Marine natural gas hydrate depressurization mining device and method

Abstract

The invention relates to the technical field of natural gas hydrate exploitation, in particular to a marine natural gas hydrate depressurization exploitation device and method. Comprising an offshore drilling platform, a drill string, a sleeve and a conduit. The drilling platform can lift or lower a drill string, a casing and a conduit. A first pipeline is arranged in the drill column, and the bottom is fixedly connected with a drill bit. The casing pipe is arranged on the peripheral side of the drill column in a sleeving mode, an annular containing space is formed between the casing pipe and the drill column, and an annular sealing device is arranged on the side wall. The guide pipe is arranged in the annular containing space and provided with a pressure adjusting device, and the side wall is provided with a slit and communicated with the sleeve and the hydrate layer through a circulation device. And fracturing fluid is guided into a hydrate layer, the pressure in the guide pipe is adjusted till the hydrate is decomposed when reaching a phase equilibrium point, and then natural gas is collected to an offshore drilling platform through the guide pipe. And after mining is finished, the drill column, the drill bit, the casing pipe and the guide pipe can be lifted and recycled by controlling the offshore drilling platform, and can be recycled after being detached and cleaned, so that the mining efficiency is improved, and the mining cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas hydrate exploitation, and in particular to a device and method for reducing the pressure of marine natural gas hydrate exploitation. Background Art

[0002] Natural gas hydrates are the most abundant methane resources in nature. The total carbon content of natural gas hydrates on Earth is twice the carbon content of proven fossil energy. my country's total reserves of marine natural gas hydrates are as high as 70 billion tons of oil equivalent. Its industrial development is of great significance to improving the energy consumption structure and ensuring energy security.

[0003] In the existing technology, many trials of marine natural gas hydrate production have been conducted. Whether it is vertical well production or horizontal well production, pressure reduction production is considered to be the primary method of natural gas hydrate production in the future. However, in the later stage of pressure reduction production, due to the reduction of hydrate layer pressure, the production work is difficult to continue and can only be forced to be suspended. On the one hand, the gas production volume cannot meet the requirements of industrialization. On the other hand, after the production is completed, the original pipelines will basically be abandoned and cannot be reused, which greatly increases the cost of natural gas hydrate production. From the perspective of large-scale, industrialized production, it is difficult to achieve this goal due to the forced suspension of production and the high cost of well construction. Summary of the invention

[0004] The object of the present invention is to provide a depressurization and production device and method for marine natural gas hydrates, which can recycle production pipelines and can be efficiently produced.

[0005] To achieve the above object, the present invention provides a marine natural gas hydrate depressurization and production device, comprising:

[0006] Offshore drilling platforms;

[0007] A drill string, the top of which is connected to the offshore drilling platform, the offshore drilling platform can lift or lower the drill string, a first pipe is provided inside the drill string, one end of the first pipe is connected to the offshore drilling platform, and the first pipe is used to discharge drilling mud;

[0008] A drill bit, the drill bit is fixedly connected to the bottom of the drill string, the drill bit is used to open a wellbore, and the diameter of the drill bit is larger than the diameter of the drill string;

[0009] A casing, wherein the casing is sleeved on the outer peripheral side of the drill string and an annular accommodation space is formed between the casing and the drill string, the top end of the casing is connected to the offshore drilling platform, and the offshore drilling platform can lift or lower the casing, and the side wall of the casing is provided with an annular sealing device, and the annular sealing device is used to isolate the overlying soil layer and the hydrate layer;

[0010] A conduit is provided in the annular accommodation space and extends toward the seabed. The top end of the conduit is connected to the offshore drilling platform. The offshore drilling platform can lift or lower the conduit. A pressure regulating device is provided on the conduit. The pressure regulating device is used to regulate the pressure in the conduit. A slit is provided on the side wall of the conduit. The slit is connected to the casing and the hydrate layer through a circulation device.

[0011] Optionally, the offshore drilling platform includes a lifting screw crane, a first lifting platform and a second lifting platform;

[0012] The lifting screw crane is detachably connected to the top of the drill string, and the lifting screw crane is used to drive the drill string to rotate and lift;

[0013] The first lifting platform is detachably connected to the top of the casing, and the first lifting platform is used to drive the casing to rise and fall along the altitude direction;

[0014] The second lifting platform is detachably connected to the top end of the conduit, and the second lifting platform is used to drive the conduit to rise and fall along the altitude direction.

