Expanding type packing system for double-gradient well drilling

Abstract

The invention discloses an expanding type packing system for double-gradient well drilling, and relates to the field of submarine natural gas hydrate exploitation. The packing system comprises a pressure extraction device, a pressure conversion device, a hydraulic device and a packing device. The pressure leading-out device is connected with an outlet pipeline of the lifting pump through a quick-change connector and communicated with a first empty 1 of the pressure conversion device. The pressure conversion device and the hydraulic device are respectively provided with an inner central pipe and an outer central pipe, and are connected with the first drill rod short section and the second drill rod short section through flanges. The packing device is arranged in a groove in the inner circumferential wall of the second inner center pipe of the hydraulic device. The packing system is ingenious in structure, the packing device is started through the pressure conversion device and the hydraulic device to form an upper fluid area and a lower fluid area which are independent in the annulus, and therefore pressure difference is formed in a closed pipeline through the lifting pump, and double-gradient well drilling is achieved. Compared with a conventional double-gradient drilling riser gas lift system, the number of layers of the casing pipes can be effectively reduced, and graded boosting can be achieved.

Description

Technical Field

[0001] The invention patent discloses an expansion isolation system for dual-gradient drilling, which relates to the field of seabed natural gas hydrate exploitation. Background Art

[0002] In the prior art, the exploitation of seabed natural gas hydrates is usually carried out by solid fluidization: on the seabed, mining equipment is used to develop the hydrate ore body in solid form, and the sediment containing hydrates is crushed into fine particles, mixed with seawater, and transported to the offshore platform by closed pipelines. The closed transport pipeline is usually composed of oil pipes and casings to form concentric cylinders. This drilling method is called dual gradient drilling. The exploited fluidized natural gas hydrate is transported to the offshore platform from the internal annulus between the oil pipe and the casing.

[0003] Due to the influence of seawater static pressure, an external lift pump is required to pump the fluidized hydrate at the closed pipeline. However, since the pipeline is connected internally, a pressure difference cannot be formed, and the high-pressure hydrate at the lift pump outlet will flow back to the low-pressure area at the lower lift pump inlet, making it impossible to achieve the pumping purpose. Summary of the invention

[0004] The purpose of the present invention is to overcome the problem that the pressure cannot be increased by the lift pump due to the internal connection of the pipeline and the pressure difference cannot be formed, and to provide an expansion isolation system for dual gradient drilling. By isolating the internal annulus between the low-pressure inlet at the lower end and the high-pressure outlet at the upper end of the lift pump of the isolation system, an upper and lower independent annulus is formed between the oil pipe and the casing. The pumping purpose is achieved, and at the same time, the isolation system can use the pump outlet liquid pressure to achieve sealing, and the sealing effect is good.

[0005] In order to achieve the above purpose, the technical solution adopted by the present invention is:

[0006] An expansion isolation system for dual gradient drilling, comprising a pumping device, a pressure conversion device, a hydraulic device and an isolation device;

[0007] The pumping device is connected to the first drill pipe short section and the second drill pipe short section through a pump outlet pipe and a pump inlet pipe respectively; the pressure conversion device is arranged at the lower end of the first drill pipe short section and is connected to the first drill pipe short section through an outer center pipe. The pressure conversion device is divided into two layers, inner and outer, and forms an annular cavity between the first inner center pipe and the outer center pipe. A rubber capsule is arranged in the annular cavity, wherein the first cavity above the rubber capsule is communicated with the pressure lead-out device through a lead-out pipe, the upper end of the first inner center pipe is connected to the first drill pipe short section, and the lower end of the first inner center pipe is connected to the upper end of the second inner center pipe through a threaded connection; the hydraulic device is arranged between the pressure conversion device and the second drill pipe short section, the hydraulic device is divided into two layers, inner and outer, and forms an annular cavity between the inner and outer layers. A piston is arranged in the annular cavity, the third cavity is above the piston, the third cavity is communicated with the second cavity of the pressure conversion device, and the fourth cavity below the piston is communicated with the fifth cavity of the isolation device through a small hole opened in the second inner center pipe; the isolation device adopts an expansion packer with a rubber skin structure, and is arranged on the inner peripheral wall of the second inner center pipe by welding.

[0008] Furthermore, the pumping device is arranged outside the casing, and is connected to the internal annulus with the second drill pipe short section through the pump inlet pipe, and is connected to the internal annulus with the first drill pipe short section through the pump outlet pipe.

