Radial well cluster mining system and method for deep sea natural gas hydrate and shallow gas

Abstract

The invention discloses a radial well cluster mining system and method for deep sea natural gas hydrate and shallow gas. The small and medium-sized workboat is sequentially connected and fixed with the seabed drilling machine and the multi-well guide suction anchor, and the small and medium-sized workboat controls the seabed drilling machine through an umbilical cable to complete drilling and arrangement of a vertical well and an inclined well of drilling and fixing integrated hydrate in the multi-well guide suction anchor; the vertical well and the inclined well are connected with a multi-pipe Christmas tree arranged on the multi-well guide suction anchor, and then the multi-pipe Christmas tree is connected with a mining and transportation pipeline through a seabed supercharging electric submersible pump to form a natural gas hydrate one-machine multi-well type radial well cluster efficient mining and transportation path. According to the hydrate anchor base radial well cluster mining system, the mining range is greatly enlarged, the mining efficiency is improved, integrated drilling and one-machine multi-well arrangement are achieved through the seabed drilling machine, the operation cost is remarkably reduced, the drilling and mining efficiency is improved, and the integrated operation process of drilling, well cementation, well completion and mining of the deep sea hydrate reservoir is achieved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of energy engineering and marine geotechnical engineering, and particularly relates to a radial well cluster mining system and method for deep-sea natural gas hydrate and shallow gas. BACKGROUND

[0002] Natural gas hydrate has a huge reserve and is considered as a clean energy with the greatest potential to replace conventional oil and gas in the 21st century. The reserve of natural gas hydrate in the South China Sea is about 19.4 billion cubic meters, and the Shenhu sea area is the current key target area. The reservoir in this area is buried at a depth of 200-300 m and occurs in a high-pressure and low-temperature deep-sea environment with a water depth of more than 1000 m. Different from traditional oil and gas reservoirs, the natural gas hydrate reservoir has the characteristics of low strength, strong particle, weak skeleton strength of hydrate decomposition, phase change, sand production, and gas production, which results in low gas production efficiency of single well mining and even induces engineering disasters such as sand production and reservoir deformation. From 2017 to 2020, China completed two tests in the Shenhu sea area of the South China Sea, verifying the technical feasibility of deep-sea hydrate mining, but showing the bottleneck problems of low gas production rate and high mining cost. At present, the drilling and mining equipment used in the global natural gas hydrate test mining has a drilling depth far exceeding the drilling and mining demand of the hydrate reservoir, resulting in high operation cost and unit gas production cost far higher than the commercial threshold.

[0003] At present, deep-sea natural gas hydrate mining faces two major challenges: one is that the low permeability of the reservoir limits the mining range, results in low gas production efficiency, and easily induces engineering disasters such as sand production and reservoir collapse during the mining process; the other is that the traditional oil and gas drilling and mining scheme is expensive, causing the cost of hydrate mining to be far beyond the commercial threshold, and this kind of method cannot be directly used for deep-sea hydrate mining.

[0004] At present, there is no method that can realize the commercial mining of marine natural gas hydrate. Therefore, it is urgent to explore a new mining system and method that can not only arrange multiple mining wells in the deep-sea natural gas hydrate reservoir to form a well cluster mining system and greatly increase the mining range, but also facilitate the construction and equipment recovery on the deep-sea bottom, significantly reduce the mining cost, and improve the construction efficiency. This is the bottleneck problem that needs to be solved to realize the efficient, safe and long-term mining of natural gas hydrate energy, and it is also the urgent need to break through the commercial mining of deep-sea natural gas hydrate. SUMMARY

[0005] The present application aims to overcome the shortcomings of the prior art and provides a radial well cluster mining system and method for deep-sea natural gas hydrate and shallow gas, which realizes the economic, safe and efficient mining of deep-sea natural gas hydrate reservoir and is suitable for solving the problems of low mining efficiency and high mining cost of deep-sea natural gas hydrate.

[0006] The present application realizes drilling and fixing integrated well completion by using a seabed drilling machine, realizes large-scale and high-efficiency exploitation of hydrates by a seabed booster electric submersible pump and a multi-tube Christmas tree, and realizes efficient drilling and exploitation and low-cost exploitation after recycling the equipment.

[0007] The purpose of the present application is achieved by the following technical solutions. One kind is a radial well cluster exploitation system of deep-sea natural gas hydrate or shallow gas: The system comprises a multi-well guided suction anchor, a drilling and fixing integrated hydrate straight well, an inclined well, a multi-tube Christmas tree, a seabed booster electric submersible pump, a seabed drilling machine, a medium and small workship and a production and transportation pipeline; the seabed drilling machine is connected with the medium and small workship through a umbilical cable, and is fixed on the top of the multi-well guided suction anchor; the medium and small workship controls the seabed drilling machine to complete the drilling and laying of the drilling and fixing integrated hydrate straight well and inclined well in the multi-well guided suction anchor through the umbilical cable; the straight well and the inclined well are connected with the multi-tube Christmas tree arranged above the multi-well guided suction anchor, and the multi-tube Christmas tree is connected with the seabed booster electric submersible pump and the production and transportation pipeline to form a radial well cluster high-efficiency production and transportation path of natural gas hydrate or shallow gas one-machine multi-well type.

[0008] The multi-well guided suction anchor comprises a top cover and a peripheral cylinder body, the top cover is fixed on the upper end of the peripheral cylinder body to form a cylinder shell structure with an open lower end and a closed upper end, and an inclined pipe, a straight pipe and a stiffener plate are arranged in the cylinder shell structure; the straight pipe is arranged on the central axis of the cylinder shell structure, the upper ends of the straight pipe and the inclined pipe are fixed to the top cover, a plurality of inclined pipes are arranged around the straight pipe, each inclined pipe is arranged obliquely, the inclined pipes are uniformly distributed in the circumferential direction, and the inclined pipes are rotationally symmetrically arranged around the straight pipe, so that each inclined pipe is distributed and extended in a radial manner from top to bottom. The upper side of each inclined pipe is fixedly connected with the top cover and the inner wall of the peripheral cylinder body through a vertical stiffener plate to form a stress unit; the top cover near the pipe openings of the inclined pipes and the straight pipe is provided with a preset leg slot for connecting and fixing a seabed drilling machine leg.

[0009] The inclined pipes are uniformly arranged at intervals at specified positions away from the central axis of the straight pipe, each inclined pipe is arranged obliquely in a direction that is not parallel to the radial direction, the axial direction and the circumferential direction, and the straight pipe and the inclined pipe are used for drilling of a seabed drilling machine.

[0010] The stiffened plate is welded between one side and the surface of the inclined guide pipe and between the other side and the bottom surface of the top cover and the inner wall of the peripheral cylinder; and the stiffened plate corresponding to each inclined guide pipe is provided with a vertical upward through slot structure at the position interfering with the inclined guide pipe above itself and the corresponding stiffened plate, forming a plug-in cooperation, and the adjacent inclined guide pipe and the corresponding stiffened plate are inserted and fixed by welding in the slot structure and are welded with each other.

[0011] The seabed drilling machine is provided with a drill for drilling, and the drill structures for drilling vertical wells and inclined wells are the same, mainly consisting of a hollow drill rod fixedly connected from top to bottom along the axial direction, a plurality of sand prevention and well cementing units and a hollow drill bit; the seabed drilling machine drives the drill to drill downward to break through the overburden layer by the hollow drill bit, and drill into the hydrate reservoir or underlying gas layer, or drill into the shallow gas reservoir layer when exploiting deep-sea shallow gas. The central axes of the hollow drill rod, the plurality of sand prevention and well cementing units and the hollow drill bit are all provided with coaxial hollow drill holes, and the underwater soil drilled from the overburden layer, the hydrate reservoir, the underlying gas layer and the shallow gas reservoir layer is transported out through the hollow drill holes.

