In-situ deep coal bed gas well casing annular pressure control process

Abstract

The application discloses an in-situ deep coal bed gas well pipe sleeve annular pressure control process and belongs to the technical field of deep coal bed gas development. The in-situ deep coal bed gas well pipe sleeve annular pressure control process is divided into six stages, i.e., pressure test and emptying, pressure rising and gas holding, pressure stabilizing and gas production, pressure adjusting and flow stabilizing, pressure reducing and production controlling, and negative pressure and production prolonging. In the pressure test and emptying stage, all the free gas generated in the fracturing construction is discharged to the pipe sleeve annulus and is pressure-hold, and then the free gas is slowly discharged from the pipe sleeve annulus. In the pressure rising and gas holding stage, the pipe sleeve annular pressure is slowly increased to the set upper limit value. In the pressure stabilizing and gas production stage, the pipe sleeve annular control pressure is maintained to be slightly lower than the set upper limit value for water drainage and gas production. In the pressure adjusting and flow stabilizing stage, the pipe sleeve annular pressure is adjusted according to the pressure fluctuation characteristics of the coal reservoir. In the pressure reducing and production controlling stage, the pipe sleeve annular pressure is stepwisely reduced to 0. In the negative pressure and production prolonging stage, the water ring vacuum pump is used to stepwisely reduce the pipe sleeve annular pressure to-0.06 MPa, so that the gas production time of the deep coal bed gas well is further prolonged.

Description

Technical Field

[0001] This invention relates to an in-situ deep coalbed methane well casing annulus pressure control technology, particularly a casing annulus control technology applicable to vertical, directional, and horizontal coalbed methane wells in high-stress, low-permeability deep coal reservoirs. By optimizing the casing annulus pressure control system, this technology avoids damage to coal reservoir permeability and improves the gas production effect of in-situ deep coalbed methane wells, belonging to the field of coalbed methane development technology. Background Technology

[0002] my country has abundant deep coalbed methane resources, with approximately 21 × 10⁻⁶ tons of coalbed methane resources at depths of 1000–1500 m. 12 m 3 The coalbed methane resource at depths of 1500–3000 m is approximately 30 × 10⁻⁶ m. 12 m 3 These represent 38.2% and 54.5% of the total coalbed methane resources at depths shallower than 3000m, respectively. Currently, coalbed methane exploration and development in my country and other major countries worldwide are concentrated at depths shallower than 1000m. Therefore, large-scale development of deep coalbed methane resources can, to some extent, address the shortage of conventional oil and gas energy in my country and generate significant economic, environmental, and social benefits.

[0003] Due to the generally high degree of coal morphology in the area, coupled with the high geostress in the deep coal reservoirs, the permeability of deep coal reservoirs is extremely low. During the drainage process of deep coalbed methane wells, improper control of drainage parameters such as casing annular pressure and bottom hole pressure drop can easily damage the coal reservoir, resulting in low gas production and short stable production periods in in-situ deep coalbed methane wells. To comprehensively promote the development of deep coalbed methane resources in the Upper Permian of Southwest my country, it is necessary to address issues such as reservoir sensitivity damage by controlling drainage parameters, thereby improving the gas production efficiency of in-situ deep coalbed methane wells and the economic benefits of deep coalbed methane development. Summary of the Invention

[0004] Technical Problem: The purpose of this invention is to overcome the problem that reservoirs are easily damaged during the drainage process of in-situ deep coalbed methane wells under high stress conditions, resulting in low gas production and short stable production time, and to provide a process for controlling the annulus pressure of in-situ deep coalbed methane well casing.

