottom pressure fluctuation during the subsequent drainage and production process can be predicted. When the coal seam liquid supply capacity is greater than 10 m³ during the liquid drainage and pressure reduction stage 3 / d, with the same daily decrease in bottom pressure, the daily water production is 5%-15%. In the subsequent drainage process of in-situ coalbed methane wells, the bottom pressure fluctuation is usually weak, which is manifested as the bottom pressure fluctuation amplitude being less than 15 KPa and the fluctuation frequency being 100 times / d. When the coal seam's liquid supply capacity is weak, less than 8 m 3 / d, with the same daily decrease in bottom pressure, the daily water production is as low as 5%-15%. In the subsequent drainage process of in-situ coalbed methane wells, the bottom pressure fluctuation is usually strong, which is manifested as the bottom pressure fluctuation amplitude is greater than 50KPa and the fluctuation frequency is less than 40 times / d.

[0008] In step (c), during the pressure-holding and drainage stage, the on-site drainage personnel reduce the pumping unit stroke frequency to control the daily decrease in bottom hole flowing pressure to 0.010–0.015 MPa / d. The casing annular pressure gradually increases to 1.5–2.0 MPa and then continues to hold the pressure. The duration of the pressure-holding and drainage stage is affected by the daily decrease in bottom hole flowing pressure of in-situ coalbed methane, the gas saturation of the coal reservoir, and the critical desorption pressure during the drainage process. When the daily drop in bottom hole flowing pressure, the gas saturation of coal-bearing gas producing layer is greater than 80%, and the critical desorption pressure is greater than 3MPa, the annular pressure rise rate is greater than 200KPa / d, and the pressure stagnation and drainage stage lasts for 5 to 10 days. When the daily drop in undercurrent pressure, the gas saturation of the coal reservoir is less than 60%, and the critical desorption pressure is less than 2 MPa, the annular pressure rise rate of the casing is 20-100 kPa, and this stage lasts for 30-50 days. During the pressure-holding and liquid-draining stage, the coalbed methane begins to desorb and does not flow out in one direction. The coal reservoir is filled with unsaturated single-phase water. The bottom pressure of the in-situ coalbed methane well only fluctuates slightly and will not damage the coal reservoir. The bottom pressure fluctuation during the subsequent drainage process can be predicted based on the difference in liquid production capacity during the pressure-holding and liquid-draining stage of the in-situ coalbed methane well. When the coal seam liquid supply capacity is greater than 10 m³ during the pressure-retaining and drainage stage 3 / d, with the same daily decrease in bottom pressure, the daily water production is greater than 100%. In the subsequent drainage process of in-situ coalbed methane wells, the bottom pressure fluctuation is usually weak, which is manifested as the bottom pressure fluctuation amplitude being less than 15 KPa and the fluctuation frequency being 100 times / d. When the coal seam's liquid supply capacity is weak, less than 5m 3 / d, when the daily water production is less than 50% under the same daily decrease in bottom pressure, the bottom pressure fluctuation of the in-situ coalbed methane well is usually strong during the subsequent drainage process, which is manifested as a bottom pressure fluctuation amplitude greater than 50KPa and a fluctuation frequency less than 40 times / d.

[0009] In step (d), the controlled production and drainage stage begins when the annular pressure of the in-situ coalbed methane well rises to the upper limit of the control value and ends when the daily decrease in bottom hole flowing pressure, daily gas production, and annular pressure of the in-situ coalbed methane well are relatively stable. The duration of the controlled production and drainage stage is controlled by the daily decrease in bottom hole flowing pressure, daily gas production, and fluctuations in annular pressure during the drainage process. If the daily decrease in bottom hole flowing pressure, daily gas production, and annular pressure of the in-situ coalbed methane well fluctuate significantly, the duration of the production control and fluid discharge phase is >30 days. If the daily decrease in bottom hole flowing pressure, daily gas production, and casing annulus pressure of the in-situ coalbed methane well are relatively stable, the duration of the production control and fluid discharge phase is 10-20 days. In step (d), the production control and fluid discharge stage begins when the annular pressure of the in-situ coalbed methane well casing rises to the upper limit of the control value and ends when the daily decrease in bottom hole flowing pressure, daily gas production and annular pressure of the in-situ coalbed methane well are relatively stable. When the bottom pressure fluctuation amplitude of an in-situ coalbed methane well is less than 15 kPa and the fluctuation frequency is greater than 100 times / day, the bottom pressure fluctuation is relatively weak. The daily bottom pressure drop is increased to 0.020 MPa / day, and the casing annulus pressure is reduced to 1.0 MPa. When the bottom hole pressure fluctuation amplitude of an in-situ coalbed methane well is 15–50 kPa and the fluctuation frequency is 50–100 times / day, it is characterized by moderate bottom hole pressure fluctuation intensity, a stable bottom hole pressure daily decrease of 0.010–0.015 MPa / day, and maintaining the casing annulus pressure at 1.5–2.0 MPa. When the fluctuation range of the bottom flow pressure (10) of the in-situ coalbed methane well is higher than 50 kPa and the fluctuation frequency is lower than 50 times / day, the bottom flow pressure (10) fluctuates strongly. It is necessary to reduce the daily drop of the bottom flow pressure (10) to 0.005 MPa / day and increase the casing annulus pressure to 2.5 MPa.

[0010] In step (e), the duration of the pressure reduction and production enhancement stage is controlled by the daily decrease in bottom hole flowing pressure during drainage and the difference in annular fluid levels corresponding to the desorption of multiple coal reservoirs. If the daily decrease in bottom hole flowing pressure is less than 10 m... 3 / d, if the difference in annular liquid level corresponding to the desorption of multiple coal reservoirs is greater than 30 m, the pressure reduction and production increase stage will last longer, usually 200-300 days; conversely, the pressure reduction and production increase stage will last shorter, usually 50-100 days. During the pressure reduction and production increase phase, the coalbed methane desorption rate increases rapidly, typically with a production increase of 10-30 m³ / s. 3 / d, at this time, the gas-water two-phase flow in the coal reservoir changes to a water-gas two-phase flow, which will lead to instability of the gas-water two-phase flow. As the daily gas production of in-situ coalbed methane wells rapidly increases during the depressurization and production enhancement phase, the bottom hole flowing pressure fluctuates dramatically.

[0011] In step (e), the pressure reduction and production increase stage begins when the daily gas production of the in-situ coalbed methane well starts to decline and ends when the daily gas production drops below the lower limit of the industrial gas flow. When the bottom hole pressure fluctuation is weak, it is manifested as the bottom hole pressure fluctuation amplitude of the in-situ coalbed methane well is less than 20 kPa and the fluctuation frequency is higher than 80 times / day. When the bottom hole pressure is gradually reduced to 0.025 MPa / day and the casing annulus pressure is reduced to 1.0 MPa. When the bottom hole pressure fluctuation amplitude of an in-situ coalbed methane well is 20–100 kPa and the fluctuation frequency is 40–80 times / day, it is characterized by moderate bottom hole pressure fluctuation intensity, a stable bottom hole pressure daily decrease of 0.015–0.020 MPa / day, and maintaining the casing annulus pressure at 1.5–2.0 MPa. When the bottom hole pressure fluctuation of an in-situ coalbed methane well exceeds 100 kPa and the fluctuation frequency is less than 40 times / day, it indicates strong bottom hole pressure fluctuation. It is necessary to reduce the daily bottom hole pressure drop to 0.010 MPa / day and increase the casing annulus pressure to 2.5 MPa.

