A system and method for synergistically reducing pressure to exploit natural gas hydrates underground

Abstract

The present invention discloses a system and method for synergistically reducing pressure to extract natural gas hydrate underground. The system includes a casing for constructing a production well. The upper end of the production well is connected to a gas production collection pipeline, which is used to connect to a gas production recovery system. The section of the casing located in the natural gas hydrate reservoir is distributed with perforated channels. A wellbore string assembly is installed in the production well, and the wellbore string assembly includes an outer cylinder column, a production string, and an auxiliary lifting pipe. A first one-way valve is installed at the bottom of the outer cylinder column, and a gas supply pipeline is connected to the upper part of the outer cylinder column. A flow controller is installed in the gas supply pipeline. The production string is installed inside the outer cylinder column, and the space between them is used as a water storage chamber. A second one-way valve is installed at the bottom of the production string. The auxiliary lifting pipe is installed inside the production string. The present invention can quickly realize the industrial exploitation and application of natural gas hydrate.

Description

Technical Field

[0001] The present invention relates to the field of natural gas hydrate exploitation, and in particular to a system and method for exploiting natural gas hydrate by coordinated gas-liquid pressure reduction in an underground well. Background Art

[0002] Natural gas hydrate is known as the new clean energy with the greatest potential to replace traditional fossil energy in the 21st century. Its huge reserves, wide distribution, high energy density, clean combustion and other advantages are attracting more and more attention from the world. my country has successfully drilled natural gas hydrate samples in the permafrost area of Qinghai on land and the Shenhu area in the northern South China Sea, proving that my country has this clean energy both on land and in the sea. As investigation and analysis continue to deepen, six gas hydrate mineralization prospective zones have been delineated in the northern continental slope of the South China Sea, with a total area of 148,400 square kilometers. The predicted prospective resource volume is equivalent to 74.4 billion tons of oil equivalent. In 2020, the second gas hydrate trial production was successfully carried out in the Shenhu area of the South China Sea at a water depth of 1,225 meters, achieving new world records in the exploitation of gas hydrate resources, such as "continuous gas production for 30 days, total gas production of 861,400 cubic meters, and average daily gas production of 28,700 cubic meters" (Ye Jianliang, Qin Xuwen, Xie Wenwei, et al. Major progress in the second trial production of gas hydrates in the South China Sea. Chinese Geology, 2020, 47(3): 557-568.).

[0003] The depressurization method and its improved scheme are considered to be the best way to achieve efficient exploitation of natural gas hydrates (Mao Peixiao, Wu Nengyou, Ning Fulong, et al. Gas and water production rules of depressurization exploitation of natural gas hydrates under different well types. Natural Gas Industry, 2020, 40(11):168-176). However, it should also be pointed out that unlike the conventional exploitation of traditional fossil energy-coal, oil and natural gas, the exploitation of natural gas hydrates involves three phases of solid, liquid and gas. In the exploitation process, there is also phase change decomposition / regeneration of hydrates. The phase change process is coupled with the fluid-solid-heat multi-physics field, making the in-situ exploitation of natural gas hydrates more complicated than traditional oil and gas exploitation. In the process of simple decompression to exploit natural gas hydrates, solid natural gas hydrates are decomposed into gaseous natural gas and liquid water, and the cementation skeleton of reservoir sediment particles is weakened, resulting in reduced hydrate reservoir strength (even reservoir destruction) and sand production in the formation; the decompressed gas, water and mobile sediment particles will also reduce the reservoir porosity, causing the permeability of the hydrate reservoir to decrease, which will slow down the decomposition of natural gas hydrates and reduce gas production efficiency. In the case of long-term exploitation, hydrate decomposition and gas and water production will lead to reservoir deficits, which will affect the instability of the formation and the stability of the production wellbore string, etc., and will cause serious damage to the seabed environment.

[0004] In addition, during the process of hydrate exploitation, a large amount of water is always accompanied (0.8 cubic meters of water can be produced after the decomposition of 1 cubic meter of hydrate). Existing research has shown that the flow of water promotes the decomposition of hydrates (Yang Mingjun, Sun Huiru, Chen Bingbing, et al. Research on the Enhancement of Depressurization Decomposition of Natural Gas Hydrates by Water Flow. Journal of Engineering Thermophysics, 2020, 41(2): 307-312.). Reasonably handling the large amount of water generated by decomposition is a practical problem that must be faced during the process of hydrate exploitation. However, there are still few relevant efforts and explorations on the combination of water treatment and depressurization at present, and it is necessary to further develop and innovate the exploitation methods of hydrates.