[0005] Technical solution: In order to achieve the above objectives, the present invention provides an in-situ deep coalbed methane well casing annulus pressure control technology, which includes six stages: pressure testing and venting stage, pressure building and gas stabilization stage, pressure regulation and flow stabilization stage, pressure reduction and production control stage, and negative pressure extended production stage. Step (a) describes the pressure testing and venting stage, which begins with the installation and start-up of the in-situ deep coalbed methane well venting equipment and ends when the pressure in the deep coal reservoir drops to the critical desorption pressure and the deep coalbed methane begins to desorb. Venting technicians continuously record the changes in annular pressure caused by the accumulation of free methane generated during hydraulic fracturing in the casing annulus, evaluating the gas content of the deep coal reservoir and predicting the production capacity of the deep coalbed methane well. This process aims to reduce the free methane generated during hydraulic fracturing in the deep coal reservoir around the wellbore. Methane enters the annulus of the well casing, causing free methane gas to continuously accumulate in the upper part of the annulus, generating and continuously increasing the annulus pressure. As fracturing fluid is continuously discharged, the fluid level in the annulus gradually decreases, and the annulus pressure rises to its maximum value at this stage. After the annulus begins to gradually decrease, the annulus pressure is gradually reduced by controlling the opening of the gas production pipeline valve at the wellhead until the annulus pressure drops to 0, and the drainage work continues until the pressure test and drainage stage ends. The pressure build-up and gas-holding stage described in step (b) begins when the in-situ coalbed methane well casing annulus pressure reappears due to the desorption of deep coalbed methane and ends when the casing annulus pressure rises to the upper limit of the casing annulus pressure set for this stage. It is controlled by the daily decrease in bottom hole flowing pressure and the difficulty of the expansion of the pressure drop funnel in the deep coal reservoir. The drainage technicians keep the casing annulus valve at the drainage wellhead closed and slowly reduce the bottom hole flowing pressure to allow the methane produced by the desorption of deep coal reservoir to accumulate in the casing annulus, generating casing annulus pressure again. The pressure is then continuously increased through gas-holding until the casing annulus pressure rises to the upper limit of the casing annulus pressure set for this stage. The pressure stabilization and gas production stage described in step (c) begins when the annular pressure of the in-situ deep coalbed methane well rises to the upper limit of the annular pressure set in the pressure-initiating and gas-blocking stage. The drainage and production technicians maintain the annular pressure and set the upper limit of the annular pressure to drain and produce gas. By slowly reducing the bottom hole pressure, the daily gas production of the in-situ deep coalbed methane well is maintained. The duration of the pressure stabilization and gas production stage is controlled by the daily decrease in bottom hole pressure and the expansion of the reservoir pressure drop funnel during the drainage and production process. The pressure regulation and flow stabilization stage described in step (d) results in a daily gas production exceeding 300 m³ from the in-situ deep coalbed methane well. 3The process begins on / d and ends when the daily production of the in-situ deep coalbed methane well stabilizes and the fluctuation of the bottom hole pressure significantly decreases. Due to the rapid increase in the desorption rate and the significant increase in the desorption amount of deep coalbed methane, a transition from gas-water two-phase flow to water-gas two-phase flow occurs in the deep coal reservoir, leading to instability in the gas-water two-phase flow. As the daily gas production of the in-situ deep coalbed methane well rapidly increases during the pressure regulation and flow stabilization phase, fluctuations in the bottom hole pressure are recorded to varying degrees. By increasing the daily reduction rate of the bottom hole pressure, the daily gas production is increased in a stepwise manner. During this phase, the pressure of the deep coal reservoir, the type, intensity, and characteristics of the bottom hole pressure fluctuations of the coalbed methane well are continuously monitored. As the degree of bottom hole pressure fluctuations weakens, the annular pressure of the in-situ deep coalbed methane well casing is gradually reduced. When the pressure regulation and flow stabilization phase ends, the annular pressure of the in-situ deep coalbed methane well casing drops to the upper limit of the casing annular pressure set during the pressure build-up and gas stagnation phase. The pressure reduction and production control stage described in step (e) begins when the annular pressure of the casing of the in-situ deep coalbed methane well drops to the upper limit of the casing annular pressure set during the pressure-initiating and gas-blocking stage, and ends when the casing annular pressure and bottom hole flowing pressure of the in-situ deep coalbed methane well drop to 0. During the drainage process, the drainage technicians maintain the casing annular pressure to gradually and slowly decrease until both the bottom hole flowing pressure and the casing annular pressure drop to 0. The negative pressure extended production stage described in step (f) begins when both the annular pressure and bottom-hole flowing pressure of the in-situ deep coalbed methane well drop to 0. The drainage technicians install a water ring vacuum pump on the gas pipeline at the drainage wellhead to perform negative pressure extraction of coalbed methane, further reducing the annular pressure to ensure that the daily gas production is higher than the lower limit of the industrial gas flow rate of the in-situ deep coalbed methane well, further extending the gas production time of the coalbed methane well, and ending when the daily gas production drops back to the lower limit of the industrial gas flow rate of the in-situ deep coalbed methane well.

[0006] In step (a), the duration of the pressure test and venting stage is relatively short, usually 20 to 50 days; the annular pressure of the pipe sleeve rises to the maximum value of 0.8 MPa in this stage, and the annular pressure of the pipe sleeve is slowly reduced to 0 kPa at a rate of 20 kPa / d while the drainage and pressure reduction work continues.