[0012] In step (f), the duration of the pressure control and stable production phase is controlled by factors such as the gas supply volume, the well-controlled resources of the in-situ coalbed methane well, and the continuity of the drainage process. For gas supply volumes greater than 15 × 10 4 m 3 Well-controlled resource quantity greater than 500×10 4 m 3 For in-situ coalbed methane wells with reasonable drainage and production control, the pressure control and stable production stage (6) should last for no less than 1.5 years; otherwise, the gas supply volume should be less than 10×10 4 m 3 Well-controlled resources less than 300×10 4 m 3 Discontinuous drainage and production will result in the pressure control and stable production phase lasting less than 1 year (6); During the pressure control and production stabilization phase, when the bottom flow pressure fluctuation amplitude of the in-situ coalbed methane well is less than 15 kPa and the fluctuation frequency is higher than 100 times / day, the bottom flow pressure fluctuation is relatively weak. The daily bottom flow pressure is gradually increased to 0.020 MPa / day, and the casing annulus pressure is reduced to 0.8 MPa. When the bottom hole pressure fluctuation amplitude of an in-situ coalbed methane well is 15–50 kPa and the fluctuation frequency is 50–100 times / day, it is characterized by moderate bottom hole pressure fluctuation intensity, a stable bottom hole pressure daily decrease of 0.010–0.015 MPa / day, and maintaining the casing annulus pressure at 0.8–1.5 MPa. When the bottom hole pressure fluctuation of an in-situ coalbed methane well exceeds 50 kPa and the fluctuation frequency is less than 50 times / day, it indicates strong bottom hole pressure fluctuation. It is necessary to reduce the daily bottom hole pressure drop to 0.005 MPa / day and increase the casing annulus pressure to 1.5 MPa.

[0013] In step (g), the process by which the on-site production personnel further increase the daily reduction of bottom hole flowing pressure during the gas production decline stage is as follows: by reducing the annular pressure of the casing and continuing to reduce the annular liquid level height, the rate of decline of daily gas production of the in-situ coalbed methane well is slowed down, the coalbed methane desorption rate is controlled to decrease and the desorption amount is reduced, thereby extending the service life of the in-situ coalbed methane well. When the bottom pressure fluctuation amplitude of the in-situ coalbed methane well is less than 15 kPa and the fluctuation frequency is higher than 100 times / day, the bottom pressure fluctuation is relatively weak. Gradually increase the daily bottom pressure drop to 0.020 MPa / day and gradually reduce the casing annulus pressure to 0.3 MPa. When the bottom pressure fluctuation amplitude of the in-situ coalbed methane well is 15-50 kPa and the fluctuation frequency is 50-100 times / day, the bottom pressure fluctuation intensity is moderate, the daily decrease of the stable bottom pressure is 0.015-0.020 MPa / day, and the casing annulus pressure is gradually reduced to 0.5 MPa. When the bottom hole pressure fluctuation of an in-situ coalbed methane well exceeds 50 kPa and the fluctuation frequency is less than 50 times / day, the bottom hole pressure fluctuation is strong. It is necessary to reduce the daily bottom hole pressure drop to 0.010 MPa / day and gradually reduce the casing annulus pressure to 0.8 MPa.

[0014] Beneficial effects: By adopting the above technical solution, the present invention overcomes the problems of severe fluctuations in bottom hole flowing pressure, velocity and pressure sensitivity damage to coal reservoirs, low resource recovery rate, and difficulty in achieving high and stable production in in-situ coalbed methane wells under traditional drainage methods. First, by utilizing the daily water production rate during the in-situ coalbed methane well test pumping and depressurization stages, the amplitude and type of bottom hole pressure fluctuations in the gas-water two-phase flow stage were inferred. Before bottom hole pressure fluctuations, reasonable production control parameters such as the daily pressure reduction and casing annulus pressure were determined. Second, during the production control and depressurization / production enhancement stages, based on reducing the amplitude and frequency of pressure fluctuations, reasonable production control parameters such as the daily pressure reduction, casing pressure, and daily gas production enhancement rate were adjusted. This ensured effective drainage and depressurization while effectively suppressing bottom hole pressure fluctuations, thus protecting the coal reservoir. Third, during the pressure control and production stabilization stages and gas production decline stages, the stable gas-water flow pattern led to a decrease in the amplitude and frequency of bottom hole pressure fluctuations. By adjusting the daily pressure reduction and lowering the casing annulus pressure, the stable production time and long-term gas production effect of the coalbed methane well were significantly extended. The main advantages compared with existing technologies are: ① Predicting bottom hole pressure fluctuations during the gas production process of in-situ coalbed methane wells using daily water production data before casing pressure is encountered, thus optimizing drainage and production control parameters before casing pressure is encountered; ② Reducing the intensity of "gas surge" in the coal reservoir and its impact on bottom hole pressure fluctuations by adjusting the daily decrease in bottom hole pressure of in-situ coalbed methane wells in stages and at high frequency; ③ Effectively suppressing large fluctuations in bottom hole pressure during drainage and gas production by utilizing the buffering effect of the higher casing pressure of in-situ coalbed methane wells on "gas surge," thus avoiding speed-sensitive and pressure-sensitive damage to the coal reservoir; ④ The drainage process is simple, the drainage project implementation cost is low, and the economic, environmental, and social benefits are good. Attached Figure Description

[0015] Figure 1 The present invention relates to a drainage process for suppressing bottom flow pressure fluctuations in in-situ coalbed methane wells.

[0016] In the diagram: 1-Trial pumping stage; 2-Drainage and pressure reduction stage; 3-Pressure holding and drainage stage; 4-Production control and drainage stage; 5-Pressure reduction and production increase stage; 6-Pressure control and stable production stage; 7-Gas production decline stage; 8-Drainage and production time axis; 9-Drainage and production control parameter change axis; 10-Bottomhole flowing pressure; 11-Casing annulus fluid level; 12-Casing annulus pressure; 13-Daily gas production; 14-Daily water production; 15-Casing annulus pressure upper limit; 16-Industrial gas flow lower limit. Detailed Implementation