[0015] Optionally, the offshore drilling platform further includes a pressure pump and a mud pump;

[0016] The pressure pump is connected to the top end of the conduit, and the pressure pump is used to suck the natural gas in the conduit and release the fracturing fluid into the conduit;

[0017] The mud pump is in communication with the first pipeline.

[0018] Optionally, a second pipeline is further provided inside the drill string, the second pipeline is spaced apart from the first pipeline, the top end of the second pipeline is connected to the pressure pump, and the second pipeline is used for introducing drilling fluid.

[0019] Optionally, the annular sealing device is a high-pressure airbag, and the high-pressure airbag is connected to the pressure pump.

[0020] Optionally, it also includes a watertight pipe, which is slidably connected to the outer wall of the drill bit.

[0021] Optionally, the cross-section of the slit is a trapezoid, and the short side of the trapezoid is arranged on a side of the casing close to the hydrate layer.

[0022] Optionally, it also includes surveying devices, which are evenly distributed along the circumference of the wellbore.

[0023] Optionally, it also includes a control system, which is communicatively connected to the offshore drilling platform.

[0024] The present invention also provides a method for reducing pressure and exploiting marine natural gas hydrates, which uses the above-mentioned marine natural gas hydrate reducing pressure and exploiting device, and comprises the following steps:

[0025] S1: Install the drill bit and the drill string, connect the drill string and the first pipeline to the offshore drilling platform, install the casing and the conduit and connect them to the offshore drilling platform;

[0026] S2: Start the drill string, so that the drill string drives the drill bit downward to form a wellbore, and at the same time lower the casing and the guide tube to complete the drilling;

[0027] S3: Pressurizing fracturing fluid into the conduit through the pressure pump platform, and adjusting the pressure regulating device in the conduit to allow the fracturing fluid to flow into the hydrate layer through the slits, the flow device and the casing to perform fracturing;

[0028] S4: After the fracturing is completed, the pressure regulating device in the conduit is adjusted to reduce the pressure in the conduit until the natural gas hydrate reaches the phase equilibrium point, and the natural gas is collected through the conduit to the offshore drilling platform;

[0029] S5: After mining is completed, the drill string, drill bit, casing and conduit are lifted and recovered by controlling the offshore drilling platform, and then disassembled, cleaned and recycled.

[0030] The technical solution provided by the present invention has the following advantages compared with the prior art:

[0031] The marine natural gas hydrate depressurization and exploitation device provided by the present invention is provided with a first pipeline on the drill string and connected to the offshore drilling platform, so that the exploited mud can be discharged to the offshore drilling platform through the first pipeline and collected and utilized during the exploitation process. A drill bit is provided to be connected to the drill string and the diameter of the drill bit is larger than the diameter of the drill string, so that the wellbore diameter is larger than the diameter of the drill string during the drill bit exploitation process. A casing is further provided to be sleeved on the outer peripheral side of the drill string and an annular accommodation space is formed between the casing and the drill string, a conduit is provided in the annular accommodation space, and both the casing and the conduit are connected to the offshore drilling platform, and a pressure regulating device and a slit are provided in the conduit. Thus, the circulation of the fracturing fluid and the collection of natural gas can be controlled by adjusting the pressure in the conduit. At the same time, an annular sealing device is provided on the outer wall of the casing, which can isolate the overlying soil layer from the hydrate layer to prevent pollutants from entering the wellbore, and can form a sealed environment in the hydrate layer, which is convenient for fracturing and exploitation. Therefore, after drilling is completed, the reservoir is fractured by introducing fracturing fluid into the hydrate layer. After fracturing, the pressure in the conduit is adjusted until the natural gas hydrate reaches the phase equilibrium point and the hydrate decomposes, and then the natural gas can be collected to the offshore drilling platform through the conduit through the pressure difference. After the mining is completed, the drill string, drill bit, casing and conduit can be lifted and recovered by controlling the offshore drilling platform, and then recycled after disassembly and cleaning, which not only improves the mining efficiency, but also realizes the integration of hydrate mining and device recycling, reducing the mining cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0033] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0034] Figure 1 This is a schematic structural diagram of a marine natural gas hydrate depressurization and production device according to an embodiment of the present invention;

[0035] Figure 2 This is a schematic diagram of the structure of a production well according to an embodiment of the present invention;

[0036] Figure 3 A schematic diagram of the local structure of a production well according to an embodiment of the present invention;

[0037] Figure 4 A top view of a production well according to an embodiment of the present invention;

[0038] Figure 5 This is a schematic diagram of a slit structure according to an embodiment of the present invention;

[0039] Figure 6 This is a schematic diagram of the recovery of the marine natural gas hydrate depressurization production device described in an embodiment of the present invention.