[0012] The sand prevention and well cementing unit mainly consists of a self-expanding packer and a pre-packed gravel screen pipe coaxially connected from top to bottom along the axial direction; the self-expanding packer includes a self-expanding packer inner rod and a water-swelling rubber sleeve, and mainly consists of the self-expanding packer inner rod and the water-swelling rubber sleeve from inside to outside, wherein the self-expanding packer inner rod and the hollow drill rod have the same inner and outer diameters, and an annular groove is arranged on the outer peripheral surface of the self-expanding packer inner rod, and the water-swelling rubber sleeve is installed in the annular groove; the pre-packed gravel screen pipe includes an inner screen pipe, a pre-packed gravel layer, an outer screen pipe and a water-soluble blocking plug; the inner screen pipe is coaxially sleeved in the outer screen pipe, the pre-packed gravel layer is fixedly filled in the annular gap between the inner screen pipe and the outer screen pipe, and a plurality of radial through perforations are arrayed and formed on the peripheral surfaces of the inner screen pipe and the outer screen pipe, and a water-soluble blocking plug is arranged at each perforation.

[0013] The self-expanding packer is arranged in the hydrate reservoir or the overburden layer, or arranged in the hydrate reservoir and the overburiden layer at the same time; in the self-expanding packer, the outer diameter of the water-swelling rubber sleeve under dry conditions is not greater than the outer diameter of the self-expanding packer inner rod; the water-swelling rubber sleeve can swell after being underwater for a preset time, and the outer diameter of the water-swelling rubber sleeve after swelling is not less than 1.1 times of the hollow drill rod, so as to realize the fixation of the well pipe and effectively block the natural gas leakage during the exploitation.

[0014] The water-soluble blocking plug is filled in the perforations of the inner screen pipe and the outer screen pipe to temporarily block, and the water-soluble blocking plug blocks the perforations under dry conditions, and hydrolyzes after being underwater for a preset time, so as to realize the unblocking of the perforations in the pre-packed gravel screen pipe.

[0015] The multi-tube Christmas tree and the subsea booster electric submersible pump are arranged on the seabed surface between seawater and overburden, or are integrated and arranged on the seabed after integration of the multi-tube Christmas tree and the subsea booster electric submersible pump. The multi-tube Christmas tree is a multi-tube integrated Christmas tree arranged above each vertical well and inclined well.

[0016] Two, a radial well cluster mining method for deep-sea natural gas hydrates, comprising the following steps: Figure 3 As shown in the figure, the method comprises the following steps: Step one: installation and fixation of multi-well directional suction anchor and subsea drilling machine The multi-well directional suction anchor, the subsea drilling machine and the drill are lowered to the seabed surface by the winch of the medium and small operation ship and are installed and fixed; Step two: drilling and completion of integrated vertical and inclined wells The drilling and completion of integrated vertical and inclined wells are carried out by cooperating with the multi-well directional suction anchor and the drill; Step three: connection of one-machine multi-well mining system The multi-tube Christmas tree and the subsea booster electric submersible pump are arranged on the seabed surface by the winch of the medium and small operation ship, and are connected with the production and transportation pipeline and the vertical and inclined wells to form a one-machine multi-well mining system arrangement; Step four: hydrate "one-machine multi-well" mining process Natural gas mining of hydrate reservoir and underlying gas layer is carried out under the one-machine multi-well mining system; Step five: recovery of drilling and mining equipment The previously arranged multi-tube Christmas tree, subsea booster electric submersible pump, multi-well directional suction anchor, subsea drilling machine and drill are recovered by the medium and small operation ship.

[0017] The step one is specifically: Firstly, the medium and small operation ship lowers the multi-well directional suction anchor to the seabed surface by the winch, positions and levels the multi-well directional suction anchor, penetrates the multi-well directional suction anchor to the designed depth, and completes the installation of the multi-well directional suction anchor; Then, the hollow drill rod, sand prevention and well cementing unit and hollow drill bit of the drill are sequentially stored in the pipe storage rack of the subsea drilling machine, the subsea drilling machine is lowered to the top of the multi-well directional suction anchor by the winch and is fixed with the multi-well directional suction anchor.

[0018] The step two specifically comprises: A) For vertical wells, the subsea drilling machine is placed at the straight conduit nozzle position of the multi-well guide suction anchor. The hollow drill bit of the drill, the pre-filled gravel screen of the sand control and cementing unit, and the self-expanding packer of the drill are sequentially spliced by the drill pipe automatic connection and disconnection technology, and then the drill is driven to drill down along the straight conduit of the multi-well guide suction anchor. The soil in the hollow drill hole of the drill is taken out in real time by the wireline coring. After drilling to the specified depth of the hydrate reservoir or underlying gas layer, the straight well of the integrated drilling and cementing hydrate is completed, and directly used as a production well, or as a pressure relief well, a hot injection well, a monitoring well, a carbon dioxide injection well, a fracturing well, or a shallow gas exploitation well, etc. The drill pipe automatic connection and disconnection technology refers to a special pipe moving machine in the subsea drilling machine to grab and splice the drill pipe in the pipe storage rack, and finally realize automatic drilling.

[0019] B) Then for inclined wells, the multi-well guide suction anchor is fixed with the subsea drilling machine, a new pipe storage rack is replaced to the subsea drilling machine, the subsea drilling machine is adjusted to the inclined conduit nozzle position of the multi-well guide suction anchor, and the inclination angle and the inclined conduit are adjusted to match the multi-well guide suction anchor. After fixing, the drill is driven to drill down along the inclined conduit of the multi-well guide suction anchor. The soil in the hollow drill hole of the drill is taken out in real time by the wireline coring. After drilling to the specified depth of the hydrate reservoir or underlying gas layer, the inclined well of the integrated drilling and cementing hydrate is completed, and used as a production well or a shallow gas exploitation well. C) Repeat step B) until all inclined wells are drilled to form a radial production well cluster.

[0020] The production well refers to a well that can be directly operated, which can include a production well, a hot injection well, a fracturing well, or a monitoring well.

[0021] The step three is specifically: after the completion of the straight well and the inclined well, the multi-pipe Christmas tree is lowered by the winch of the medium and small work ship, and is locked and sealed with the radial production well cluster by the underwater robot, manned deep submersible vehicle or other subsea control equipment. The subsea booster electric submersible pump is lowered to the seabed surface and connected to the multi-pipe Christmas tree by a hydraulic quick connector. The production and transportation pipeline is connected in sequence through the subsea booster electric submersible pump, the multi-pipe Christmas tree, the straight well and the multiple inclined wells. The subsea booster electric submersible pump is used in combination with the multi-pipe Christmas tree, as shown in Figure 4 , to form a one-machine multi-well efficient exploitation layout of a subsea booster electric submersible pump and multiple production wells connected to a hydrate reservoir.

[0022] The straight well at the center axis of the multi-well guide suction anchor can be flexibly laid as a monitoring well, a hot injection well, a pressure relief well, a carbon dioxide injection well, a fracturing well, etc.

