A dual-gradient drilling system and its drilling method

Abstract

The present invention discloses a dual-gradient drilling system and a drilling method thereof. The system mainly consists of a top drive adapter device, double-walled drill pipes, a transition sub, a data monitoring and control system, and an auxiliary pump device. The drilling pump unit injects a first fluid through the double-walled drill pipes and the transition sub, and then reaches the downstream mud treatment system through the discharge pipeline; the auxiliary pump device is used to inject a second fluid into the annulus A to adjust the pressure gradient in the annulus A; the data monitoring and control system is used to monitor the flow rate of the second fluid injected by the auxiliary pump device so as to determine the height of the second fluid in the annulus A. The present invention has strong adaptability and can be applied to onshore drilling and offshore drilling; it can achieve deepwater oil and gas exploration and development at a lower cost, with a shorter well construction time and safer operation.

Description

Technical Field

[0001] The present invention relates to a drilling system and a drilling method thereof, and particularly to a dual-gradient drilling system based on double-walled drill pipes and a drilling method thereof. Background Art

[0002] With the continuous and rapid development of China's national economy, the demand for oil and gas is increasing continuously, and it has become an inevitable trend to march into deep sea for oil and gas resource exploration and development. Compared with onshore and shallow sea drilling, the deep sea drilling environment is more complex, and technical problems that are difficult to overcome by conventional drilling equipment and methods are likely to occur.

[0003] Currently, the main drilling technology used in offshore drilling is single-gradient drilling technology. In a wellbore of the same size, the single-gradient drilling technology has only one liquid column gradient, that is, the bottom hole pressure is generated by the drilling fluid column pressure from the sea surface to the bottom hole, and the drilling fluid column pressure gradient is referenced to the sea surface. In deep water drilling, due to the influence of loose sediments on the seabed and the seawater column, the gap between the formation pressure and the fracture pressure is very small, making drilling very difficult. The dual-gradient drilling (DGD) technology developed abroad in the 1990s has well solved this problem. When using this technology, the fluid density in the riser above the seabed is similar to that of seawater, and the pressure calculation of the drilling fluid column is referenced to the seabed, so the area between the formation pore pressure and the fracture pressure becomes relatively wider. Well kick, blowout and lost circulation accidents are greatly reduced, and there is no need for a multi-layer casing system, saving casing time and cementing time, shortening the well construction period, reducing drilling time and the time and cost for dealing with drilling accidents, and greatly reducing the requirements for drilling equipment such as drilling platforms and rigs.

[0004] Currently, foreign countries have carried out research and application on multiple dual-gradient drilling systems: the Subsea Mudlift Drilling (SMD) system studied by the industrial joint project team led by Conoco, the DeepVision dual-gradient drilling system studied by Baker Hughes and Transocean companies in the United States, the Subsea Pumping System (SSPS) of Shell, the Hollow Glass Spheres (HGS) dual-gradient drilling system of Maurer company, the Riserless Mud Recovery System (1ZM) of AGR Subsea company, the riser gas lift and dilution dual-gradient drilling system studied by Louisiana State University, etc. Among them, the subsea mudlift drilling system was field-tested in the Gulf of Mexico at the end of 2001 and was successful. Currently, it has been put into industrial application and has good industrial application prospects.

[0005] It can be seen from this that the dual-gradient drilling technology is a deepwater drilling technology with broad application prospects. It can overcome the problems brought about by the special natural environment and reservoir conditions in deep water, and can achieve deepwater oil and gas exploration and development at a lower cost, with a shorter well construction period, safer operation, and higher production. This technology has developed various implementation methods and equipment systems and has been industrially applied in the field. To accelerate the pace of deepwater oil development in China, it is necessary to track and study foreign advanced DGD technologies in light of the actual situation of deepwater blocks in the Chinese ocean, draw on their successful experiences, develop deepwater drilling technologies suitable for China's national conditions and with independent intellectual property rights, and form a complete set of drilling technology systems to guide deepwater oil development in China, which is of great strategic significance for improving the level of deepwater drilling technology in China. Summary of the Invention

[0006] The object of the present invention is to provide a complete set of dual-gradient drilling systems and drilling methods for dual-gradient drilling systems to guide deepwater oil development in China in light of the actual situation of deepwater blocks in the Chinese ocean so as to accelerate the pace of deepwater oil development in China.