[0007] In step (b), the pressure-initiating and air-holding phase lasts for a relatively short period, typically 10 to 20 days; until the annular pressure of the casing rises to the maximum value of 5 MPa in this phase.

[0008] In step (b), the upper limit of the casing annulus pressure set in this stage is 3 to 5 MPa; for in-situ deep coalbed methane wells with bottom flow pressure greater than 8 MPa during deep coalbed methane desorption, the upper limit of the casing annulus pressure set in this stage (13) is 4 to 5 MPa.

[0009] In step (c), the drainage technicians maintain the casing annular pressure (9) at 85% to 95% of the upper limit value (13) of the casing annular pressure set in the pressurization and gas-blocking stage (2) for drainage and gas production. By slowly reducing the bottom hole flowing pressure (10), the daily gas production (12) of the in-situ deep coalbed methane well is maintained at 200 to 300 m³ / h. 3 / d.

[0010] In step (d), the daily gas production of the in-situ deep coalbed methane well rapidly increases during the pressure regulation and flow stabilization stage, and the bottom hole flowing pressure fluctuates to varying degrees. When the fluctuation range of the bottom flow pressure of the in-situ coalbed methane well is 50-120 kPa and the fluctuation frequency is 30-60 times / day, it is characterized by moderate bottom flow pressure fluctuation intensity. It is necessary to maintain the casing annulus pressure under moderate bottom flow pressure fluctuation at 85%-95% of the upper limit of casing annulus pressure set in the pressurization and gas-blocking stage for drainage and gas production. When the bottom pressure fluctuation amplitude of an in-situ coalbed methane well is higher than 120 kPa and the fluctuation frequency is lower than 30 times / day, it is characterized by strong bottom pressure fluctuation. It is necessary to increase the casing annulus pressure under strong bottom pressure fluctuation in a stepwise manner to 130% to 150% of the upper limit of casing annulus pressure set in the pressure-starting and gas-blocking stage for drainage and gas production. When the bottom pressure fluctuation amplitude of an in-situ deep coalbed methane well is less than 50 kPa and the fluctuation frequency is higher than 120 times / day, it is characterized by weak bottom pressure fluctuation. It is necessary to reduce the casing annulus pressure under the weak bottom pressure fluctuation in a stepwise manner to 60% to 70% of the upper limit of casing annulus pressure set in the pressurization and gas production stage for drainage and gas production.

[0011] In step (e), the duration of the pressure reduction and production control phase is jointly controlled by the well control area, the amount of deep coalbed methane resources, and the continuity of the drainage process. During the pressure reduction and production control phase, the casing annulus pressure is slowly reduced at a rate of 2 to 4 kPa, and at the end of the pressure reduction and production control phase, the bottom flow pressure of the in-situ deep coalbed methane well and the casing annulus pressure are simultaneously reduced to 0.

[0012] In step (f), the casing annular pressure is further reduced to -0.06 MPa. Then, by adjusting the speed of the water ring vacuum pump, the casing annular pressure is maintained at -0.06 MPa for an extended period. The daily gas production of the deep coalbed methane well decreases to the lower limit (14) of the in-situ deep coalbed methane well industrial gas flow rate, which is 104 m³. 3 / d.

[0013] Beneficial Effects: By adopting the above technical solution, this invention overcomes the problems of large fluctuations in the annulus fluid level, easy exposure of the upper producing layer, velocity and pressure sensitivity damage to the coal reservoir, low resource recovery rate, and difficulty in achieving high and stable production in in-situ deep coalbed methane wells under traditional casing annulus pressure control methods. First, it utilizes the free gas generated during hydraulic fracturing to evaluate the gas content of deep coal reservoirs and predict the long-term production capacity of deep coalbed methane wells. Second, through controlled pressure suppression, it increases the daily gas production, improves the flowback rate of the fracturing fluid, and expands the pressure drop funnel. Third, it adjusts the casing annulus pressure according to the coal reservoir pressure fluctuations to eliminate the damage caused by coal reservoir pressure fluctuations. Finally, it uses a water-ring vacuum pump for negative pressure extraction to extend the long-term production time of in-situ deep coalbed methane wells and improve the gas production effect, demonstrating broad applicability. The main advantages compared to existing technologies are: ① By utilizing the changes in annular pressure in the casing before desorption of deep coal reservoirs, long-term drainage performance can be predicted, and this can be used as the basis for controlling annular pressure in the casing during the drainage process. ② Based on the characteristics of coal reservoir pressure fluctuations, the annular pressure of the casing during the pressure regulation and flow stabilization stage is quantitatively adjusted to suppress the adverse effects of reservoir pressure fluctuations on gas production. ③ By adopting negative pressure extraction, the gas production time of in-situ deep coalbed methane wells can be further extended; ④ The process for controlling the annular pressure of the casing is simple, the engineering implementation cost is low, and the economic, environmental and social benefits are good. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the in-situ deep coalbed gas well casing annulus pressure control process of the present invention.