[0023] The step four is specifically: continuously pumping out the formation water in the straight well and inclined well of the drilling and cementing integrated hydrate through the seabed pressurized electric submersible pump, or injecting hot fluid, carbon dioxide, fracturing fluid and the like into the well, so that the hydrate reservoir and underlying gas layer are depressurized, thermally stimulated, depressurized-thermal stimulation combined exploitation, carbon dioxide displacement exploitation, and fracturing exploitation, and the property change of the hydrate reservoir in the exploitation process is monitored in real time, the natural gas obtained after the exploitation of the hydrate reservoir and underlying gas layer is connected to the seabed manifold system through the production and transportation pipeline, and is connected to the oil and gas production and transportation ship for centralized gas production.

[0024] The step five is specifically: after the exploitation is completed, the small-step throttling is implemented by using the multi-tube Christmas tree, and the seabed pressurized electric submersible pump is simultaneously controlled to be reduced at a small amplitude according to steps until the flow rate reaches the minimum stable value, and then the seabed pressurized electric submersible pump is turned off; until the bottom hole pressure rises to the reservoir pressure, the seabed pressurized electric submersible pump, the multi-tube Christmas tree and the multi-well guide suction anchor are recovered in sequence by using the medium and small work ship through setting the sequence, and then the drilling and exploitation is carried out at the next location after the recovery is completed.

[0025] The small-step throttling and the reduction at a small amplitude according to steps refer to that the rotating speed of the electric submersible pump is reduced by 2-5% each time, and the water line throttling valve of the multi-tube Christmas tree is opened by 5-10% at the same time, and the operation is maintained for tens of minutes to hours, and then the next step operation is performed after the pressure and flow rate are observed to be stable; and the above steps are repeated according to the stepwise repressuring and well closing.

[0026] The present application realizes the integration of the exploitation system, realizes the integrated operation process of drilling, cementing and well completion, one multi-well guide suction anchor matches multiple (production) wells, and one seabed pressurized electric submersible pump matches multiple (production) wells, so that the operation efficiency of offshore drilling and exploitation is improved, the exploitation range is significantly increased, the drilling and equipment cost of hydrate reservoir exploitation is reduced, and the effect / advantage of economic and efficient exploitation of deep sea natural gas hydrate is realized.

[0027] In the present application, preset leg clamping grooves are arranged on the multi-well guide suction anchor and used for fixing the seabed drilling machine; the seabed drilling machine is controlled by the umbilical cable to complete the drilling and cementing integrated hydrate straight well arrangement, the angle of the seabed drilling machine is adjusted according to the design inclination angle of the multi-well guide suction anchor inclined pipe, and the drilling and cementing integrated hydrate inclined well drilling and arrangement are completed; after the well arrangement is completed, the multi-tube Christmas tree and the seabed pressurized electric submersible pump are lowered to the seabed, connected with the drilling and cementing integrated hydrate straight well and inclined well, and finally connected to the production and transportation pipeline, so that the radial well cluster exploitation system of natural gas hydrate is arranged, the efficient exploitation of the hydrate reservoir in the sea area is realized, and the equipment can be recycled after the exploitation is completed.

[0028] Compared with the prior art, the present application has the following beneficial effects: The present application is suitable for the field of marine gas hydrate exploitation, and the exploitation range is greatly increased and the exploitation efficiency is improved through the hydrate anchor base radial well cluster exploitation system; the operation cost is significantly reduced and the drilling and exploitation efficiency is improved through the integrated drilling process of the subsea drilling machine and the joint use of the subsea booster electric submersible pump and the multi-tube Christmas tree, and the present application is suitable for the drilling and exploitation of deep-sea shallow gas with similar occurrence environment.

[0029] The present application is novel in method and clear in idea, fills the blank of the deep-sea gas hydrate radial well cluster exploitation system and method, provides a new method for the commercial exploitation of deep-sea gas hydrate, and is expected to solve the problems of low gas production efficiency and high production cost in the current gas hydrate exploitation.

[0030] The present application greatly increases the exploitation range and improves the exploitation efficiency through the hydrate anchor base radial well cluster exploitation system, significantly reduces the operation cost and improves the drilling and exploitation efficiency through the integrated drilling of the subsea drilling machine and the one-machine multi-well layout, and provides a new exploitation system and method for the energy safety, high efficiency and long-term of marine gas hydrate, which is a potential path to break through the target of commercial exploitation of gas hydrate. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is the structural schematic diagram of the multi-well guided suction anchor in the present application; wherein, Figure 1 (a) is the top view of the multi-well guided suction anchor; Figure 1 (b) is the radial section view of the multi-well guided suction anchor; Figure 1 (c) is the front view of the multi-well guided suction anchor; Figure 2 is the structural schematic diagram of the drilling and fixing integrated vertical shaft in the present application; wherein, Figure 2 (a) is the front view of the drilling and fixing integrated vertical shaft; Figure 2 (b) is the radial section view of the drilling and fixing integrated vertical shaft; Figure 3 is the subsea drilling machine drilling and fixing integrated completion process in the present application; wherein Figure 3 (a)-(b) of is the installation and fixing process of the multi-well guided suction anchor and the subsea drilling machine; Figure 3 (c)-(d) of is the drilling and completion process of the drilling and fixing integrated hydrate vertical well; Figure 3 (e)-(f) in is the drilling and completion process of the drilling and fixing integrated hydrate inclined well; Figure 3 (g) in is the recovery schematic diagram of the subsea drilling machine after the drilling of the production well is completed; Figure 3 (h) in is the schematic diagram of the radial well cluster exploitation system; Figure 4 is the schematic diagram of the radial well cluster exploitation system in the present application; Figure 5The flow chart of the method for mining deep-sea natural gas hydrate radial well cluster in the application.

[0032] In the figure, the preset leg clamping groove 101, the inclined guide pipe 102, the straight guide pipe 103, the outer wall of the suction anchor 104, the inner wall of the suction anchor 105, the stiffener 106, the hollow drill pipe 201, the self-expanding packer 202, the pre-packed gravel screen pipe 203, the hollow drill bit 204, the inner rod of the self-expanding packer 205, the water-swelling self-expanding rubber rubber cylinder 206, the inner screen pipe 207, the pre-packed gravel layer 208, the outer screen pipe 209, the hydrolyzable plugging plug 210, seawater 301, overburden 302, hydrate reservoir 303, underlying gas layer 304, multi-well guide suction anchor 305, small and medium-sized work ship 306, winch 307, subsea drilling machine 308, storage pipe rack 309, drilling leg 310, drilling and cementing integrated hydrate straight well 311, umbilical cable 312, drilling machine inclination 313, inclined well 314, multi-tube production tree 315, subsea booster electric submersible pump 316, production and transportation pipeline 317. DETAILED DESCRIPTION

[0033] The exemplary embodiments will be described in detail herein with reference to the attached drawings. In the following description, the same numbers are used to indicate the same or similar elements, unless otherwise represented. The embodiments described in the following exemplary embodiments do not represent all the embodiments consistent with the present application. Rather, they are merely examples of systems and methods consistent with some aspects of the present application as detailed in the appended claims. It is understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the application.

[0034] The technical solutions of the application will be described in detail below with reference to the drawings. The features in the following examples and embodiments can be combined with each other without conflict.

[0035] As shown in Figure 3 and Figure 4 , the system includes a multi-well guide suction anchor 305, a drilling and cementing integrated hydrate straight well 311, an inclined well 314, a multi-tube production tree 315, a subsea booster electric submersible pump 316, a subsea drilling machine 308, a small and medium-sized work ship 306 and a production and transportation pipeline 317.

