Double-gradient well drilling system and method based on double-wall drill rod

Abstract

The invention relates to a double-gradient well drilling system and method based on a double-wall drill pipe, a well drilling device comprises a double-gradient well drilling pipe column, a drilling fluid circulating device, a power fluid driving device and a well mouth device, the well mouth device is fixedly installed at a well mouth of a shaft, and a liquid inlet and a liquid outlet are formed in the well mouth device; the double-gradient well drilling pipe column is vertically installed in the shaft, the upper end of the double-gradient well drilling pipe column penetrates through the well head device and extends to the position above the well head device, and the double-gradient well drilling pipe column can vertically rotate around the axial direction of the double-gradient well drilling A center channel and an annular channel are sequentially arranged in the double-gradient well drilling pipe column from inside to outside, the two ends of the center channel are open, the upper end of the annular channel is open, and a liquid outlet is formed in the lower portion of the annular channel. The drilling fluid circulating device is installed at the upper end of the double-gradient drilling pipe column, and the power fluid driving device is matched with the drilling fluid circulating device and used for injecting power fluid into the annular channel. The pressure of the shaft can be adjusted by adjusting parameters such as displacement and pressure of drilling fluid and power fluid, and real-time flexible adjustment of the pressure at the bottom of the shaft is achieved.

Description

Technical Field

[0001] The present invention relates to the technical fields of oil and gas drilling and completion and gas hydrate drilling and production, and particularly relates to a dual-gradient drilling system and a drilling method based on double-walled drill pipes. Background Art

[0002] With the continuous development of conventional and easily exploitable oil and gas resources in China, the grades of newly discovered oil and gas reserves have been continuously declining, the geological conditions of oil and gas exploration and development targets have become increasingly harsh, and the complexity, difficulty and safety risks of drilling have increased exponentially. The oil and gas resources in complex formations are an important area for tackling key problems in the development of world oil and gas resources. Strengthening the exploration and development of oil and gas resources in complex formations can not only alleviate the contradiction between oil and gas supply and demand in China and ensure national energy and resource security, but also be an important measure to promote petroleum scientific and technological innovation.

[0003] However, due to the difficulty in predicting the geological risks of drilling in complex formations, there are many problems such as multiple formation pressure systems, narrow safety density windows, and mutual constraints between high-pressure layers and weak layers. The geological conditions such as lost circulation zones, fractured zones, easily collapsible zones, and abnormal high pressures are very complex. Drilling often faces "encounter battles", and the accident time of drilling in complex formations is significantly longer than that of conventional drilling, and it is extremely difficult to respond to engineering risks and ensure safe drilling.

[0004] To solve the problem of narrow safety density windows, it is necessary to control the bottom hole pressure. Especially when drilling and completing wells in deep formations and complex formations, it is particularly crucial to control a reasonable bottom hole pressure. In conventional drilling operations, there is only one dynamic fluid system of drilling fluid, which limits the regulation methods of annulus flow and pressure. The single wellbore fluid drive mode and drilling fluid flow system may face limitations in pressure regulation under the conditions of multi-pressure complex formations. Therefore, it is urgent to carry out cutting-edge engineering and technical research in the field of new wellbore controlled pressure drilling, break through the drilling limitations of a single dynamic fluid system, form new technologies and new processes, and support flexible regulation of wellbore pressure and efficient drilling and completion operations.

[0005] In order to break through the limitations of the single flow system of wellbore drilling fluid and achieve real-time and flexible adjustment of the bottom hole pressure, it is necessary to fully consider the narrow space environment of the wellbore and the requirements of well control safety, and innovatively propose a wellbore pressure regulation technology and equipment that can be used in environments such as land and sea. On the basis of the existing conventional drilling processes, a new type of dual-gradient drilling system and drilling method are formed to enhance China's voice in related technical fields and help China achieve safe and efficient drilling in complex formations and increase oil and gas reserves and production. Summary of the Invention

[0006] The present invention provides a dual-gradient drilling system and a drilling method based on double-walled drill pipes, aiming to solve the problems in the prior art.