[0017] The present invention will be further described below with reference to the embodiments shown in the accompanying drawings: The present invention provides a drainage and production process for suppressing bottom pressure fluctuations in in-situ coalbed methane wells, which mainly consists of seven stages: trial pumping stage 1, drainage and pressure reduction stage 2, pressure stagnation and drainage stage 3, production control and drainage stage 4, pressure reduction and production increase stage 5, pressure control and stable production stage 6, and gas production decline stage 7. Step a: In the trial pumping stage 1, the on-site drainage personnel start the process from the installation and start-up of the drainage equipment in the in-situ coalbed methane well to the stable descent of the annular fluid level 11 in the casing of the in-situ coalbed methane well. The pumping frequency of the pumping unit is gradually adjusted from low to high until the end of this stage. By continuously monitoring and analyzing the bottom flow pressure 10 and the position of the annular fluid level 11 in the casing of the in-situ coalbed methane well, the coal seam fluid supply capacity is comprehensively evaluated, the range of daily water production 14 of the in-situ coalbed methane well during the drainage process is predicted, and the amplitude and type of bottom flow pressure fluctuation in the gas-water two-phase flow stage of the in-situ coalbed methane well are inferred. In the later stage of the trial pumping stage 1, the drainage parameters of daily water production 14, daily decrease of bottom flow pressure 10, and descent rate of the annular fluid level 11 in the casing are determined, providing a basis for the determination of the drainage parameters in the early stage of the drainage and pressure reduction stage 2. The duration of the trial pumping stage 1, from the installation and start-up of the in-situ coalbed methane well drainage equipment to the stable decline of the annular liquid level 11 in the casing of the in-situ coalbed methane well, is 3 to 5 days. The pumping frequency of the pumping unit in the first stage of the trial pumping phase was controlled by on-site production personnel, and the bottom hole flowing pressure 10 and the annular fluid level 11 of the in-situ coalbed methane well were adjusted. In the first stage of the trial pumping phase, the coal reservoir contained saturated single-phase water flow, and there was no fluctuation in the bottom hole flowing pressure 10 in the in-situ coalbed methane well. Therefore, the bottom hole flowing pressure fluctuation 10 during the subsequent production process was predicted based on the difference in the production capacity of the in-situ coalbed methane well to comprehensively evaluate the coal seam fluid supply capacity. During the trial pumping phase, the liquid supply capacity of coal seam 1 is strong, exceeding 5 m. 3 / d, the daily water production under the same 10-day decrease in bottom pressure is 14, which is 20-30%. In the subsequent drainage process of in-situ coalbed methane wells, the fluctuation of bottom pressure 10 is usually weak, which is manifested as small fluctuation amplitude of bottom pressure 10, below 15 KPa, and high fluctuation frequency, which is 100 times / d. When the coal seam liquid supply capacity is less than 4 m³ 3 / d, the daily water production under the same bottom pressure decrease over 10 days is low, at 20-30%. In the subsequent drainage process of in-situ coalbed methane wells, the bottom pressure fluctuation is usually strong, which is manifested as a bottom pressure fluctuation amplitude greater than 50 kPa and a fluctuation frequency less than 40 times / day.

[0018] Step b: The aforementioned drainage and pressure reduction stage 2 begins with the stable and slow descent of the annular fluid level 11 in the casing and ends when the annular pressure 12 in the casing of the in-situ coalbed methane well just begins to appear, starting from the end of the test pumping stage 1 and ending at the end of this stage. The on-site drainage personnel will drain water according to the daily water production 14 determined in the test pumping stage 1 and maintain the relative stability of the daily water production 14. During the drainage process, they will closely monitor the changes in the bottom hole flowing pressure 10 and further speculate on the change law of the bottom hole flowing pressure of the in-situ coalbed methane well in the gas-water two-phase flow stage. They will try to keep the daily decrease of the bottom hole flowing pressure 10 within 0.025 to 0.035 MPa / d, increase the fracturing fluid return rate before the casing annular pressure 12 appears in the in-situ coalbed methane well, and increase the expansion range of the coal reservoir pressure drop funnel. The described drainage and pressure reduction stage 2 begins with a stable and slow descent of the annular liquid level 11 in the casing and ends when the annular pressure 12 in the casing just begins to appear at the critical desorption of the coal reservoir in the in-situ coalbed methane well. The duration of the drainage and pressure reduction stage 2 is controlled by the gas saturation of the coal reservoir in the in-situ coalbed methane well, the critical desorption pressure, and the magnitude of the 10-day decrease in the bottom flow pressure during the drainage process of the in-situ coalbed methane well. For coal reservoirs with high gas saturation, the second stage of liquid drainage and depressurization is short, lasting from 10 to 30 days. For coal reservoirs with low gas saturation, the second stage of liquid drainage and depressurization lasts for a long time, reaching 80 to 140 days. In the second stage of liquid drainage and pressure reduction, the coal reservoir is filled with saturated single-phase water flow. Therefore, there is no fluctuation in the bottom pressure 10 in the in-situ coalbed methane well. However, the fluctuation of the bottom pressure 10 during the subsequent drainage process can be predicted based on the difference in production capacity in the second stage of liquid drainage and pressure reduction of the in-situ coalbed methane well. During the drainage and depressurization stage, the coal seam has a strong liquid supply capacity, exceeding 10 m. 3 / d, when the daily water production is high under the same bottom pressure decrease of 10 days, which is 5%-15%, the bottom pressure fluctuation of the in-situ coalbed methane well is usually weak during the subsequent drainage process. It is characterized by small bottom pressure fluctuation amplitude, below 15 KPa, and high fluctuation frequency, 100 times / day. When the coal seam's liquid supply capacity is weak, less than 8 m 3 / d, the daily water production under the same bottom pressure decrease over 10 days is low, at 5%-15%. In the subsequent drainage process of in-situ coalbed methane wells, the bottom pressure fluctuation is usually strong, which is manifested as a large fluctuation amplitude of 50KPa and a low fluctuation frequency of 40 times / day.

[0019] Step c: The pressure-holding and drainage stage 3 begins after the annular pressure 12 of the in-situ coalbed methane well is observed, and ends when the annular pressure 12 rises to the upper limit 15. The duration of the pressure-holding and drainage stage 3 is affected by the 10-day decrease in bottom pressure of the in-situ coalbed methane well, the gas saturation of the coal reservoir, and the critical desorption pressure during the drainage process. When the 10-day decrease in bottom pressure, the gas saturation of the coal-bearing gas producing layer, and the critical desorption pressure are relatively high, the annular pressure 12 rises rapidly, and the duration of the pressure-holding and drainage stage 3 is very short. On-site drainage personnel control the 10-day decrease in bottom pressure by reducing the number of pumping cycles, so that the annular pressure 12 gradually increases and continues to hold pressure. During the pressure-holding and drainage process, the fluctuation of bottom pressure 10 is closely observed, and the type and characteristics of bottom pressure fluctuation are compared and determined to predict the fluctuation characteristics of bottom pressure 10 in the subsequent production control and drainage stage 4 (pressure reduction and production increase stage 5) of the in-situ coalbed methane well. In the aforementioned pressure-holding and drainage stage 3, the on-site drainage personnel reduced the pumping unit stroke frequency to control the 10-day decrease in bottom hole flowing pressure to 0.010–0.015 MPa / d. The casing annular pressure 12 gradually increased to 1.5–2.0 MPa and then continued to hold pressure. The duration of pressure-holding and drainage stage 3 is affected by the 10-day decrease in bottom hole flowing pressure of in-situ coalbed methane during drainage, the gas saturation of the coal reservoir, and the critical desorption pressure. When the 10-day drop in bottom hole flowing pressure, the gas saturation of coal-bearing gas producing layer is greater than 80%, and the critical desorption pressure is greater than 3MPa, the casing annular pressure rises rapidly, exceeding 200KPa / d, and the pressure stagnation and drainage stage 3 lasts for a very short time, from 5 to 10 days. When the 10-day drop in bottom pressure, the gas saturation of the coal reservoir is below 60%, and the critical desorption pressure is less than 2 MPa, the annular pressure rise rate is 20-100 kPa, and this stage can last for 30-50 days. During the pressure-holding and liquid-draining stage 3, the coalbed methane begins to desorb and does not flow in one direction. The coal reservoir is filled with unsaturated single-phase water. The bottom pressure 10 in the in-situ coalbed methane well only fluctuates slightly and will not damage the coal reservoir. The fluctuation of the bottom pressure 10 during the subsequent drainage process can be predicted based on the difference in liquid production capacity during the pressure-holding and liquid-draining stage 3 of the in-situ coalbed methane well. During the pressure buildup and drainage stage, the 3rd coal seam has a strong liquid supply capacity, exceeding 10 m. 3 / d, with the same 10-day decrease in bottom pressure, the daily water production is 14 high. When it is greater than 100%, the fluctuation of bottom pressure 10 in the subsequent drainage process of the in-situ coalbed methane well is usually weak, which is manifested as low fluctuation amplitude of bottom pressure 10, less than 15 kPa, and high fluctuation frequency, 100 times / day. When the coal seam's liquid supply capacity is weak, less than 5m 3 / d, when the daily water production is less than 50% under the same 10-day decrease in bottom pressure, the bottom pressure 10 of the in-situ coalbed methane well usually fluctuates strongly during subsequent drainage, which is manifested as a fluctuation amplitude of bottom pressure 10 greater than 50 kPa and a fluctuation frequency of less than 40 times / day.