[0005] In summary, it is urgent to propose an exploitation scheme that can handle the large amount of water generated by hydrate decomposition and prevent large-scale reservoir depletion and instability during the process of gas and water production during exploitation. Combining with the depressurization method of natural gas hydrates, which is considered the most economical and has the most industrial prospects, to achieve the economy, safety, and high efficiency of long-term exploitation of natural gas hydrates. Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies of the above-mentioned existing technologies, and provide a system and method for downhole gas-liquid collaborative depressurization exploitation of natural gas hydrates, to realize the comprehensive utilization and treatment of the liquid water generated by the decomposition of a large amount of hydrates downhole, to control the problems of reservoir depletion and instability during the process of hydrate decomposition and gas and water production, and finally to achieve the purpose of safely and continuously depressurizing and exploiting natural gas hydrates.

[0007] To achieve the above purpose, the technical solution of the present invention is as follows:

[0008] In the first aspect, the present invention provides a system for downhole gas-liquid collaborative depressurization exploitation of natural gas hydrates, including:

[0009] A casing, which is used to penetrate the seawater layer, the sediment overlying layer, the natural gas hydrate reservoir, and the sediment underlying layer to construct a production well. The upper end of the production well is connected to a gas production collection pipeline, and the gas production collection pipeline is used to connect to a gas production recovery system; perforated channels are distributed in the section of the casing located in the natural gas hydrate reservoir; a filtering device is arranged around the section of the casing located in the natural gas hydrate reservoir;

[0010] A wellbore string assembly is installed in the production well. The wellbore string assembly includes an outer cylinder column, a production string, and an auxiliary lifting pipe; a first one-way valve is installed at the bottom of the outer cylinder column, and a gas supply pipeline is connected to the upper part of the outer cylinder column. A flow controller is installed in the gas supply pipeline to adjust the flow rate of gas entering the outer cylinder column; the production string is installed inside the outer cylinder column, and the space between the outer cylinder column and the production string is used as a water storage chamber. A second one-way valve is installed at the bottom of the production string; the auxiliary lifting pipe is installed inside the production string to discharge the liquid water.

[0011] Furthermore, the system for synergistically reducing pressure to exploit natural gas hydrates downhole further includes a monitoring well, which is independent of the production well and is used to monitor the pressure change of the natural gas hydrate reservoir.

[0012] Furthermore, the top end of the auxiliary lift pipe is connected to the gas-water separation device, and the gas-water separation device is used to separate natural gas and water in the liquid-phase water; the gas-water separation device is also connected with a water outlet pipeline for conveying the separated water to the water outlet pipeline, and the water outlet pipeline is connected with the water return pipeline. A switching valve is installed in the water return pipeline, and part of the discharged water enters the water return pipeline through the switching valve when needed, and after being heated by the heating device, it is reinjected into the outer cylinder column.

[0013] Furthermore, the gas-water separation device is connected to the gas production collection pipeline for transmitting the separated natural gas into the gas production collection pipeline.

[0014] Furthermore, a flow valve is installed in the pipeline connecting the gas-water separation device and the gas production collection pipeline; a gas flow detector is installed in the gas production collection pipeline.

[0015] Furthermore, the filtering device is gravel; a gravel settling pit is arranged in the part of the sediment subjacent layer penetrated by the casing.

[0016] Furthermore, a sand filtering device is arranged in the gravel settling pit.

[0017] In a second aspect, the present invention provides a method for synergistically reducing pressure to exploit natural gas hydrates downhole. The method is based on the above system and includes the following steps:

[0018] Step 1: Construct a production casing that penetrates the seawater layer, the sediment upper cover layer, the natural gas hydrate reservoir, and the sediment subjacent layer in the formation of the natural gas hydrate mining area, and perform cementing operations; perform drilling operations on the casing section of the natural gas hydrate reservoir, arrange perforation channels, and fill gravel around the casing wall of the natural gas hydrate reservoir; arrange a gravel settling pit in the part of the sediment subjacent layer penetrated by the casing; correspondingly arrange a monitoring well near the hydrate production well to monitor the pressure change of the hydrate reservoir in real time;

[0019] Step 2: Lower and install a wellbore pipe string assembly in the wellbore of the production well constructed by the casing; perform pressure reduction exploitation production according to the pressure of the natural gas hydrate reservoir, the gas and water production situation of hydrate decomposition, and the gas-water pressure situation in the production well;

[0020] Step 3: The gas produced by the pressure-reducing decomposition of natural gas hydrates in the reservoir is recovered and utilized through the gas production collection pipeline at the upper end of the casing production well; the produced water is discharged externally after being gradually regulated according to the overall requirements of hydrate pressure-reducing production, passing through the casing production well - water storage chamber - annulus area inside the production string - auxiliary lifting pipe, and finally being separated by the gas-water separation device on the operating platform and then recovered and utilized.