[0040] Among them, 1. offshore drilling platform; 11. lifting screw crane; 12. first lifting platform; 13. second lifting platform; 14. pressure pump; 15. mud pump; 2. drill string; 21. first pipeline; 22. second pipeline; 3. drill bit; 4. casing; 41. annular sealing device; 5. guide tube; 51. pressure regulating device; 52. thermometer; 53. flow meter; 54. slit; 6. survey device; 7. control system; 8. circulation device; 9. watertight pipe. DETAILED DESCRIPTION

[0041] In order to more clearly understand the above-mentioned objectives, features and advantages of the present invention, the scheme of the present invention will be further described below. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0042] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present invention, rather than all of the embodiments.

[0043] The following is a detailed description of the marine natural gas hydrate depressurization and mining device through a specific embodiment:

[0044] like Figures 1 to 3 As shown, the marine natural gas hydrate depressurization exploitation device provided by the present invention includes an offshore drilling platform 1, a drill string 2, a drill bit 3, a casing 4 and a conduit 5. The top of the drill string 2 is connected to the offshore drilling platform 1 and the offshore drilling platform 1 can lift or lower the drill string 2, and the bottom is fixedly connected to the drill bit 3. The diameter of the drill bit 3 is larger than the diameter of the drill string 2, and then after the drill bit 3 exploits the wellbore, the casing 4 is sleeved on the outer peripheral side of the drill string 2, and an annular accommodation space is formed between the casing 4 and the drill string 2. The conduit 5 is arranged in the annular accommodation space and extends toward the seabed. A pressure regulating device 51 is provided in the conduit 5 to adjust the pressure in the conduit 5 so that a pressure difference is formed inside and outside the conduit 5, which is convenient for the collection of natural gas. At the same time, a slit 54 is provided on the side wall of the conduit 5, and the slit 54 is connected to the casing 4 and the hydrate layer through the circulation device 8, so that the fracturing fluid can flow to the casing 4 through the slit 54 on the conduit 5, and then flow to the hydrate layer for fracturing. When the natural gas hydrate reaches the phase equilibrium point, the hydrate begins to decompose and release natural gas, and then the pressure in the conduit 5 is adjusted to be lower than the pressure outside the conduit 5, and the natural gas is pressed into the conduit 5 by the pressure difference and collected to the offshore drilling platform 1. After the collection is completed, the drill string 2, the drill bit 3, the casing 4 and the conduit 5 can be lifted and recovered by controlling the offshore drilling platform 1, and they can be disassembled and cleaned for recycling, which not only improves the mining efficiency but also reduces the mining cost.

[0045] The above-mentioned offshore drilling platform 1 may include a lifting screw crane 11, a first lifting platform 12, and a second lifting platform 13. The lifting screw crane 11 is detachably connected to the top of the drill string 2 and can drive the drill string 2 to rotate and lift. When drilling is required, the drill string 2 can be lowered by the lifting screw crane 11. After mining is completed, the drill string 2 can be lifted, disassembled, cleaned, and recycled. It can be understood that the lifting screw crane 11 can simultaneously drive the drill bit 3 to be lifted or lowered. The first lifting platform 12 is detachably connected to the top of the casing 4 and can drive the casing 4 to be lifted and lowered along the altitude direction. During the drilling process, the casing 4 is lowered to the outer peripheral side of the drill string 2 by the first lifting platform 12, and the casing 4 can be lifted after the end, and the casing can be removed and cleaned for recycling. The second lifting platform 13 is detachably connected to the top of the conduit 5 and can drive the conduit 5 to be lifted and lowered along the altitude direction. During the drilling process, the conduit 5 is lowered to the annular accommodation space between the casing 4 and the drill string 2 by the second lifting platform 13, and the conduit 5 can be lifted after the end, and the conduit can be removed and cleaned for recycling. In order to facilitate the fixation of the conduit 5 during the drilling and recovery process, a mechanical ring lock can be set on the inner wall of the casing 4, and the mechanical ring lock is connected to the inner wall of the casing 4 through a mechanical claw, and the mechanical claw can slide and lock along the circumference of the inner wall of the casing 4. The diameter of the mechanical ring lock is slightly larger than the diameter of the conduit 5, so as to fix the conduit 5 in the ring lock. The above-mentioned casing 4 and drill string 2 can be set in multiple sections, and the multiple sections of the casing 4 can be threadedly connected. The connection between the multiple sections of the casing 4 can be controlled by the rotation of the first lifting platform 12. The connection method between the multiple sections of the drill string 2 can be the same as that of the casing 4, and is specifically designed according to actual needs.

