An expandable isolation system for dual-gradient drilling

Abstract

The patent of the present invention discloses an expandable isolation system for dual-gradient drilling, which relates to the field of seabed natural gas hydrate exploitation. The isolation system includes a pressure lead-out device, a pressure conversion device, a hydraulic device and an isolation device. The pressure lead-out device is connected to the outlet pipe of the lifting pump through a quick-change joint and is connected to the first air 1 of the pressure conversion device. Both the pressure conversion device and the hydraulic device have an inner center pipe and an outer center pipe structure, and are connected to the first and second drill pipe short sections through flanges. The isolation device is arranged in a groove on the inner peripheral wall of the second inner center pipe of the hydraulic device. The isolation system has an ingenious structure. The isolation device is activated by the pressure conversion device and the hydraulic device to form two independent fluid areas in the annulus, thereby forming a pressure difference in the closed pipeline through the lifting pump, thereby realizing dual-gradient drilling. Compared with the conventional dual-gradient drilling watertight pipe gas lift system, it can effectively reduce the number of casing layers and can increase the pressure in stages.

Description

Technical Field

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

[0002] In existing technology, subsea gas hydrate extraction typically uses solid-state fluidization. Mining equipment is used to develop the hydrate ore body in solid form on the seabed. The hydrate-containing sediment is crushed into fine particles, mixed with seawater, and transported to an offshore platform via a closed pipeline. This closed transport pipeline typically consists of concentric cylinders formed by tubing and casing. This drilling method is known as dual-gradient drilling. The extracted fluidized gas hydrate is transported to the offshore platform through the internal annulus between the tubing and casing.

[0003] Due to the influence of seawater static pressure, an external lift pump is required to pump the fluidized hydrate through the closed pipeline. However, since the pipeline is internally connected, no pressure differential can be generated. The high-pressure hydrate at the lift pump outlet will flow back into the low-pressure area at the lift pump inlet at the lower end, making pumping impossible. Summary of the Invention

[0004] The present invention aims to overcome the problem of being unable to increase pressure through a lift pump due to the lack of pressure differential created by internally connected pipelines. This invention provides an expandable 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, the isolation system creates upper and lower independent annuli between the tubing and casing, achieving pumping. Furthermore, the isolation system utilizes the liquid pressure at the pump outlet to achieve a strong seal.

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

[0006] An expandable 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 sub and the second drill pipe sub via a pump outlet pipe and a pump inlet pipe, respectively. The pressure conversion device is disposed at the lower end of the first drill pipe sub and connected to the first drill pipe sub via an outer central pipe. The pressure conversion device is divided into an inner and outer layer, forming an annular cavity between the first inner central pipe and the outer central pipe. A rubber capsule is disposed within the annular cavity, wherein a first cavity above the rubber capsule is connected to the pressure outlet device via an outlet pipe. The upper end of the first inner central pipe is connected to the first drill pipe sub, and the lower end of the first inner central pipe is threadedly connected to the upper end of the second inner central pipe. The hydraulic device is disposed between the pressure conversion device and the second drill pipe sub. The hydraulic device is divided into an inner and outer layer, forming an annular cavity between the inner and outer layers. A piston is disposed within the annular cavity. A third cavity is disposed above the piston, which is connected to the second cavity of the pressure conversion device. A fourth cavity below the piston is connected to the fifth cavity of the isolation device via a small hole in the second inner central pipe. The isolation device utilizes an expandable packer with a rubber skin structure and is welded to the inner circumferential wall of the second inner central pipe.

[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 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 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, driving 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 short section 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 short section to form a flange seal with the first drill pipe short section, the first drill pipe short section 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 short section.

[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 lifting pump stably enters the first cavity of the pressure conversion device through the lead-out pipe, preventing leakage and pressure loss.

[0014] Furthermore, a rubber capsule is provided in the annular cavity, and 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 groove of the first inner center tube and the outer center tube by an adhesive; the locking block is provided with a thread in the circumferential direction, and is connected to the first inner center tube and the outer center tube by a thread, 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 this 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 at the same time isolating the two media to effectively prevent 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 in the guide device to prevent the piston from offsetting 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 threaded connection. 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 above and below 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 hydraulically drives the radial expansion of a rubber skin, creating upper and lower isolated sealing sections within the wellbore annulus, achieving dual-gradient pressure control. Simultaneously, pressure from the hydraulic system is transmitted to the packer through a small hole in the second inner central tube, triggering expansion.

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

[0023] 1. The expandable isolation system for dual-gradient drilling of the present invention activates the isolation device through a pressure conversion device and a hydraulic device to form two independent fluid zones in the annulus, thereby forming a pressure difference in the closed pipeline through a lift pump, thereby achieving dual-gradient drilling.

[0024] 2. This expandable isolation system for dual-gradient drilling utilizes a bidirectional differential pressure control valve to achieve dynamic pressure regulation, enabling automatic switching between injection and release of high-pressure fluids and ensuring precise triggering of packer expansion and contraction. Compared to conventional dual-gradient drilling riser gas lift systems, this system offers customizable pressure trigger thresholds to accommodate operations at varying well depths and formation pressure gradients, effectively reducing the number of casing layers and enabling staged pressure increases.