[0020] Step d: The controlled production and drainage stage 4 begins when the annular pressure 12 of the in-situ coalbed methane well rises to the upper limit of the control range, and ends when the bottomhole flowing pressure 10-day decrease, daily gas production 13, and annular pressure 12 of the in-situ coalbed methane well become relatively stable. The in-situ coalbed methane well maintains the controlled annular pressure 12 and the bottomhole flowing pressure 10-day decrease relatively stable until the bottomhole flowing pressure 10-day decrease of the in-situ coalbed methane well becomes stable. At the end of this stage, the on-site drainage personnel open the gas pipeline valves to gradually increase the daily gas production of the in-situ coalbed methane well from 0 to 100 m³. 3 / d, then gradually increased to 200 m 3 / d During this process, the bottom hole pressure fluctuation begins to appear and intensifies. The bottom hole pressure fluctuation of the in-situ coalbed methane well should be closely observed to determine the type and characteristics of the bottom hole pressure fluctuation. The evolution characteristics of the bottom hole pressure fluctuation of the in-situ coalbed methane well in the pressure reduction and production increase stage 5 should be predicted, and this should be used as the basis for the daily reduction of bottom hole pressure 10 and the adjustment of casing annulus pressure 12 in the production control and fluid discharge stage 4. The production control and fluid drainage stage 4 begins when the annular pressure 12 of the in-situ coalbed methane well rises to the upper limit of the control value and ends when the bottom hole flowing pressure 10-day decrease, daily gas production 13, and annular pressure 12 of the in-situ coalbed methane well become relatively stable. The duration of the production control and fluid drainage stage 4 is controlled by the fluctuation of the bottom hole flowing pressure 10-day decrease, daily gas production 13, and annular pressure 12 during the drainage process. If the bottom hole flowing pressure of the in-situ coalbed methane well shows a significant 10-day decrease, daily gas production of 13, and casing annular pressure of 12, then the duration of the production control and liquid discharge stage 4 is >30 days. If the bottom hole flowing pressure of the in-situ coalbed methane well has a relatively stable 10-day decrease, daily gas production of 13, and casing annular pressure of 12, then the duration of the production control and liquid discharge stage 4 is 10-20 days. The production control and fluid discharge stage 4 begins when the casing annulus pressure 12 of the in-situ coalbed methane well rises to the upper limit of the control value, and ends when the bottom flow pressure 10-day decrease, daily gas production 13, and casing annulus pressure 12 of the in-situ coalbed methane well are relatively stable. When the fluctuation amplitude of the bottom hole flowing pressure 10 in an in-situ coalbed methane well is less than 15 kPa and the fluctuation frequency is higher than 100 times / day, the bottom hole flowing pressure 10 fluctuation is relatively weak. The bottom hole flowing pressure 10 can be gradually increased to 0.020 MPa / day, and the casing annulus pressure can be reduced by 12 to 1.0 MPa. When the bottom hole flowing pressure 10 of the in-situ coalbed methane well fluctuates by 15 to 50 kPa and by 50 to 100 times / day, it indicates that the bottom hole flowing pressure 10 fluctuates in a moderate intensity and can stabilize the daily decrease of the bottom hole flowing pressure 10 at 0.010 to 0.015 MPa / day, maintaining the casing annulus pressure 12 at 1.5 to 2.0 MPa. When the fluctuation range of the bottom hole flowing pressure 10 in an in-situ coalbed methane well is higher than 50 kPa and the fluctuation frequency is lower than 50 times / day, it indicates that the bottom hole flowing pressure 10 fluctuates strongly. It is necessary to reduce the bottom hole flowing pressure 10 daily to 0.005 MPa / day and increase the casing annulus pressure by 12 to 2.5 MPa.

[0021] Step e: The pressure reduction and production enhancement stage 5 begins when the bottom hole flowing pressure 10-day decrease, daily gas production 13, and casing annular pressure 12 of the in-situ coalbed methane well are relatively stable, and ends when the daily gas production 13 of the in-situ coalbed methane well reaches a relatively stable state under the condition of stable casing annular pressure 12. The duration of the pressure reduction and production enhancement stage 5 is controlled by factors such as the 10-day decrease in bottom hole flowing pressure during the drainage process and the difference in casing annular liquid level 11 corresponding to the desorption of multiple coal reservoirs. The bottom hole flowing pressure 10 of the coalbed methane well fluctuates. As the pressure gradually increases, the 10-day decrease in bottom-hole flowing pressure and the casing annulus pressure 12 should be determined in the early stage of the pressure reduction and production enhancement stage 5 based on the type and characteristics of bottom-hole flowing pressure fluctuations in the later stage of production control and fluid discharge stage 4. During the process of step-by-step reduction of bottom-hole flowing pressure 10 to increase the daily gas production of in-situ coalbed methane wells 13, the changes in the type and characteristics of bottom-hole flowing pressure fluctuations should be closely observed, and this should be used as the basis for adjusting the 10-day decrease in bottom-hole flowing pressure, the casing annulus pressure 12, and the rate of increase in daily gas production 13 in the pressure reduction and production enhancement stage 5. The duration of the pressure reduction and production enhancement stage 5 is controlled by the 10-day decrease in bottom hole flowing pressure during drainage and the difference in annular fluid level 11 corresponding to the desorption of multiple coal reservoirs. If the 10-day decrease in bottom hole flowing pressure is less than 10 m... 3 / d, if the difference in annular liquid level 11 corresponding to the desorption of multiple coal reservoirs is greater than 30 m, then the duration of the pressure reduction and production improvement stage 5 is relatively long, usually 200-300 days; conversely, the duration of the pressure reduction and production improvement stage 5 is relatively short, usually 50-100 days. During stage 5 of pressure reduction and production increase, the coalbed methane desorption rate increases rapidly, typically with a gas production increase of 10-30 m³ / h. 3 / d, at this time, the gas-water two-phase flow in the coal reservoir changes to a water-gas two-phase flow, which will lead to instability of the gas-water two-phase flow. As the daily gas production of the in-situ coalbed methane well 13 rapidly increased during the pressure reduction and production increase stage 5, the bottom hole flowing pressure fluctuated violently.