[0015] In the diagram: 1-Pressure testing and venting stage; 2-Pressure building and gas holding stage; 3-Pressure stabilization and gas production stage; 4-Pressure regulation and flow stabilization stage; 5-Pressure reduction and production control stage; 6-Negative pressure extended production stage; 7-Casing annular pressure control time axis; 8-Volume control parameter change axis; 9-Casing annular pressure; 9-1-Casing annular pressure under moderate fluctuations in bottom hole flowing pressure; 9-2-Casing annular pressure under strong fluctuations in bottom hole flowing pressure; 9-3-Casing annular pressure under weak fluctuations in bottom hole flowing pressure; 10-Bottom hole flowing pressure; 11-Daily water production; 12-Daily gas production; 13-Upper limit of casing annular pressure; 14-Lower limit of industrial gas flow in in-situ deep coalbed methane wells. Detailed Implementation

[0016] The present invention will be further described below with reference to an embodiment in the accompanying drawings: like Figure 1As shown, the in-situ deep coalbed methane well casing annulus pressure control technology of the present invention mainly consists of six stages: pressure testing and venting stage 1, pressure building and gas stabilization stage 2, pressure stabilization and gas production stage 3, pressure regulation and flow stabilization stage 4, pressure reduction and production control stage 5, and negative pressure extended production stage 6. Step (a) of the pressure test and degassing phase 1 begins with the installation and start-up of the in-situ deep coalbed methane well drainage equipment and ends when the pressure in the deep coal reservoir drops to the critical desorption pressure, at which point the deep coalbed methane begins to desorb. Drainage technicians depressurize the well by draining water, allowing free methane generated during the hydraulic fracturing of the deep coal reservoir around the wellbore to enter the annulus of the well casing. This free methane gas continuously accumulates in the upper part of the annulus, generating and continuously increasing the annulus pressure 9. When the annulus pressure 9 reaches its maximum value, it is slowly reduced to 0 kPa at a rate of 20 kPa / d, and drainage and depressurization continue. The pressure test and degassing phase 1 is relatively short, typically lasting 20–50 days. By continuously recording the changes in annular pressure 9 caused by the accumulation of free methane in the annulus during hydraulic fracturing, the drainage technicians evaluate the gas content of deep coal reservoirs and predict the production capacity of deep coalbed methane wells. As fracturing fluid is continuously discharged, the fluid level in the annulus gradually decreases, and the annular pressure 9 reaches its maximum value in this stage. After the annulus begins to gradually decrease, the gas production pipeline valve at the drainage wellhead is opened in a controlled manner to gradually reduce the annular pressure 9 until it drops to 0 MPa, and drainage work continues until the pressure test and drainage stage 1 ends.

[0017] Step (b) describes the pressure build-up and gas-suppressing stage 2, which begins when the in-situ coalbed methane well casing annulus pressure reappears due to deep coalbed methane desorption and ends when the casing annulus pressure rises to the upper limit of the casing annulus pressure set for this stage. This stage is controlled by the daily decrease in bottom hole flowing pressure and the difficulty of expanding the pressure drop funnel in the deep coal reservoir. The drainage technicians keep the casing annulus valve at the drainage wellhead closed and slowly reduce the bottom hole flowing pressure 10 to allow the methane produced by the desorption of the deep coal reservoir to continuously accumulate in the casing annulus, generating casing annulus pressure 9 again. This pressure build-up and gas-suppressing stage 2 begins when the in-situ coalbed methane well casing annulus pressure 9 reappears due to deep coalbed methane desorption and ends when the casing annulus pressure 9 rises to the upper limit of the casing annulus pressure set for this stage. The second stage of pressure build-up and gas stagnation is relatively short, typically lasting 10–20 days, and is controlled by the 10-day decrease in bottomhole flowing pressure and the difficulty of pressure drop funnel expansion in deep coal reservoirs. The upper limit of the casing annulus pressure 13 for this stage is usually set at 3–5 MPa. For in-situ deep coalbed methane wells where the bottomhole flowing pressure 10 during desorption is greater than 8 MPa, the upper limit of the casing annulus pressure 13 for this stage is set at 4–5 MPa; for in-situ deep coalbed methane wells where the bottomhole flowing pressure during desorption is greater than 8 MPa, the upper limit of the casing annulus pressure 13 for this stage is set at 3–4 MPa.