[0007] The dual-gradient drilling system of the present invention is implemented as follows: The first dual-gradient drilling system includes a drilling pump set 1, a top drive 2, an auxiliary pump device 3, a data monitoring and control system 4, a top drive adapter device 5, a riser 20, a conventional blowout preventer set 7, a casing 8, a dual-wall drill pipe 9, a transition sub 10, a conventional drill string 11, and a bit 12. Among them: The dual-wall drill pipe 9 consists of a concentric inner drill pipe 91 and an outer drill pipe 90. An annulus B17 is formed between the inner drill pipe 91 and the outer drill pipe 90, and a passage A18 is formed inside the inner drill pipe 91; The annulus B17 is used to deliver the first fluid to the bottom of the well, and the passage A18 is the return flow path for the first fluid. The upper part of the dual-wall drill pipe 9 is connected to the top drive adapter device 5, and the lower part is connected to the transition sub 10; The top drive adapter device 5 is a rotary joint installed between the top drive 2 and the dual-wall drill pipe 9. The top drive adapter device 5 is machined with a passage B50 and a passage C51. The passage B50 connects the top drive mud passage and the annulus B17, and the passage C51 connects the passage A18 and the wellhead discharge pipeline; The transition sub 10 is machined with a passage D100, a passage E101, and a check valve 102. The lower part of the transition sub 10 is connected to the conventional drill string 11 and the bit 12; The dual-wall drill pipe 9, the transition sub 10, the conventional drill string 11, and the bit 12 are connected in sequence from top to bottom to form a downhole drill string assembly; The drilling pump set 1 is connected to the bottom of the well through the top drive 2, the passage B50 of the top drive adapter device, the annulus B17 of the dual-wall drill pipe, the passage E101 of the transition sub, the conventional drill string 11, and the bit 12 in sequence; Then, from the bottom of the well, it returns to the wellhead discharge pipeline through the passage D100 of the transition sub, the passage A18 of the dual-wall drill pipe, and the passage C51 of the top drive adapter device; The riser 20 is connected above the conventional blowout preventer set 7, and the auxiliary pump device 3 is connected to the annulus outside the dual-wall drill pipe 9 through the riser 20; The data monitoring and control system 4 is connected to the auxiliary pump device 3 and monitors the flow rate of the auxiliary pump device 3.

[0008] The second dual-gradient drilling system includes a drilling pump unit 1, a top drive 2, an auxiliary pump device 3, a data monitoring and control system 4, a top drive adapter device 5, a rotating blowout preventer 6, a conventional blowout preventer group 7, a casing 8, a dual-wall drill pipe 9, a transition sub 10, a conventional drill string 11, and a drill bit 12, where: The dual-wall drill pipe 9 consists of a concentric inner drill pipe 91 and an outer drill pipe 90. An annulus B17 is formed between the inner drill pipe 91 and the outer drill pipe 90, and a passage A18 is formed inside the inner drill pipe 91; The annulus B17 is used to convey the first fluid to the bottom of the well, and the passage A18 is the return flow path for the first fluid. The upper part of the dual-wall drill pipe 9 is connected to the top drive adapter device 5, and the lower part is connected to the transition sub 10; The top drive adapter device 5 is a rotary joint installed between the top drive 2 and the dual-wall drill pipe 9. The top drive adapter device 5 is machined with a passage B50 and a passage C51. The passage B50 connects the top drive mud passage and the annulus B17, and the passage C51 connects the passage A18 and the wellhead discharge pipeline; The transition sub 10 is machined with a passage D100, a passage E101, and a check valve 102. The lower part of the transition sub 10 is connected to the conventional drill string 11 and the drill bit 12; The dual-wall drill pipe 9, the transition sub 10, the conventional drill string 11, and the drill bit 12 are connected in sequence from top to bottom to form a downhole drill string assembly; The drilling pump unit 1 is connected to the bottom of the well through the top drive 2, the passage B50 of the top drive adapter device, the annulus B17 of the dual-wall drill pipe, the passage E101 of the transition sub, the conventional drill string 11, and the drill bit 12 in sequence; Then, from the bottom of the well, it returns to the wellhead discharge pipeline through the passage D100 of the transition sub, the passage A18 of the dual-wall drill pipe, and the passage C51 of the top drive adapter device; The rotating blowout preventer 6 is connected above the conventional blowout preventer group 7, and the auxiliary pump device 3 is connected to the annulus outside the dual-wall drill pipe 9 through the rotating blowout preventer 6; The data monitoring and control system 4 is connected to the auxiliary pump device 3 and monitors the flow rate of the auxiliary pump device 3.