[0022] The pressure reduction and production increase stage 5 begins with a decrease in the daily gas production of the in-situ coalbed methane well, starting from 13, and ends when the daily gas production of 13 drops below the lower limit of industrial gas flow, 16. When the bottom hole pressure 10 fluctuates weakly, it is characterized by a fluctuation amplitude of less than 20 kPa and a fluctuation frequency of more than 80 times / day. This means that the daily decrease in bottom hole pressure 10 can be gradually increased to 0.025 MPa / day, and the casing annulus pressure can be reduced from 12 to 1.0 MPa. When the bottom hole flowing pressure 10 of the in-situ coalbed methane well fluctuates within an amplitude of 20–100 kPa and a frequency of 40–80 times / day, it indicates that the bottom hole flowing pressure 10 fluctuates in a moderate intensity and can stabilize the daily decrease of the bottom hole flowing pressure 10 within 0.015–0.020 MPa / day, maintaining the casing annulus pressure 12 at 1.5–2.0 MPa. When the fluctuation range of the bottom hole flowing pressure 10 in an in-situ coalbed methane well is higher than 100 kPa and the fluctuation frequency is lower than 40 times / day, it indicates that the bottom hole flowing pressure 10 fluctuates strongly. It is necessary to reduce the bottom hole flowing pressure 10 daily to 0.010 MPa / day and increase the casing annulus pressure by 12 to 2.5 MPa.

[0023] Step f: The pressure control and stable production stage 6 begins with the desorption and gas supply from multiple coal reservoirs in the in-situ coalbed methane well and stabilizes gas production, and ends when the daily gas production of the in-situ coalbed methane well reaches a high level and stabilizes. The duration of the pressure control and stable production stage 6 is controlled by factors such as the gas supply volume, the well-controlled resources of the in-situ coalbed methane well, and the continuity of the drainage process. The gas-water production rate of the in-situ coalbed methane well and the gas-water two-phase flow state in the coal reservoir have stabilized. The fluctuation of the bottom hole flowing pressure 10 gradually weakens. The on-site drainage personnel reduce the coal reservoir pressure by gradually reducing the daily decrease of the bottom hole flowing pressure 10 and the casing annulus pressure 12, thereby releasing the coal reservoir's desorption and gas supply capacity and maintaining the long-term stable production of the in-situ coalbed methane well. The duration of the pressure control and production stabilization stage 6 is controlled by factors such as the gas supply area, the well-controlled resources of the in-situ coalbed methane well, and the continuity of the drainage process. For gas supply volumes greater than 15 × 10 4 m 3 Well-controlled resource quantity greater than 500×10 4 m 3 For in-situ coalbed methane wells with reasonable drainage and production control, the pressure control and stable production phase should last for no less than 1.5 years; conversely, if the gas supply volume is less than 10×10⁶, the wells should be properly managed. 4 m 3 Well-controlled resources less than 300×10 4 m 3 Discontinuous drainage and extraction can lead to the duration of the pressure control and stable production phase being less than 1 year. In the pressure control and production stabilization stage 6, when the fluctuation amplitude of the bottom flow pressure 10 of the in-situ coalbed methane well is less than 15 kPa and the fluctuation frequency is higher than 100 times / day, the bottom flow pressure 10 fluctuation is relatively weak. The bottom flow pressure 10 is gradually increased to 0.020 MPa / day, and the casing annulus pressure is reduced by 12 to 0.8 MPa. When the bottom hole flowing pressure 10 of the in-situ coalbed methane well fluctuates by 15 to 50 kPa and by 50 to 100 times / day, it indicates that the bottom hole flowing pressure 10 fluctuates in a moderate intensity, and the daily decrease in the stable bottom hole flowing pressure 10 is 0.010 to 0.015 MPa / day, maintaining the casing annulus pressure 12 at 0.8 to 1.5 MPa. When the fluctuation range of the bottom flow pressure (10) of the in-situ coalbed methane well is higher than 50 kPa and the fluctuation frequency is lower than 50 times / day, the bottom flow pressure (10) fluctuates strongly. It is necessary to reduce the bottom flow pressure (10) to 0.005 MPa / day and increase the casing annulus pressure (12 to 1.5 MPa).

[0024] Step g: The gas production decline stage 7 begins with the daily gas production 13 of the in-situ coalbed methane well decreasing and ends when the daily gas production 13 drops below the lower limit of industrial gas flow 16. On-site drainage personnel need to further increase the daily decrease in bottom hole pressure 10, reduce the annular pressure 12 and bottom hole pressure 10, and slow down the rate of decrease in daily gas production 13 of the in-situ coalbed methane well as much as possible, ensuring that the daily gas production 13 is higher than the lower limit of industrial gas flow 16 for in-situ coalbed methane wells in the region. The duration of the gas production decline stage 7 is controlled by factors such as the gas supply volume of the in-situ coalbed methane well and the drainage control process; generally, this stage should last 4 to 6 years.

[0025] The process of further increasing the bottom flow pressure reduction by 10 days during the gas production decline stage 7: by reducing the casing annulus pressure 12 and continuing to reduce the casing annulus liquid level 11, the decline rate of the daily gas production 13 of the in-situ coalbed methane well is slowed down as much as possible, the coalbed methane desorption rate is controlled to decrease and the desorption amount is reduced, and the service life of the in-situ coalbed methane well is extended. As the desorption rate and amount of coalbed methane decrease, the bottom pressure fluctuation of in-situ coalbed methane wells will be further weakened. When the bottom pressure fluctuation of in-situ coalbed methane wells is less than 15 kPa and the fluctuation frequency is higher than 100 times / day, the bottom pressure fluctuation is relatively weak. Gradually increase the bottom pressure fluctuation to 0.020 MPa / day and gradually reduce the casing annulus pressure to 0.3 MPa. When the bottom hole flowing pressure 10 of the in-situ coalbed methane well fluctuates by 15 to 50 kPa and by 50 to 100 times / day, the fluctuation intensity of the bottom hole flowing pressure 10 is moderate, and the daily decrease of the stable bottom hole flowing pressure 10 is 0.015 to 0.020 MPa / day, gradually reducing the casing annulus pressure by 12 to 0.5 MPa. When the fluctuation range of the bottom hole flowing pressure 10 in an in-situ coalbed methane well is higher than 50 kPa and the fluctuation frequency is lower than 50 times / day, the fluctuation of the bottom hole flowing pressure 10 is relatively strong. It is necessary to reduce the bottom hole flowing pressure 10 daily to 0.010 MPa / day and gradually reduce the casing annulus pressure 12 to 0.8 MPa.

