Marine natural gas hydrate decompression mining device and method

Abstract

The present invention relates to the technical field of natural gas hydrate extraction, and specifically to a device and method for depressurizing and extracting marine natural gas hydrates. The device comprises an offshore drilling platform, a drill string, a casing, and a conduit. The drilling platform can lift or lower the drill string, the casing, and the conduit. A first pipe is provided inside the drill string, and a drill bit is fixedly connected to the bottom. The casing is sleeved on the outer peripheral side of the drill string and forms an annular accommodation space between the casing and the drill string, and an annular sealing device is provided on the side wall. The conduit is arranged in the annular accommodation space and is provided with a pressure regulating device. The side wall is provided with a slit and is connected to the casing and the hydrate layer through a circulation device. Fracturing fluid is introduced into the hydrate layer, and the pressure in the conduit is adjusted until the hydrate reaches the phase equilibrium point and decomposes, and then the natural gas is collected to the offshore drilling platform through the conduit. After the extraction is completed, the drill string, the drill bit, the casing, and the conduit can be lifted and recovered by controlling the offshore drilling platform, and then recycled after disassembly and cleaning, which not only improves the extraction efficiency but also reduces the extraction cost.

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 depressurizing and exploiting marine natural gas hydrate. Background Art

[0002] Natural gas hydrates are the most abundant methane resource in nature. The total carbon content of natural gas hydrates on Earth is twice that of proven fossil fuels. my country's total marine natural gas hydrate reserves amount to 70 billion tons of oil equivalent. Their industrial development is crucial for improving energy consumption and ensuring energy security.

[0003] Existing technologies have conducted numerous trials of marine natural gas hydrate production, using both vertical and horizontal wells. Pressure reduction is considered the primary method for future natural gas hydrate extraction. However, in the later stages of pressure reduction, the pressure in the hydrate layer decreases, making it difficult to sustain extraction and forcing it to be suspended. This results in insufficient gas production to meet industrial requirements, and also in the fact that existing pipelines are essentially abandoned and cannot be reused, significantly increasing the cost of natural gas hydrate extraction. From the perspective of large-scale, industrialized extraction, the forced suspension of extraction and the high well construction costs make it difficult to achieve this goal. Summary of the Invention

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

[0005] To achieve the above-mentioned 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, so that the offshore drilling platform can lift or lower the drill string; a first pipe is provided inside the drill string, one end of which is connected to the offshore drilling platform, and the first pipe is used to discharge drilling mud;

[0008] a drill bit, the drill bit being fixedly connected to the bottom of the drill string and used for opening a wellbore, the diameter of the drill bit being larger than the diameter of the drill string;

[0009] a casing, the casing being sleeved on the outer circumference of the drill string and forming an annular accommodation space between the casing and the drill string; the top end of the casing being connected to the offshore drilling platform, the offshore drilling platform being capable of lifting or lowering the casing; and an annular sealing device being provided on the sidewall of the casing, the annular sealing device being used to isolate the overlying soil layer from 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, and the offshore drilling platform can lift or lower the conduit. The conduit is provided with a pressure regulating device for regulating the pressure in the conduit. A slit is provided on the side wall of the conduit, and 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 is used to drive the casing to move up and down 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 move up and down 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 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 pipe is further provided inside the drill string. The second pipe is spaced apart from the first pipe, and the top of the second pipe is connected to the pressure pump. The second pipe is used to introduce 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, a watertight pipe is also included, which is slidably connected to the outer wall of the drill bit.

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

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

[0023] Optionally, a control system is further included, and the control system is communicatively connected to the offshore drilling platform.

[0024] The present invention also provides a method for depressurizing and mining marine natural gas hydrates, which uses the above-mentioned marine natural gas hydrate depressurization and mining device, and includes the following steps:

[0025] S1: Install the drill bit and 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 guide pipe to complete the drilling;