[0009] The further optimized solution of the above first or second dual-gradient drilling system further includes: Connecting a dedicated choke manifold 19 to the wellhead discharge pipeline of the top drive adapter device 5. The dedicated choke manifold 19 is equipped with a drilling choke valve, and the data monitoring and control system 4 is connected to and monitors the drilling choke valve.

[0010] The drilling method using the aforementioned first or second dual-gradient drilling system includes the following steps: The double-wall drill pipe 9, the transition sub 10, the conventional drill string 11, and the drill bit 12 form a downhole drill string assembly and extend into the wellbore. The upper part is connected to the top drive adapter device 5, and an annulus A14 is formed between the downhole drill string assembly and the wellbore. The drilling pump unit 1 injects the first fluid through the top drive 2, the passage B50 of the top drive adapter device, the annulus B17 of the double-wall drill pipe, the passage E101 of the transition sub, the conventional drill string 11, and the drill bit 12, and enters the annulus A14. The first fluid returning from the annulus A14 passes through the passage D100 of the transition sub, the passage A18 of the double-wall drill pipe, the passage C51 of the top drive adapter device, and then reaches the downstream mud treatment system through the discharge pipeline. The auxiliary pump device 3 injects the second fluid into the annulus A14 to adjust the pressure gradient in the annulus A14. The data monitoring and control system 4 is used to monitor the flow rate of the second fluid injected by the auxiliary pump device 3, so as to determine the height of the second fluid in the annulus A14.

[0011] The dual-gradient drilling method using the further optimized scheme of the first or second dual-gradient drilling system includes the following steps: The double-wall drill pipe 9, the transition sub 10, the conventional drill string 11, and the drill bit 12 form a downhole drill string assembly and extend into the wellbore. The upper part is connected to the top drive adapter device 5, and an annulus A14 is formed between the downhole drill string assembly and the wellbore. The drilling pump unit 1 injects the first fluid through the top drive 2, the passage B50 of the top drive adapter device, the annulus B17 of the double-wall drill pipe, the passage E101 of the transition sub, the conventional drill string 11, and the drill bit 12, and enters the annulus A14. The first fluid returning from the annulus A14 passes through the passage D100 of the transition sub, the passage A18 of the double-wall drill pipe, the passage C51 of the top drive adapter device, and then reaches the downstream mud treatment system through the discharge pipeline and the special choke manifold 19. The auxiliary pump device 3 injects the second fluid into the annulus A14 to adjust the pressure gradient in the annulus A14. The data monitoring and control system 4 is used to monitor the flow rate of the second fluid injected by the auxiliary pump device 3, so as to determine the height of the second fluid in the annulus A14. At the same time, the data monitoring and control system 4 realizes the backpressure adjustment of the first fluid returning from the double-wall drill pipe by controlling the drilling choke valve of the special choke manifold 19, and adjusts the bottomhole pressure by changing the height of the second fluid annular liquid column while adjusting the backpressure of the first fluid, so as to adjust the fluid pressure at different openhole section depths in the annulus A14.

[0012] The further optimized scheme of the aforementioned dual-gradient drilling method includes: The density of the second fluid is greater than that of the first fluid.

[0013] The bottomhole pressure is adjusted by changing the density of the first fluid and simultaneously changing the height of the second fluid annular liquid column, so as to adjust the fluid pressure at different openhole section depths in the annulus A14.

[0014] The second fluid within the annulus A14 forms an annular liquid column, with the lower end of the second-fluid annular liquid column located above the upper part of the transition sub 10. The second fluid does not participate in the entire circulation process of the first fluid; within the open-hole section of the annulus A14, the hydrostatic pressure of the second fluid at different depths lies between the formation pressure and the fracture pressure at the corresponding depths.

[0015] During the circulation process of the first fluid, the pressure formed in the annulus A14 below the transition sub 10 lies between the formation pressure and the fracture pressure at the corresponding depth.