[0007] The technical solution of the present invention for solving the above technical problems is as follows:

[0008] A dual-gradient drilling system based on double-wall drill pipes, comprising a dual-gradient drilling string, a drilling fluid circulation device, a power fluid driving device and a wellhead device. The wellhead device is fixedly installed at the wellhead of the wellbore, and is provided with a liquid inlet and a liquid outlet thereon; the dual-gradient drilling string is vertically installed in the wellbore, its upper end passes through the wellhead device and extends above the wellhead device, and it can rotate vertically around its own axis; a central channel and an annular channel are sequentially arranged from inside to outside in the dual-gradient drilling string, both ends of the central channel are open, the upper end of the annular channel is open, and a liquid discharge port is arranged at the lower part; the drilling fluid circulation device is installed at the upper end of the dual-gradient drilling string for injecting drilling fluid into the central channel; the power fluid driving device cooperates with the drilling fluid circulation device for injecting power fluid into the annular channel. The drilling fluid and the power fluid are mixed in the wellbore and enter the wellhead device through the liquid inlet and are discharged through the liquid outlet.

[0009] The beneficial effects of the present invention are as follows: During the operation process, the drilling fluid circulation device injects drilling fluid into the central channel in the dual-gradient drilling string, and the drilling fluid enters the wellbore through the central channel to establish a drilling fluid circulation;

[0010] Meanwhile, the power fluid driving device injects power fluid into the annular channel in the dual-gradient drilling string to output power. The power fluid enters the wellbore through the annular channel and the liquid discharge port and is mixed with the drilling fluid. The mixed liquid enters the wellhead device through the liquid inlet and is discharged through the liquid outlet.

[0011] The present invention can adjust the wellbore pressure by regulating parameters such as the displacement and pressure of the drilling fluid and the power fluid, and realize the real-time and flexible regulation of the bottom hole pressure.

[0012] Based on the above technical solutions, the present invention can also be improved as follows.

[0013] Further, the dual-gradient drilling string includes double-wall drill pipes, a turbine drive short section and an axial flow lift pump short section. The double-wall drill pipes, the turbine drive short section and the axial flow lift pump short section are vertically installed in the wellbore from top to bottom and are fixedly connected in sequence, and the upper end of the double-wall drill pipes extends outside the wellbore and is connected with the drilling fluid circulation device; the central channel is distributed in the double-wall drill pipes, the turbine drive short section and the axial flow lift pump short section, the annular channel is distributed in the double-wall drill pipes, and the liquid discharge port is located on the turbine drive short section.

[0014] The beneficial effects of adopting the above further solution are that during the operation process, the drilling fluid circulation device injects drilling fluid into the central channel in the dual-gradient drilling string, and the drilling fluid enters the wellbore through the central channel to establish a drilling fluid circulation;

[0015] Meanwhile, power fluid is injected into the annular channel inside the double-gradient drilling string through the power fluid driving device to output power. The power fluid enters the wellbore through the annular channel and the drain port and mixes with the drilling fluid. The mixed liquid enters the wellhead device through the liquid inlet and is discharged through the liquid outlet.

[0016] Furthermore, a transition joint is fixedly installed between the double-wall drill pipe and the turbine drive sub. The central channel and the annular channel are distributed inside the transition joint.

[0017] The beneficial effect of adopting the above further scheme is that the structure is simple and the design is reasonable. The double-wall drill pipe and the turbine drive sub are connected by the transition joint, which is convenient for assembly.

[0018] Furthermore, a hydraulic packer tool is installed outside the axial flow lift pump sub.

[0019] The beneficial effect of adopting the above further scheme is that the pressure changes of the upper sealing part and the lower sealing part of the wellbore annulus separated by the hydraulic packer tool are realized. The pressure of the lower sealing part decreases under the assistance of the axial flow lift pump sub, and the pressure of the upper sealing part increases under the pressure energy mixing action of the drilling fluid and the power fluid, thereby establishing a double-gradient drilling mode.

[0020] Furthermore, it also includes downhole motor drill tools, which are fixedly installed at the lower end of the axial flow lift pump sub.

[0021] The beneficial effect of adopting the above further scheme is that during the operation, drilling fluid is injected into the central channel inside the double-gradient drilling string through the drilling fluid circulation device. The drilling fluid enters the wellbore through the central channel to establish a drilling fluid circulation and assist in the rock breaking operation.

[0022] Furthermore, the liquid outlet is connected to a solids control device through a surface manifold.

[0023] The beneficial effect of adopting the above further scheme is that after the drilling fluid and the power fluid are mixed, they enter the wellhead device and are discharged by the solids control device after solid-liquid separation, which is convenient for discharge.