[0036] The small and medium-sized work vessel 306 is sequentially connected and fixed to the subsea drilling rig 308 and the multi-well directional suction anchor 305. The subsea drilling rig 308 is connected to the small and medium-sized work vessel 306 via an umbilical cable 312. The subsea drilling rig 308 is fixed to the top of the multi-well directional suction anchor 305. The multi-well directional suction anchor 305 contains multiple drilling wells, including vertical wells 311 and inclined wells 314. The small and medium-sized work vessel 306 controls the subsea drilling rig 308 to complete the multi-well directional drilling through the umbilical cable 312. The drilling and layout of the vertical well 311 and the inclined well 314 of the integrated drilling and solidification hydrate in the suction anchor 305; the vertical well 311 and the inclined well 314 are connected to the multi-tube production tree 315 arranged on the multi-well guide suction anchor 305 in the multi-well guide suction anchor 305, and the multi-tube production tree 315 is then connected to the subsea booster electric submersible pump 316 and the production and transportation pipeline 317 to form a radial well cluster high-efficiency production and transportation path for natural gas hydrate or shallow gas with one machine and multiple wells.

[0037] From top to bottom, the layers are: seawater 301, overlying layer 302, hydrate reservoir 303, and underlying gas layer 304. A multi-well directional suction anchor 305 is anchored on the overlying layer 302. Vertical well 311 and inclined well 314 extend downward from the overlying layer 302 to the hydrate reservoir 303 and the underlying gas layer 304.

[0038] like Figure 1 As shown, the multi-well guide suction anchor 305 includes a top cover and a cylindrical outer shell. The top cover can be welded to the upper end of the outer shell to form an integral shell structure with an open lower end and a closed upper end. An inclined guide tube 102, a straight guide tube 103, and a stiffening plate 106 are installed inside the shell structure. A straight guide tube 103 is arranged on the central axis of the shell structure. The straight guide tube 103 is vertically arranged along the direction of gravity. The upper ends of both the straight guide tube 103 and the inclined guide tube 102 can be welded to the bottom surface of the top cover. Multiple guide tubes are arranged around the straight guide tube 103. The inclined guide pipes 102 are arranged at an angle, and are evenly spaced circumferentially to form an approximate spiral array. The inclined guide pipes 102 and straight guide pipes 103 are spaced apart and do not contact each other. Each inclined guide pipe 102 is arranged rotationally symmetrically around the straight guide pipe 103, resulting in the inclined guide pipes 102 of each inclined well 314 being radially distributed and extending from top to bottom. The inclined guide pipes 102 are staggered, and their extension directions sequentially point to the adjacent wellhead positions. The straight guide pipe 103 of the straight well 311 is placed at the center of each inclined guide pipe 102. This forms a radial well cluster structure of "central straight well + circumferential inclined wells" within a multi-well directional suction anchor 305.

[0039] Each oblique guide pipe 102 is fixedly connected between the upper side and the outer side of the top cover and the inner wall of the outer peripheral cylinder (i.e. the inner wall of the suction anchor 105) through a vertical stiffener 106 to form a force receiving unit, the stiffener 106 is located on a vertical plane parallel to gravity, and is used to keep the relative position between the oblique guide pipe 102 and the cylinder shell structure of the multi-well guide suction anchor 305 stable. The multi-well guide suction anchor 305 is provided with a preset leg clamping groove 101 on the top cover near the pipe orifice of the oblique guide pipe 102 and the pipe orifice of the straight guide pipe 103, the preset leg clamping groove 101 is specifically designed as an annular groove or a chamfer structure, and is used to be connected with the bottom of the subsea drilling machine 308 to realize the quick connection and separation between the subsea drilling machine 308 and the multi-well guide suction anchor 305.

[0040] In specific implementation, the oblique guide pipes 102 are uniformly arranged at intervals from the central axis of the straight guide pipe 103, each oblique guide pipe 102 is arranged in a direction that is not parallel to the radial direction, the axial direction and the circumferential direction of the outer peripheral cylinder, and the upper end of each oblique guide pipe 102 is directed to the pipe orifice of the adjacent oblique guide pipe 102 at a preset inclination angle, and the straight guide pipe 103 and the oblique guide pipe 102 are used for the drilling guide of the drill-in of the subsea drilling machine 308.

[0041] The outer wall and the inner wall of the outer peripheral cylinder are respectively used as the outer wall 104 of the suction anchor and the inner wall 105 of the suction anchor, the inner wall 105 of the suction anchor is connected with the oblique guide pipe 102 through the stiffener 106, the stiffener 106 is arranged along the direction of the oblique guide pipe 102, and the outer wall 104 of the suction anchor relies on the friction between the outer wall 104 of the suction anchor and the soil to provide the uplift bearing capacity of the suction anchor; the outer wall of the suction anchor also bears the lateral force generated by the pressure difference between the inside and the outside under the action of the negative pressure penetration.

[0042] One side of the stiffener 106 is welded with the surface of the oblique guide pipe 102, and the other side is welded with the bottom surface of the top cover and the inner wall of the outer peripheral cylinder (i.e. the inner wall 105 of the suction anchor); and the stiffener 106 corresponding to each oblique guide pipe 102 is provided with a vertical upward through slot structure at the position interfering with the oblique guide pipe 102 above itself and the stiffener 106 corresponding to the adjacent oblique guide pipe 102 to form a plug-in cooperation, and the adjacent oblique guide pipe 102 and the stiffener 106 corresponding to the adjacent oblique guide pipe 102 are inserted into the slot structure and are welded and fixed with each other.

[0043] The subsea drilling machine 308 has a pipe storage rack 309, and the pipe storage rack 309 is provided with drill-in devices for the oblique guide pipes 102 and the straight guide pipes 103 of the multi-well guide suction anchor 305, and the drill-in devices for the straight well 311 and the inclined well 314 are of the same structure, such as Figure 2As shown, it mainly consists of a hollow drill pipe 201 fixedly connected from top to bottom along the axial direction, several consecutive sand control and cementing units, and a hollow drill bit 204; the hollow drill pipe 201 is connected to the power head and drill pipe unloading and storage system of the subsea drilling rig 308, and the subsea drilling rig 308 drives the drilling tool to drill downwards, and the hollow drill bit 204 breaks through the overlying layer 302 and the hydrate reservoir 303 to drill into the underlying gas layer 304; Hollow drill holes are opened on the central axis of the hollow drill pipe 201, several consecutive sand control and cementing units and the hollow drill bit 204, and the underwater soil of the overlying layer 302, hydrate reservoir 303 and underlying gas layer 304 are transported out through the hollow drill holes.

[0044] like Figure 2 As shown, the sand control and cementing unit is a repeatable modular unit structure, mainly composed of a self-expanding packer 202 and a pre-filled gravel screen 203 connected coaxially from top to bottom along the axial direction. The self-expanding packer 202 is located at the upper end of the pre-filled gravel screen 203. The hollow drill pipe 201 and the sand control and cementing unit, the sand control and cementing unit and the hollow drill bit 204, and the self-expanding packer 202 and the pre-filled gravel screen 203 are all connected coaxially by threads. Hollow drill holes are opened on both the self-expanding packer 202 and the pre-filled gravel screen 203.

[0045] The drilling tools for the vertical well 311 and the inclined well 314 of the integrated drilling and cementing hydrate are all assembled from hollow drill pipe 201, self-expanding packer 202, pre-filled gravel screen 203 and hollow drill bit 204 in a specified sequence. Based on the automatic drill pipe connection and unloading technology of the subsea drilling rig 308, the hollow drill pipe 201, self-expanding packer 202 and pre-filled gravel screen 203 are connected in different sections according to the mining requirements, and drilled along the inclined guide pipe 102 and the straight guide pipe 103 of the multi-well guide suction anchor 305 until drilling to the specified depth of the hydrate reservoir 303 or the underlying gas layer 304, finally forming the integrated drilling and cementing hydrate vertical well 311 and the inclined well 314.