[0021] Further, step 2 includes:

[0022] Open the outlet end and pipeline of the gas production recovery system for gas production collection. After the gas hydrate reservoir is evacuated and depressurized, the gas and water produced by the decomposition of natural gas hydrates flow into the production well through the perforation channel after filtering out particulate sediments in the casing gravel packing area, and the first gas-water natural separation occurs here. The gas gradually accumulates in the upper part of the casing production well, while the corresponding liquid water slowly accumulates at the bottom of the production well; during the above process, the liquid water level entering the casing production well always remains not lower than the safety level.

[0023] According to the gas and water production and pressure change conditions during the decomposition of natural gas hydrates in the reservoir, the gas produced and accumulated in the upper part of the casing production well flows through the gas production collection pipeline connected to the production well to the outlet end for metering, collection, and utilization. The first one-way valve at the bottom of the outer cylinder column and the second one-way valve at the bottom of the production string are opened in a timely manner, so that the liquid water in the production well exceeding the safety level is gradually regulated and discharged under the condition of ensuring safe and effective pressure-reducing production, thereby forming an underground gas-liquid collaborative pressure-reducing production operation among the natural gas hydrate reservoir - casing production well - water storage chamber - annulus area inside the production string.

[0024] Further, in step 3, the discharged gas enters the gas storage reservoir after being metered by a gas flow detector, or is stored in a liquefied state. Part of it enters the water storage chamber through the supply pipeline and the flow controller to boost the pressure and drain water when there is a pressure compensation requirement in the water storage chamber; the produced water discharged from the water storage chamber passes through the gas-water separation device. Part of it enters the outlet pipe for collection, and part enters the return water pipeline and then returns to the water storage chamber after being heated by the heating device.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] The characteristics of the present invention lie in exploiting the synergistic effect of downhole gas-liquid discharge for pressure reduction exploitation of natural gas hydrate resources. While avoiding drastic fluctuations in reservoir pressure during the exploitation process, preventing large-scale reservoir depletion, and maintaining reservoir stability, a treatment solution for the water produced by the decomposition of natural gas hydrate is provided. The specific advantages of the present invention include: The technical solution of downhole gas-liquid synergistic pressure reduction exploitation of natural gas hydrate proposed by the present invention eliminates the electric submersible pump conventionally used for downhole water pumping and gas extraction, saving equipment costs and corresponding operation and maintenance costs; The technical solution of downhole gas-liquid synergistic pressure reduction exploitation of natural gas hydrate proposed by the present invention conducts hierarchical regulation of drainage. While ensuring the safety and stability of the reservoir during the pressure reduction exploitation process, it cooperates with gas exhaust for synergistic pressure reduction, promotes the effective flow of gas and liquid during the hydrate exploitation process, enhances the decomposition efficiency and production capacity of hydrates, and effectively extends the pressure reduction exploitation cycle; The technical solution of downhole gas-liquid synergistic pressure reduction exploitation of natural gas hydrate proposed by the present invention can comprehensively solve the problem of a large amount of water treatment generated during the exploitation process of natural gas hydrate; The present invention adopts gravel filling around the casing wall of the hydrate reservoir + sediment pits formed by the casing penetrating through the underlying layer of the sediment to stepwise prevent the production of large and small particle sediments, which meets and is applicable to the sand control treatment during the downhole gas-liquid synergistic pressure reduction exploitation of natural gas hydrate of the present invention, and can effectively prevent sediment gravel from entering the production string; The present invention adopts the recycling and heating reuse of the water produced by the reservoir, and the hot water returns to the outer cylinder column, which can effectively prevent the secondary formation of hydrates in the production wellbore, etc.

[0027] In summary, the solution of the present invention is easy to implement and the relevant application equipment technology is mature, and it can quickly realize the industrial exploitation and application of natural gas hydrate. It is an innovative, safe, economic and effective hydrate exploitation method. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is the schematic diagram of the system for downhole gas-liquid synergistic pressure reduction exploitation of natural gas hydrate provided by the embodiment of the present invention;

[0029] Figure 2 It is implemented as the schematic flow diagram of downhole gas-liquid synergistic pressure reduction exploitation of natural gas hydrate;