[0024] Furthermore, the drilling fluid circulation device includes a drilling top drive, a top drive adapter, and a drilling pump. The top drive adapter is installed at the upper end of the double-gradient drilling string. An inner cavity and an outer cavity are sequentially arranged inside it from the inside to the outside. The inner cavity and the outer cavity are respectively communicated with the central channel and the annular channel. The drilling top drive is installed at the upper end of the top drive adapter and is used to drive the double-gradient drilling string to rotate. A flow channel communicated with the inner cavity is arranged inside it. The drilling pump is communicated with the flow channel through a surface manifold. The power fluid driving device is communicated with the outer cavity through a surface manifold.

[0025] The beneficial effect of adopting the above further solution is that during the operation process, functions such as rotation, lifting and lowering of the double-gradient drilling string are realized through the drilling top drive, and at the same time, a power fluid flow channel is constructed in cooperation with the top drive adapter.

[0026] Further, the power fluid driving device includes a power fluid pump, and the power fluid pump is communicated with the outer cavity through a surface manifold.

[0027] The beneficial effect of adopting the above further solution is that during the operation process, the power fluid is sent into the annular channel by the power fluid pump, which is convenient for liquid delivery.

[0028] Further, a regulating valve is fixedly installed on the surface manifold between the outer cavity and the power fluid pump.

[0029] The beneficial effect of adopting the above further solution is that the regulating valve can adjust the power fluid displacement in real time according to the on-site actual bottom hole pressure control requirements and in combination with the drilling fluid displacement.

[0030] The present invention also relates to a double-gradient drilling method based on double-walled drill pipes, which is realized by using the double-gradient drilling system based on double-walled drill pipes as described above, and includes the following specific steps:

[0031] Inject drilling fluid into the central channel in the double-gradient drilling string through the drilling fluid circulation device to establish drilling fluid circulation;

[0032] At the same time, inject power fluid into the annular channel in the double-gradient drilling string through the power fluid driving device to output power.

[0033] The beneficial effect of adopting the above further solution is that the present invention also provides a double-gradient drilling method based on double-walled drill pipes. This method can adjust the wellbore pressure by regulating parameters such as the displacement and pressure of the drilling fluid and the power fluid, and realize real-time and flexible control of the bottom hole pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a structural schematic diagram of the present invention.

[0035] In the drawings, the list of components represented by each reference numeral is as follows:

[0036] 1, wellbore; 2, double-walled drill pipe; 3, turbine drive sub; 4, axial flow lift pump sub; 5, hydraulic packoff tool; 6, drilling pump; 7, power fluid pump; 8, regulating valve; 9, drilling top drive; 10, top drive adapter; 11, wellhead device; 12, adapter; 13, solids control equipment; 14, downhole motor; 15, surface manifold. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0037] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments may be combined with each other.

[0038] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be understood as a limitation to the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first", "second", etc. may explicitly or implicitly include one or more of such features. In the description of the present invention, unless otherwise specified, the meaning of "a plurality" is two or more.

[0039] In the description of the present invention, it should be noted that unless otherwise clearly defined and limited, the terms "installed", "connected", "connected to" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and it may be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood through specific situations.

[0040] The present invention will be described in detail below with reference to the drawings and in combination with embodiments.

[0041] Embodiment 1

[0042] As Figure 1As shown in the figure, this embodiment provides a dual-gradient drilling system based on a double-walled drill pipe, which includes a dual-gradient drilling string, a drilling fluid circulation device, a power fluid drive device, and a wellhead device 11. The wellhead device 11 is fixedly installed at the wellhead of the wellbore 1, and is provided with a liquid inlet and a liquid outlet thereon; the dual-gradient drilling string is vertically installed in the wellbore 1, its upper end passes through the wellhead device 11 and extends above the wellhead device 11, and it can rotate vertically around its own axis; a central channel and an annular channel are sequentially arranged inside the dual-gradient drilling string from inside to outside. Both ends of the central channel are open, the upper end of the annular channel is open, and a liquid discharge port is provided at the lower part; the drilling fluid circulation device is installed at the upper end of the dual-gradient drilling string for injecting drilling fluid into the central channel; the power fluid drive device cooperates with the drilling fluid circulation device for injecting power fluid into the annular channel. After the drilling fluid and the power fluid are mixed in the wellbore, they enter the wellhead device 11 through the liquid inlet and are discharged through the liquid outlet.