Claims

1. A drainage process for suppressing bottomhole flowing pressure fluctuations in in-situ coalbed methane wells, characterized in that: It includes seven stages: trial pumping stage (1), liquid drainage and pressure reduction stage (2), pressure holding and liquid drainage stage (3), production control and liquid drainage stage (4), pressure reduction and production increase stage (5), pressure control and stable production stage (6), and gas production decline stage (7). Step (a): In the trial pumping stage (1), the on-site pumping personnel start the installation and start the pumping of the in-situ coalbed methane well pumping equipment and continue until the annular fluid level (11) of the in-situ coalbed methane well stabilizes and ends. The pumping frequency of the pumping unit is gradually adjusted from low to high until the end of this stage. By continuously monitoring and analyzing the bottom flow pressure (10) and the position of the annular fluid level (11) of the in-situ coalbed methane well, the coal seam liquid supply capacity is comprehensively evaluated, the range of change of the daily water production (14) of the in-situ coalbed methane well during the pumping process is predicted, the amplitude and type of bottom flow pressure fluctuation of the in-situ coalbed methane well in the gas-water two-phase flow stage are inferred, and the pumping parameters of the daily water production (14), the daily decrease of bottom flow pressure (10), and the rate of decrease of the annular fluid level (11) of the pipe casing are determined in the later stage of the trial pumping stage (1). The coal seam fluid supply capacity is comprehensively evaluated by predicting the bottom hole flowing pressure fluctuations during the subsequent drainage process based on the differences in the production capacity of in-situ coalbed methane wells. When the coal seam liquid supply capacity is greater than 5 m during the trial pumping stage (1) 3 / d, with the same daily decrease in bottom pressure (10), the daily water production (14) is 20-30%. The fluctuation of bottom pressure (10) in the subsequent drainage process of in-situ coalbed methane well is usually weak, which is manifested as the fluctuation amplitude of bottom pressure (10) is less than 15 KPa and the fluctuation frequency is 100 times / d. When the coal seam liquid supply capacity is less than 4 m³ 3 / d, with the same daily decrease in bottom pressure (10), the daily water production (14) is as low as 20-30%. The bottom pressure (10) fluctuation in the subsequent drainage process of in-situ coalbed methane wells is usually strong, which is manifested as a bottom pressure (10) fluctuation amplitude greater than 50KPa and a fluctuation frequency less than 40 times / d. Step (b): The aforementioned drainage and pressure reduction stage (2) begins from the stable and slow decline of the annular fluid level (11) in the casing and ends when the annular pressure (12) in the casing begins to appear at the critical desorption of the coal reservoir in the in-situ coalbed methane well. From the end of the trial pumping stage (1) to the end of this stage, the on-site drainage personnel carry out drainage according to the daily water production (14) determined in the trial pumping stage (1) and maintain the relative stability of the daily water production (14). During the drainage process, they closely monitor the changes in the bottom hole pressure (10) and further speculate on the change law of the bottom hole pressure in the in-situ coalbed methane well in the gas-water two-phase flow stage. They maintain the daily drop of the bottom hole pressure (10) at 0.025 to 0.035 MPa / d, increase the fracturing fluid return rate before the annular pressure (12) in the casing of the in-situ coalbed methane well appears, and increase the expansion range of the coal reservoir pressure drop funnel. Step (c): The pressure-holding and liquid-draining stage (3) begins after the annular pressure (12) of the in-situ coalbed methane well is revealed. From the beginning until the annular pressure (12) rises to the upper limit of the annular pressure (15), the on-site production personnel control the daily decrease of the bottom flow pressure (10) by reducing the pumping unit stroke, so that the annular pressure (12) gradually increases and continues to hold pressure. During the pressure-holding and liquid-draining process, the fluctuation of the bottom flow pressure (10) is closely observed, and the type and characteristics of the bottom flow pressure fluctuation are compared and determined. The fluctuation characteristics of the bottom flow pressure (10) of the in-situ coalbed methane well in the subsequent production control and liquid-draining stage (4) and pressure reduction and production increase stage (5) are predicted. Step (d): The controlled production and drainage stage (4) begins when the annular pressure (12) of the in-situ coalbed methane well rises to the upper limit of the control range, and ends when the daily decrease of the bottom hole flowing pressure (10), the daily gas production (13), and the annular pressure (12) of the in-situ coalbed methane well are relatively stable; the in-situ coalbed methane well maintains the controlled annular pressure (12) and the daily decrease of the bottom hole flowing pressure (10) relatively stable, and ends when the daily decrease of the bottom hole flowing pressure (10) of the in-situ coalbed methane well is stable. The on-site drainage personnel open the gas pipeline valves to gradually increase the daily gas production of the in-situ coalbed methane well from 0 to 100 m³. 3 / d, then gradually increased to 200 m 3 / d, during this process, the bottom pressure (10) fluctuation begins to appear and intensifies. The bottom pressure (10) fluctuation of the in-situ coalbed methane well should be closely observed to determine the type and characteristics of the bottom pressure fluctuation. Predict the evolution characteristics of the bottom pressure (10) fluctuation of the in-situ coalbed methane well during the pressure reduction and production increase stage (5), and use this as the basis for adjusting the daily decrease of bottom pressure (10) and the casing annulus pressure (12) during the production control and liquid discharge stage (4). Step (e): The pressure reduction and production enhancement stage (5) begins when the daily decrease in bottom hole flowing pressure (10), daily gas production (13), and casing annular pressure (12) of the in-situ coalbed methane well are relatively stable, and ends when the daily gas production (13) of the in-situ coalbed methane well reaches a relatively stable state under the condition of stable casing annular pressure (12); the duration of the pressure reduction and production enhancement stage (5) is controlled by the daily decrease in bottom hole flowing pressure (10) during the drainage process and the difference in casing annular liquid level (11) corresponding to the desorption of multiple coal reservoirs. As the fluctuations gradually increase, the daily decrease of bottom pressure (10) and casing annular pressure (12) in the early stage of the pressure reduction and production increase stage (5) should be determined based on the type and characteristics of bottom pressure fluctuations in the later stage of production control and liquid discharge (4). During the process of step-by-step reduction of bottom pressure (10) to increase the daily gas production (13) of the in-situ coalbed methane well, the changes in the type and characteristics of bottom pressure fluctuations should be closely observed, and this should be used as the basis for adjusting the daily decrease of bottom pressure (10), casing annular pressure (12), and daily gas production (13) increase rate in the pressure reduction and production increase stage (5). When the bottom flow pressure (10) fluctuates weakly, it is manifested that the fluctuation amplitude of the bottom flow pressure (10) of the in-situ coalbed methane well is less than 20 kPa and the fluctuation frequency is higher than 80 times / day. It is manifested that the daily decrease of the bottom flow pressure (10) is gradually increased to 0.025 MPa / day and the casing annulus pressure (12) is reduced to 1.0 MPa. When the fluctuation range of the bottom flow pressure (10) of the in-situ coalbed methane well is 20-100 kPa and the fluctuation frequency is 40-80 times / day, the bottom flow pressure (10) fluctuation intensity is moderate, the daily drop of the stable bottom flow pressure (10) is 0.015-0.020 MPa / day, and the casing annulus pressure (12) is maintained at 1.5-2.0 MPa. When the fluctuation amplitude of the bottom flow pressure (10) of the in-situ coalbed methane well is higher than 100 kPa and the fluctuation frequency is lower than 40 times / day, it is characterized by strong fluctuation of the bottom flow pressure (10). It is necessary to reduce the daily drop of the bottom flow pressure (10) to 0.010 MPa / day and increase the casing annulus pressure (12) to 2.5 MPa. Step (f): The pressure control and stable production stage (6) begins with the desorption and gas supply of multiple coal reservoirs in the in-situ coalbed methane well and stable gas production, and ends when the daily gas production (13) of the in-situ coalbed methane well reaches a high level and stabilizes. The duration of the pressure control and stable production stage (6) is controlled by factors such as the gas supply volume, the well control resources of the in-situ coalbed methane well, and the continuity of the drainage process. The gas-water production rate of the in-situ coalbed methane well and the gas-water two-phase flow state in the coal reservoir have stabilized. The fluctuation of the bottom flow pressure (10) gradually weakens. The on-site drainage personnel reduce the pressure of the coal reservoir by gradually reducing the daily decrease of the bottom flow pressure (10) and the annular pressure (12) of the casing, thereby releasing the desorption and gas supply capacity of the coal reservoir and maintaining the long-term stable production of the in-situ coalbed methane well. Step (g): The gas production decline stage (7) begins to decline from the daily gas production (13) of the in-situ coalbed methane well until the daily gas production (13) drops below the lower limit of industrial gas flow (16). On-site drainage personnel need to further increase the daily decrease of bottom hole pressure (10), reduce the casing annulus pressure (12) and bottom hole pressure (10), slow down the decline rate of daily gas production (13) of the in-situ coalbed methane well, and make the daily gas production (13) higher than the lower limit of industrial gas flow (16) of the regional in-situ coalbed methane well.