[0030] In the figure: 1, seawater layer; 2, gravel sediment pit; 3, upper cover layer; 4, natural gas hydrate reservoir; 5, underlying layer; 6, casing; 7, perforation channel; 8, outer cylinder column; 9, production string; 10, annulus area; 11, auxiliary lifting pipe; 12, second one-way valve; 13, first one-way valve; 14, water storage chamber; 15, gas-liquid separation device; 16, water outlet pipeline; 17, water return pipeline; 18, heating device; 19, flow control valve; 20, gas production collection pipeline; 21, gas flow detector; 22, flow controller; 23, gas supply pipeline; 24, production well. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] In the description of the present invention, it should be noted that unless otherwise clearly defined and limited, the terms "installation" and "connection" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, a direct connection, or an indirect connection through an intermediate medium. It can be said that there is a connection inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations. The technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0032] Embodiment:

[0033] Refer to Figure 1 As shown, when the system for underground gas-liquid collaborative pressure reduction to exploit natural gas hydrates provided in this embodiment is applied, first, a casing 6 that penetrates the seawater layer 1, the sediment overlying layer 3, the natural gas hydrate reservoir 4, and the sediment underlying layer 5 is constructed in the formation of the natural gas hydrate mining area, and a cementing operation is carried out to form a production well 24. The upper end of the production well 24 is connected to a gas production collection pipeline 20, and the gas production collection pipeline 20 is used to connect to a gas production recovery system to collect natural gas. Then, a perforation operation is carried out on the casing section of the natural gas hydrate reservoir 4 to arrange a perforation channel 7, and gravel is filled around the pipe wall of the casing section of the natural gas hydrate reservoir to filter larger sediment particles and prevent large sediment particles from entering the casing 6. In addition, a gravel settling pit 2 can also be arranged in the part of the sediment underlying layer 5 penetrated by the casing.

[0034] In the wellbore of the production well 24 constructed by the casing 6, a wellbore pipe string assembly for gas-liquid collaborative pressure reduction is lowered and installed. This wellbore pipe string assembly includes an outer cylinder column 8, a production pipe string 9, and an auxiliary lifting pipe 11. A first one-way valve 13 is installed at the bottom of the outer cylinder column 8, and the first one-way valve 13 is required to have a certain sand prevention function. A gas supply pipeline 23 is connected to the upper part of the outer cylinder column 8, and a flow controller 22 is installed in the gas supply pipeline 23 to adjust the flow rate of gas entering the outer cylinder column 8. The upper part of the outer cylinder column 8 is also connected to a return water pipeline 17 to heat part of the return water and then reinject it into the outer cylinder column 8 (for adjusting and compensating the hydrostatic pressure in the outer cylinder column and preventing the secondary formation of hydrates). The production pipe string 9 is installed inside the outer cylinder column 8, and the space between the outer cylinder column 8 and the production pipe string 9 is used as a water storage chamber 1. A second one-way valve 12 is installed at the bottom of the production pipe string 9. The auxiliary lifting pipe 11 is installed inside the production pipe string 9, and the gap between the two is used as an annulus area 10 to discharge liquid water.

[0035] As a preference of this system, monitoring wells are also arranged near the production well 24 to monitor the pressure change of the natural gas hydrate reservoir 4 in real time. The pressure data of the natural gas hydrate reservoir is used for the stability judgment of the natural gas hydrate reservoir 4 and the subsequent drainage of the natural gas hydrate reservoir and the regulation of hydrate pressure reduction and decomposition.

[0036] As another preference of the present system, the top end of the auxiliary riser 11 is connected to the gas-water separation device 15, and the gas-water separation device 15 is used to separate natural gas and water in the liquid-phase water; the gas-water separation device 15 is also connected to a water outlet pipeline 16 for outputting the separated water. The water outlet pipeline 16 is connected to the water return pipeline 17, and a switching valve is installed in the water return pipeline 17. Part of the outlet water enters the water return pipeline 17 through the switching valve when needed, and after being heated by the heating device 18, it is reinjected into the outer cylinder column 8. In addition, the gas-water separation device 15 is also connected to the gas production collection pipeline 20 for transmitting the separated natural gas into the gas production collection pipeline 20, and a flow valve 19 is installed in the pipeline connecting the gas-water separation device 15 and the gas production collection pipeline 20; a gas flow detector 21 is installed in the gas production collection pipeline 20 for counting the amount of natural gas collected.