[0043] During the operation, the drilling fluid circulation device injects drilling fluid into the central channel inside the dual-gradient drilling string. The drilling fluid enters the wellbore 1 through the central channel to make the dual-gradient drilling string operate;

[0044] At the same time, the power fluid drive device injects power fluid into the annular channel inside the dual-gradient drilling string to output power. The power fluid enters the wellbore 1 through the annular channel and the liquid discharge port to be mixed with the drilling fluid. The mixed liquid enters the wellhead device through the liquid inlet and is discharged through the liquid outlet.

[0045] It should be noted that the above wellhead device 11 adopts the existing technology, and its specific structure and principle will not be elaborated here.

[0046] This embodiment can adjust the wellbore pressure by regulating parameters such as the displacement and pressure of the drilling fluid and the power fluid, and realize the real-time and flexible regulation of the bottom hole pressure.

[0047] Embodiment 2

[0048] On the basis of Embodiment 1, in this embodiment, the dual-gradient drilling string includes a double-walled drill pipe 2, a turbine drive sub 3, and an axial flow lift pump sub 4. The double-walled drill pipe 2, the turbine drive sub 3, and the axial flow lift pump sub 4 are vertically installed in the wellbore 1 from top to bottom and are fixedly connected in sequence. The upper end of the double-walled drill pipe 2 extends outside the wellbore 1 and is connected to the drilling fluid circulation device; the central channel is distributed in the double-walled drill pipe 2, the turbine drive sub 3, and the axial flow lift pump sub 4, the annular channel is distributed in the double-walled drill pipe 2, and the liquid discharge port is located on the turbine drive sub 3.

[0049] During the operation, drilling fluid is injected into the central channel inside the dual-gradient drilling string through the drilling fluid circulation device. The drilling fluid enters the wellbore 1 through the central channel to establish a drilling fluid circulation.

[0050] Meanwhile, power fluid is injected into the annular channel inside the dual-gradient drilling string through the power fluid driving device to output power. The power fluid enters the wellbore 1 through the annular channel and the drain port to mix with the drilling fluid. The mixed liquid enters the wellhead device 11 through the liquid inlet and is discharged through the liquid outlet.

[0051] Moreover, the above-mentioned dual-gradient drilling string may also include multiple double-wall drill pipes 2, which are connected in series from bottom to top, and the upper end of the double-wall drill pipe 2 located at the uppermost position extends to the wellhead.

[0052] Embodiment 3

[0053] Based on Embodiment 2, in this embodiment, a transition joint 12 is fixedly installed between the double-wall drill pipe 2 and the turbine drive sub 3, and the central channel and the annular channel are distributed inside the transition joint 12.

[0054] This solution has a simple structure and reasonable design. The double-wall drill pipe 2 and the turbine drive sub 3 are connected through the transition joint 12, which is convenient for assembly.

[0055] Preferably, in this embodiment, both ends of the above-mentioned transition joint 12 are threadedly connected to the lower end of the double-wall drill pipe 2 and the upper end of the turbine drive sub 3 respectively.

[0056] Embodiment 4

[0057] Based on any one of Embodiments 2 to 3, in this embodiment, a hydraulic packoff tool 5 is installed outside the axial flow lift pump sub 4.

[0058] This solution realizes the pressure change of the upper sealing part and the lower sealing part of the wellbore annulus with the hydraulic packoff tool 5 as the boundary. The pressure of the lower sealing part decreases under the assistance of the axial flow lift pump sub 4, and the pressure of the upper sealing part increases under the pressure energy mixing action of the drilling fluid and the power fluid, thereby establishing a dual-gradient drilling mode.

[0059] Based on the above solution, the hydraulic packoff tool 5 is in a ring structure. It can be fixedly sleeved on the axial flow lift pump sub 4, and at this time, the hydraulic packoff tool 5 rotates with the axial flow lift pump sub 4; alternatively, the hydraulic packoff tool 5 is installed on the axial flow lift pump sub 4 through a bearing, and at this time, the hydraulic packoff tool 5 does not rotate with the axial flow lift pump sub 4.