[0009] Furthermore, the pressure lead-out device is provided with a two-way pressure difference control valve, which is composed of two one-way valves in opposite directions. The one-way valves are provided with an opening pressure to prevent the sealing performance of the isolation device from being affected when the outlet pressure of the lifting pump fluctuates.

[0010] This scheme limits the pressure lead-out device. When the pump outlet pressure is greater than the sum of the first cavity pressure and the forward valve opening pressure, the forward valve opens and the reverse valve closes, and the high-pressure liquid enters the pressure conversion device to drive the packer to expand and seal. Conversely, when the first cavity pressure is greater than the sum of the pump outlet pressure and the reverse valve pressure, the reverse valve opens and the forward valve closes, the high-pressure fluid of the pressure conversion device is released, and the packer is reset.

[0011] In addition, this solution sets a certain opening pressure for the one-way valve to block the transmission of pressure fluctuations, avoid sealing failure caused by pressure fluctuations, and reduce fatigue loss of the packer.

[0012] Furthermore, the lead-out pipe is connected to the first drill pipe pup joint and is connected to the first cavity of the pressure conversion device; a flat flange is welded on the end face of the first drill pipe pup joint to form a flange seal with the first drill pipe pup joint, the first drill pipe pup joint is connected to the first inner center pipe through threads and is sealed with a sealing ring, and a flat flange is welded on the end face of the outer center pipe to form a flange seal with the first drill pipe pup joint.

[0013] This solution achieves safe transmission and sealing of high-pressure fluid through flat flanges, threaded connections, sealing ring multi-stage sealing design and high-light mechanical connections, ensuring that the high-pressure fluid at the outlet of the lift pump stably enters the first cavity of the pressure conversion device through the lead-out pipe to prevent leakage and pressure loss.

[0014] Furthermore, a rubber capsule is provided in the annular cavity, and the rubber capsule and a matching locking block are arranged between the first inner center tube and the outer center tube, and the lead-out pipe is connected to the first cavity; hydraulic oil is under the capsule, which is used to convert the pressure of the natural gas hydrate fluid into the pressure of the hydraulic oil; the end of the rubber capsule is in a "T" shape, and its end is connected to the grooves of the first inner center tube and the outer center tube by an adhesive; the locking block is provided with threads in the circumferential direction, and is connected to the first inner center tube and the outer center tube by threads, and the upper end of the locking block is a "7-shaped" structure, and the upper end has 4 petals.

[0015] In this solution, the locking block cooperates with the end face of the rubber capsule, and the first inner center tube outer peripheral wall groove and the outer center tube inner peripheral wall groove cooperate with the end face of the rubber capsule and are connected with an adhesive to lock the end face of the rubber capsule.

[0016] In addition, the rubber capsule of the present scheme serves as a pressure transmission medium, converting the natural gas hydrate fluid pressure at the outlet of the lift pump into hydraulic oil pressure, driving the packer to expand, and isolating the two media at the same time, effectively preventing sand and gravel from entering the hydraulic system.

[0017] Furthermore, the upper end of the second inner center tube of the hydraulic device and the lower end of the first inner center tube, the lower end of the second inner center tube and the upper end of the second drill rod short section, and the lower end of the outer center tube and the upper end of the second drill rod short section are all connected by threads and sealed with a sealing ring; a guide device is provided between the second inner center tube and the outer center tube, and 8 stepped holes are distributed in an annular manner inside the guide device to prevent the piston from deviating during movement.

[0018] Furthermore, the lower end of the piston rod connected to the piston is connected to the return spring, and the return spring is pre-tightened by the gravity of the guide device to ensure that the rubber skin is deformed and restored when the pressure is released.

[0019] This solution imposes certain restrictions on the structure of the hydraulic system. The hydraulic device is arranged between the second inner center tube and the outer center tube. A sealing surface is formed between the piston, the second inner center tube and the outer center tube. The piston rod is connected to the piston through a thread. The piston rod is limited by the guide device and connected to the return spring in the guide device. The return spring is pre-tightened by the gravity of the guide device.

[0020] Furthermore, the isolation device is connected to the hydraulic system through a small hole opened in the second inner center tube, and the isolation device adopts the structure of the existing expansion type isolation device; under the action of the hydraulic oil, the rubber skin of the isolation device expands radially and contacts the oil pipe, thereby forming two independent fluid areas in the annulus; the fluidized natural gas hydrate mined below can be pumped to the upper drilling platform through the lifting pump.