[0037] After the above-mentioned relevant well layout and downhole equipment installation are completed, the outlet end of the gas production collection pipeline 20 of the gas-water recycling system and the flow valve 19 on its pipeline are opened, and the pressure of the natural gas hydrate reservoir 4 connected to the production well through it will decrease. The natural gas hydrate in the natural gas hydrate reservoir 4 will start to decompose due to the destruction of the phase equilibrium. The decomposed gas and water flow into the production well 24 along the perforation channel 7. The gas and water flowing into the production well 24 will first pass through the casing gravel filling area for filtration to filter out large particle sediments that may be carried in the gas and water from the natural gas hydrate reservoir 4; the filtered gas and water flow into the production well 24 and undergo the first natural gas-water separation here. The gas gradually accumulates in the upper part of the production well 24, and the corresponding liquid-phase water slowly accumulates at the lower end of the production well 24; at the same time, the small particle sediments flowing out of the natural gas hydrate reservoir with the gas and water undergo natural sedimentation in the gravel settling pit 2. If necessary, a sand filtering device can also be arranged and opened here in the gravel settling pit 2 for large-area and rapid filtration of small particle sediments. In the above process, the liquid-phase water level entering the production well 24 always remains not lower than a certain position, so as to keep the pressure change between the production well and the hydrate reservoir not too drastic, and the outflow of gas and water will not cause large-area depletion and instability of the hydrate reservoir.

[0038] According to the gas-water production and pressure change conditions during the decomposition of hydrates in the natural gas hydrate reservoir, the first one-way valve 13 at the bottom of the outer cylinder column 8 is opened in a timely manner, so that the liquid-phase water exceeding the water level requirement in the production well 24 enters the water storage chamber 14 through the channel of the first one-way valve 13 with a sand prevention function under the action of the pressure difference. This process of draining water from the production well 24 into the water storage chamber 14, like the opening of the gas production collection pipeline 20 to collect gas production, will cause a certain decrease in the pressure in the production well 24 and the pressure of the natural gas hydrate reservoir 4, thereby continuously forming an effective driving force for hydrate pressure reduction decomposition, promoting decomposition, and at the same time promoting the flow of gas and water in the natural gas hydrate reservoir 4.

[0039] The closing of the first one-way valve 13 is determined according to the sand control capability, the pressure in the production well 24 and the water storage chamber 14. Generally, when the sand control capability is not weakened and the pressure in the production well 24 and the water storage chamber 14 can be dynamically kept stable, the first one-way valve 13 can be kept in the open state. In the above process, the pressure in the outer cylinder 8 is jointly determined by the gas pressure in the upper part and the hydrostatic pressure in the water storage chamber 14. When the water storage chamber 14 is in the water storage state, the gas supply pipeline 23 and the flow controller 22 connected to the upper part of the outer cylinder 8 adjust the gas pressure in the upper part of the water storage chamber 14 in the outer cylinder 8 by regulating the gas in and out of the outer cylinder 8, so that the water storage chamber 14 can continue to smoothly receive the produced water discharged from the production well 24 from the first one-way valve 13. When the water level in the water storage chamber 14 rises to a certain height, the second one-way valve 12 at the bottom of the production pipe string 9 is opened according to the overall prevention and control requirements, so that the water in the water storage chamber 14 enters the annular area 10 in the production pipe string 9. In this process, the flow rate of gas entering the outer cylinder 8 can be adjusted through the gas supply pipeline 23 and the flow controller 22, thereby increasing the gas pressure in the upper part of the water storage chamber 14, so that the produced water in the water storage chamber 14 can be smoothly discharged into the production string 9 through the second one-way valve 12 at the bottom of the production string 9, and at the same time, the water volume in the water storage chamber 14 is regulated. The produced water entering the production string 9 is finally discharged to the outside through the auxiliary lifting pipe 11 under the action of siphon.

[0040] The gas separated by the gas-water separation device 15 is introduced into the gas production collection pipeline 20 by opening the flow valve 19, and enters the gas flow detector 21 through the gas production collection pipeline 20 together with the gas naturally separated from the production well 24 for metering and collection.

[0041] The following combination Figure 2 , the present invention further explains the underground gas-liquid coordinated pressure reduction and safety control:

[0042] After the gas production recovery system opens the relevant pipeline loop channel, the natural gas hydrate in the natural gas hydrate reservoir connected to it through the production well will start the decompression and decomposition production process when the pressure balance of the stable existence conditions is broken. After the decompression and decomposition of natural gas hydrates begins, the gas and water produced by the decomposition will cause a sharp increase in the pressure in the natural gas hydrate reservoir. Under the pressure difference between the natural gas hydrate reservoir, the production well and the gas recovery end, the gas and water in the natural gas hydrate reservoir will gradually flow to the production well end; the gas and water flowing into the production well will naturally separate due to their different densities. The produced gas will gradually gather in the upper part of the production well due to its low density, while the produced water with relatively large density will slowly accumulate in the lower part of the production well; under the condition of maintaining a certain water level (safe water level) of the produced water in the production well, the corresponding pressure can be maintained between the lower part of the production well and the part connected to the natural gas hydrate reservoir, so that the pressure change between the production well and the hydrate reservoir will not be too drastic to damage the reservoir, and it can also ensure that the normal and smooth flow of gas and water is not hindered, and the large outflow of gas and water will not cause large-scale deficits in the hydrate reservoir and then instability, affecting the continuous decomposition of hydrates; in the above process, the pressure change in the reservoir is detected and data collected by the monitoring well in real time, and the reservoir stability is judged and the drainage coordinated decompression operation is regulated.