[0046] The self-expanding packer 202 includes a tubular self-expanding packer inner rod 205 and a water-swellable rubber sleeve 206. The self-expanding packer 202 is mainly composed of the self-expanding packer inner rod 205 and the water-swellable rubber sleeve 206 from the inside to the outside. The inner rod 205 has the same inner and outer diameter as the hollow drill rod 201. An annular groove is provided on the outer circumference, and the water-swellable rubber sleeve 206 is installed in the annular groove.

[0047] The self-expanding packer 202 is arranged in the hydrate reservoir 303 or the overlying layer 302, or both the hydrate reservoir 303 and the overlying layer 302 according to requirements; in the self-expanding packer 202, the outer diameter size of the water-swelling self-expanding rubber sleeve 206 under dry conditions is not greater than the outer diameter size of the self-expanding packer inner rod 205; the water-swelling self-expanding rubber sleeve 206 can swell after being soaked in water for a preset time under water, and the outer diameter size of the water-swelling self-expanding rubber sleeve 206 after swelling is not less than 1.1 times of the hollow drill rod 201, so as to realize the fixation of the well pipe and effectively block the natural gas leakage in the mining process.

[0048] The pre-packed gravel screen pipe 203 is mainly composed of the inner screen pipe 207, the pre-packed gravel layer 208, and the outer screen pipe 209 from inside to outside; the inner screen pipe 207 and the outer screen pipe 209 are arranged inside and outside, the inner screen pipe 207 is coaxially sleeved in the outer screen pipe 209, the pre-packed gravel layer 208 is fixedly filled in the annular gap between the inner screen pipe 207 and the outer screen pipe 209, the pre-packed gravel layer 208 is formed into an integral structure with the inner screen pipe 207 and the outer screen pipe 209 by mechanical fixation and cementation curing after the gravel is filled in the inner screen pipe 207 and the outer screen pipe 209, a plurality of radial through perforations are arranged on the circumferential surface of the inner screen pipe 207 and the outer screen pipe 209, the plurality of perforations are arranged in two row directions of the circumferential direction and the axial direction on the circumferential surface, and the same number of perforations are arranged on the circumferential surface of the inner screen pipe 207 and the outer screen pipe 209, and the water-soluble blocking plug 210 is arranged at each perforation.

[0049] The water-soluble blocking plug 210 is filled in the perforation of the inner screen pipe 207 and the outer screen pipe 209 to temporarily block, so as to prevent the soil particles from blocking the pre-packed gravel screen pipe 203 in the drilling process; the water-soluble blocking plug 210 blocks the perforation under dry conditions, and is hydrolyzed after being soaked in water for a preset time under water, so as to realize the unblocking of the perforation in the pre-packed gravel screen pipe 203, and facilitate the smooth mining of the natural gas hydrate or the shallow gas.

[0050] The water-soluble blocking plug 210 can be a water-soluble material such as polyvinyl alcohol PVOH / PVA, sodium polyacrylate, and polyglycolic acid PGA; the material can be dissolved in seawater 301 after a certain time, so as to effectively prevent the soil particles from invading in the drilling process, and ensure the unblocking of the perforation in the pre-packed gravel screen pipe 203.

[0051] The multi-tube wellhead 315 and the subsea booster ESP 316 can be installed separately on the seabed surface between the seawater 301 and the overlying layer 302, or they can be integrated and deployed on the seabed. The multi-tube wellhead 315 adopts a single-tube distributed interconnection or multi-tube integrated wellhead, and is deployed above each vertical well 311 and inclined well 314.

[0052] like Figure 3 The drilling-and-solidification integrated well completion process of this invention is shown below. After the multi-well directional suction anchor 305 is installed, the subsea drilling rig 308 is lowered into the top of the multi-well directional suction anchor 305 and fixed by a small-to-medium-sized work vessel 306 and a winch 307. The small-to-medium-sized work vessel 306 controls the subsea drilling rig 308 to complete the drilling and layout of the drilling-and-solidification integrated hydrate vertical well 311 through the umbilical cable 312. After the drilling and layout of the drilling-and-solidification integrated hydrate vertical well 311 is completed, a new storage rack 309 is replaced on the subsea drilling rig 308, the subsea drilling rig 308 is adjusted to the position of the inclined guide pipe 102, and the extension of the drilling rig outriggers 310 is adjusted so that the subsea drilling rig drills and lays the inclined well 314 at the set inclination angle 313. The above process is repeated until all production wells are drilled, the subsea drilling rig 308 is retrieved, and the multi-tube production tree 315 and the subsea booster electric submersible pump 316 are lowered.

[0053] like Figure 4 As shown, in the radial well cluster production system of the present invention, the vertical well 311 and the inclined well 314 are connected to the gas pipeline 317 via the multi-tube production tree 315 and the subsea booster pump 316, forming a high-efficiency radial well cluster production and transportation path for natural gas hydrates with one machine and multiple wells.

[0054] like Figure 3 and Figure 5 As shown, the embodiments of the present invention and their implementation process include the following steps: Step 1: Installation and securing of multi-well directional suction anchors and subsea drilling rigs The small and medium-sized work vessel 306 lowers the multi-well directional suction anchor 305 to the seabed surface via winch 307. After positioning and leveling the multi-well directional suction anchor 305, it is driven into the multi-well directional suction anchor 305 to the design depth, and the installation of the multi-well directional suction anchor 305 is completed. The hollow drill pipe 201, the self-expanding packer 202, the pre-filled gravel screen pipe 203, and the hollow drill bit 204 are stored in the storage rack 309 of the subsea drilling rig 305 in a specified order. The subsea drilling rig 308 is lowered to the top of the multi-well directional suction anchor 305 via winch 307 and fixed to the multi-well directional suction anchor 305.

[0055] Specifically, the small and medium-sized workboat 306 places the multi-well guide suction anchor 305 to the seabed surface through the winch 307. After positioning and leveling the multi-well guide suction anchor 305, the multi-well guide suction anchor 305 is self-gravitationally penetrated. Finally, the multi-well guide suction anchor 305 is penetrated by pumping water until the designed depth is reached, and the installation of the multi-well guide suction anchor 305 is completed. Before the subsea drilling rig 308 is lowered to the top of the multi-well guide suction anchor 205, the hollow drill pipe 201, the self-expanding packer 202, the pre-packed gravel screen pipe 203, and the hollow drill bit 204 are stored in the pipe rack 309 of the subsea drilling rig 308 in the specified order. Based on the plane coordinates of the orifice calibrated by the GPS-RTK technology, after the small and medium-sized workboat 306 is positioned stably, the subsea drilling rig 308 is slowly lowered by using the winch 307. During the installation process, the underwater robot, manned submersible, or other subsea control equipment is used for real-time monitoring of the underwater attitude. When the subsea drilling rig 308 approaches the multi-well guide suction anchor 305, the underwater robot, manned submersible, or other subsea control equipment starts the side thrust system for fine adjustment and guides the drill rig leg 310 pad to accurately align the preset leg clamping groove 101 at the top of the multi-well guide suction anchor 305. After confirming the position, the opening clasp is released. The clasp is reset and embedded in the preset leg clamping groove 101 under the action of elasticity, realizing the connection between the subsea drilling rig 308 and the multi-well guide suction anchor 305.