[0018] Step (c) of the pressure stabilization and gas production stage 3 begins when the annular pressure of the in-situ deep coalbed methane well rises to the upper limit of the annular pressure set in the pressure-initiating and gas-suppressing stage. The drainage and production technicians maintain the annular pressure 9 at 85% to 95% of the upper limit of the annular pressure 13 set in the pressure-initiating and gas-suppressing stage 2 for drainage and gas production. By slowly reducing the bottom hole flowing pressure 10, the daily gas production 12 of the in-situ deep coalbed methane well is maintained at 200 to 300 m³ / h. 3 / d; The pressure stabilization and gas production stage 3 begins when the casing annulus pressure 9 of the in-situ deep coalbed methane well rises to the upper limit value 13 of the casing annulus pressure set in the pressure building and gas stabilization stage 2, and continues until the daily gas production 12 of the in-situ deep coalbed methane well exceeds 300 m³. 3 / d ends; the duration of stage 3 of pressure stabilization and gas production is controlled by the 10-day decrease in bottom hole flowing pressure and the expansion of the reservoir pressure drop funnel during the drainage process.

[0019] In step (d), the pressure regulation and flow stabilization stage 4 requires the daily gas production of the in-situ deep coalbed methane well to exceed 300 m³. 3The process begins on / d and ends when the daily production of the in-situ deep coalbed methane well stabilizes and the fluctuation of the bottom hole pressure significantly decreases. Due to the rapid increase in the desorption rate and the significant increase in the desorption amount of deep coalbed methane, a transition from gas-water two-phase flow to water-gas two-phase flow occurs in the deep coal reservoir, leading to instability in the gas-water two-phase flow. As the daily gas production of the in-situ deep coalbed methane well rapidly increases during the pressure regulation and flow stabilization phase, fluctuations in the bottom hole pressure are recorded to varying degrees. By increasing the 10-day decrease in bottom hole pressure, the daily gas production is increased in a stepwise manner. During this phase, the pressure of the deep coal reservoir, the fluctuation type, intensity, and characteristics of the bottom hole pressure of the coalbed methane well are continuously monitored, and the casing annulus pressure is adjusted accordingly. As the fluctuation of the bottom hole pressure decreases, the casing annulus pressure of the in-situ deep coalbed methane well is gradually reduced. When the pressure regulation and flow stabilization phase ends, the casing annulus pressure of the in-situ deep coalbed methane well drops to the upper limit of the casing annulus pressure set during the pressure build-up and gas stagnation phase. The pressure regulation and flow stabilization stage 4 resulted in a daily gas production of over 300 m³ from the in-situ deep coalbed methane well. 3 The process begins at / d and ends when the daily production of the in-situ deep coalbed methane well stabilizes at 12 and the fluctuation of the bottom hole flowing pressure at 10 significantly decreases. During the pressure regulation and flow stabilization stage 4, the desorption rate of deep coalbed methane increases rapidly and the desorption amount increases significantly. A transition from gas-water two-phase flow to water-gas two-phase flow occurs in the deep coal reservoir, leading to instability in the gas-water two-phase flow. As the daily gas production of the in-situ deep coalbed methane well rapidly increases at the pressure regulation and flow stabilization stage 4, the bottom hole flowing pressure fluctuates to varying degrees. During the pressure regulation and flow stabilization stage 4, when the fluctuation range of the bottom flow pressure 10 of the in-situ coalbed methane well is 50-120 kPa and the fluctuation frequency is 30-60 times / day, it is characterized by a moderate fluctuation intensity of the bottom flow pressure 10. It is necessary to maintain the casing annulus pressure 9-1 under the moderate fluctuation intensity of the bottom flow pressure at 85%-95% of the upper limit value 13 of the casing annulus pressure set in the pressure-initiating and gas-blocking stage 2 for drainage and gas production. When the fluctuation amplitude of the bottom flow pressure 10 in the in-situ coalbed methane well is higher than 120 kPa and the fluctuation frequency is lower than 30 times / day, it is characterized by strong fluctuation of the bottom flow pressure 10. It is necessary to increase the casing annulus pressure 9-2 under strong fluctuation of bottom flow pressure in a stepwise manner to 130% to 150% of the upper limit value 13 of the casing annulus pressure set in the pressure-starting and gas-blocking stage 2 for drainage and gas production. When the fluctuation amplitude of the bottom flow pressure 10 in the in-situ deep coalbed methane well is less than 50 kPa and the fluctuation frequency is higher than 120 times / day, it is characterized by weak fluctuation of the bottom flow pressure 10. It is necessary to reduce the casing annulus pressure 9-3 under the weak fluctuation of the bottom flow pressure in a stepwise manner to 60% to 70% of the upper limit value 13 of the casing annulus pressure set in the pressure-starting and gas-blocking stage 2 for drainage and gas production. In the later stage of pressure regulation and flow stabilization, as the fluctuation of the bottom hole pressure 10 weakens, the annular pressure 9 of the in-situ deep coalbed methane well casing needs to be gradually reduced. When the pressure regulation and flow stabilization stage 4 ends, the annular pressure 9 of the in-situ deep coalbed methane well casing drops to 60% to 70% of the upper limit value 13 of the casing annular pressure set in the pressure-initiating and gas-blocking stage 2.