[0027] S3: Fracturing fluid is injected into the conduit through the pressure pump 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 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 guide pipe 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 mining device provided by the present invention is configured to be connected to an offshore drilling platform by arranging a first pipeline on the drill string, so that the mined mud can be discharged to the offshore drilling platform through the first pipeline and collected and utilized during the mining process. A drill bit is 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 drilling process. A casing is further configured to be sleeved on the outer peripheral side of the drill string and to form an annular accommodation space between the casing and the drill string. A conduit is configured in the annular accommodation space, and both the casing and the conduit are connected to the offshore drilling platform. A pressure regulating device and a slit are configured 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 to facilitate 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 is adjusted until the natural gas hydrate reaches a phase equilibrium point, causing the hydrate to decompose. The pressure differential then allows the natural gas to be collected through the conduit to the offshore drilling platform. Once extraction is complete, the offshore drilling platform is controlled to lift and recover the drill string, drill bit, casing, and conduit, disassemble and clean them, and then recycle them. This not only improves extraction efficiency but also integrates hydrate extraction and equipment recycling, reducing extraction costs. 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 following briefly introduces the drawings required for use in the embodiments or the description of the prior art. 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 This is 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 and production device according to 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. Conduit; 51. Pressure regulating device; 52. Thermometer; 53. Flow meter; 54. Slit; 6. Survey device; 7. Control system; 8. Circulation device; 9. Water-repellent riser. 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, in the absence of conflict, the embodiments of the present invention and the features therein can be combined with each other.

[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 the embodiments.

[0043] The following describes the marine natural gas hydrate decompression extraction device in detail through specific embodiments:

[0044] like Figures 1 to 3 As shown, the marine natural gas hydrate decompression extraction 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. The bottom is fixedly connected to the drill bit 3. The diameter of the drill bit 3 is larger than that of the drill string 2. After the drill bit 3 has produced a wellbore, a casing 4 is installed on the outer periphery 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 disposed within the annular accommodation space and extends toward the seabed. A pressure regulating device 51 is provided within the conduit 5 to adjust the pressure within the conduit 5, thereby creating a pressure differential between the inside and outside of the conduit 5, facilitating the collection of natural gas. Furthermore, a slit 54 is provided on the sidewall of the conduit 5. The slit 54 is connected to the casing 4 and the hydrate layer through a circulation device 8, allowing fracturing fluid to flow through the slit 54 in the conduit 5 to the casing 4, and then to the hydrate layer for fracturing. When the natural gas hydrate reaches its phase equilibrium point, it begins to decompose, releasing natural gas. The pressure inside conduit 5 is then adjusted to be lower than the pressure outside. This pressure differential forces the natural gas into conduit 5 and collects it on offshore drilling platform 1. Once collected, offshore drilling platform 1 controls the drill string 2, drill bit 3, casing 4, and conduit 5 to be hoisted, recovered, disassembled, cleaned, and reused, improving extraction efficiency and reducing costs.

[0045] The offshore drilling platform 1 may include a lifting screw gantry 11, a first lifting platform 12, and a second lifting platform 13. The lifting screw gantry 11 is detachably connected to the top of the drill string 2 and can rotate and raise the drill string 2. When drilling is required, the drill string 2 can be lowered using the lifting screw gantry 11. After drilling is complete, the drill string 2 can be lifted, disassembled, cleaned, and recycled. It should be understood that the lifting screw gantry 11 can also simultaneously lift and lower the drill bit 3. The first lifting platform 12 is detachably connected to the top of the casing 4 and can raise and lower the casing 4 in the vertical direction. During drilling, the casing 4 is lowered to the outer periphery of the drill string 2 via the first lifting platform 12. After drilling is complete, the casing 4 can be lifted, disassembled, cleaned, and recycled. The second lifting platform 13 is detachably connected to the top of the guide tube 5 and can raise and lower the guide tube 5 in the vertical direction. During drilling, the guide tube 5 is lowered into the annular space between the casing 4 and the drill string 2 via the second lifting platform 13. After drilling is complete, the guide tube 5 can be lifted, disassembled, cleaned, and recycled. To facilitate the fixation of the conduit 5 during drilling and recovery, a mechanical ring lock can be provided on the inner wall of the casing 4. The mechanical ring lock is connected to the inner wall of the casing 4 via mechanical claws, and the mechanical claws can slide along the circumference of the inner wall of the casing 4 and lock. The diameter of the mechanical ring lock is slightly larger than the diameter of the conduit 5 to facilitate the fixation of the conduit 5 within the ring lock. The casing 4 and drill string 2 can be provided with multiple sections, and the multiple sections of casing 4 can be connected by threads. The connection between the multiple sections of casing 4 can be controlled by the rotation of the first lifting platform 12. The connection method between the multiple sections of drill string 2 can be the same as that of the casing 4, and the specific design is based on actual needs.