[0056] Step two: Drilling and completion of integrated vertical and inclined wells The subsea drilling rig 308 connects, drills, and penetrates the hollow drill bit 204, the pre-packed gravel screen pipe 203, the self-expanding packer 202, and the hollow drill pipe 201 in the specified order by using the automatic pipe connection and disconnection technology. The soil in the borehole is taken out by using the rope coring technology. After drilling to the specified depth of the hydrate reservoir 303 or the underlying gas layer 304, the integrated drilling and cementing hydrate vertical well 311 is laid out, which can be directly used as a production well, a depressurization well, a hot injection well, a monitoring well, a carbon dioxide injection well, or a fracturing well, etc.

[0057] After releasing the fixation between the multi-well guide suction anchor 305 and the subsea drilling rig 308, the new pipe rack 309 is replaced to the subsea drilling rig 308. The subsea drilling rig 308 is adjusted to the position of the inclined guide pipe 102 nozzle, the inclination angle 313 is adjusted, and the fixation with the multi-well guide suction anchor 305 is performed. The integrated inclined well drilling and cementing is performed until the specified depth of the hydrate reservoir 303 or the underlying gas layer 304 is reached, and the inclined well 314 is laid out.

[0058] Specifically, when the subsea drilling rig 308 starts drilling, the hollow drill pipe 201, the self-expanding packer 202, the pre-packed gravel screen pipe 203 and the hollow drill bit 204 are first taken out by the manipulator and moved above the designated drilling hole position. The power head is connected above the drill pipe, and the rotation of the drill pipe is realized by hydraulic drive. Combined with the propulsion system, drilling is completed. When the drill pipe drills to the specified depth, the drilling is completed, the power head is controlled to move upward, and the rope coring method is used. The drilling power head is separated from the drill pipe, and the manipulator places the inner pipe carrying the soil sample on the storage rack to realize soil sampling during drilling. Before the next drilling starts, the manipulator will grab a new drill pipe from the storage pipe rack 309, and the power head will complete the precise tightening connection between the drill pipes. The above process is repeated to realize continuous drilling. When the subsea drilling rig 308 drills, the hollow drill bit 204, the pre-packed gravel screen pipe 203, the self-expanding packer 202 and the hollow drill pipe 201 are connected and drilled in the specified order until the specified depth of the hydrate reservoir 303 or the underlying gas layer 304 is reached. The drilling and cementing integrated hydrate straight well 311 is completed and can be directly used as a production well.

[0059] Specifically, after the drilling and cementing integrated hydrate straight well 311 is drilled, the mechanical locking between the subsea drilling rig 308 and the multi-well guide suction anchor 305 is released, and the two are quickly separated. The new hollow drill pipe 201, the self-expanding packer 202, the pre-packed gravel screen pipe 203 and the hollow drill bit 204 are stored in the storage pipe rack 309 of the subsea drilling rig 308 in the specified order for subsequent inclined well drilling; adjust the subsea drilling rig 308 to the inclined guide pipe 102 pipe opening position, and connect and fix it with the pre-set leg clamping groove 101 around the inclined guide pipe 102 pipe opening; after the connection is completed, the drilling rig legs 310 are driven to extend by the hydraulic motor, and the extension amount difference of the three drilling rig legs 310 is controlled to accurately adjust the drilling rig inclination 312; after the subsea drilling rig 303 is adjusted to the designed inclination angle, drilling and cementing integrated inclined well drilling is performed until the specified depth of the hydrate reservoir 303 or the underlying gas layer 304 is reached. The inclined well 314 is completed, and the above steps are repeated until all the production wells are drilled. The subsea drilling rig 308 is recovered to the small and medium-sized workboat 306.

[0060] Step three: one machine and multiple well production system connection After the completion of the straight well 311 and the inclined well 314, the small and medium-sized work ship 306 lowers the multi-tube Christmas tree 315 through the winch 307, and the multi-tube Christmas tree 315 is locked and sealed with the radial production well cluster through the underwater robot, manned deep submersible or other deep sea control equipment, and then is placed into the seabed booster electric submersible pump 316 to the seabed surface, and is connected and fixed with the multi-tube Christmas tree 315 through the hydraulic quick connector; the production and transportation pipeline 317 is connected with the drill-solid integrated hydrate straight well 311 and the inclined well 314 through the seabed booster electric submersible pump 316 and the multi-tube Christmas tree 315; the seabed booster electric submersible pump 316 and the multi-tube Christmas tree 315 are used in combination to realize the efficient exploitation of one seabed booster electric submersible pump and multiple production wells; wherein, the drill-solid integrated hydrate straight well 311 at the central axis of the multi-well guide suction anchor 305 can be used as a depressurization well, a heat injection well, a monitoring well, a carbon dioxide injection well or a fracturing well according to requirements.

[0061] Specifically, after the completion of the straight well 311 and the inclined well 314, the small and medium-sized work ship 306 slowly lowers the multi-tube Christmas tree 315 by using the winch 307, and the six-degree-of-freedom fine adjustment and guided docking are performed by the underwater robot, manned deep submersible or other deep sea control equipment when approaching the wellhead, so as to ensure that the multi-tube Christmas tree 315 is locked and sealed with the drill-solid integrated hydrate straight well 311 and the inclined well 314. After the multi-tube Christmas tree 315 is in place, the seabed booster electric submersible pump 316 is continuously lowered to the seabed surface, and is rigidly connected with the multi-tube Christmas tree 315 through the hydraulic quick connector, and finally is connected to the production and transportation pipeline 317, so as to realize the efficient exploitation of natural gas hydrate with one machine and multiple wells. Among them, the multi-tube Christmas tree 315 and the seabed booster electric submersible pump 316 can be separately arranged on the seabed surface; or the multi-tube Christmas tree 315 and the seabed booster electric submersible pump 316 can be integrated and arranged on the seabed; the multi-tube Christmas tree 315 can be a single-tube distributed type or a multi-tube integrated type and is arranged above each straight well 311 and inclined well 314; the multi-tube Christmas tree 315 needs to be integrated with a pressure-temperature composite sensor, a safety cut-off valve with intelligent valve position feedback and an electric throttle valve to form a multi-parameter online monitoring node, so as to obtain key production data such as pressure, temperature, valve opening degree and opening and closing state in real time; the drill-solid integrated hydrate straight well 311 at the central axis of the multi-well guide suction anchor 305 can be flexibly arranged as a monitoring well, a heat injection well or a production well according to requirements.

[0062] Step four: hydrate "one machine and multiple wells" exploitation process The formation water in the drill-solid integrated hydrate vertical well 311 and inclined well 314 is continuously pumped out by the subsea booster electric submersible pump 316, or hot fluid is injected into it, to realize the depressurization, thermal stimulation, combined depressurization-thermal stimulation, carbon dioxide displacement mining, and fracturing mining of the hydrate reservoir 303 and underlying gas layer 304. The characteristics of the hydrate reservoir 303 and underlying gas layer 304 are monitored in real time during the mining process. The mined natural gas can be connected to the subsea manifold system through the production and transportation pipeline 317 and concentrated on the oil and gas production and transportation ship for gas production.

[0063] Specifically, the subsea booster electric submersible pump 316 is remotely controlled to implement controllable pumping or injection operations: by discharging the wellbore fluid column in the drill-solid integrated hydrate vertical well 311 and inclined well 314 or injecting hot fluid, carbon dioxide, or fracturing fluid into it, the pressure or temperature of the hydrate reservoir 303 and underlying gas layer 304 is decreased according to the designed gradient to below the hydrate phase equilibrium line, to induce hydrate decomposition and continuous gas production. During the mining process, the characteristics of the hydrate reservoir 303 and underlying gas layer 304 are monitored in real time to realize safe mining. The mined natural gas can be connected to the subsea manifold system through the production and transportation pipeline 317 and concentrated on the oil and gas production and transportation ship for gas production.