[0020] In step (e), during the pressure reduction and production increase stage 5, and the pressure reduction and production control stage 5, the drainage technicians gradually and slowly reduce the casing annular pressure 9 during the drainage process of reducing the bottom hole flowing pressure 10, until both the bottom hole flowing pressure 10 and the casing annular pressure 9 decrease to 0. The pressure reduction and production control stage 5 begins when the casing annular pressure 9 of the in-situ deep coalbed methane well decreases to the upper limit value 13 of the casing annular pressure set in the pressure-increasing and gas-blocking stage 2, and ends when both the casing annular pressure 9 and the bottom hole flowing pressure 10 of the in-situ deep coalbed methane well decrease to 0 MPa. The duration of the pressure reduction and production control stage 5 is jointly controlled by factors such as the well control area, the amount of deep coalbed methane resources, and the continuity of the drainage process. During the pressure reduction and production control stage 5, the casing annular pressure 9 is slowly reduced at a rate of 2–4 kPa, and at the end of the pressure reduction and production control stage 5, the bottom hole flowing pressure 10 and the casing annular pressure 9 of the in-situ deep coalbed methane well simultaneously decrease to 0 MPa.

[0021] Step (f) describes the negative pressure extended production stage 6, which begins when both the annular pressure 9 and the bottom-hole flowing pressure 10 of the in-situ deep coalbed methane well casing drop to 0. Drainage technicians install a water ring vacuum pump on the drainage wellhead gas pipeline to perform negative pressure extraction of coalbed methane, further reducing the annular pressure 9 to -0.06 MPa. This ensures that the daily gas production 12 is higher than the lower limit of the industrial gas flow rate of the in-situ deep coalbed methane well, further extending the gas production time of the coalbed methane well. The negative pressure extended production stage 6 begins when both the annular pressure 9 and the bottom-hole flowing pressure 10 of the in-situ deep coalbed methane well casing drop to 0 MPa, and ends when the annular pressure 9 further decreases to -0.06 MPa under the negative pressure extraction condition of the water ring vacuum pump, and the daily gas production 12 decreases again to the lower limit of the industrial gas flow rate 14 of the in-situ deep coalbed methane well. In the negative pressure extended production stage 6, a water ring vacuum pump is used to create negative pressure in the annulus of the casing. As the pumping time of the water ring vacuum pump increases, the annulus pressure 9 gradually decreases, and the daily gas production 12 increases. When the annulus pressure 9 drops to -0.06 MPa, the speed of the water ring vacuum pump is adjusted to maintain the annulus pressure 9 at -0.06 MPa for a long period of time. When the daily gas production 12 of the deep coalbed methane well drops to the lower limit value 14 of the industrial gas flow of the in-situ deep coalbed methane well, the well abandonment and sealing operation is carried out.