[0043] When the water volume in the mining well exceeds the safe water level, the water storage chamber is opened to perform graded drainage of the produced water in the mining well. The low-pressure environment of the water storage chamber is used to drive the produced water in the mining well to enter the water storage chamber through the first one-way valve 13 under the pressure difference, adjust the water volume in the mining well, reduce the water level and hydrostatic pressure in the mining well, and realize drainage and coordinated pressure reduction on the basis of gas extraction and pressure reduction caused by gas recovery in the upper part of the mining well; as the produced water in the mining well is continuously discharged, the water level in the water storage chamber also increases continuously, and the pressure in the water storage chamber also gradually rises. Then, according to the overall hydrate pressure reduction production and safety prevention and control situation, the high-pressure environment of the water storage chamber is used in time to discharge the produced water in the water storage chamber into the production string, and finally the siphon effect is used to achieve the final drainage to the outside with the assistance of the auxiliary lifting pipe in the production string. At the same time, the next graded drainage and coordinated pressure reduction process is started. In the above process, the low-pressure / high-pressure environment of the water storage chamber is assisted by the flow controller to control the inlet and outlet of gas, and this part of the gas source comes from the gas recovery system.

[0044] In addition, the water produced by the decomposition of hydrates discharged outward through the water storage chamber is recycled after gas-water separation treatment; part of the water is reinjected into the water storage chamber through a heating device to prevent the formation of secondary hydrates in the wellbore column and eliminate the risk of blockage.

[0045] Embodiment 2:

[0046] This embodiment provides a method for synergistically reducing pressure to extract natural gas hydrate underground. This method is based on the system of Embodiment 1 and specifically includes the following steps:

[0047] Step 1: Arrangement of production wells and related production equipment: Construct a production casing that penetrates the seawater layer, sediment upper cover layer, natural gas hydrate reservoir, and sediment lower cover layer in the formation of the natural gas hydrate ore-forming area, and perform cementing operations; Drill holes in the casing section of the hydrate reservoir to arrange perforation channels, and fill gravel around the wall of the production casing in the hydrate reservoir; Arrange gravel settling pits in the part of the sediment lower cover layer penetrated by the casing, and a sand filtration device can also be arranged here if necessary; Arrange monitoring wells near the hydrate production wells to monitor the pressure changes in the hydrate reservoir in real time;

[0048] Step 2: Lower and install the relevant wellbore string components for synergistically reducing pressure of gas and liquid in the wellbore of the production well constructed by the production casing, and carry out pressure reduction production according to the reservoir pressure, gas and water production and decomposition conditions of hydrates, and gas and water pressure conditions in the production well.

[0049] Step 3: The gas produced by the pressure reduction and decomposition of natural gas hydrates in the natural gas reservoir is recovered and utilized through the gas production collection pipeline at the upper end of the casing production well; The produced water is gradually regulated according to the overall requirements of hydrate pressure reduction production, and then discharged to the offshore operation platform through the casing production well - water storage chamber - production string - auxiliary lifting pipe, and finally recovered and utilized after passing through the gas - water separation device.

[0050] Specifically, Step 2 specifically includes:

[0051] After the outlet end of the gas production collection pipeline of the gas - water recovery and utilization system is opened, the pressure of the hydrate reservoir connected to it through the production well will decrease, and the natural gas hydrates in the hydrate reservoir will start to decompose due to the destruction of phase equilibrium. The decomposed gas and water flow into the production well along the perforation channels of the casing in the hydrate reservoir section. The gas and water flowing into the casing production well are filtered in the gravel - filled area of the casing to filter out the large - particle sediments that may be carried; The filtered gas and water flow into the casing production well and undergo the first natural gas - water separation here. The gas gradually accumulates in the upper part of the casing production well, and the corresponding liquid water slowly accumulates at the bottom of the production well; At the same time, the small - particle sediments flowing out of the hydrate reservoir with the gas and water undergo natural sedimentation in the gravel settling pit where the casing penetrates the sediment lower cover layer, and a sand filtration device can also be arranged and opened here if necessary to quickly filter the small - particle sediments over a large area. During the above process, the liquid - phase water level entering the casing production well always remains not lower than a certain position to keep the pressure between the production well and the hydrate reservoir from changing violently, and the hydrate reservoir will not become unstable due to large - area depletion.