[0016] The advantages of the present invention include: (1) strong adaptability, capable of being applied to onshore drilling and offshore drilling; (2) being able to effectively solve the complex drilling problems in deepwater drilling caused by the small gap between the formation pressure and the fracture pressure due to the influence of loose seabed sediments and seawater columns; (3) achieving deepwater oil and gas exploration and development with lower costs, shorter well construction times, and safer operations; (4) the drilling system and drilling method proposed by the present invention are applicable not only to deepwater drilling but also to drilling under complex drilling conditions such as narrow density windows in onshore drilling. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is the overall equipment and connection schematic diagram of Embodiment 1 of the first dual-gradient drilling system of the present invention.

[0018] Figure 2 is the overall equipment and connection schematic diagram of Embodiment 2 of the second dual-gradient drilling system of the present invention.

[0019] Figure 3 is the wellbore structure design diagram of conventional drilling (single-gradient drilling).

[0020] Figure 4 is the wellbore structure design diagram of a dual-gradient drilling method of the present invention.

[0021] In the figures: 1. Drilling pump unit, 2. Top drive, 3. Auxiliary pump device, 4. Data monitoring and control system, 5. Top drive adapter device, 6. Rotary blowout preventer, 7. Conventional blowout preventer group, 8. Casing, 9. Dual-wall drill pipe, 10. Transition sub, 11. Conventional drill string, 12. Bit, 13. Open hole, 14. Annulus A, 15. Drill string check valve, 16. Lower end of the second-fluid annular liquid column, 17. Annulus B, 18. Passage A, 19. Special choke manifold, 20. Riser, 50. Passage B, 51. Passage C, 90. Outer drill pipe, 91. Inner drill pipe, 100. Passage D, 101. Passage E, 102. Check valve. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The present invention will be further described below in conjunction with the accompanying drawings of the specification: During the offshore drilling process, in the case of using the conventional single-gradient drilling technology, the hydrostatic pressure curve of the drilling fluid (mud) and the wellbore structure are as shown in the attached Figure 3 figures. Due to the loose sediment on the deep-sea floor and the action of the seawater column, the gap between the formation pressure curve and the fracture pressure curve is narrow. The hydrostatic pressure curve of the conventional drilling fluid (i.e., the mud static pressure in the figure) is a straight line extending from the offshore drilling ship on the sea surface. This hydrostatic pressure passes through the drilling fluid density window over a very short vertical distance, making it difficult to maintain the annulus pressure in the open hole between these two curves and prone to well leakage accidents. In order to ensure the quality of the wellbore, it is necessary to run multiple casing strings (such as Figure 3 even designing 7 casing strings cannot fully ensure the smooth drilling of the first open hole section at the seabed).

[0023] Example 1, as shown in FIG. 2, the dual-gradient drilling system is applied to offshore drilling without a riser. A dual-gradient drilling system includes a drilling pump set 1, a top drive 2, an auxiliary pump device 3, a data monitoring and control system 4, a top drive adapter device 5, a rotating blowout preventer 6, a conventional blowout preventer group 7, a casing 8, a dual-wall drill pipe 9, a transition sub 10, a conventional drill string 11, and a drill bit 12. The dual-wall drill pipe 9 is composed of a concentric inner drill pipe 91 and an outer drill pipe 90. An annulus B17 is formed between the inner drill pipe 91 and the outer drill pipe 90, and a passage A18 is formed inside the inner drill pipe 91; the annulus B17 is used to deliver the first fluid to the bottom of the well, and the passage A18 is the return flow path for the first fluid; the top drive adapter device 5 is a rotary joint installed between the top drive 2 and the dual-wall drill pipe 9 for connecting the dual-wall drill pipe 9 to the top drive 2; the top drive adapter device 5 is machined with a passage B50 and a passage C51; the passage B50 connects the top drive mud passage and the annulus B17, and the passage C51 connects the passage A18 to a dedicated choke manifold 19; the transition sub 10 is machined with a passage D100, a passage E101, and a check valve 102 for the flow path conversion of the injected fluid and the opening and closing of the passage A18. A subsea rotating blowout preventer 6 is installed above the conventional blowout preventer group 7 on the seabed, and the auxiliary pump device 3 reaches the seabed through a pipeline and is connected to the annulus A14.