[0046] Furthermore, a first pipe 21 can be provided within the drill string 2, extending axially along the drill string 2. Thus, during the drilling process, drilling mud can be discharged to the offshore drilling platform 1 for recycling via the first pipe 21. Specifically, a mud pump 15 can be provided in communication with the first pipe 21, pumping the drilling mud up through the mud pump 15. A sedimentation tank can also be provided, connected to the mud pump 15, allowing the pumped drilling mud to be discharged into the sedimentation tank for sedimentation and stratification.

[0047] Furthermore, a second pipe 22 can be provided inside the drill string 2, spaced apart from the first pipe 21. The second pipe 22 can be used to introduce drilling fluid. The diameter of the first pipe 21 can be larger than the diameter of the second pipe 22, and can be set specifically according to actual needs. At the same time, a pressure pump 14 is provided at the outlet 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, to save resources, the sedimentation tank can be connected to the pressure pump 14, so that the liquid after sedimentation in the sedimentation tank enters the pressure pump 14 and serves as a circulating drilling fluid. The outlet end of the conduit 5 can also be connected to the pressure pump 14, so that the fracturing fluid can be pumped into the conduit 5 through the pressure pump 14, and then flow into the hydrate layer through the slits 54 on the conduit 5 for fracturing. At the same time, the pressure pump 14 can suck up the natural gas collected in the conduit 5 and pass it into a storage device, 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 installed 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 via a four-way joint.

[0049] The sealing device provided on the casing 4 is intended to isolate the overlying soil layer from the hydrate layer, preventing contaminants such as seawater from the overlying soil layer from entering the hydrate layer and impacting extraction. 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 via an interface. Furthermore, the pressure pump 14 can inflate and deflate the airbag. When extraction is required, the airbag is inflated, the pressure gauge reading is monitored, and inflation is stopped when a predetermined pressure is reached, completing the seal between the casing 4 and the wellbore. To further enhance the sealing effect, multiple annular sealing devices 41 can be provided, spaced apart along the axial direction of the casing 4. After extraction is completed, the pressure is released by the pressure pump 14, releasing the gas in the airbag and facilitating the recovery of the casing 4. The outer surface of the airbag can be wrapped with aluminum foil to prevent puncture by the hard rock in the wellbore. This not only reduces the loss of disposable raw materials and mining costs, but also facilitates the recycling of the casing 4, preventing damage to the pipeline or the inability to disassemble it due to cement grouting. Furthermore, the annular seal 41 forms a closed space around the hydrate layer, enabling high-pressure fracturing of the hydrate layer when fracturing fluid is introduced. Furthermore, when regulating the pressure within the conduit 5, a pressure differential is created between the inside and outside of the conduit 5, thereby enabling the collection of natural gas.

[0050] The pressure regulating device 51 can be a miniature pressure cell. A thermometer 52 and a flowmeter 53 can also be installed within the conduit 5 to monitor the temperature and natural gas flow rate within the conduit 5. When the natural gas flow rate is low, the pressure can be further reduced to allow production or production can be stopped. Furthermore, an inclination sensor and a multi-function probe can be installed on the outer wall of the casing 4 to monitor reservoir changes in real time.

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

[0052] The offshore drilling platform can be installed within a drillship, allowing for rapid movement to the next mining area after completion, utilizing the equipment recovered from the previous mining area to continue mining. Furthermore, a survey and positioning system can be installed on the bottom of the drillship to facilitate the positioning and survey of the mining area, further improving mining efficiency.

[0053] like Figure 4 and Figure 5 As shown, the slits 54 in the sidewall of the conduit 5 are trapezoidal in shape, with the short side of the trapezoid positioned on the side of the casing 4 near the hydrate layer. A filtration device and a fracturing device can be installed within the slits 54. The filtration device can be located near the short side of the trapezoid, and the fracturing device can be located on the long side. The filtration device can be arranged, from the inside to the outside, with a stainless steel filter screen, permeable stone, sand-control screen, and gravel layer, wrapped with wire mesh to prevent it from falling out. The trapezoidal shape of the slits 54 provides a sand control mechanism, allowing small sand particles of a certain size that can be carried by the water flow to pass through, while retaining larger sand particles outside the slits 54. The large sand particles form a "sand bridge" outside the removable casing 4, thereby enhancing the sand control function. The slits 54 can be connected to the casing 4 via a circulation device 8. Multiple slits 54 can be provided around the casing 4 on the same horizontal plane and connected by 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 annular openings are respectively connected to the slits 54 on the casing 4. It will be understood that the pipelines of the annular multi-pass correspond one-to-one 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 circumference of the casing 4. Accordingly, the natural gas on the circumference of the casing 4 can be collected and concentrated into the conduit 5, achieving efficient extraction. Of course, multiple conduits 5 can be installed in the annular accommodation space, and the specific arrangement can be based on actual needs.