[0021] This solution imposes certain restrictions on the isolation system. The isolation device: drives the rubber skin to expand radially through hydraulic pressure, forming a sealing section with upper and lower isolation in the wellbore annulus, realizing dual gradient pressure control. At the same time, the pressure of the hydraulic system is transmitted to the isolation device through the small hole of the second inner center pipe, triggering the expansion action.

[0022] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0023] 1. An expandable isolation system for dual-gradient drilling of the present invention forms two independent fluid areas in the annulus by starting the isolation device through a pressure conversion device and a hydraulic device, thereby forming a pressure difference in a closed pipeline through a lift pump, thereby realizing dual-gradient drilling.

[0024] 2. The expandable isolation system for dual-gradient drilling of the present invention realizes dynamic pressure regulation through a two-way pressure differential control valve, enables automatic switching of injection and release of high-pressure fluid, and ensures accurate triggering of expansion and contraction of the isolation device. Compared with the conventional dual-gradient drilling riser gas lift system, the pressure trigger threshold can be customized to adapt to the operational requirements of different well depths and formation pressure gradients, and can effectively reduce the number of casing layers and can increase pressure in stages.

[0025] 3. The expandable isolation system for dual-gradient drilling of the present invention can avoid sealing failure caused by pressure fluctuations. Compared with the traditional method, the opening pressure is set by a one-way valve to block the transmission of pressure fluctuations, reduce fatigue loss of the isolation device, and increase the service life of the isolation device.

[0026] 4. The expandable isolation system for dual-gradient drilling of the present invention can effectively prevent pressure leakage and damage, and realizes safe transmission and sealing of high-pressure fluid through multi-stage sealing design of flat flange, threaded connection and sealing ring, ensuring that the high-pressure fluid at the outlet of the lift pump stably enters the first cavity of the pressure conversion device. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention.

[0028] Figure 1It is a schematic diagram of the structure of an expandable isolation system for dual-gradient drilling in an embodiment of the present invention;

[0029] Figure 2 This is a schematic diagram of the structure of the pressure lead-out device of the embodiment of the present invention;

[0030] Figure 3 This is a schematic diagram of the structure of the pressure conversion device of the embodiment of the present invention;

[0031] Figure 4 This is a partial enlarged view of the locking block and rubber capsule of the pressure conversion device of the embodiment of the patent of the present invention;

[0032] Figure 5 This is a schematic diagram of the structure of the hydraulic device of the embodiment of the present invention;

[0033] Figure 6 This is a schematic diagram of the structure of the sealing device of the embodiment of the present invention;

[0034] Figure 7 This is a partial enlarged view of the expandable packer structure of the embodiment of the present invention.

[0035] Marks and corresponding parts names in the attached drawings:

[0036] 1- pumping device; 2- pressure lead-out device; 3- pressure conversion device; 4- hydraulic device; 5- isolation device; 11- pump outlet pipeline; 12- lifting pump; 13- pump inlet pipeline; 21- quick-change joint; 22- two-way pressure difference control valve; 23- lead-out pipeline; 31- first drill pipe short section; 32- first inner center pipe; 33- outer center pipe; 34- locking block; 35- rubber capsule; 36- first cavity; 37- second cavity; 41- second inner center pipe; 42- piston; 43- piston rod; 44- guide device; 45- reset spring; 46- second drill pipe short section; 47- third cavity; 48- fourth cavity; 51- expansion type packer structure; 52- oil pipe; 53- fifth cavity; 54- internal annulus. DETAILED DESCRIPTION

[0037] Example 1

[0038] This embodiment provides an expandable isolation system for dual-gradient drilling. The isolation system can achieve sealing by utilizing the liquid pressure at the pump outlet, and has a good sealing effect.

[0039] Please refer to Figure 1 , Figure 1 The specific structure of the expandable isolation system for dual-gradient drilling in this embodiment is shown.

[0040] An expansion isolation system for dual-gradient drilling comprises a pumping device 1, a pressure lead-out device 2, a pressure conversion device 3, a hydraulic device 4 and an isolation device 5.

[0041] The inlet of the pumping device 1 is connected to the inner annulus 54 of the second drill pipe short section 46 through the pump inlet pipe 13 , and the outlet of the lift pump 12 is connected to the inner annulus 54 of the first drill pipe short section 31 through the pump outlet pipe 11 .