[0024] The dual-wall drill pipe 9 extends into the wellbore, is connected to the top drive adapter device 5 at the upper part, and is connected to the transition sub 10 at the lower part; the transition sub 10 is connected to the conventional drill string 11 and the drill bit 12 at the lower part; the dual-wall drill pipe 9, the transition sub 10, the conventional drill string 11, and the drill bit 12 form a downhole drill string assembly; an annulus A14 is formed between the downhole drill string assembly and the wellbore. The wellbore is composed of a casing 8 and an open hole 13. The upper liquid level of the second fluid can also be located in the pipeline connecting the auxiliary pump device 3 and the annulus A14 on the seabed. The conventional drill string 11 may include a conventional single-wall drill pipe, a single-wall drill collar, a drill string check valve 15, downhole measurement and control tools, etc.

[0025] The drilling pump unit 1 injects the first fluid through the top drive 2, the orifice B50 of the top drive adapter device, the annulus B17 of the double-wall drill pipe, the orifice E101 of the transition sub, the conventional drill string 11, and the drill bit 12, and enters the annulus A14; the first fluid returned from the annulus A14 passes through the orifice D100 of the transition sub, the orifice A18 of the double-wall drill pipe, the orifice C51 of the top drive adapter device, and then reaches the downstream mud treatment system through the discharge pipeline.

[0026] The auxiliary pump device 3 is used to inject the second fluid into the annulus A14 to adjust the pressure gradient in the annulus A14; the data monitoring and control system 4 is used to monitor the flow rate of the second fluid injected by the auxiliary pump device 3 to determine the height of the second fluid in the annulus A14.

[0027] Example 2, as Figure 1 shown, the dual-gradient drilling system is applied to riser offshore drilling. The dual-gradient drilling system includes a drilling pump unit 1, a top drive 2, an auxiliary pump device 3, a data monitoring and control system 4, a top drive adapter device 5, a riser 20, a conventional blowout preventer group 7, a casing 8, a double-wall drill pipe 9, a transition sub 10, a conventional drill string 11, and a drill bit 12. Among them, the auxiliary pump device 3 is connected to the upper part of the riser 20 through a pipeline, so as to be connected to the annulus A14. Therefore, the annulus A14 also includes the annulus formed by the downhole drill string assembly and the riser 20. The rest of the implementation content is the same as that of Example 1.

[0028] In Example 1 and Example 2, the second fluid in the annulus A14 forms an annular liquid column. The lower end of the second-fluid annular liquid column is located above the transition sub 10. The second fluid does not participate in the entire circulation process of the first fluid; in the open-hole section in the annulus A14, the static pressure of the second fluid at different depths is between the formation pressure and the fracture pressure at the corresponding depths; during the circulation process of the first fluid, the pressure formed in the annulus A14 below the transition sub 10 is between the formation pressure and the fracture pressure at the corresponding depths; by changing the density of the first fluid and simultaneously changing the height of the second-fluid annular liquid column to adjust the bottom-hole pressure, so as to adjust the fluid pressure at different open-hole section depths in the annulus A14; a special choke manifold 19 is installed between the discharge pipeline of the top drive adapter device 5 and the downstream mud treatment system. The special choke manifold is equipped with a drilling choke valve. The data monitoring and control system 4 realizes the back-pressure adjustment of the first fluid returning from the double-wall drill pipe by controlling the drilling choke valve and simultaneously changes the height of the second-fluid annular liquid column to adjust the bottom-hole pressure, so as to adjust the fluid pressure at different open-hole section depths in the annulus A14.

[0029] Example 3. A dual-gradient drilling method includes the following steps: A double-wall drill pipe 9 extends into a wellbore, with a top drive adapter device 5 connected to its upper part and a transition sub 10 connected to its lower part; A conventional drill string 11 and a drill bit 12 are connected to the lower part of the transition sub 10; The double-wall drill pipe 9, the transition sub 10, the conventional drill string 11, and the drill bit 12 form a downhole drill string assembly; An annulus A14 is formed between the downhole drill string assembly and the wellbore; A drilling pump unit 1 injects a first fluid through the top drive 2, the orifice B50 of the top drive adapter device, the annulus B17 of the double-wall drill pipe, the orifice E101 of the transition sub, the conventional drill string 11, and the drill bit 12, and enters the annulus A14; The first fluid returning from the annulus A14 passes through the orifice D100 of the transition sub, the orifice A18 of the double-wall drill pipe, and the orifice C51 of the top drive adapter device, and then reaches the downstream mud treatment system through the discharge pipeline; An auxiliary pump device 3 is used to inject a second fluid into the annulus A14 to adjust the pressure gradient in the annulus A14; A data monitoring and control system 4 is used to monitor the flow rate of the second fluid injected by the auxiliary pump device 3 to determine the height of the second fluid in the annulus A14.