2. The drainage process for suppressing bottom-hole flowing pressure fluctuations in in-situ coalbed methane wells as described in claim 1, characterized in that: In step (a), the trial pumping stage (1) lasts for 3 to 5 days from the installation and start-up of the in-situ coalbed methane well drainage equipment to the stable drop of the annulus liquid level (11) in the casing of the in-situ coalbed methane well. By controlling the number of pumping operations of the pumping unit during the trial pumping stage (1) and adjusting the bottom flow pressure (10) and casing annulus fluid level (11) parameters of the in-situ coalbed methane well by the on-site production personnel; during the trial pumping stage (1), the coal reservoir is saturated with single-phase water flow, and there is no bottom flow pressure (10) fluctuation in the in-situ coalbed methane well. Therefore, the bottom flow pressure fluctuation during the subsequent production process is predicted based on the difference in the production capacity of the in-situ coalbed methane well to comprehensively evaluate the coal seam fluid supply capacity.

3. The drainage process for suppressing bottom-hole flowing pressure fluctuations in in-situ coalbed methane wells as described in claim 1, characterized in that: In step (b), the drainage and pressure reduction stage (2) begins when the annular liquid level (11) of the casing is stable and slowly decreases, and ends when the annular pressure (12) of the casing just begins to appear at the critical desorption of the coal reservoir in the in-situ coalbed methane well. The duration of the drainage and pressure reduction stage (2) is controlled by the gas saturation of the coal reservoir in the in-situ coalbed methane well, the critical desorption pressure, and the daily decrease in the bottom flow pressure (10) during the drainage process of the in-situ coalbed methane well. For coal reservoirs with high gas saturation, the duration of the liquid drainage and depressurization stage (2) is 10 to 30 days. For coal reservoirs with low gas saturation, the duration of the liquid drainage and depressurization stage (2) reaches 80 to 140 days; In the drainage and depressurization stage (2), the coal reservoir is filled with saturated single-phase water flow. Therefore, there is no bottom pressure (10) fluctuation in the in-situ coalbed methane well. Based on the difference in production capacity in the drainage and depressurization stage (2) of the in-situ coalbed methane well, the bottom pressure (10) fluctuation in the subsequent drainage process is predicted. When the coal seam liquid supply capacity is greater than 10 m3 / d during the drainage and pressure reduction stage (2), the daily water production (14) under the same bottom pressure (10) daily reduction is 5%-15%. The bottom pressure (10) fluctuation in the subsequent drainage process of the in-situ coalbed methane well is usually weak, which is manifested as the bottom pressure (10) fluctuation amplitude is less than 15 KPa and the fluctuation frequency is 100 times / d. When the coal seam's liquid supply capacity is weak, less than 8 m3 / d, the daily water production (14) under the same bottom pressure (10) daily decrease is as low as 5%-15%. During the subsequent drainage process of the in-situ coalbed methane well, the bottom pressure (10) usually fluctuates strongly, which is manifested as the bottom pressure (10) fluctuation amplitude being greater than 50KPa and the fluctuation frequency being less than 40 times / d.

4. The drainage process for suppressing bottom-hole flowing pressure fluctuations in in-situ coalbed methane wells as described in claim 1, characterized in that: In step (c), during the pressure-holding and drainage stage (3), the on-site drainage personnel reduce the pumping stroke to control the daily decrease of the bottom flow pressure (10) to 0.010-0.015 MPa / d. The casing annular pressure (12) gradually increases to 1.5-2.0 MPa and then continues to hold the pressure. The duration of the pressure-holding and drainage stage (3) is affected by the daily decrease of the bottom flow pressure (10) of the in-situ coalbed methane well during the drainage process, the gas saturation of the coal reservoir, and the critical desorption pressure. When the daily decrease of bottom hole pressure (10), the gas saturation of coal-bearing gas producing layer is greater than 80% and the critical desorption pressure is greater than 3MPa, the annular pressure (12) of the casing rises at a rate greater than 200KPa / d, and the duration of the pressure stagnation and liquid discharge stage (3) is 5 to 10 days. When the daily decrease of the underflow pressure (10), the gas saturation of the coal reservoir is less than 60%, and the critical desorption pressure is less than 2MPa, the annular pressure (12) of the casing rises at a rate of 20-100 KPa, and this stage lasts for 30-50 days. During the pressure-holding and liquid-draining stage (3), the coalbed methane begins to desorb and does not flow out in one direction. The coal reservoir is filled with unsaturated single-phase water. The bottom pressure (10) of the in-situ coalbed methane well only fluctuates slightly and will not damage the coal reservoir. Based on the difference in liquid production capacity during the pressure-holding and liquid-draining stage (3) of the in-situ coalbed methane well, the fluctuation of the bottom pressure (10) during the subsequent drainage process can be predicted. When the coal seam liquid supply capacity is greater than 10 m3 / d during the pressure-holding and liquid-draining stage (3), the daily water production (14) under the same bottom pressure (10) daily decrease is greater than 100%. In the subsequent drainage process of the in-situ coalbed methane well, the bottom pressure (10) fluctuation is usually weak, which is manifested as the bottom pressure (10) fluctuation amplitude is less than 15 KPa and the fluctuation frequency is 100 times / d. When the coal seam liquid supply capacity is weak (less than 5 m3 / d) and the daily water production (14) is less than 50% under the same bottom pressure (10) daily decrease, the bottom pressure (10) fluctuation during the subsequent drainage process of the in-situ coalbed methane well is usually strong, which is manifested as the bottom pressure (10) fluctuation amplitude being greater than 50 kPa and the fluctuation frequency being less than 40 times / d.

5. The drainage process for suppressing bottom-hole flowing pressure fluctuations in in-situ coalbed methane wells as described in claim 1, characterized in that: In step (d), the production control and liquid discharge stage (4) begins when the annular pressure (12) of the in-situ coalbed methane well rises to the upper limit of the control value and ends when the daily decrease of the bottom flow pressure (10), the daily gas production (13), and the annular pressure (12) of the in-situ coalbed methane well are relatively stable; the duration of the production control and liquid discharge stage (4) is controlled by the daily decrease of the bottom flow pressure (10), the daily gas production (13), and the fluctuation of the annular pressure (12) during the discharge process; If the daily decrease of bottom pressure (10), daily gas production (13), and annular pressure (12) of the in-situ coalbed methane well are significantly fluctuating, then the duration of the production control and liquid discharge stage (4) is >30 days. If the daily decrease in bottom pressure (10), daily gas production (13), and casing annulus pressure (12) of the in-situ coalbed methane well are relatively stable, the duration of the production control and liquid discharge stage (4) is 10-20 days.