[0042] Please refer to Figure 2 , please refer to Figure 2 , Figure 2 A schematic structural diagram of the pressure lead-out device 2 is given.

[0043] The pressure lead-out device 2 connects the pump outlet pipe 13 and the first cavity 36 of the pressure conversion device 3 via the lead-out pipe 23 .

[0044] The outlet pipe 23 is provided with a bidirectional pressure difference control valve 22 and is connected to the pump outlet pipe 13 via a quick-change joint 21 .

[0045] Part of the pipeline of the outlet pipe 23 is constructed inside the first drill pipe sub 31 .

[0046] For further information, please refer to Figure 3 and Figure 4 , Figure 3 The specific structure of the pressure conversion device 3 in this embodiment is shown. Figure 4 The enlarged structure of the locking block 34 and the rubber capsule 35 of the pressure conversion device 3 in this embodiment is shown.

[0047] The pressure conversion device is composed of a first inner center tube 32 and an outer center tube 33 to form an annular cavity, wherein the first inner center tube 32 is connected to the first drill pipe short section 31 through threads and O-rings, and a flat flange is welded between the upper end surface of the outer center tube 33 and the relative position of the first drill pipe short section 31, and the outer center tube 33 and the first drill pipe short section 31 are connected through a flat flange.

[0048] A rubber capsule 35 is provided in the annular cavity constructed by the first inner center tube 32 and the outer center tube 33, dividing the annular cavity into a first cavity 36 and a second cavity 37. The first cavity 36 is connected to the outlet of the lead-out pipe 23, and the second cavity 37 is connected to the third cavity 47 of the hydraulic device 4. The second cavity 37 and the third cavity 47 are filled with hydraulic oil.

[0049] An annular groove is provided at the same axial position on the outer peripheral wall of the first inner central tube 32 and the inner peripheral wall of the outer central tube 33, and a thread is also provided on the upper part of the groove.

[0050] The rubber capsule 35 is annular, and its end is set to a "T"-shaped structure. One end of the "T"-shaped structure is connected to the grooves of the outer peripheral wall of the first inner central tube 32 and the inner peripheral wall of the outer central tube 33 by adhesive.

[0051] The locking block 34 is a "7"-shaped structure, and is set to a four-petal structure at one end close to the rubber capsule 35, and is connected to the outer peripheral wall of the first inner center tube 32 and the inner peripheral wall of the outer center tube 33 by threads; the "7"-shaped structure of the locking block 37 cooperates with the "T"-shaped structure of the rubber capsule 35 to limit the movement of the end of the rubber capsule 35.

[0052] For further information, please refer to Figure 5 , Figure 5 The specific structure of the hydraulic device 4 in this embodiment is given.

[0053] The hydraulic device is composed of a second inner center tube 41 and an outer center tube 33 to form an annular cavity. The upper end surface of the second inner center tube 41 is connected to the lower end surface of the first inner center tube 32 by means of a thread + an O-ring. Similarly, the lower end surface of the second inner center tube 41 and the lower end surface of the outer center tube 33 are connected to the second drill pipe short section 46 by means of a thread + an O-ring.

[0054] An annular piston 42 is provided in the annular cavity constructed by the second inner center tube 41 and the outer center tube 33, and is sealed with the outer peripheral wall of the second inner center tube 41 and the inner peripheral wall of the outer center tube 33 by O-rings, thereby dividing the annular cavity into a third cavity 47 and a fourth cavity 48. Similarly, the fourth cavity 48 is filled with hydraulic oil.

[0055] Eight piston rods 43 are evenly arranged at circumferential positions of the piston. The piston rod 43 is in the form of a stepped shaft, and the piston 42 is connected to the upper end of the piston rod 43 by threads.

[0056] The guide device 44 is an annular structure and has 8 stepped holes evenly arranged along the circumferential direction. The piston rod 43 is connected with the small hole of the guide device 44. The reset spring 45 is arranged in the large hole of the guide device 44 and is matched with the large end of the piston rod 44. The reset spring 45 is preloaded by the gravity of the guide device 44.

[0057] For further information, please refer to Figure 6 , Figure 6 The specific structure of the sealing device 5 in this embodiment is given.

[0058] A groove is provided on the inner peripheral wall of the second inner center tube 41, and four through holes are provided in the circumferential direction. An expandable packer structure 51 is provided at the groove. The expandable packer structure 51 is connected to the second inner center tube 41 by welding. A fifth cavity 52 is formed inside the groove of the second inner center tube 41. The fifth cavity 53 is connected to the fourth cavity 48 through a small hole. Similarly, the fifth cavity 53 is filled with hydraulic oil.