[0054] In some other embodiments, a riser 9 is also provided, slidably connected to the outer wall of the drill bit 3. This arrangement ensures that during drilling, as the drill bit 3 descends into the overburden, the riser 9 is subjected to friction from the overburden, with the frictional force directed upward. As drilling continues, the frictional force gradually increases, causing the riser 9 to slide upward and settle within the overburden. A flexible connection between the riser 9 and the drill bit 3 can be provided, allowing the riser 9 to detach from the drill bit 3 and settle within the overburden. This not only serves to isolate the drill bit 3 from the seawater, but also allows monitoring devices to be installed within the riser 9 to monitor changes in the overburden in real time. After mining is completed, the riser 9 can be lifted during the drill bit 3's lifting process, using the same principle as during lowering. The diameter of the riser 9 can be the same as or slightly smaller than that of the drill bit 3, as long as it can be fitted over the drill bit 3 and lifted. Of course, it is understood that if the drill bit 3 is conical, the diameter of the riser 9 is the same as the larger 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 and production apparatus provided by the present invention also includes a surveying device 6, which is evenly distributed along the circumference of the wellbore. The surveying device 6 can be a survey well, which can be arranged in the same manner as the production well. Of course, other surveying devices are also possible. Furthermore, a multi-beam detector can be installed on the bottom of the drilling vessel to detect the hydrate reservoir, and a GPS system can be used to locate the production area.

[0056] The present invention also provides a method for depressurizing and mining marine natural gas hydrates, which uses the above-mentioned marine natural gas hydrate depressurization and mining device, and comprises the following steps:

[0057] S1: Install the drill bit and 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 extraction can be selected. Specifically, the GPS system on the bottom of the drillship can be used to detect and locate suitable natural gas hydrate reservoir locations. Once the extraction location is located, satellite coordinates can be determined, and stratigraphic information can be analyzed to determine the thickness of the overburden and hydrate layers. The drillship is then launched to provide a fixed platform for the drilling platform and subsequent natural gas hydrate extraction.

[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, a pressure pump continuously delivers drilling fluid to the drill bit. At the same time, a mud pump sucks the drilling fluid carrying rock cuttings into a sedimentation tank. The sedimentation and filtration of the sedimentation tank then delivers the cleaned drilling fluid back to the pressure pump, thereby reusing the drilling fluid. At the same time, the control system is turned on to receive the various 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 guide pipe downward. When the watertight pipe reaches the upper part of the overlying soil layer, it will fall off due to friction. After the drilling of the production well is completed, the drilling work is completed.

[0061] S3: Fracturing fluid is injected 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 circulation device and the casing to perform fracturing.

[0062] Before introducing the fracturing fluid, cementing can be performed. Specifically, connect the pressure pump to the high-pressure airbag tube, begin inflating the airbag, observe the pressure indicator, and when the desired pressure is reached, close the connection between the pressure pump and the airbag tube. This seals the casing and the wellbore. Then, connect the pressure pump to the conduit and inject fracturing fluid into the pump. The fracturing fluid flows through the conduit and the micro-pressure cell to the slits, where the hydrate layer is hydraulically fractured by 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 hydraulic fracturing is completed, the interface connecting the pressure pump to the conduit is closed, and the interface connecting the conduit to the natural gas separation device is opened. The micro-pressure cell is controlled to reduce the pressure in the conduit of the hydrate layer until the natural gas hydrate phase equilibrium point is reached. The resulting natural gas mixture is then pumped to the top of the conduit through the micro-pressure cell via the pressure pump 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 via pressure swing adsorption. Once it meets sales standards, the purified natural gas is then transported to the natural gas storage device and distributed and stored in various types of gas cylinders, awaiting transportation and sale. During the drilling and production process, the control system analyzes and stores the transmitted real-time data. This data is used to determine whether the riser or casing is excessively bent, whether there is large-scale soil settlement, whether the casing is broken or damaged, and whether the temperature, pressure, and flow rate in the conduit are normal. If any abnormality occurs, the production rate is promptly controlled or the system is shut down for emergency treatment.