[0046] Furthermore, a first pipe 21 may be provided inside the drill string 2, and the first pipe 21 is distributed along the axial direction of the drill string 2. Therefore, the drilling mud may be discharged to the offshore drilling platform 1 through the first pipe 21 during the drilling process for recycling. Specifically, a mud pump 15 may be provided to be connected to the first pipe 21, and the drilling mud may be sucked up through the mud pump 15. At the same time, a sedimentation tank may be provided to be connected to the mud pump 15, and the sucked drilling mud may be discharged to the sedimentation tank through the mud pump 15 for sedimentation and stratification.

[0047] Furthermore, a second pipe 22 may be provided inside the drill string 2, and the second pipe 22 is spaced apart from the first pipe 21. The second pipe 22 may be used to pass drilling fluid. The diameter of the first pipe 21 may be greater than the diameter of the second pipe 22, and may be set specifically according to actual needs. At the same time, a pressure pump 14 is provided at the outlet end of the second pipe 22, so that the drilling fluid is pumped into the second pipe 22 through the pressure pump 14, thereby accelerating the drilling process. Of course, in order to save resources, the sedimentation tank may be connected to the pressure pump 14, so that the liquid after sedimentation in the sedimentation tank enters the pressure pump 14 to serve as a drilling fluid for circulation. The outlet end of the conduit 5 may also be connected to the pressure pump 14, so that the fracturing fluid may be pumped into the conduit 5 through the pressure pump 14, and then flow into the hydrate layer through the slit 54 on the conduit 5 for fracturing. At the same time, the pressure pump 14 may draw up the natural gas collected in the conduit 5 and pass it into the storage device, thereby accelerating the collection of natural gas. Of course, it is understandable that a four-way joint can be installed on the pressure pump 14, one of which is connected to the sedimentation tank, one is connected to the fracturing fluid pipeline, and one is connected to the natural gas storage device.

[0048] In some embodiments, a natural gas separation device, a natural gas purification device and a natural gas storage device may be provided on the offshore drilling platform 1, and the natural gas separation device, the natural gas purification device and the natural gas storage device are sequentially connected by pipelines. The pressure pump 14 is connected to the natural gas separation device through a four-way joint.

[0049] The sealing device is provided on the casing 4 to isolate the overburden layer from the hydrate layer, and prevent pollutants such as seawater in the overburden layer from entering the hydrate layer to affect the exploitation. The sealing device can be annular, and the annular sealing device 41 can be bonded to the outer wall of the casing 4. The annular sealing device 41 can be a high-pressure airbag, which can be connected to the pressure pump 14 through an interface. Furthermore, the high-pressure airbag can be inflated or deflated by the pressure pump 14. When exploitation is required, the high-pressure airbag is inflated, the reading of the pressure gauge is observed, and the inflation is stopped after reaching a predetermined pressure, and the sealing between the casing 4 and the wellbore is completed at this time. Of course, in order to further enhance the sealing effect, multiple annular sealing devices 41 can be provided, and the multiple annular sealing devices 41 are distributed at intervals along the axial direction of the casing 4. When the exploitation is completed, the pressure is released by the pressure pump 14 to release the gas in the high-pressure airbag, thereby facilitating the recovery of the casing 4. The outer surface of the high-pressure airbag can be wrapped with aluminum foil to prevent it from being punctured by the hard rock in the wellbore. It can not only reduce the loss of disposable raw materials and mining costs, but also facilitate the recycling of the casing 4, and will not cause pipeline damage or non-removable due to cement grouting. In addition, the annular sealing device 41 can form a closed space for the hydrate layer, and then the hydrate layer can be high-pressure fractured when the fracturing fluid is introduced. Moreover, when adjusting the pressure in the conduit 5, it is convenient to form a pressure difference inside and outside the conduit 5, thereby realizing the collection of natural gas.