Claims

1. A process for controlling the annular pressure of casing in in-situ deep coalbed methane wells, characterized in that: It includes six stages: pressure testing and venting stage (1), pressure building and gas holding stage (2), pressure stabilization and gas production stage (3), pressure regulation and flow stabilization stage (4), pressure reduction and production control stage (5), and negative pressure extended production stage (6). Step (a): The pressure test and venting stage (1) begins with the installation and start-up of the in-situ deep coalbed methane well drainage equipment, and ends when the pressure in the deep coal reservoir drops to the critical desorption pressure and the deep coalbed methane begins to desorb. Drainage technicians continuously record the changes in the annular pressure (9) caused by the accumulation of free methane in the casing annulus during hydraulic fracturing, through drainage and pressure reduction, to evaluate the gas content of the deep coal reservoir, predict the production capacity of the deep coalbed methane well, and allow the free methane generated in the deep coal reservoir during hydraulic fracturing around the wellbore to enter the annulus. In the well casing annulus, free methane gas continuously accumulates in the upper part of the well casing annulus, generating and continuously increasing the casing annulus pressure (9). As the fracturing fluid is continuously discharged, the fluid level in the casing annulus gradually decreases, and the casing annulus pressure (9) rises to the maximum value of this stage. When the casing annulus begins to gradually decrease, the gas production pipeline valve at the wellhead is opened in a controlled manner to gradually decrease the casing annulus pressure (9) until the casing annulus pressure (9) drops to 0, and the drainage work continues until the pressure test and drainage stage (1) ends. Step (b): The pressure build-up and gas-holding stage (2) begins when the annular pressure (9) of the in-situ coalbed methane well reappears due to the desorption of deep coalbed methane, and ends when the annular pressure (9) rises to the upper limit of the annular pressure (13) set for this stage. It is controlled by the daily decrease of the bottom flow pressure (10) and the difficulty of the expansion of the pressure drop funnel in the deep coal reservoir. The drainage technicians keep the annular valve of the drainage wellhead closed and slowly reduce the bottom flow pressure (10) so that the methane produced by the desorption of deep coal reservoir can accumulate in the annular space, generate the annular pressure (9) again, and continuously increase the pressure until the annular pressure (9) rises to the upper limit of the annular pressure (13) set for this stage. Step (c): The pressure stabilization gas production stage (3) begins when the casing annulus pressure (9) of the in-situ deep coalbed methane well rises to the upper limit value (13) of the casing annulus pressure set in the pressure stabilization and gas blocking stage (2). The drainage technicians maintain the casing annulus pressure (9) and set the upper limit value (13) of the casing annulus pressure to drain and produce gas. By slowly reducing the bottom flow pressure (10), the daily gas production (12) of the in-situ deep coalbed methane well is maintained. The duration of the pressure stabilization gas production stage (3) is controlled by the daily decrease of the bottom flow pressure (10) and the expansion of the reservoir pressure drop funnel during the drainage process. Step (d): During the pressure regulation and flow stabilization stage (4), the daily gas production (12) of the in-situ deep coalbed methane well exceeds 300 m³. 3 / d begins, and ends when the daily gas production (12) of the in-situ deep coalbed methane well stabilizes and the fluctuation of the bottom hole flowing pressure (10) significantly decreases; due to the rapid increase in the desorption rate and the significant increase in the desorption amount of deep coalbed methane, a transition from gas-water two-phase flow to water-gas two-phase flow occurs in the deep coal reservoir, leading to instability of the gas-water two-phase flow. As the pressure regulation and flow stabilization stage (4) progresses, the daily gas production (12) of the in-situ deep coalbed methane well rapidly increases, and the bottom hole flowing pressure is recorded to have different degrees of fluctuation; by increasing the bottom hole flowing pressure ( 10) Daily decrease, so that the daily gas production (12) increases in a stepwise manner. During this stage, the pressure of the deep coal reservoir, the type, intensity and characteristics of the bottom flow pressure (10) of the coalbed methane well are continuously monitored. As the bottom flow pressure (10) decreases, the casing annulus pressure (9) of the in-situ deep coalbed methane well is gradually reduced. When the pressure regulation and flow stabilization stage (4) ends, the casing annulus pressure (9) of the in-situ deep coalbed methane well drops to the upper limit value (13) of the casing annulus pressure set in the pressure-initiating and gas-blocking stage (2). During the pressure regulation and flow stabilization stage (4), the daily gas production (12) of the in-situ deep coalbed methane well rapidly increased, and the bottom hole flowing pressure showed varying degrees of fluctuation. When the fluctuation range of the bottom flow pressure (10) of the in-situ coalbed methane well is 50-120 KPa and the fluctuation frequency is 30-60 times / d, it is characterized by a moderate fluctuation intensity of the bottom flow pressure (10). It is necessary to maintain the casing annulus pressure (9-1) under the moderate fluctuation intensity of the bottom flow pressure at 85%-95% of the upper limit value (13) of the casing annulus pressure set in the pressure-starting and gas-blocking stage (2) for drainage and gas production. When the fluctuation amplitude of the bottom flow pressure (10) of the in-situ coalbed methane well is higher than 120 KPa and the fluctuation frequency is lower than 30 times / day, it is characterized by strong fluctuation of the bottom flow pressure (10). It is necessary to increase the casing annulus pressure (9-2) under strong fluctuation of bottom flow pressure in a stepwise manner to 130% to 150% of the upper limit value (13) of the casing annulus pressure set in the pressure-starting and gas-blocking stage (2) for drainage and gas production. When the fluctuation amplitude of the bottom flow pressure (10) of the in-situ deep coalbed methane well is less than 50 KPa and the fluctuation frequency is higher than 120 times / day, the bottom flow pressure (10) fluctuates weakly. It is necessary to reduce the casing annulus pressure (9-3) under the weak fluctuation of the bottom flow pressure in a stepwise manner to 60% to 70% of the upper limit value (13) of the casing annulus pressure set in the pressure-starting and gas-blocking stage (2) for drainage and gas production. Step (e): The pressure reduction and production control stage (5) begins when the casing annulus pressure (9) of the in-situ deep coalbed methane well decreases to the pressure build-up and gas-blocking stage (2) and the upper limit value (13) of the casing annulus pressure is set, and ends when the casing annulus pressure (9) and bottom hole flowing pressure (10) of the in-situ deep coalbed methane well decrease to 0; the drainage technicians maintain the casing annulus pressure (9) gradually and slowly decreasing during the drainage process of reducing the bottom hole flowing pressure (10) until both the bottom hole flowing pressure (10) and the casing annulus pressure (9) decrease to 0; Step (f): The negative pressure extended production stage (6) begins when the casing annulus pressure (9) and bottom flow pressure (10) of the in-situ deep coalbed methane well both drop to 0. The drainage technicians install a water ring vacuum pump on the gas pipeline at the drainage wellhead to perform negative pressure extraction of coalbed methane, so that the casing annulus pressure (9) is further reduced to ensure that the daily gas production (12) is higher than the lower limit of the industrial gas flow of the in-situ deep coalbed methane well, and further extend the gas production time of the coalbed methane well. When the daily gas production (12) of the deep coalbed methane well drops to the lower limit of the industrial gas flow of the in-situ deep coalbed methane well (14), the well abandonment and sealing operation is carried out.