6. The drainage process for suppressing bottomhole flowing pressure fluctuations in in-situ coalbed methane wells as described in claim 1 or 5, characterized in that: In step (d), the production control and liquid discharge stage (4) begins when the annular pressure (12) of the in-situ coalbed methane well casing rises to the upper limit of the control value and ends when the daily decrease of the bottom flow pressure (10), daily gas production (13), and annular pressure (12) of the in-situ coalbed methane well are relatively stable. When the fluctuation amplitude of the bottom flow pressure (10) of the in-situ coalbed methane well is less than 15 KPa and the fluctuation frequency is higher than 100 times / day, the bottom flow pressure (10) fluctuation is relatively weak, the daily drop of the bottom flow pressure (10) is increased to 0.020 MPa / day, and the casing annulus pressure (12) is reduced to 1.0 MPa; When the bottom pressure (10) of the in-situ coalbed methane well fluctuates by 15 to 50 kPa and the fluctuation frequency is 50 to 100 times / day, the bottom pressure (10) fluctuates by a moderate intensity, the daily decrease of the stable bottom pressure (10) is 0.010 to 0.015 MPa / day, and the casing annulus pressure (12) is maintained at 1.5 to 2.0 MPa. When the fluctuation range of the bottom flow pressure (10) of the in-situ coalbed methane well is higher than 50 kPa and the fluctuation frequency is lower than 50 times / day, the bottom flow pressure (10) fluctuates strongly. It is necessary to reduce the daily drop of the bottom flow pressure (10) to 0.005 MPa / day and increase the casing annulus pressure (12) to 2.5 MPa.

7. The drainage process for suppressing bottomhole flowing pressure fluctuations in in-situ coalbed methane wells as described in claim 1, characterized in that: In step (e), the duration of the pressure reduction and production increase stage (5) is controlled by the daily decrease in bottom hole flowing pressure (10) during the drainage process and the difference in the annular fluid level (11) corresponding to the desorption of multiple coal reservoirs. If the daily decrease in bottom hole flowing pressure (10) is less than 10 m 3 / d, if the difference in the annular liquid level (11) corresponding to the desorption of multiple coal reservoirs is greater than 30 m, then the pressure reduction and production improvement stage (5) will last for a long time, usually 200-300 days; otherwise, the pressure reduction and production improvement stage (5) will last for a short time, usually 50-100 days. During the pressure reduction and production increase stage (5), the coalbed methane desorption rate increases rapidly, and the gas production increase is usually 10-30 m³ / s. 3 / d, at this time, the gas-water two-phase flow in the coal reservoir changes to a water-gas two-phase flow, which will lead to instability of the gas-water two-phase flow. As the daily gas production (13) of the in-situ coalbed methane well rapidly increases during the pressure reduction and production increase stage (5), the bottom hole flowing pressure fluctuates violently.

8. The drainage process for suppressing bottomhole flowing pressure fluctuations in in-situ coalbed methane wells as described in claim 1, characterized in that: In step (e), the pressure reduction and production increase stage (5) begins to decrease from the daily gas production (13) of the in-situ coalbed methane well until the daily gas production (13) drops below the industrial gas flow lower limit (16).

9. The drainage process for suppressing bottomhole flowing pressure fluctuations in in-situ coalbed methane wells as described in claim 1, characterized in that: In step (f), the duration of the pressure control and stable production stage (6) is controlled by factors such as the gas supply volume, the well-controlled resources of the in-situ coalbed methane well, and the continuity of the drainage process. For gas supply volumes greater than 15 × 10 4 m 3 Well-controlled resource quantity greater than 500×10 4 m 3 For in-situ coalbed methane wells with reasonable drainage and production control, the pressure control and stable production stage (6) should last for no less than 1.5 years; otherwise, the gas supply volume should be less than 10×10 4 m 3 Well-controlled resources less than 300×10 4 m 3 Discontinuous drainage and production will result in the pressure control and stable production phase lasting less than 1 year (6); During the pressure control and stable production stage (6), when the fluctuation range of the bottom flow pressure (10) of the in-situ coalbed methane well is less than 15 KPa and the fluctuation frequency is higher than 100 times / day, the bottom flow pressure (10) fluctuation is relatively weak, and the daily decrease of the bottom flow pressure (10) is gradually increased to 0.020 MPa / day, and the casing annulus pressure (12) is reduced to 0.8 MPa; When the bottom pressure (10) fluctuation amplitude of the in-situ coalbed methane well is 15-50 kPa and the fluctuation frequency is 50-100 times / day, the bottom pressure (10) fluctuation intensity is moderate, the daily drop of the stable bottom pressure (10) is 0.010-0.015 MPa / day, and the casing annulus pressure (12) is maintained at 0.8-1.5 MPa. When the fluctuation range of the bottom flow pressure (10) of the in-situ coalbed methane well is higher than 50 kPa and the fluctuation frequency is lower than 50 times / day, the bottom flow pressure (10) fluctuates strongly. It is necessary to reduce the daily drop of the bottom flow pressure (10) to 0.005 MPa / day and increase the casing annulus pressure (12) to 1.5 MPa.

10. The drainage process for suppressing bottom-hole flowing pressure fluctuations in in-situ coalbed methane wells as described in claim 1, characterized in that: In step (g), the process of further increasing the daily decrease of bottom hole flowing pressure (10) by the on-site production personnel in the gas production attenuation stage (7) is as follows: by reducing the casing annulus pressure (12) and continuing to reduce the casing annulus liquid level (11), the rate of decrease of daily gas production (13) of the in-situ coalbed methane well is slowed down, the coalbed methane desorption rate is controlled to decrease, the desorption amount is reduced, and the service life of the in-situ coalbed methane well is extended. When the fluctuation range of the bottom flow pressure (10) of the in-situ coalbed methane well is less than 15 kPa and the fluctuation frequency is higher than 100 times / day, the fluctuation of the bottom flow pressure (10) is relatively weak. The daily decrease of the bottom flow pressure (10) is gradually increased to 0.020 MPa / day, and the annulus pressure (12) of the casing is gradually reduced to 0.3 MPa. When the bottom pressure (10) of the in-situ coalbed methane well fluctuates by 15 to 50 kPa and the fluctuation frequency is 50 to 100 times / day, the bottom pressure (10) fluctuation intensity is moderate, the daily decrease of the stable bottom pressure (10) is 0.015 to 0.020 MPa / day, and the casing annulus pressure (12) is gradually reduced to 0.5 MPa; When the fluctuation range of the bottom flow pressure (10) of the in-situ coalbed methane well is higher than 50 kPa and the fluctuation frequency is lower than 50 times / day, the bottom flow pressure (10) fluctuates strongly. It is necessary to reduce the daily drop of the bottom flow pressure (10) to 0.010 MPa / day and gradually reduce the casing annulus pressure (12) to 0.8 MPa.

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