[0050] The pressure regulating device 51 can be a micro pressure box. A thermometer 52 and a flowmeter 53 can also be set in the conduit 5 to monitor the temperature in the conduit 5 and the flow rate of natural gas. When the flow rate of natural gas is low, the pressure can be further reduced for production or production can be stopped. In addition, an inclination sensor and a multifunctional probe can be set on the outer wall of the casing 4 to monitor reservoir changes in real time.

[0051] Furthermore, a control system 7 may be provided to transmit the real-time monitoring data back to the control system 7 for analysis. Of course, the control system 7 may be connected in communication with the offshore drilling platform 1 to control the lifting and lowering of the drill string 2, the casing 4 and the guide tube 5. At the same time, the control system 7 may be connected in communication with the pressure regulating device 51, the thermometer 52, the flow meter 53 and the pressure pump 14 to control the operation of each device.

[0052] The above-mentioned offshore drilling platform can be set in a drilling ship, so that it can quickly move to the next mining area after mining is completed, and continue mining using the equipment recovered from the previous mining area. Furthermore, a survey and positioning system can be set at the bottom of the drilling ship to facilitate the positioning and survey of the mining area, further improving the mining efficiency.

[0053] like Figure 4 and Figure 5 As shown, the slit 54 on the side wall of the conduit 5 is a trapezoid, and the short side of the trapezoid is arranged on the side of the casing 4 close to the hydrate layer. A filtering device and a fracturing device can be arranged in the slit 54, and the filtering device can be arranged near the short side of the trapezoid, and the fracturing device can be arranged at the long side of the trapezoid. The filtering device can be arranged with a stainless steel filter screen, a permeable stone, a sand-proof screen and a gravel layer from the inside to the outside, and the outside is wrapped with a wire mesh to prevent falling off. The trapezoidal style of the slit 54 has a sand-proof mechanism, which can allow fine gravel of a certain size that can be carried by the water flow to pass through, and block larger sand particles outside the slit 54. The large gravel forms a "sand bridge" outside the detachable casing 4, thereby enhancing the sand-proof function. The slit 54 can be connected to the casing 4 through the circulation device 8. Multiple slits 54 can be opened on the peripheral side of the same horizontal plane of the casing 4, and connected through the circulation device 8. Specifically, it can be an annular multi-pass, one end of which is connected to the slit 54 of the conduit 5, and the other openings of the annular are respectively connected to the slits 54 on the casing 4. It can be understood that the pipelines of the annular multi-pass are arranged in a one-to-one correspondence with the number of slits 54 on the casing 4. Therefore, the fracturing fluid can pass through the slits 54 on the conduit 5 and flow into the slits 54 on the casing 4 through the annular multi-pass, and then flow into the peripheral side of the casing 4. Accordingly, the natural gas on the peripheral side of the casing 4 can be collected and concentrated into the conduit 5 to achieve efficient exploitation. Of course, multiple conduits 5 can be arranged in the annular accommodation space, which can be arranged specifically according to actual needs.

[0054] In some other embodiments, a watertight pipe 9 is also provided, and the watertight pipe 9 is slidably connected to the outer wall of the drill bit 3. This is provided so that during the drilling process, when the drill bit 3 drills downward into the overburden layer, the watertight pipe 9 is subjected to friction from the overburden layer, and the direction of the friction force is upward. When drilling continues, the friction force gradually increases, causing the watertight pipe 9 to slide upward and stay in the overburden layer. The watertight pipe 9 can be provided with a movable connection with the drill bit 3 so that the watertight pipe 9 is separated from the drill bit 3 and stays in the overburden layer. On the one hand, it plays a role in isolating seawater, and on the other hand, a monitoring device can be provided in the watertight pipe 9 to monitor the changes of the overburden layer in real time. When mining is completed, the watertight pipe 9 can be driven to be lifted during the lifting process of the drill bit 3, and the principle is the same as when it is lowered. The diameter of the watertight pipe 9 can be set to be the same as or slightly smaller than the diameter of the drill bit 3, as long as the watertight pipe 9 can be sleeved on the drill bit 3 and lifted. Of course, it can be understood that when the drill bit 3 is conical, the diameter of the watertight pipe 9 is the same as the large end of the cone. The conical drill bit 3 may be a roller cone drill bit 3 .