[0064] Step five: recovery of drilling and production equipment After the mining is completed, the multi-string Christmas tree 315 is used to implement small-step throttling, and the subsea booster electric submersible pump 316 is simultaneously controlled to decrease the speed in steps until the flow rate reaches the minimum stable value, and then the subsea booster electric submersible pump 316 is turned off. After the bottom hole pressure rises to the reservoir pressure, the subsea booster electric submersible pump 315, the multi-string Christmas tree 316, and the multi-well directional suction anchor 305 are recovered in sequence according to the set order.

[0065] Specifically, after the mining is completed, the multi-string Christmas tree 315 is used to implement small-step throttling on each production line, and the electric submersible pump 316 is simultaneously instructed to decrease the speed in steps. In each step, the parameters are kept stable based on real-time pressure and flow rate feedback before entering the next step. When the total flow rate decreases to the minimum stable value, the subsea booster electric submersible pump 316 is turned off. The bottom hole pressure is increased to the target value according to the pre-set pressure recovery curve to complete the stop depressurization process. The multi-well directional suction anchor 305 is disconnected from the multi-string Christmas tree 315, the hydraulic quick connector of the subsea booster electric submersible pump 316 is disconnected from the multi-string Christmas tree 315, the multi-string Christmas tree 315, the subsea booster electric submersible pump 316, and the multi-well directional suction anchor 308 are recovered, and the winch 307 is used to lift them to the deck of the medium and small work ship 306. After all the equipment is high-pressure washed and functionally rechecked, it is transported to the next well site with the ship to realize the modular circulation of the whole system.

[0066] Thus, it can be seen that the present application innovatively adopts the natural gas hydrate radial well cluster mining system and method, has the advantages of novel method, simple operation, clear process and the like, provides a new method for economic, safe and efficient mining of natural gas hydrate in a sea area, can solve the problems of low gas production rate and high production cost of natural gas hydrate mining, and is expected to promote the commercial mining process of hydrate.

[0067] The above specific embodiments are used to explain and illustrate the present application, rather than limiting the present application, and any modification and change made to the present application within the spirit and protection scope of the claims of the present application shall fall within the protection scope of the present application.

[0068] The above only describes the preferred embodiments of the present application, and any equivalent changes or modifications made to the structure, features and principles described in the scope of the present application shall be included in the scope of the present application.

Claims

1. A radial well cluster exploitation system for deep-sea natural gas hydrates and shallow gas, characterized in that: The system includes a multi-well directional suction anchor (305), a drill-solid integrated hydrate vertical well (311), an inclined well (314), a multi-tube wellhead (315), a subsea booster electric submersible pump (316), a subsea drilling rig (308), a small and medium-sized work vessel (306), and a production and transportation pipeline (317). The subsea drilling rig (308) is connected to the medium and small working vessel (306) via the umbilical cable (312). The subsea drilling rig (308) is fixed on the top of the multi-well directional suction anchor (305). The medium and small working vessel (306) controls the subsea drilling rig (308) via the umbilical cable (312) to complete the drilling and layout of the vertical well (311) and inclined well (314) of the integrated drilling and solidification hydrate in the multi-well directional suction anchor (305). The vertical well (311) and inclined well (314) are connected to the multi-tube production tree (315) arranged on the multi-well directional suction anchor (305). The multi-tube production tree (315) is then connected to the subsea booster electric submersible pump (316) and the production and transportation pipeline (317) to form a radial well cluster efficient production and transportation path for natural gas hydrate or deep-sea shallow gas in a multi-well configuration.

2. The radial well cluster exploitation system for deep-sea natural gas hydrates and shallow gas as described in claim 1, characterized in that: The multi-well guide suction anchor (305) includes a top cover and an outer peripheral cylinder. The top cover is fixed to the upper end of the outer peripheral cylinder to form an integral cylindrical shell structure with an open lower end and a closed upper end. An inclined guide tube (102), a straight guide tube (103) and a stiffening plate (106) are arranged inside the cylindrical shell structure. A straight guide tube (103) is arranged on the central axis of the cylindrical shell structure of the multi-well guide suction anchor (305). The upper ends of the straight guide tube (103) and the inclined guide tube (102) are fixed to the top cover. Multiple inclined guide tubes (102) are arranged around the straight guide tube (103). Each inclined guide tube (102) is arranged at an inclination. Each inclined guide tube (102) is evenly distributed along the circumference. Each inclined guide tube (102) is arranged symmetrically about the straight guide tube (103) as the center, so that each inclined guide tube (102) is radially distributed and extends from top to bottom. Each inclined guide pipe (102) is fixedly connected to the top cover and the inner wall of the outer peripheral cylinder through a vertical stiffening plate (106) to form a force-bearing unit; the multi-well guide suction anchor (305) is provided with a preset support leg slot (101) for connecting and fixing the support legs of the subsea drilling rig (308) on the top cover near the pipe opening of the inclined guide pipe (102) and the pipe opening of the straight guide pipe (103).

3. A radial well cluster exploitation system for deep-sea natural gas hydrates and shallow gas as described in claim 1, characterized in that: The subsea drilling rig (308) is equipped with a drilling tool for drilling. The drilling tool for drilling vertical wells (311) and inclined wells (314) has the same structure. It mainly consists of a hollow drill rod (201) fixedly connected from top to bottom along the axial direction, several consecutive sand control and cementing units (AAA) and a hollow drill bit (204). The subsea drilling rig (308) drives the drilling tool to drill downwards, and the hollow drill bit (204) breaks through the overlying layer (302) and drills into the hydrate reservoir (303) or the underlying gas layer (304), or drills into the shallow gas reservoir during deep-sea shallow gas extraction. Hollow drill holes are opened on the central axis of the hollow drill rod (201), several consecutive sand control and cementing units and hollow drill bit (204), and the underwater soil of the overlying layer (302), hydrate reservoir (303) and underlying gas layer (304) are transported out through the hollow drill holes.

4. A radial well cluster exploitation system for deep-sea natural gas hydrates and shallow gas as described in claim 3, characterized in that: The sand control and cementing unit is mainly composed of a self-expanding packer (202) and a pre-filled gravel screen (203) connected coaxially from top to bottom along the axial direction; The self-expanding packer (202) includes a tubular self-expanding packer inner rod (205) and a water-swellable rubber sleeve (206). It is mainly composed of the self-expanding packer inner rod (205) and the water-swellable rubber sleeve (206) from the inside to the outside. The inner rod (205) of the self-expanding packer has the same inner and outer diameter as the hollow drill rod (201). An annular groove is provided on the outer circumference, and the water-swellable rubber sleeve (206) is installed in the annular groove. The pre-filled gravel screen tube (203) includes an inner screen tube (207), a pre-filled gravel layer (208), an outer screen tube (209), and a water-repellent plug (210). The inner screen tube (207) is coaxially fitted inside the outer screen tube (209). The annular gap between the inner screen tube (207) and the outer screen tube (209) is fixedly filled with the pre-filled gravel layer (208). Multiple radially penetrating perforations are arranged in an array on the circumferential surface of the inner screen tube (207) and the outer screen tube (209). A water-repellent plug (210) is provided at each perforation.

5. A radial well cluster exploitation system for deep-sea natural gas hydrates and shallow gas as described in claim 4, characterized in that: The self-expanding packer (202) is disposed in the hydrate reservoir (303) or the overlying layer (302), or simultaneously disposed in the hydrate reservoir (303) and the overlying layer (302). In the self-expanding packer (202), the outer diameter of the water-swelling rubber sleeve (206) under dry conditions is not greater than the outer diameter of the inner rod (205) of the self-expanding packer; the water-swelling rubber sleeve (206) can self-expand after a preset time when it comes into contact with water underwater, and the outer diameter of the expanded water-swelling rubber sleeve (206) is not less than 1.1 times that of the hollow drill pipe (201), thereby achieving the fixation of the well pipe and effectively blocking the leakage of natural gas or shallow gas during the mining process.

