A drilling system with a controllable mud level in a surface section

Abstract

This invention relates to the field of offshore drilling technology and discloses a drilling system for controllable mud level in the surface well section, comprising: a guide pipe, a drilling fluid tank, a return pump, and a control system; the guide pipe is disposed in the wellbore; the guide pipe has a gas chamber and a drilling fluid chamber, the drilling fluid chamber being connected to the wellbore, and the top of the guide pipe can seal the gas chamber; the guide pipe has a return outlet connected to the drilling fluid chamber; a return pipe is connected between the drilling fluid tank and the return outlet; a supply pipe is connected between the drilling fluid tank and the gas chamber, and the supply pipe is equipped with an injection pump; the return pump is disposed on the return pipe, and the return pipe is connected to a return pipe; the control system is electrically connected to the return pump and the injection pump respectively; when shallow gas overflows into the wellbore, the control system receives a signal indicating an increase in the rotational speed of the return pump and controls the return pump to shut down and the injection pump to start, so as to replenish drilling fluid into the drilling fluid chamber, and the drilling fluid level rises after replenishment to reach a new equilibrium. This drilling system can effectively cope with sudden changes in bottom hole pressure and improve the safety of drilling operations.

Description

Technical Field

[0001] This invention relates to the field of offshore drilling technology, and in particular to a drilling system with controllable mud level in the surface well section. Background Technology

[0002] In offshore surface drilling operations, low-density drilling fluids are typically used. However, when drilling passes through seabed sedimentary layers containing shallow gas or groundwater deposits, the shallow gas or gas-containing water can intrude into the wellbore. The gas expands within the fluid column in the wellbore, which can easily lead to drilling platform safety accidents.

[0003] For example, Chinese invention patent CN104763333B discloses a drilling system and method based on subsea pump pressure control. The drilling system includes a drilling vessel equipped with a drilling fluid supply device and a control device. The drilling fluid supply device is connected to the inlet of a faucet via a manifold, and the outlet of the faucet is rotatably connected to the top of a drill pipe. A suction module is fitted onto the lower part of the drill pipe, and two pipes are installed on the outer wall of the suction module. A throttle valve is installed on one of the pipes. The control device is electrically connected to an electrical device, which is electrically connected to a motor via a control and power line. The output of the motor is connected to a subsea pump, and a hose with a pressure and flow sensor is connected to the inlet of the subsea pump. When the pressure at the subsea drilling wellhead increases due to the influence of free gas or other gaseous media, the control device adjusts the opening of the throttle valve and the speed of the subsea pump via the control and power line to stabilize the pressure at the subsea drilling wellhead.

[0004] However, during surface drilling, if high-pressure shallow gas overflows rapidly into the wellhead, it is difficult to effectively cope with the sudden change in bottom hole pressure by simply controlling the opening of the throttle valve and the speed of the subsea pump. This makes it impossible to ensure a relative balance between drilling fluid pressure and formation pore pressure, resulting in poor operational safety. Summary of the Invention

[0005] The technical problem this invention aims to solve is that when high-pressure shallow gas rapidly overflows into the wellhead during surface drilling, it is difficult to effectively cope with the sudden change in bottom hole pressure by simply controlling the opening of the throttle valve and the speed of the subsea pump. This makes it impossible to ensure a relative balance between drilling fluid pressure and formation pore pressure, resulting in poor operational safety.

[0006] To address the aforementioned technical problems, this invention provides a technical solution for a drilling system with controllable mud level in the surface well section: Drilling systems with controllable mud levels in the surface section include: A guide pipe is installed in a wellbore on the seabed; the guide pipe has a gas chamber and a drilling fluid chamber that are connected vertically, the drilling fluid chamber is connected to the wellbore, and the top of the guide pipe can seal the gas chamber; the guide pipe also has a return outlet connected to the drilling fluid chamber; A drilling fluid tank is located outside the conduit, and a return pipe is connected between the drilling fluid tank and the return outlet; a supply pipe is connected between the drilling fluid tank and the gas chamber, and the supply pipe is equipped with an injection pump; A return pump is provided on the return pipe, and a return pipe is connected to the side of the return pipe away from the return outlet, and the return pipe is in communication with the gas chamber; The control system is electrically connected to the return pump and the injection pump, respectively. The control system is configured to receive a signal indicating an increase in the rotational speed of the return pump when shallow gas overflows into the wellbore, and to control the return pump to shut down and the injection pump to start, so as to inject drilling fluid into the drilling fluid chamber.

[0007] Furthermore, the conduit is also provided with an injection port, which is spaced apart above the return outlet; the drilling system for controllable mud level in the surface well section also includes an inert gas device, and an injection pipe is connected between the inert gas device and the injection port.

[0008] Furthermore, the gas injection pipe is equipped with a gas injection control valve, and the control system is electrically connected to the gas injection control valve to regulate the amount of inert gas injected into the gas chamber.