[0065] S5: After mining is completed, the drill string, drill bit, casing and guide pipe 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 atmospheric pressure so that it is close to the outer wall of the casing. Figure 6 As shown, the offshore drilling platform twists and lifts the drill string, raising the drill string, casing, and guide pipe. The components are then disassembled, cleaned, and recycled. The recovery method for components in the exploration well is similar to that for the production well. After disassembly, the dynamic positioning system is shut down, the drillship is moved to the next production point, and the above operation is repeated to resume production.

[0067] The marine natural gas hydrate decompression extraction device provided by the present invention comprises a first pipe 21 disposed on a drill string 2 and connected to an offshore drilling platform 1, so that during the extraction process, the extracted mud can be discharged to the offshore drilling platform 1 through the first pipe 21 for collection and utilization. A drill bit 3 is connected to the drill string 2, and the diameter of the drill bit 3 is larger than that of the drill string 2, so that during the extraction process, the wellbore diameter is larger than that of the drill string 2. A casing 4 is further disposed and sleeved on the outer periphery of the drill string 2, forming an annular accommodation space between the casing 4 and the drill string 2. A conduit 5 is disposed in the annular accommodation space, and both the casing 4 and the conduit 5 are connected to the offshore drilling platform 1. A pressure regulating device 51 and a slit 54 are provided in the conduit 5. Thus, the flow of fracturing fluid and the collection of natural gas can be controlled by adjusting the pressure within the conduit 5. Furthermore, an annular sealing device 41 is provided on the outer wall of the casing 4, which isolates the overlying soil layer from the hydrate layer to prevent contaminants from entering the wellbore and creates a sealed environment within the hydrate layer, facilitating fracturing and extraction. 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 a phase equilibrium point, causing the hydrate to decompose. The pressure differential allows the natural gas to be collected through the conduit 5 and brought to the offshore drilling platform 1. After extraction is complete, the offshore drilling platform 1 is controlled to lift and recover the drill string 2, drill bit 3, casing 4, and conduit 5. These can then be disassembled, cleaned, and recycled, improving extraction efficiency and reducing extraction costs.

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

[0069] The foregoing description is intended only to provide specific embodiments of the present invention, which will enable those skilled in the art to understand and implement the present invention. Various modifications to these embodiments will be readily 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 is not intended to be limited to the embodiments described herein, but is to be construed in the widest manner consistent with the principles and novel features disclosed herein.

Claims

1. A marine natural gas hydrate depressurization and mining device, characterized in that: include: Offshore drilling platform (1); A drill string (2), the top end 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, the diameter of the drill bit (3) being larger than the diameter of the drill string (2); A casing (4), the casing (4) is sleeved on the outer peripheral side of the drill string (2) and forms an annular accommodation space between the casing (4) and the drill string (2), the top end of the casing (4) is connected to the offshore drilling platform (1), and the offshore drilling platform (1) can lift or lower the casing (4), and the side wall of the casing (4) is provided with an annular sealing device (41), and the annular sealing device (41) is used to isolate the overlying soil layer and the hydrate layer; A conduit (5), the conduit (5) being arranged in the annular accommodation space and extending toward the seabed, the top end of the conduit (5) being connected to the offshore drilling platform (1), the offshore drilling platform (1) being capable of lifting or lowering the conduit (5), the conduit (5) being provided with a pressure regulating device (51), the pressure regulating device (51) being used to regulate the pressure in the conduit (5), the side wall of the conduit (5) being provided with a slit (54), the slit (54) being in communication with the casing (4) and the hydrate layer through a circulation device (8); 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 move up and down along the altitude direction; The offshore drilling platform (1) further comprises a pressure pump (14) and a mud pump (15); The pressure pump (14) is in communication with 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).

2. The marine natural gas hydrate depressurization and mining device according to claim 1, characterized in that: A second pipe (22) is further 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 introduce drilling fluid.

3. The marine natural gas hydrate depressurization and mining device according to claim 1, 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).

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

5. The marine natural gas hydrate depressurization and mining 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.

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

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

8. A method for depressurizing and mining marine natural gas hydrates, using the marine natural gas hydrate depressurizing and mining device according to any one of claims 1 to 7, characterized in that: The steps include: S1: Install the drill bit and 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 guide pipe to complete the drilling; S3: injecting fracturing fluid into the conduit through a pressure pump, 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; 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 guide pipe are lifted and recovered by controlling the offshore drilling platform, and then disassembled, cleaned and recycled.

Images