6. A radial well cluster exploitation system for deep-sea natural gas hydrates and shallow gas as described in claim 4, characterized in that: The water-dissolving plug (210) is used to temporarily plug the perforations of the inner screen tube (207) and the outer screen tube (209). The water-dissolving plug (210) blocks the perforations under dry conditions and hydrolyzes after a preset time when it comes into contact with water underwater, thus ensuring the smooth flow of the perforations in the pre-filled gravel screen tube (203).

7. A radial well cluster exploitation system for deep-sea natural gas hydrates and shallow gas as described in claim 4, characterized in that: The multi-tube production tree (315) and the subsea booster electric submersible pump (316) are separately installed on the seabed surface between the seawater (301) and the overlying layer (302), or the multi-tube production tree (315) and the subsea booster electric submersible pump (316) are integrated and deployed on the seabed. The multi-tube wellhead (315) is an integrated multi-tube wellhead, which is installed above each vertical well (311) and inclined well (314).

8. A radial well cluster extraction method for deep-sea natural gas hydrates, applied to any of the radial well cluster extraction systems described in claims 1-7, characterized in that, The method includes the following steps: Step 1: Installation and securing of multi-well directional suction anchors and subsea drilling rigs The multi-well directional suction anchor (305), the subsea drilling rig (308), and the drilling equipment are lowered to the seabed and installed and fixed by a medium-sized working vessel (306); Step Two: Drilling and Completion of Vertical and Inclined Wells (Integrated Drilling and Completion) The drilling and completion of vertical wells (311) and inclined wells (314) using a multi-well directional suction anchor (305) and drilling tools for integrated drilling and cementing; Step 3: Connecting the multi-well mining system The multi-tube wellhead (315) and the subsea booster electric submersible pump (316) are deployed on the seabed by a medium-sized and small-sized work vessel (306), and connected to the production and transportation pipeline (317) and the vertical well (311) and the inclined well (314) to form a multi-well production system. Step 4: Hydrate "One Machine, Multiple Wells" Extraction Process Natural gas extraction from hydrate reservoirs (303) and underlying gas layers (304) or deep-sea shallow gas extraction is carried out under a multi-well extraction system. Step 5: Drilling and Production Equipment Recovery The small and medium-sized work vessel (306) was used to recover the previously deployed multi-tube wellhead (315), subsea booster electric submersible pump (316), multi-well directional suction anchor (305), subsea drilling rig (308) and drilling equipment.

9. A radial well cluster exploitation method for deep-sea natural gas hydrates according to claim 8, characterized in that, Step one specifically involves: First, the small and medium-sized work vessel (306) places the multi-well guide suction anchor (305) on the seabed. After positioning and leveling the multi-well guide suction anchor (305), it is driven into the multi-well guide suction anchor (305) to the design depth, and the installation of the multi-well guide suction anchor (305) is completed. Then, the hollow drill pipe (201), sand control and cementing unit and hollow drill bit (204) of the drilling rig are stored in the storage rack (309) of the subsea drilling rig (305) in sequence, and the subsea drilling rig (308) is lowered to the top of the multi-well guide suction anchor (305) and fixed therewith by the winch (307).

10. A radial well cluster exploitation method for deep-sea natural gas hydrates according to claim 8, characterized in that, Step two specifically includes: A) For a vertical well (311), the subsea drilling rig (308) is placed at the opening of the straight guide pipe (103) of the multi-well guide suction anchor (305). The subsea drilling rig (308) uses the drill pipe automatic connection and unloading technology to connect the hollow drill bit (204), the pre-filled gravel screen pipe (203) of the sand control and cementing unit, the self-expanding packer (202), and the hollow drill pipe (201) in sequence. Then, the drilling rig is driven to drill downward along the straight guide pipe (103) of the multi-well guide suction anchor (305). The soil in the hollow borehole of the drilling rig is extracted in real time by wireline coring until the specified depth of the hydrate reservoir (303) or the underlying gas layer (304) is reached. The vertical well (311) of the integrated drilling and cementing hydrate is then laid out and used directly as a production well, or as a depressurization well, heat injection well, monitoring well, carbon dioxide injection well, fracturing well, or shallow gas production well, etc., as needed. B) For the deviated well (314), release the multi-well guide suction anchor (305) from the subsea drilling rig (308), replace the new storage rack (309) with the subsea drilling rig (308), adjust the subsea drilling rig (308) to the position of the inclined guide pipe (102) of the multi-well guide suction anchor (305), adjust the inclination angle (313) and fix it with the multi-well guide suction anchor (305); drive the drilling rig downward along the inclined guide pipe (102) of the multi-well guide suction anchor (305), and take out the soil in the hollow borehole of the drilling rig in real time through wireline core sampling until the drilling reaches the specified depth of the hydrate reservoir (303) or the underlying gas layer (304), and complete the drilling and solidification integrated hydrate deviated well (314) as a production well or shallow gas extraction well; C) Repeat step B) until all deviated wells (314) are drilled, forming a radial production well cluster.

11. A radial well cluster exploitation method for deep-sea natural gas hydrates according to claim 8, characterized in that, Step three specifically involves: After the vertical well (311) and the inclined well (314) are laid out, the small and medium-sized working vessel (306) lowers the multi-tube production tree (315) through the winch (307) and locks and seals the multi-tube production tree (315) with the radial production well cluster; lowers the subsea booster electric submersible pump (316) to the seabed surface and connects and fixes it with the multi-tube production tree (315); and connects the production pipeline (317) to the vertical well (311) and multiple inclined wells (314) in sequence through the subsea booster electric submersible pump (316), the multi-tube production tree (315), and the multi-well production layout under the connection of one subsea booster electric submersible pump (316) and multiple production wells.

12. A radial well cluster exploitation method for deep-sea natural gas hydrates according to claim 8, characterized in that, Step four specifically involves: By continuously pumping out formation water from the vertical well (311) and inclined well (314) of the integrated drilling and solidification hydrate by submersible pump (316), or by injecting hydrothermal fluid, carbon dioxide, fracturing fluid, etc., the hydrate reservoir (303) and the underlying gas layer (304) can be depressurized, thermally stimulated, depressurized-thermally stimulated combined mining, carbon dioxide replacement mining, and fracturing mining. The characteristics of the hydrate reservoir (303) can be monitored in real time during the mining process. The natural gas obtained after mining the hydrate reservoir (303) and the underlying gas layer (304) can be connected to the subsea manifold system through the production and transportation pipeline (317) and then connected to the oil and gas production and transportation vessel for centralized gas production.

13. A radial well cluster exploitation method for deep-sea natural gas hydrates according to claim 8, characterized in that, Step five specifically involves: After extraction is completed, a multi-tube wellhead (315) is used to implement small-step flow throttling, and the subsea booster electric submersible pump (316) is simultaneously controlled to reduce its speed step by step until the flow rate reaches the minimum stable value. Then the subsea booster electric submersible pump (316) is shut down. Until the bottom hole pressure rises back to the reservoir pressure, a small to medium-sized work vessel (306) is used to retrieve the subsea booster electric submersible pump (316), the multi-tube wellhead (315) and the multi-well directional suction anchor (305) in a set sequence. After retrieval, the vessel is moved to the next location for drilling and extraction.

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