[0055] Furthermore, the marine natural gas hydrate depressurization production device provided by the present invention also includes a survey device 6, and the survey device 6 is evenly distributed along the circumference of the wellbore. The above-mentioned survey device 6 can be a survey well, and the arrangement of the survey well can be the same as the arrangement of the above-mentioned production well. Of course, it can also be other devices with survey functions. Furthermore, a multi-beam detector can also be set at the bottom of the drilling ship to detect the situation of the hydrate reservoir, and a GPS system can be set to locate the production area.

[0056] The present invention also provides a method for reducing pressure and exploiting marine natural gas hydrates, using the marine natural gas hydrate reducing pressure exploitation device as described above, comprising the following steps:

[0057] S1: Install the drill bit and the drill string, connect the drill string and the first pipeline to the offshore drilling platform, install the casing and the conduit and connect them to the offshore drilling platform.

[0058] Before installing the drill bit and drill string, the target area for hydrate exploitation can be selected first. Specifically, the GPS system at the bottom of the drilling ship can be used to detect and find gas hydrate reservoir points that meet the exploitation conditions. After finding the exploitation point, the satellite coordinates can be determined, the formation information can be analyzed, and the thickness of the overlying soil layer and the hydrate layer can be determined. The drilling ship is started to provide a fixed platform for the drilling platform and the subsequent gas hydrate exploitation.

[0059] S2: Start the drill string, so that the drill string drives the drill bit downward to form a wellbore, and at the same time lower the casing and guide pipe to complete the drilling.

[0060] Before drilling a well, you can first drill a survey well. Specifically, when drilling downward, the drilling fluid is continuously delivered to the drill bit through a pressure pump, and the drilling fluid carrying cuttings is sucked into a sedimentation tank through a mud pump. The cleaned drilling fluid is then delivered to the pressure pump again through the sedimentation and filtration of the sedimentation tank, thereby reusing the drilling fluid. At the same time, the control system is turned on to receive the detected information. After the drilling of the survey well is completed, the drilling of the production well is carried out in the same way. When drilling a production well, the drill bit and drill string simultaneously carry the watertight pipe, casing and conduit downward. When the watertight pipe reaches the upper part of the overlying soil layer, it will fall off automatically due to friction. After the drilling of the production well is completed, the drilling work is completed.

[0061] S3: Fracturing fluid is pressed into the conduit through the offshore drilling platform, and the pressure regulating device in the conduit is adjusted to allow the fracturing fluid to flow into the hydrate layer through the slits, the flow device and the casing for fracturing.

[0062] Before the fracturing fluid is introduced, cementing work can be performed in advance. Specifically, the pressure pump is connected to the high-pressure airbag tube, the high-pressure airbag is inflated, and the pressure indicator is observed. After reaching the predetermined pressure, the interface connecting the pressure pump and the high-pressure airbag tube is closed. At this time, the sealing between the casing and the wellbore is completed. Then the pressure pump is connected to the catheter, and the fracturing fluid is injected into the pressure pump. The fracturing fluid reaches the slit through the catheter and the micro pressure box, and the hydrate layer is hydraulically fractured under the action of the hydraulic fracturing device.

[0063] S4: After the fracturing is completed, the pressure regulating device in the conduit is adjusted to reduce the pressure in the conduit until the natural gas hydrate reaches the phase equilibrium point, and the natural gas is collected through the conduit to the offshore drilling platform.