[0030] The second fluid in the annulus A14 forms an annular liquid column, and the lower end of the second-fluid annular liquid column is located above the transition sub 10. The second fluid does not participate in the entire circulation process of the first fluid; In the open-hole section within the annulus A14, the hydrostatic pressure of the second fluid at different depths is between the formation pressure and the fracture pressure at the corresponding depths. During the circulation process of the first fluid, the pressure formed in the annulus A14 below the transition sub 10 is between the formation pressure and the fracture pressure at the corresponding depths. The bottom hole pressure is adjusted by changing the density of the first fluid and simultaneously changing the height of the second-fluid annular liquid column, thereby adjusting the fluid pressure at different open-hole section depths in the annulus A14. A special choke manifold 19 is installed between the discharge pipeline of the top drive adapter device 5 and the downstream mud treatment system. The special choke manifold is equipped with a drilling choke valve. The data monitoring and control system 4 adjusts the back pressure of the first fluid returning from the double-wall drill pipe by controlling the drilling choke valve and simultaneously changes the height of the second-fluid annular liquid column to adjust the bottom hole pressure, thereby adjusting the fluid pressure at different open-hole section depths in the annulus A14. The density of the second fluid is greater than that of the first fluid.

[0031] After adopting a dual-gradient drilling system and its drilling method proposed by the present invention, the hydrostatic pressure curve of the drilling fluid (the first fluid and the second fluid) and the wellbore structure are as shown in the appendix Figure 4As shown. The second fluid is within the annulus A14, and the hydrostatic pressure curve formed by the second fluid is a straight line. The hydrostatic pressure curve of the second fluid intersects with the upper window line segment and the lower window formation line segment (the upper window line segment is the connection line of the formation pressure and the fracture pressure at the uppermost end of the open hole section, and the lower window line segment is the connection line of the formation pressure and the fracture pressure at the lowermost end of the open hole section), and this straight line is also located between the formation pressure and the fracture pressure at different depths of the open hole wellbore. There are two calculation methods for the bottom hole pressure: one is that the bottom hole pressure is equal to the sum of the circulation pressure loss of the first fluid in the channel A18, the annular circulation pressure loss of the first fluid in the lower section of the annulus A14 (the annulus A below the crossover sub 10), and the hydrostatic pressure of the first fluid in the entire wellbore; the other is that the bottom hole pressure is equal to the sum of the hydrostatic pressure of the second fluid, the circulation pressure loss of the first fluid in the lower section of the annulus A14 (the annulus A below the crossover sub 10), and the hydrostatic pressure of the first fluid in the lower section of the annulus A14. The bottom hole pressures calculated by the two methods are equal and are located between the formation pressure and the fracture pressure at the bottom of the drilled open hole section. During the drilling process, continuously adjust the density of the first fluid and the height of the annular liquid column of the second fluid in the annulus A14 to ensure that the pressure in the open hole section of the annulus A is between the formation pressure and the fracture pressure at the corresponding depth. When a special choke manifold 19 is installed between the discharge pipeline of the top drive adapter device 5 and the downstream mud treatment system, the first calculation method of the bottom hole pressure can also be expressed as the sum of the circulation pressure loss of the first fluid in the channel A18, the annular circulation pressure loss of the first fluid in the lower section of the annulus A14 (the annulus A below the crossover sub 10), the back pressure of the first fluid, and the hydrostatic pressure of the first fluid in the entire wellbore; therefore, the back pressure of the first fluid returning from the double-wall drill pipe can be adjusted by controlling the drilling choke valve through the data monitoring and control system 4 while changing the height of the annular liquid column of the second fluid to adjust the bottom hole pressure, thereby adjusting the fluid pressure at different depths of the open hole section in the annulus A14.

[0032] The dual-gradient drilling system and its drilling method of the present invention can reduce the annulus pressure in the open hole section to a straight line starting from below the sea surface, and the slope of the straight line (relative to the sea surface or the seabed) is greatly reduced, so the gap between the pore pressure and the fracture pressure becomes relatively wider, and there is a relatively large vertical distance to ensure safe drilling, reduce the number of casing layers used, and make drilling under deepwater complex conditions possible.