[0060] In addition, the hydraulic packer tool 5 is a packer tool that can be driven by liquid pressure energy to open and close. Its main function is to block the fluid suction port and discharge port of the axial flow lifting pump sub - section, so as to form an upper sealing part and a lower sealing part in the wellbore, preventing the short - range backflow of drilling fluid; the main function of the axial flow lifting pump sub - section is to convert the rotational mechanical energy transmitted by the turbine drive sub - section into fluid kinetic energy, generate additional lifting force to drive the drilling fluid in the wellbore annulus to rise. The drilling fluid is sucked in from the lower suction port, discharged from the upper discharge port after being driven, and the bottom - hole pressure is regulated.

[0061] Embodiment 5

[0062] Based on any one of Embodiments 2 to 4, this embodiment further includes a downhole motor 14, and the downhole motor 14 can be directly connected to the axial flow lifting pump sub - section 4 or connected through multiple double - wall drill pipes 2 or ordinary drill pipes.

[0063] During the operation, drilling fluid is injected into the central channel in the dual - gradient drilling string through the drilling fluid circulation device. The drilling fluid enters the wellbore 1 through the central channel to establish a drilling fluid circulation and assist in the rock - breaking operation.

[0064] Or, in some operating environments, the downhole motor 14 can be not installed. Therefore, the installation of the downhole motor 14 can be designed according to requirements.

[0065] Embodiment 6

[0066] Based on the above - mentioned embodiments, in this embodiment, the liquid outlet is connected to a solids control device 13 through a surface manifold 15.

[0067] After the drilling fluid and the power fluid are mixed, they enter the wellhead device 11, and are subjected to solid - liquid separation and discharged by the solids control device 13, which is convenient for discharge.

[0068] It should be noted that the above - mentioned solids control device 13 adopts existing technology, and its specific structure and principle will not be elaborated here.

[0069] Embodiment 7

[0070] Based on the above - mentioned embodiments, in this embodiment, the drilling fluid circulation device includes a drilling top drive 9, a top - drive adapter 10 and a drilling pump 6. The top - drive adapter 10 is installed at the upper end of the dual - gradient drilling string. An inner cavity and an outer cavity are sequentially arranged inside it from inside to outside, and the inner cavity and the outer cavity are respectively communicated with the central channel and the annular channel; the drilling top drive 9 is installed at the upper end of the top - drive adapter 10 and is used to drive the dual - gradient drilling string to rotate. A flow channel communicated with the inner cavity is arranged inside it, and the drilling pump 6 is communicated with the flow channel through a surface manifold 15; the power - fluid driving device is communicated with the outer cavity through a surface manifold 15.

[0071] During the operation process, functions such as rotation, hoisting and lowering of the double-gradient drilling string are realized through the drilling top drive 9, and at the same time, a power fluid flow channel is constructed in cooperation with the top drive adapter 10.

[0072] When the drilling fluid circulates, it is first pumped out by the drilling pump, enters the central flow channel of the double-wall drill pipe after passing through the drilling top drive and the top drive adapter, and then passes through the turbine drive sub-section, the hydraulic packer tool and the axial flow lift pump sub-section to reach the bottom of the well. After cleaning the bottom of the well and carrying the cuttings, it flows through the outer flow channel of the axial flow lift pump sub-section, obtains additional driving force and then mixes with the power fluid, reaches the wellhead from the wellbore annulus and returns to the drilling fluid pit to complete the circulation.

[0073] It should be noted that the above-mentioned drilling top drive 9 adopts the existing technology, and its specific structure and principle will not be elaborated here.

[0074] Embodiment 8

[0075] On the basis of Embodiment 7, in this embodiment, the power fluid driving device includes a power fluid pump 7, and the power fluid pump 7 is connected to the outer cavity through a surface manifold 15.

[0076] During the operation process, the power fluid is sent into the annular channel by the power fluid pump 7, which is convenient for liquid delivery.

[0077] The main function of the power fluid pump 7 is to provide high-pressure power fluid and pump the power fluid into the annular channel of the double-wall drill pipe through a regulating valve and a top drive adapter to drive the turbine drive sub-section; the main function of the regulating valve is to adjust the power fluid displacement in real time according to the on-site actual bottom hole pressure regulation requirements and in combination with the drilling fluid displacement; the main function of the surface manifold is to connect the power fluid pump and the top drive adapter, as well as the top drive adapter and the power fluid pit, and provide a surface channel for the power fluid flow.

[0078] In addition, the main function of the special structure of the double-wall drill pipe 2 is to jointly provide the main flow channel for the power fluid flow with the top drive adapter 10, so that the power fluid can drive the turbine drive sub-section 3.