[0009] Furthermore, a return control valve is provided on the side of the return pipe near the return outlet. The control system is electrically connected to the return control valve. The control system is used to control the return control valve to close when it receives a signal that the speed of the return pump increases.

[0010] Furthermore, a flow meter is also provided on the side of the return pipe away from the return outlet. The control system is electrically connected to the flow meter and is used to receive the drilling fluid return flow signal detected by the flow meter.

[0011] Furthermore, a reflux control valve is provided on the side of the reflux pipe near the conduit, and / or a supply control valve is provided on the side of the supply pipe near the conduit, and the control system is electrically connected to the reflux control valve and the supply control valve.

[0012] Furthermore, the conduit is also equipped with a liquid level sensor, which is used to detect the drilling fluid level inside the conduit, and the control system is electrically connected to the liquid level sensor.

[0013] Furthermore, the liquid level sensor includes a first probe and a second probe. The first probe is inserted into the drilling fluid chamber to detect the pressure of the drilling fluid, and the second probe is located on the outside of the conduit to detect the pressure of the seawater. The liquid level sensor is used to calculate the drilling fluid level in the conduit based on the pressure difference between the drilling fluid and the seawater.

[0014] Furthermore, the drilling system for controllable mud level in the surface well section also includes a drilling string and a fluid supply pump. The drilling string is located on the guide tube and passes through the gas chamber and the drilling fluid chamber. The top of the drilling string protrudes from the guide tube. A fluid supply pipe is connected between the drilling string and the drilling fluid tank. The fluid supply pump is located on the fluid supply pipe. The control system is electrically connected to the fluid supply pump.

[0015] Furthermore, the drilling system with controllable mud level in the surface well section also includes a drilling platform and a drilling rig. The drilling platform is located on the seabed, and the drilling rig is located on the drilling platform and is vertically opposite to the guide pipe. The drilling rig is also equipped with lifting equipment, which is connected to the drilling string.

[0016] Furthermore, the drilling system with controllable mud level in the surface well section also includes a drilling fluid purifier, which is connected to the return pipe and the drilling fluid tank. The return pipe is located on the side of the drilling fluid purifier away from the drilling fluid tank.

[0017] Furthermore, the conduit has a segmented structure, comprising a first pipe section, a connector sleeve, and a second pipe section connected sequentially from bottom to top. The first pipe section is inserted into the wellbore of the seabed, the connector sleeve is located on the upper side of the seabed, and the return outlet, the gas injection port, and the liquid level sensor are all located on the connector sleeve.

[0018] Furthermore, the top of the conduit is provided with a rotating control head and a flow divider. The flow divider is connected to the gas chamber. The rotating control head is used to close the top of the gas chamber, and the flow divider is used to guide the gas in the gas chamber outward. The control system is electrically connected to the rotating control head and the flow divider.

[0019] Furthermore, the top of the duct is also connected to an exhaust pipe, which is in communication with the gas chamber, and the exhaust pipe is equipped with an outboard exhaust valve.

[0020] Compared with the prior art, the drilling system for controllable mud level in the surface section of the present invention has the following advantages: The drilling system for controllable mud level in the surface section adopts a structural design of a guide tube, a drilling fluid tank, a return pump, and a control system. The guide tube is located in the wellbore on the seabed; the guide tube has a gas chamber and a drilling fluid chamber connected vertically, with the drilling fluid chamber connected to the wellbore; the guide tube also has a return outlet connected to the drilling fluid chamber; the drilling fluid tank is located outside the guide tube, and a return pipe connects the drilling fluid tank and the return outlet; a supply pipe connects the drilling fluid tank and the gas chamber, and the supply pipe is equipped with an injection pump; the return pump is located on the return pipe, and a return pipe is connected to the side of the return pipe away from the return outlet, and the return pipe is connected to the gas chamber; the control system is electrically connected to the return pump and the injection pump respectively.

[0021] The guide pipe provides guidance for the drill bit in the surface section of the well, while ensuring the isolation of seawater and the normal circulation of drilling fluid. The drilling fluid chamber of the guide pipe is connected to the wellbore, allowing drill cuttings to be carried out of the well during drilling. A return pipe connects the drilling fluid tank and the return outlet of the guide pipe, and a return pipe is connected to the side of the return pipe away from the return outlet. During normal operation, the drilling fluid flows sequentially from the return outlet through the return pump, the return pipe, and the return pipe, finally entering the gas chamber of the guide pipe, thus achieving the purpose of internal and external circulation of drilling fluid.

[0022] When drilling reaches a shallow gas-bearing layer, if high-pressure shallow gas overflows into the wellbore, it will increase the pressure in the drilling fluid chamber. At this time, the control system receives a signal indicating an increase in the rotational speed of the return pump and controls it to shut down and start the injection pump, injecting drilling fluid into the drilling fluid chamber through the replenishment pipe. After replenishment, the increased drilling fluid level in the conduit generates greater pressure, allowing the drilling fluid pressure and formation pore pressure to reach a new equilibrium, preventing continuous shallow gas overflow and accumulation in the conduit, which could lead to safety risks.