[0064] Specifically, after the hydraulic fracturing is completed, the interface connecting the pressure pump and the conduit is closed, the interface connecting the conduit and the natural gas separation device is opened, and the micro pressure box is controlled to reduce the pressure in the conduit of the hydrate layer until the natural gas hydrate phase equilibrium point is reached. Then, the generated natural gas mixture is pumped to the top of the conduit through the pressure pump through the micro pressure box and then transported to the natural gas separation device. After multi-layer membrane separation, the initially filtered natural gas is transported to the natural gas purification device for purification by pressure swing adsorption. After reaching the sales standard, the purified natural gas is transported to the natural gas storage device and stored in gas cylinders of various models according to the amount, waiting for transportation and sale. During the drilling and mining process, the real-time data transmitted is analyzed and stored by the control system. By analyzing the data, it is determined whether the riser and casing are excessively bent, whether the soil layer has large-scale settlement, whether the casing is broken or damaged, whether the temperature, pressure and flow in the conduit are normal, and if an abnormality occurs, the mining rate is controlled in time or shut down for emergency treatment.

[0065] S5: After mining is completed, the drill string, drill bit, casing and conduit are lifted and recovered by controlling the offshore drilling platform, and then disassembled, cleaned and recycled.

[0066] Specifically, after the mining is completed, the pressure regulating device and the natural gas collection system are turned off, and the pressure in the high-pressure airbag is reduced to below the atmospheric pressure so that it is close to the outer wall of the casing. Figure 6 As shown in the figure, the drill string, casing and guide tube are lifted by twisting and lifting the drill string by the offshore drilling platform, and then the devices are disassembled, cleaned and recycled. The device recovery method in the detection well is the same as that in the production well. After the disassembly is completed, the dynamic positioning system is turned off, the drilling ship is driven to the next production point, and the above operation is repeated to carry out the production work.

[0067] The marine natural gas hydrate depressurization mining device provided by the present invention is provided with a first pipeline 21 on the drill string 2 and connected to the offshore drilling platform 1, so that the mined mud can be discharged to the offshore drilling platform 1 through the first pipeline 21 during the mining process and collected and utilized. The drill bit 3 is provided to be connected to the drill string 2 and the diameter of the drill bit 3 is larger than the diameter of the drill string 2, so that the wellbore diameter is larger than the diameter of the drill string 2 during the mining process of the drill bit 3. The casing 4 is further provided to be sleeved on the outer peripheral side of the drill string 2 and an annular accommodation space is formed between the casing 4 and the drill string 2, and the conduit 5 is provided in the annular accommodation space, and the casing 4 and the conduit 5 are both connected to the offshore drilling platform 1, and the conduit 5 is provided with a pressure regulating device 51 and a slit 54. Therefore, the circulation of the fracturing fluid and the collection of natural gas can be controlled by adjusting the pressure in the conduit 5. At the same time, the outer wall of the casing 4 is provided with an annular sealing device 41, which can isolate the overlying soil layer from the hydrate layer to prevent pollutants from entering the wellbore, and can form a sealed environment in the hydrate layer, which is convenient for fracturing and mining. Therefore, after drilling is completed, the reservoir is fractured by introducing fracturing fluid into the hydrate layer. After fracturing, the pressure in the conduit 5 is adjusted until the natural gas hydrate reaches the phase equilibrium point and the hydrate decomposes, and then the natural gas can be collected to the offshore drilling platform 1 through the conduit 5 by the pressure difference. After the exploitation is completed, the drill string 2, the drill bit 3, the casing 4 and the conduit 5 can be lifted and recovered by controlling the offshore drilling platform 1, and they can be disassembled and cleaned for recycling, which not only improves the exploitation efficiency but also reduces the exploitation cost.

[0068] It should be noted that, in this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0069] The above description is only a specific embodiment of the present invention, so that those skilled in the art can understand or implement the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments described herein, but should conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A marine natural gas hydrate depressurization and production device, characterized in that: include: Offshore drilling platform (1); A drill string (2), the top of which is connected to the offshore drilling platform (1), and the offshore drilling platform (1) can lift or lower the drill string (2). A first pipe (21) is provided inside the drill string (2), one end of which is connected to the offshore drilling platform (1), and the first pipe (21) is used to discharge drilling mud; A drill bit (3), the drill bit (3) being fixedly connected to the bottom of the drill string (2), the drill bit (3) being used to open a wellbore, and the diameter of the drill bit (3) being larger than the diameter of the drill string (2); A casing (4), the casing (4) being sleeved on the outer peripheral side of the drill string (2) and forming an annular accommodation space between the casing (4) and the drill string (2), the top end of the casing (4) being connected to the offshore drilling platform (1), the offshore drilling platform (1) being able to lift or lower the casing (4), the side wall of the casing (4) being provided with an annular sealing device (41), the annular sealing device (41) being used to isolate the overlying soil layer and the hydrate layer; A conduit (5), wherein the conduit (5) is arranged in the annular accommodation space and extends toward the seabed, the top end of the conduit (5) is connected to the offshore drilling platform (1), and the offshore drilling platform (1) can lift or lower the conduit (5), the conduit (5) is provided with a pressure regulating device (51), and the pressure regulating device (51) is used to regulate the pressure in the conduit (5), and the side wall of the conduit (5) is provided with a slit (54), and the slit (54) is connected to the casing (4) and the hydrate layer through a circulation device (8).