[0079] Moreover, the main function of the turbine drive sub-section 3 is to receive the driving energy of the power fluid and convert the fluid kinetic energy into rotational mechanical energy and transmit it to the axial flow lift pump sub-section 4.

[0080] Embodiment 9

[0081] On the basis of Embodiment 8, in this embodiment, a regulating valve 8 is fixedly installed on the surface manifold 15 between the outer cavity and the power fluid pump 7.

[0082] The regulating valve 8 can adjust the power fluid displacement in real time according to the on-site actual bottom hole pressure regulation requirements and in combination with the drilling fluid displacement.

[0083] Embodiment 10

[0084] Based on the above embodiments, the present embodiment further provides a dual-gradient drilling method based on a double-walled drill pipe, which is implemented by using the dual-gradient drilling system based on a double-walled drill pipe as described above, and includes the following specific steps:

[0085] Inject drilling fluid into the central channel in the dual-gradient drilling string through the drilling fluid circulation device to establish a drilling fluid circulation;

[0086] At the same time, inject power fluid into the annular channel in the dual-gradient drilling string through the power fluid driving device to output power.

[0087] The present embodiment further provides a dual-gradient drilling method based on a double-walled drill pipe. This method can adjust the wellbore pressure by regulating parameters such as the displacement and pressure of the drilling fluid and the power fluid, so as to realize real-time and flexible control of the bottom hole pressure.

[0088] The specific steps of the drilling operation of the present invention are as follows:

[0089] Step 1: Check the working reliability of the connection structures of each component, and ensure that the hydraulic packer tool can be switched flexibly and can be in a state of fitting with the inner wall of the wellbore, so that the upper and lower parts are in an approximately sealed environment;

[0090] Step 2: Subsequently, inject drilling fluid into the central channel of the double-walled drill pipe to establish a drilling fluid circulation to assist in the rock breaking operation;

[0091] Step 3: Inject power fluid into the drill pipe annulus channel of the double-walled drill pipe, monitor the pressure of the lower seal part of the wellbore and the formation pressure, and adjust the output power of the power fluid pump through the regulating valve at the outlet of the power fluid pump;

[0092] Step 4: Based on the pressure monitoring in Step 3, when the pressure of the lower seal part at the bottom of the wellbore is greater than the formation pressure, increase the output power of the power fluid through the regulating valve; when the pressure of the lower seal part at the bottom of the wellbore is less than the formation pressure, decrease the output power of the power fluid through the regulating valve.

[0093] On the ground of the drilling site, a drilling pump and a power fluid pump are installed. The drilling fluid is pumped out by the drilling pump. After the high-pressure drilling fluid passes through the surface manifold, the top drive, and the top drive adapter, it is finally injected into the central flow channel of the double-wall drill pipe. The drilling fluid drives the downhole downhole motor to rotate, and after passing through the drill bit, it carries the cuttings back to the surface. At the same time, the power fluid is pumped out by the power fluid pump. A regulating valve is installed on the outlet manifold of the power fluid pump to control the displacement of the power fluid. After the high-pressure power fluid passes through the surface manifold and the top drive adapter, it is finally injected into the annular channel of the double-wall drill pipe. After passing through the adapter, the power fluid drives the turbine drive sub to rotate. After the turbine drive sub rotates, it transmits the power to the axial flow lift pump sub, driving the axial flow lift pump sub to rotate. At the same time, the power fluid relies on its own pressure to push the hydraulic packer tool to seal the wellbore annulus, separating the suction port and the discharge port of the axial flow lift pump sub. Finally, the power fluid is discharged into the wellbore annulus from the lower end outlet of the turbine drive sub and mixes with the drilling fluid to return to the surface. During the upward return of the drilling fluid, due to the lifting force of the axial flow lift pump sub, the drilling fluid is sucked into the axial flow lift pump sub through the suction port, and after obtaining additional driving force, it is discharged into the wellbore annulus through the discharge port and mixes with the discharged power fluid to jointly return to the wellhead.