[0052] As the hydrate continues to decompose, the decomposed gas and water produced in the hydrate reservoir continuously flow into the casing production well. The gas produced in the upper part of the casing production well flows through the gas collection pipeline connected to the production well to the outlet end for collection and utilization. The gas production rate / flow rate at the outlet end is determined according to the pressure regulation requirements in the production well and production economy. On the basis of ensuring a certain water level, the water produced in the lower part of the casing production well flows into the water storage chamber through the combined action of the reservoir pressure, the pressure in the casing production well, and the pressure in the water storage chamber via the check valve at the bottom of the outer cylinder column. The check valve here is required to have a certain sand prevention function to prevent a large area of small particle sediments suspended in the liquid-phase water from entering the water storage chamber. As the liquid-phase water in the casing production well flows into the water storage chamber, the pressure in the production well will further decrease, promoting the flow of gas-liquid fluids in the reservoir and further promoting the pressure reduction and decomposition of the hydrate. At the same time, the water volume / water level in the water storage chamber will gradually increase accordingly. According to the overall progress and pressure reduction requirements of the hydrate pressure reduction production, the water volume control arrangement in the water storage chamber can inject air and pressurize the water storage chamber through the air supply circuit at the top of the water storage chamber, so that the liquid-phase water in the water storage chamber flows into the production string through the check valve at the bottom of the production string and is then discharged through the auxiliary lift pipe, thus forming an underground gas-liquid synergy (exhaust and drainage) pressure reduction system among the reservoir - casing production well - water storage chamber - production string.

[0053] Further, step (3) specifically includes:

[0054] The discharged gas enters the gas storage reservoir after being detected and metered for gas flow, or is stored in a liquefied form. Among them, when there is a pressure compensation requirement in the underground water storage chamber, a part of the gas enters the underground water storage chamber through the gas supply pipeline via the flow (pressure) control equipment to increase the pressure and drain the water. The water produced from the underground passes through the gas-liquid separation device on the offshore operation platform. A part of the water enters the outlet pipe for collection, and a part enters the return water pipeline and then returns to the underground water storage chamber after being heated by the heating device according to the safety production requirements, where the safety production requirements include reservoir safety pressure regulation and prevention and control of secondary hydrate formation in the wellbore, etc.

[0055] The above embodiments are only used to illustrate the technical concept and features of the present invention. The purpose is to enable ordinary technicians in the field to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the essence of the content of the present invention should be covered within the protection scope of the present invention.

Claims

1. A system for synergistically reducing pressure to exploit natural gas hydrates underground, characterized in that, Comprising: A casing pipe, which is used to penetrate through the seawater layer, the sediment overlying layer, the natural gas hydrate reservoir and the sediment underlying layer to construct a production well. The upper end of the production well is connected to a gas production collection pipeline, and the gas production collection pipeline is used to connect to a gas production recovery system; perforation channels are distributed in the section of the casing pipe located in the natural gas hydrate reservoir; a filtering device is arranged around the section of the casing pipe located in the natural gas hydrate reservoir; A wellbore pipe string assembly is installed in the production well. The wellbore pipe string assembly includes an outer cylinder column, a production pipe string and an auxiliary lifting pipe; a first one-way valve is installed at the bottom of the outer cylinder column, and a gas supply pipeline is connected to the upper part of the outer cylinder column. A flow controller is installed in the gas supply pipeline to adjust the flow rate of gas entering the outer cylinder column; the production pipe string is installed inside the outer cylinder column, and the space between the outer cylinder column and the production pipe string is used as a water storage chamber. A second one-way valve is installed at the bottom of the production pipe string; the auxiliary lifting pipe is installed inside the production pipe string to discharge liquid water; The top end of the auxiliary lifting pipe is connected to a gas-liquid separation device, and the gas-liquid separation device is used to separate natural gas and water in the liquid water; the gas-liquid separation device is also connected to a water outlet pipeline for transporting the separated water to the water outlet pipeline. The water outlet pipeline is connected to a water return pipeline, and a switching valve is installed in the water return pipeline. Part of the discharged water enters the water return pipeline through the switching valve when needed, and after being heated by a heating device, it is reinjected into the outer cylinder column.