2. The marine natural gas hydrate depressurization and production device according to claim 1, characterized in that: The offshore drilling platform (1) comprises a lifting screw crane (11), a first lifting platform (12) and a second lifting platform (13); The lifting screw crane (11) is detachably connected to the top of the drill string (2), and the lifting screw crane (11) is used to drive the drill string (2) to rotate and lift; The first lifting platform (12) is detachably connected to the top end of the casing (4), and the first lifting platform (12) is used to drive the casing (4) to rise and fall along the altitude direction; The second lifting platform (13) is detachably connected to the top end of the conduit (5), and the second lifting platform (13) is used to drive the conduit (5) to rise and fall along an altitude direction.

3. The marine natural gas hydrate depressurization and production device according to claim 2, characterized in that: The offshore drilling platform (1) further comprises a pressure pump (14) and a mud pump (15); The pressure pump (14) is connected to the top end of the conduit (5), and the pressure pump (14) is used to suck the natural gas in the conduit (5) and release the fracturing fluid into the conduit (5); The mud pump (15) is in communication with the first pipeline (21).

4. The marine natural gas hydrate depressurization and production device according to claim 3, characterized in that: A second pipe (22) is also provided inside the drill string (2). The second pipe (22) is spaced apart from the first pipe (21). The top end of the second pipe (22) is connected to the pressure pump (14). The second pipe (22) is used to pass drilling fluid.

5. The marine natural gas hydrate depressurization and production device according to claim 3, characterized in that: The annular sealing device (41) is a high-pressure airbag, and the high-pressure airbag is connected to the pressure pump (14).

6. The marine natural gas hydrate depressurization and production device according to claim 1, characterized in that: It also comprises a watertight pipe (9), wherein the watertight pipe (9) is slidably connected to the outer wall of the drill bit (3).

7. The marine natural gas hydrate depressurization and production device according to claim 1, characterized in that: The cross section of the slit (54) is a trapezoid, and the short side of the trapezoid is arranged on a side of the casing (4) close to the hydrate layer.

8. The marine natural gas hydrate depressurization and production device according to claim 1, characterized in that: It also comprises surveying devices (6), wherein the surveying devices (6) are evenly distributed along the circumference of the wellbore.

9. The marine natural gas hydrate depressurization and production device according to claim 1, characterized in that: It also comprises a control system (7), wherein the control system (7) is communicatively connected with the offshore drilling platform (1).

10. A method for depressurizing and exploiting marine natural gas hydrates, using the depressurizing and exploiting marine natural gas hydrate device according to any one of claims 1 to 9, characterized in that: The steps include: S1: Install the drill bit and the drill string, connect the drill string and the first pipeline to the offshore drilling platform, install the casing and the conduit and connect them to the offshore drilling platform; S2: Start the drill string, so that the drill string drives the drill bit downward to form a wellbore, and at the same time lower the casing and the guide tube to complete the drilling; S3: injecting fracturing fluid into the conduit through a pressure pump, and adjusting a pressure regulating device in the conduit to allow the fracturing fluid to flow into the hydrate layer through the slits, the flow device and the casing to perform fracturing; S4: After the fracturing is completed, the pressure regulating device in the conduit is adjusted to reduce the pressure in the conduit until the natural gas hydrate reaches the phase equilibrium point, and the natural gas is collected through the conduit to the offshore drilling platform; S5: After mining is completed, the drill string, drill bit, casing and conduit are lifted and recovered by controlling the offshore drilling platform, and then disassembled, cleaned and recycled.

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