[0094] Therefore, under the drilling conditions where the drilling fluid and the power fluid are injected simultaneously, the pressure of the drilling fluid in the wellbore annulus changes with the hydraulic packer tool as the boundary. Due to the assisting lifting effect of the axial flow lift pump sub in the lower sealing part, the driving force of part of the drilling fluid that should be driven by the drilling pump is provided by the axial flow lift pump sub, resulting in a decrease in the bottom hole pressure; in the upper sealing part, due to the mixing of the power fluid and the drilling fluid in the double-wall drill pipe, the total fluid displacement increases, and the power hydraulic energy and the drilling hydraulic energy are mixed, resulting in an increase in the pressure of the upper wellbore. Eventually, two pressure systems are formed, namely dual gradient drilling. According to the above method, it can be obtained that when the displacement and pressure of the power fluid are increased, the bottom hole pressure gradually decreases, and the pressure of the upper sealing part gradually increases; conversely, when the displacement and pressure of the power fluid are decreased, the bottom hole pressure gradually increases, and the pressure of the upper sealing part gradually decreases. During the actual dual gradient drilling operation, the operation parameters can be determined according to the actual on-site requirements. In the present invention, the power fluid used needs to be adapted to the drilling fluid to avoid interfering with the normal performance of the drilling fluid.

[0095] The dual-gradient drilling system and method based on double-walled drill pipes proposed by the present invention have certain innovation. This technology first proposed a dual-gradient drilling mode that combines double-walled drill pipes with axial flow lift pump short joints, and comprehensively applies the assisting lift effect of the axial flow lift pump short joints and the pressure energy mixing effect of the drilling fluid power fluid to realize the pressure change of the upper and lower sealed parts of the wellbore annulus separated by a hydraulic packer tool. The pressure of the lower sealed part decreases under the assisting lift effect of the axial flow lift pump short joints, and the pressure of the upper sealed part increases under the pressure energy mixing effect of the drilling fluid power fluid, thereby establishing a dual-gradient drilling mode. This technical solution provides new ideas for the development of wellbore pressure control drilling technology in various environments such as land and sea. General designers and R & D personnel in the oil and gas industry can form a suitable dual-gradient drilling technology according to the system and method of the present invention based on actual site needs. This technical solution can provide technical support and safety guarantee for solving accidents such as wellbore collapse, stuck pipe, well kick, blowout, and coexistence of leakage and blowout in the drilling operation field in China.

[0096] The dual-gradient drilling system and method based on double-walled drill pipes proposed by the present invention have the following beneficial effects:

[0097] 1. The drilling system has a compact structure and can be used in the narrow space environment of general wellbores. The equipment technologies such as double-walled drill pipes and axial flow lift pump short joints adopted are mature and have high reliability;

[0098] 2. The dual-gradient drilling system and method based on double-walled drill pipes proposed by the present invention are the first at home and abroad, providing new ideas for international wellbore pressure control technology, and enhancing China's voice and leading advantage in the technical field of dual-gradient drilling systems and methods;

[0099] 3. The dual-gradient drilling system based on double-walled drill pipes proposed by the present invention has good adaptability and high integration with the existing drilling system, basically does not change the existing drilling equipment and processes, and has good on-site acceptability;

[0100] 4. The present invention uses an axial flow lift pump short joint to lift the annulus fluid and regulate the pressure. Its operation is stable, the rotation speed can be steplessly adjusted, and the wellbore pressure can be accurately and stably regulated. At the same time, due to the mixing of the drilling fluid power fluid in the upper sealed section of the wellbore, the rock-carrying efficiency and wellbore cleanliness are improved;

[0101] 5. The dual-gradient drilling method proposed by the present invention can regulate the wellbore pressure in real time by regulating parameters such as the drilling fluid displacement, drilling fluid pressure, power fluid displacement, and power fluid pressure, and can determine the installation position of the hydraulic packer tool and customize the pressure demarcation point according to needs;

[0102] 6. The dual-gradient drilling method proposed by the present invention can address complex downhole conditions such as losses and overflows caused by narrow density windows, and can provide a new solution and technical guarantee for safe and efficient drilling and completion operations in complex formations.

[0103] It should be noted that all the electronic components involved in the present invention adopt existing technologies, and the above-mentioned components are electrically connected to the controller. The control circuits between the controller and each component are existing technologies.

[0104] In addition, the arrows in the drawings only indicate the flow directions of the drilling fluid and the power fluid, and have no other substantial meanings.

[0105] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention. Any reference signs in the claims should not be regarded as limiting the claimed rights.