[0059] For further information, please refer to Figure 7 , Figure 7 A partial enlarged view of the expandable packer structure 51 in this embodiment is given.

[0060] When working, the expandable packer structure 51 is a rubber skin structure, which expands under pressure and cooperates with the oil pipe 52 to form a sealing surface, thereby separating the internal annulus 54.

[0061] The sealing system has an ingenious structure and can utilize the liquid pressure at the pump outlet to achieve sealing, with good sealing effect.

[0062] The specific implementation steps of this embodiment are as follows:

[0063] Step 1: When transporting natural gas hydrate, the lift pump 12 pumps the fluid in the annulus of the second drill pipe short section 46 to the annulus of the first drill pipe short section 31, and the fluid pressure increases.

[0064] Step 2: Use the pressure outlet device 2 to lead out the high-pressure fluid in the pump outlet pipe 11. When the fluid pressure P 11 The pressure P of the first cavity 36 36 When the difference is greater than the opening pressure of the bidirectional pressure differential control valve 22 , the bidirectional pressure differential control valve 22 opens in the forward direction, and the fluid flows from the pump outlet pipe 11 through the lead-out pipe 23 to the first cavity 36 .

[0065] Step 3: At this time, the fluid pressure P in the first cavity 36 36 Greater than the hydraulic oil pressure P of the second cavity 37 37 The rubber capsule 35 is elastically deformed toward the second cavity 37 under pressure, and the hydraulic oil pressure P in the second cavity 37 is 37 Increase, at this time, the locking block and the grooves of the first inner center tube 32 and the outer center tube 33 jointly limit the axial displacement of the end of the rubber capsule 35.

[0066] Step 4: Since the second cavity 37 is connected to the third cavity 47, the hydraulic oil pressure P 47 =P 37 , but is greater than the hydraulic oil pressure P of the fourth cavity 48 48 , which pushes the piston 42 downward, and the return spring 45 is compressed and elastically deformed, and the hydraulic oil pressure P of the fourth cavity 48 is 48 Increase.

[0067] Step 5: Since the fourth cavity 48 is connected to the fifth cavity 53, the hydraulic oil pressure P 48 =P 53 , the hydraulic oil pressure P of the fifth cavity 53 53 Increase, so that the rubber skin of the expandable packer structure 51 undergoes radial elastic deformation until it contacts the oil pipe 52 to form an effective sealing surface, thereby forming two independent upper and lower areas in the internal annulus 54, and the high-pressure fluid from the pump outlet pipe 11 cannot flow back to the low-pressure area of ​​the pump inlet pipe 13, thereby transporting the fluidized natural gas hydrate to the offshore platform.

[0068] Step 6: When the delivery stops, the lift pump 12 stops working, and the pressure of the fluid 11 in the pump outlet pipeline is P 11 When the pressure P of the first cavity 36 decreases, 36 The fluid pressure P of the pump outlet pipe 11 11 When the difference is greater than the opening pressure of the bidirectional pressure differential control valve 22 , the bidirectional pressure differential control valve 22 opens in the reverse direction, and the fluid flows back from the first cavity 36 to the pump outlet pipe 11 through the outlet pipe 23 .

[0069] Step 7: At this time, the fluid pressure P in the first cavity 36 36 Less than the hydraulic oil pressure P of the second cavity 37 37 , the rubber capsule 35 rebounds, and the hydraulic oil pressure P of the second cavity 37 37 Reduce, at the same time, the third cavity 47 hydraulic oil pressure P 47 Decreases, less than the hydraulic oil pressure P of the fourth cavity 48 48 The piston 42 moves upward under the pressure difference and the push of the return spring 45, and the hydraulic oil pressure P of the fourth cavity 48 48 Reduce.

[0070] Step 8: At this time, the hydraulic oil pressure P of the fifth cavity 53 53 The radial elastic deformation of the rubber skin of the expandable packer structure 51 is restored, the internal annulus 54 is connected, and the pumping stops.