Claims

1. A dual-gradient drilling method, comprising a dual-gradient drilling system, which mainly consists of a drilling pump set (1), a top drive (2), an auxiliary pump device (3), a data monitoring and control system (4), a top drive adapter device (5), a riser (20), a conventional blowout preventer set (7), a casing (8), a double-wall drill pipe (9), a transition sub (10), a conventional drill string (11) and a drill bit (12); the double-wall drill pipe (9) is composed of a concentric inner drill pipe (91) and an outer drill pipe (90), an annulus B (17) is formed between the inner drill pipe (91) and the outer drill pipe (90), and a passage A (18) is formed inside the inner drill pipe (91); the upper part of the double-wall drill pipe (9) is connected to the top drive adapter device (5), and the lower part is connected to the transition sub (10); the top drive adapter device (5) is a rotary joint installed between the top drive (2) and the double-wall drill pipe (9), and the top drive adapter device (5) is machined with a passage B (50) and a passage C (51), the passage B (50) communicates with the top drive mud passage and the annulus B (17), and the passage C (51) communicates with the passage A (18) and the wellhead discharge pipeline; the transition sub (10) is machined with a passage D (100), a passage E (101) and a check valve (102), and the lower part of the transition sub (10) is connected to the conventional drill string (11) and the drill bit (12); the riser (20) is connected above the conventional blowout preventer set (7), and the auxiliary pump device (3) communicates with the annulus outside the double-wall drill pipe (9) through the riser (20); the data monitoring and control system (4) is connected to the auxiliary pump device (3), including the following steps: The double-wall drill pipe (9), the transition sub (10), the conventional drill string (11) and the drill bit (12) form a downhole drill string assembly and extend into the wellbore, and the upper part is connected to the top drive adapter device (5), and an annulus A (14) is formed between the downhole drill string assembly and the wellbore; the drilling pump set (1) injects the first fluid through the top drive (2), the passage B (50) of the top drive adapter device, the annulus B (17) of the double-wall drill pipe, the passage E (101) of the transition sub, the conventional drill string (11) and the drill bit (12), and enters the annulus A (14); the first fluid returned from the annulus A (14) passes through the passage D (100) of the transition sub, the passage A (18) of the double-wall drill pipe, the passage C (51) of the top drive adapter device, and then reaches the downstream mud treatment system through the discharge pipeline; the auxiliary pump device (3) injects the second fluid into the annulus A (14) to adjust the pressure gradient in the annulus A (14); the data monitoring and control system (4) is used to monitor the flow rate of the second fluid injected by the auxiliary pump device (3) to determine the height of the second fluid in the annulus A (14); The second fluid in the annulus A (14) forms an annular liquid column, and the lower end of the second fluid annular liquid column is located above the transition sub (10), and the second fluid does not participate in the entire circulation process of the first fluid; In the open hole section in the annulus A (14), the static pressure of the second fluid at different depths is between the formation pressure and the fracture pressure at the corresponding depths.

2. A dual-gradient drilling method according to claim 1, characterized in that: Connect a dedicated choke manifold (19) to the wellhead discharge pipeline of the top drive adapter device (5).

3. A dual-gradient drilling method according to claim 2, characterized in that: The dedicated choke manifold (19) is equipped with a drilling choke valve, and the data monitoring and control system (4) is connected to and monitors the drilling choke valve.

4. A dual-gradient drilling method according to claim 3, wherein the first fluid returned from the annulus A (14) passes through the conversion stub hole D (100), the double-wall drill pipe hole A (18), and the top drive adapter device hole C (51), and then reaches the downstream mud treatment system through the discharge pipeline and the dedicated choke manifold (19); the data monitoring and control system (4) adjusts the back pressure of the first fluid returning from the double-wall drill pipe by controlling the drilling choke valve of the dedicated choke manifold (19), and adjusts the bottom hole pressure by changing the height of the second fluid annular liquid column while adjusting the back pressure of the first fluid, so as to adjust the fluid pressure at different open hole section depths in the annulus A (14).