[0106] In addition, it should be understood that although this specification is described according to embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0107] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A dual-gradient drilling system based on double-wall drill pipe, Features: The invention comprises a dual-gradient drilling pipe, a drilling fluid circulation device, a power fluid driving device and a wellhead device (11), wherein the wellhead device (11) is fixedly installed at the wellhead of a wellbore (1) and is provided with a liquid inlet and a liquid outlet; the dual-gradient drilling pipe is vertically installed in the wellbore (1), and its upper end passes through the wellhead device (11) and extends to the top of the wellhead device (11), and can rotate vertically around its own axis; a central channel and an annular channel are sequentially provided in the dual-gradient drilling pipe from the inside to the outside, both ends of the central channel are open, the upper end of the annular channel is open, and a drainage port is provided at the lower part; the drilling fluid circulation device is installed at the upper end of the dual-gradient drilling pipe and is used to inject drilling fluid into the central channel; The power fluid driving device cooperates with the drilling fluid circulation device to inject the power fluid into the annular channel. The drilling fluid and the power fluid are mixed in the wellbore and then enter the wellhead device (11) through the liquid inlet and are discharged from the liquid outlet.

2. The dual gradient drilling system based on double wall drill pipe according to claim 1, Features: The dual-gradient drilling pipe string comprises a double-wall drill pipe (2), a turbine-driven short section (3) and an axial-flow lift pump short section (4); the double-wall drill pipe (2), the turbine-driven short section (3) and the axial-flow lift pump short section (4) are vertically installed in the wellbore (1) from top to bottom and fixedly connected in sequence, and the upper end of the double-wall drill pipe (2) extends outside the wellbore (1) and is connected to the drilling fluid circulation device; the central channel is distributed in the double-wall drill pipe (2), the turbine-driven short section (3) and the axial-flow lift pump short section (4); the annular channel is distributed in the double-wall drill pipe (2); and the drainage port is located on the turbine-driven short section (3).

3. The dual gradient drilling system based on double wall drill pipe according to claim 2, Features: A conversion joint (12) is fixedly installed between the double-wall drill pipe (2) and the turbine drive short section (3), and the central channel and the annular channel are distributed in the conversion joint (12).

4. The dual gradient drilling system based on double wall drill pipe according to claim 2, Features: A hydraulic isolation tool (5) is installed outside the axial flow lift pump nipple (4).

5. The dual gradient drilling system based on double wall drill pipe according to claim 2, Features: It also comprises a downhole power drilling tool (14), wherein the downhole power drilling tool (14) is fixedly mounted on the lower end of the axial flow lift pump short section (4).

6. A dual gradient drilling system based on a double wall drill pipe according to any one of claims 1 to 5, Features: The liquid outlet is connected to a solid control device (13) via a surface manifold (15).

7. A dual gradient drilling system based on a double wall drill pipe according to any one of claims 1 to 5, Features: The drilling fluid circulation device comprises a drilling top drive (9), a top drive adapter (10) and a drilling pump (6); the top drive adapter (10) is installed at the upper end of the dual-gradient drilling pipe column, and is provided with an inner cavity and an outer cavity from the inside to the outside, and the inner cavity and the outer cavity are respectively connected to the central channel and the annular channel; the drilling top drive (9) is installed at the upper end of the top drive adapter (10) and is used to drive the dual-gradient drilling pipe column to rotate, and is provided with a flow channel connected to the inner cavity, and the drilling pump (6) is connected to the flow channel through a ground manifold (15); the power fluid drive device is connected to the outer cavity through the ground manifold (15).

8. The dual gradient drilling system based on double wall drill pipe according to claim 7, Features: The power fluid drive device comprises a power fluid pump (7), and the power fluid pump (7) is connected to the outer cavity through a ground manifold (15).

9. The dual gradient drilling system based on double wall drill pipe according to claim 8, Features: A regulating valve (8) is fixedly installed on the ground manifold (15) between the outer cavity and the power fluid pump (7).

10. A dual gradient drilling method based on double wall drill pipe, Features: The double-gradient drilling system based on the double-wall drill pipe as claimed in any one of claims 1 to 9 is used, comprising the following specific steps: Inject drilling fluid into the central channel of the dual-gradient drilling string through the drilling fluid circulation device to establish drilling fluid circulation; At the same time, power fluid is injected into the annular channel in the dual-gradient drilling pipe string through a power fluid driving device to output power.

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