[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 one, 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 the safe transmission and sealing of high-pressure fluid through the multi-stage sealing design of flat flange, threaded connection and sealing ring, ensuring that the high-pressure fluid at the outlet of the lifting 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 1This is a schematic structural diagram of an expandable isolation system for dual-gradient drilling in an embodiment of the present invention;

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

[0030] Figure 3 This is a schematic structural diagram of a pressure conversion device according to an 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 in the embodiment 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 structural diagram of a sealing device according to an embodiment of the present invention;

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

[0035] Markings and corresponding parts names in the accompanying drawings:

[0036] 1- Pumping device; 2- Pressure lead-out device; 3- Pressure conversion device; 4- Hydraulic device; 5- Packing device; 11- Pump outlet pipe; 12- Lifting pump; 13- Pump inlet pipe; 21- Quick-change connector; 22- Two-way pressure differential control valve; 23- Lead-out pipe; 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- Return spring; 46- Second drill pipe short section; 47- Third cavity; 48- Fourth cavity; 51- Expandable 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 expandable isolation system for dual-gradient drilling comprises a pumping device 1, a pressure outlet 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 sub 46 via the pump inlet pipe 13 , while the outlet of the lift pump 12 is connected to the inner annulus 54 of the first drill pipe sub 31 via the pump outlet pipe 11 .

[0042] Please refer to Figure 2 , please refer to Figure 2 , Figure 2 A structural schematic 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 through the lead-out pipe 23 .

[0044] The outlet pipe 23 is provided with a two-way pressure differential 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 outgoing 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 at 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 the 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, which divides 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 further provided on the upper portion 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 has a "7"-shaped structure and is set as a four-petal structure at one end close to the rubber capsule 35. It 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 through 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 an annular cavity formed by the second inner center tube 41 and the outer center tube 33. The upper end face of the second inner center tube 41 is connected to the lower end face of the first inner center tube 32 by means of a thread + O-ring. Similarly, the lower end face of the second inner center tube 41 and the lower end face of the outer center tube 33 are connected to the second drill pipe short section 46 by means of a thread + O-ring.

[0054] An annular piston 42 is provided in the annular cavity formed 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 an O-ring, 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 around the piston. The piston rods 43 are in the form of stepped shafts. The piston 42 is connected to the upper ends of the piston rods 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 set in the large hole of the guide device 44 and cooperates 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 isolation 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 in 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 provided.

[0060] During operation, the expandable packer structure 51 is a rubber skin structure that 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 achieve sealing by utilizing the liquid pressure at the pump outlet, 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 sub 46 to the annulus of the first drill pipe sub 31 , at which time the fluid pressure increases.

[0064] Step 2: Through the pressure outlet device 2, the high-pressure fluid is drawn out of the pump outlet pipe 11. When the fluid pressure P 11 and 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 At this time, the locking block and the grooves of the first inner central tube 32 and the outer central 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 The increase causes the rubber skin of the expandable packer structure 51 to undergo 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. The high-pressure fluid coming out of 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 lifting pump 12 stops working, and the pressure of the fluid 11 in the pump outlet pipeline is P 11 When the pressure P in the first cavity 36 decreases 36 The fluid pressure P in 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 in the second cavity 37 37 Decrease, at the same time, the hydraulic oil pressure P of the third cavity 47 47 Decreases to 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 in the fourth cavity 48 is 48 Decrease.

[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 merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection 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 central 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 central tube and the outer central tube. A rubber capsule is arranged in the annular cavity. The first cavity located above the rubber capsule is connected to the pressure outlet device (2) through an outlet 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. Above the piston is a third cavity. Below the piston is a fourth cavity. The third cavity is connected to the second cavity of the pressure conversion device (3). The fourth cavity below the piston is connected to the fifth cavity of the isolation device (5) through a small hole opened in the second inner central tube. The sealing device (5) adopts an expandable sealing device with a rubber skin structure and is arranged on the inner peripheral wall of the second inner central tube by welding; The pressure outlet 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); The rubber capsule and the matching locking block are disposed between the first inner central tube and the outer central tube, with the lead-out pipe communicating with the first cavity; hydraulic oil is contained below the capsule, for converting 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 the end is connected to the grooves of the first inner central tube and the outer central tube by an adhesive; 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; The isolation device (5) is connected to the hydraulic device (4) through a small hole opened in the second inner central tube, 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 by a lifting pump.

2. The 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 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 a threaded connection and is sealed with a sealing ring; a flat flange is welded at the upper end surface position of the outer center pipe and the relative position of the first drill pipe short section, and the outer center pipe and the first drill pipe short section are connected through the flat flange.

4. The expandable isolation system for dual gradient drilling according to claim 1, 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.

5. 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.

6. The expandable isolation system for dual-gradient drilling according to claim 1, characterized in that: A guide device is provided between the second inner center tube and the outer center tube. Eight stepped holes are distributed in an annular pattern inside the guide device to prevent the piston from deflecting during movement.

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