5. A dual-gradient drilling method, including a dual-gradient drilling system, which mainly consists of a drilling pump unit (1), a top drive (2), an auxiliary pump device (3), a data monitoring and control system (4), a top drive adapter device (5), a rotating blowout preventer (6), a conventional blowout preventer group (7), a casing (8), a double-wall drill pipe (9), a conversion stub (10), a conventional drill string (11), and a drill bit (12); the double-wall drill pipe (9) consists of a concentric inner drill pipe (91) and an outer drill pipe (90), and an annulus B (17) is formed between the inner drill pipe (91) and the outer drill pipe (90), and a hole A (18) is formed inside the inner drill pipe (91); the upper part of the double-wall drill pipe (9) is connected to the top drive adapter device (5), and the lower part is connected to the conversion stub (10); the top drive adapter device (5) is a rotary joint installed between the top drive (2) and the double-wall drill pipe (9), and the top drive adapter device (5) is machined with a hole B (50) and a hole C (51), the hole B (50) communicates with the top drive mud channel and the annulus B (17), and the hole C (51) is connected to the hole A (18) and the wellhead discharge pipeline; the conversion stub (10) is machined with a hole D (100), a hole E (101), and a check valve (102), and the lower part of the conversion stub (10) is connected to the conventional drill string (11) and the drill bit (12); the rotating blowout preventer (6) is connected above the conventional blowout preventer group (7), and the auxiliary pump device (3) communicates with the annulus outside the double-wall drill pipe (9) through the rotating blowout preventer (6); the data monitoring and control system (4) is connected to the auxiliary pump device (3), comprising the following steps: The double-wall drill pipe (9), the transition sub (10), the conventional drill string (11) and the drill bit (12) form a downhole drill string assembly and extend into the wellbore, and are connected to the top drive adapter device (5) at the upper part. An annular gap A (14) is formed between the downhole drill string assembly and the wellbore; the drilling pump unit (1) injects the first fluid through the top drive (2), the passage B (50) of the top drive adapter device, the annular gap B (17) of the double-wall drill pipe, the passage E (101) of the transition sub, the conventional drill string (11) and the drill bit (12), and enters the annular gap A (14); the first fluid returning from the annular gap A (14) passes through the passage D (100) of the transition sub, the passage A (18) of the double-wall drill pipe, the passage C (51) of the top drive adapter device, and then reaches the downstream mud treatment system through the discharge pipeline; the auxiliary pump device (3) injects the second fluid into the annular gap A (14) to adjust the pressure gradient in the annular gap A (14); the data monitoring and control system (4) is used to monitor the flow rate of the second fluid injected by the auxiliary pump device (3) to determine the height of the second fluid in the annular gap A (14). The second fluid in the annular gap A (14) forms an annular liquid column, and the lower end of the second fluid annular liquid column is located above the transition sub (10). The second fluid does not participate in the entire circulation process of the first fluid. In the open hole section in the annular gap A (14), the static pressure of the second fluid at different depths is between the formation pressure and the fracture pressure at the corresponding depths.

6. A dual-gradient drilling method according to claim 5, wherein:[[]]END]] A special choke manifold (19) is connected to the wellhead discharge pipeline of the top drive adapter device (5).

7. A dual-gradient drilling method according to claim 6, wherein:[[]]END]] The special choke manifold (19) is equipped with a drilling choke valve, and the data monitoring and control system (4) is connected to and monitors the drilling choke valve.

8. A dual-gradient drilling method according to claim 7, the first fluid returning from the annular gap A (14) passes through the passage D (100) of the transition sub, the passage A (18) of the double-wall drill pipe, the passage C (51) of the top drive adapter device, and then reaches the downstream mud treatment system through the discharge pipeline and the special choke manifold (19); the data monitoring and control system (4) adjusts the back pressure of the first fluid returning from the double-wall drill pipe by controlling the drilling choke valve of the special choke manifold (19), and adjusts the bottom hole pressure by changing the height of the second fluid annular liquid column while adjusting the back pressure of the first fluid, so as to adjust the fluid pressure at different open hole section depths in the annular gap A (14).

9. A dual-gradient drilling method according to claim 8, wherein:[[]]END]] The density of the second fluid is greater than the density of the first fluid.

10. A dual-gradient drilling method according to claim 9, wherein:[[]]END]] The bottom hole pressure is adjusted by changing the density of the first fluid and simultaneously changing the height of the second fluid annular liquid column, so as to adjust the fluid pressure at different open hole section depths in the annular gap A (14).

11. A dual-gradient drilling method according to claim 10, wherein:[[]]END]] During the first fluid circulation process described above, the pressure formed in the annulus A (14) below the switching jumper (10) lies between the formation pressure and the fracture pressure at the corresponding depth.

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