[0071] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An expandable isolation system for dual gradient drilling, characterized in that: include: A pumping device (1), a pressure outlet device (2), a pressure conversion device (3), a hydraulic device (4) and a sealing device (5); The pumping device (1) is connected to the first drill pipe short section and the second drill pipe short section respectively through a pump outlet pipe and a pump inlet pipe; The pressure outlet device (2) is connected to the pump outlet pipe via a quick-change joint; The pressure conversion device (3) is arranged at the lower end of the first drill pipe short section and is connected to the first drill pipe short section through an outer center tube. The pressure conversion device (3) is divided into two layers, an inner layer and an outer layer. An annular cavity is formed between the first inner center tube and the outer center tube. A rubber capsule is arranged in the annular cavity. The first cavity located above the rubber capsule is connected to the pressure lead-out device (2) through a lead-out pipe. The hydraulic device (4) is arranged between the pressure conversion device (3) and the second drill pipe short section. The hydraulic device (4) is divided into two layers, an inner layer and an outer layer. An annular cavity is formed between the inner layer and the outer layer. A piston is arranged in the annular cavity. The third cavity is above the piston and the fourth cavity is below the piston. The third cavity is connected to the second cavity of the pressure conversion device (2). The fourth cavity below the piston is connected to the fifth cavity of the isolation device (5) through a small hole provided in the second inner center tube. The sealing device (5) adopts an expansion type sealing device with a rubber skin structure, and is arranged on the inner peripheral wall of the second inner central tube by welding.

2. An expandable isolation system for dual gradient drilling according to claim 1, characterized in that: The pumping device (1) is arranged outside the casing, connected to the internal annulus of the second drill pipe short section through a pump inlet pipe, and connected to the internal annulus of the first drill pipe short section through a pump outlet pipe.

3. The expandable isolation system for dual gradient drilling according to claim 1, characterized in that: The pressure lead-out device (2) is provided with a bidirectional pressure differential control valve, which is composed of two one-way valves in opposite directions. The one-way valves are provided with an opening pressure to prevent the lifting pump outlet pressure from fluctuating and affecting the sealing performance of the isolation device (5).

4. The expandable isolation system for dual gradient drilling according to claim 3, characterized in that: The lead-out pipe in the pressure lead-out device (2) is connected to the first drill pipe short section and communicates with the first cavity of the pressure conversion device (3); the first drill pipe short section is connected to the first inner center pipe through threads and sealed with a sealing ring; a flat flange is welded at the upper end surface of the outer center pipe and the relative position of the first drill pipe short section; the outer center pipe and the first drill pipe short section are connected through the flat flange.

5. The expandable isolation system for dual gradient drilling according to claim 4, characterized in that: The rubber capsule and the matching locking block are arranged between the first inner center tube and the outer center tube, and the lead-out pipe is connected to the first cavity; hydraulic oil is under the capsule, which is used to convert the pressure of the natural gas hydrate fluid into the pressure of the hydraulic oil; the end of the rubber capsule is "T" shaped, and its end is connected to the grooves of the first inner center tube and the outer center tube by an adhesive.

6. An expandable isolation system for dual gradient drilling according to claim 5, characterized in that: The locking block is provided with threads in the circumferential direction and is connected to the first inner center tube and the outer center tube through threads. The upper end of the locking block is a "7-shaped" structure and has 4 petals.

7. The expandable isolation system for dual gradient drilling according to claim 1, characterized in that: The upper end of the second inner center tube of the hydraulic device (4) and the lower end of the first inner center tube, the lower end of the second inner center tube and the upper end of the second drill rod short section, and the lower end of the outer center tube and the upper end of the second drill rod short section are all connected by threads and sealed with a sealing ring.

8. An expandable isolation system for dual gradient drilling according to claim 7, characterized in that: A guide device is provided between the second inner center tube and the outer center tube, and 8 stepped holes are distributed in an annular manner in the guide device to prevent the piston from deviating during movement.

9. An expandable isolation system for dual gradient drilling according to claim 8, characterized in that: It also includes a piston rod connected to the piston, the lower end of the piston rod is connected to a return spring, and the return spring is pre-tightened by the gravity of the guide device to ensure that the rubber skin is deformed and restored when the pressure is released.

10. The expandable isolation system for dual gradient drilling according to claim 1, characterized in that: The isolation device (5) is connected to the hydraulic device (4) through a small hole opened in the second inner center pipe, and the isolation device (5) adopts an expansion type isolation device structure; under the action of the hydraulic oil, the rubber skin of the isolation device (5) expands radially and contacts the oil pipe, thereby forming two independent fluid areas in the annulus; the fluidized natural gas hydrate mined below is pumped to the upper drilling platform through a lifting pump.

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