2. The in-situ deep coalbed methane well casing annulus pressure control technology as described in claim 1, characterized in that: In step (a), the duration of the pressure test and venting stage (1) is relatively short, usually 20 to 50 days; the annular pressure (9) of the pipe sleeve rises to the maximum value of 0.8 MPa in this stage, and the annular pressure of the pipe sleeve is slowly reduced to 0 kPa at a rate of 20 kPa / d while the drainage and pressure reduction work continues.

3. The in-situ deep coalbed methane well casing annulus pressure control technology as described in claim 1, characterized in that: In step (b), the pressure-inducing and air-holding stage (2) lasts for a short period of time, usually 10 to 20 days; until the annular pressure (9) of the casing rises to the maximum value of 5 MPa in this stage.

4. The in-situ deep coalbed methane well casing annulus pressure control technology as described in claim 1, characterized in that: In step (b), the upper limit value (13) of the casing annulus pressure set in this stage is 3 to 5 MPa; for in-situ deep coalbed methane wells where the bottom flow pressure (10) during deep coalbed methane desorption is greater than 8 MPa, the upper limit value (13) of the casing annulus pressure set in this stage is 4 to 5 MPa.

5. The in-situ deep coalbed methane well casing annulus pressure control technology as described in claim 1, characterized in that: In step (c), the drainage technicians maintain the casing annular pressure (9) at 85% to 95% of the upper limit value (13) of the casing annular pressure set in the pressurization and gas-blocking stage (2) for drainage and gas production. By slowly reducing the bottom hole flowing pressure (10), the daily gas production (12) of the in-situ deep coalbed methane well is maintained at 200 to 300 m³ / h. 3 / d.

6. The in-situ deep coalbed gas well casing annulus pressure control technology as described in claim 1, characterized in that: In step (e), the duration of the pressure reduction and production control stage (5) is jointly controlled by the well control area, the amount of deep coalbed methane resources, and the continuity of the drainage process. During the pressure reduction and production control stage (5), the casing annulus pressure (9) is slowly reduced at a rate of 2 to 4 kPa, and at the end of the pressure reduction and production control stage (5), the bottom flow pressure (10) of the in-situ deep coalbed methane well and the casing annulus pressure (9) are simultaneously reduced to 0.

7. The in-situ deep coalbed methane well casing annulus pressure control technology as described in claim 1, characterized in that: In step (f), the casing annular pressure (9) is further reduced to -0.06 MPa. At this time, the casing annular pressure (9) is maintained at -0.06 MPa for a long period by adjusting the speed of the water ring vacuum pump. The daily gas production (12) of the deep coalbed methane well decreases to the lower limit (14) of the in-situ deep coalbed methane well industrial gas flow rate, which is 104 m³. 3 / d.

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