2. The system for synergistically reducing pressure of gas and liquid for downhole exploitation of natural gas hydrate according to claim 1, wherein It also includes a monitoring well, which is independent of the production well and is used to monitor the pressure change of the natural gas hydrate reservoir.

3. The system for synergistically reducing pressure to exploit natural gas hydrate underground according to claim 1, wherein The gas-liquid separation device is connected to the gas production collection pipeline to transmit the separated natural gas into the gas production collection pipeline.

4. The system for synergistically reducing pressure of gas and liquid to exploit natural gas hydrate underground according to claim 3, wherein, A flow-through valve is installed in the pipeline connecting the gas-liquid separation device and the gas production collection pipeline; a gas flow detector is installed in the gas production collection pipeline.

5. The system for underground gas-liquid collaborative pressure reduction for natural gas hydrate exploitation according to claim 1, characterized in that, The filtering device is gravel; a gravel settling pit is arranged in the part of the sediment underlying layer penetrated by the casing pipe.

6. The system for synergistically reducing pressure to exploit natural gas hydrate underground according to claim 5, characterized in that, A sand filtering device is arranged in the gravel settling pit.

7. A method for synergistically reducing pressure to extract natural gas hydrates underground, the method being based on the system described in claim 5, characterized in that, The method includes the following steps: Step 1: Construct a production casing pipe that penetrates through the seawater layer, the sediment overlying layer, the natural gas hydrate reservoir and the sediment underlying layer in the formation of the natural gas hydrate mining area, and perform a cementing operation; perform a drilling operation on the casing section in the natural gas hydrate reservoir, arrange perforation channels, and fill gravel around the casing wall of the natural gas hydrate reservoir; arrange a gravel settling pit in the part of the sediment underlying layer penetrated by the casing pipe; correspondingly arrange a monitoring well near the hydrate production well to monitor the pressure change of the hydrate reservoir in real time; Step 2: Lower and install a wellbore pipe string assembly in the wellbore of the production well constructed by the casing pipe; perform pressure reduction mining production according to the pressure of the natural gas hydrate reservoir, the gas and water production situation of hydrate decomposition, and the gas-water pressure situation in the production well; Step 3: The gas produced by the pressure reduction decomposition of the natural gas hydrate in the reservoir is recovered and utilized through the gas production collection pipeline at the upper end of the casing production well; the produced water is discharged externally after being gradually regulated according to the overall requirements of the hydrate pressure reduction production, passing through the casing production well - the water storage chamber - the annulus area inside the production string - the auxiliary lifting pipe, and finally being separated by the gas - liquid separation device on the operating platform and then recovered and utilized.

8. The method for jointly reducing pressure by gas and liquid for underground exploitation of natural gas hydrate according to claim 7, characterized in that, The said Step 2 includes: Open the outlet end and pipeline of the gas production recovery system for gas production collection. After the gas extraction and pressure reduction of the hydrate reservoir are realized, the gas and water generated by the decomposition of the natural gas hydrate pass through the casing gravel packing area to filter out particulate sediments and then flow into the production well through the perforated channel, and the first natural gas - water separation occurs here. The gas gradually accumulates in the upper part of the casing production well, and the corresponding liquid water slowly accumulates at the bottom of the production well; during the above process, the water level of the liquid water entering the casing production well always remains not lower than the safety water level. According to the gas - water production and pressure change situation during the decomposition of the natural gas hydrate in the reservoir, the gas produced and accumulated in the upper part of the casing production well flows through the gas production collection pipeline connected to the production well to the outlet end for metering, collection and utilization. The first one - way valve at the bottom of the outer cylinder column and the second one - way valve at the bottom of the production string are opened timely, so that the liquid water in the production well exceeding the safety water level can be gradually regulated and discharged under the condition of ensuring safe and effective pressure reduction production, thus forming an underground gas - liquid collaborative pressure reduction production operation among the natural gas hydrate reservoir - the casing production well - the water storage chamber - the annulus area inside the production string.

9. The method for jointly reducing pressure of gas and liquid in the wellbore to exploit natural gas hydrate according to claim 7, wherein, In the said Step 3, the discharged gas enters the gas storage reservoir after being metered by the gas flow detector, or is stored in a liquefied state. Among them, a part of the gas enters the water storage chamber through the supply pipeline and the flow controller to increase the pressure and drain water when there is a pressure compensation requirement in the water storage chamber; the produced water discharged from the water storage chamber passes through the gas - liquid separation device, a part of it enters the outlet pipe for collection, a part enters the return water pipeline, and then returns to the water storage chamber after being heated by the heating device.

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