[0023] More importantly, the gas chamber at the top of the guide pipe provides space for the drilling fluid chamber to rise, allowing for the smooth injection of drilling fluid into the guide pipe to generate greater pressure. This ensures that the bottom hole pressure and formation pore pressure are in balance. Furthermore, the top of the guide pipe can seal the gas chamber, preventing drilling fluid from being directly ejected from the guide pipe when high-pressure shallow gas overflows into the wellbore. This avoids potential leakage of drilling fluid into the surrounding seawater, ensuring the stability of the guide pipe and drilling platform. Additionally, the sealed gas chamber at the top generates auxiliary pressure, which works in conjunction with the drilling fluid level pressure to effectively cope with sudden changes in bottom hole pressure, thereby improving the safety of drilling operations. Attached Figure Description

[0024] Figure 1 This is an overall structural diagram of the drilling system with controllable mud level in the surface well section according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the drilling system with controllable mud level in the surface section (when the drilling fluid is at a low level) according to an embodiment of the present invention. Figure 3 This is a schematic diagram of the drilling system with controllable mud level in the surface section (when entering a shallow gas deposition layer) according to an embodiment of the present invention. Figure 4 This is a schematic diagram of the drilling system with controllable mud level in the surface section (when shallow gas overflows into the wellbore) according to an embodiment of the present invention. Figure 5 This is a schematic diagram of the drilling system for controllable mud level in the surface section of the present invention (when the drilling fluid is replenished to a new height); In the diagram: 1. Drilling platform; 2. Seawater; 3. Seabed; 4. Seafloor sediments; 5. Shallow gas deposits; 6. Lifting equipment; 7. Drilling derrick; 8. Drill bit; 9. Wellbore; 10. Conductor; 11. First pipe section; 12. Connecting casing; 13. Second pipe section; 14. Rotary control head; 15. Diverter; 16. Drilling string; 17. Fluid supply pump; 18. Injection pump; 19. Drilling fluid tank; 20. Drilling fluid purifier; 21. Inert gas device; 22. Injection... 23. Gas pipe; 24. Return pipe; 25. Return pump; 26. Liquid level sensor; 27. Outboard exhaust valve; 28. Gas injection port; 29. ​​Return outlet; 30. Return control valve; 31. Gas injection control valve; 32. Return control valve; 33. Flow meter; 34. Switching control valve; 35. Replenishment control valve; 36. Liquid supply pipe; 47. Replenishment pipe; 48. Return pipe; 49. Gas chamber; 40. Drilling fluid chamber; 41. Shallow gas; 42. Drilling fluid level; 43. Shallow gas top surface. Detailed Implementation

[0025] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0026] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise" used to indicate orientations or positional relationships are based on the orientations or positional relationships shown in the accompanying drawings and are only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0027] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0028] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0029] like Figure 1 , Figure 2 As shown, an embodiment of the present invention provides a drilling system for controllable mud level in the surface section, comprising: a guide pipe 10, a drilling fluid tank 19, a return pump 24, and a control system; the guide pipe 10 is disposed in a wellbore 9 in a seabed 3; the guide pipe 10 is provided with a gas chamber 42 and a drilling fluid chamber 43 connected vertically, the drilling fluid chamber 43 being connected to the wellbore 9, and the top of the guide pipe 10 being able to seal the gas chamber 42; the guide pipe 10 is also provided with a return outlet 28 connected to the drilling fluid chamber 43; the drilling fluid tank 19 is disposed outside the guide pipe 10, and a return pipe 23 is connected between the drilling fluid tank 19 and the return outlet 28; a supply pipe 38 is connected between the drilling fluid tank 19 and the gas chamber 42, and the supply pipe 38 is provided with an injection pump 18.

[0030] The return pump 24 is located on the return pipe 23. The return pipe 23 is connected to the return pipe 41 on the side away from the return outlet 28, and the return pipe 41 is connected to the gas chamber 42. The control system is electrically connected to the return pump 24 and the injection pump 18 (not shown in the figure). The control system is configured to receive a signal that the rotation speed of the return pump 24 increases when the shallow gas 44 overflows into the wellbore 9, and control the return pump 24 to close and the injection pump 18 to open, so as to inject drilling fluid into the drilling fluid chamber 43.

[0031] The drilling system for controllable mud level in the surface section employs a structural design consisting of a guide pipe 10, a drilling fluid tank 19, a return pump 24, and a control system. The guide pipe 10 provides guidance for the drill bit 8 in the surface section while ensuring the isolation of seawater 2 and the normal circulation of drilling fluid. The drilling fluid chamber 43 of the guide pipe 10 is connected to the wellbore 9, allowing drill cuttings to be carried out of the well during drilling. A return pipe 23 connects the drilling fluid tank 19 and the return outlet 28 of the guide pipe 10, with a return pipe 41 connected to the side of the return pipe 23 furthest from the return outlet 28. During normal operation, the drilling fluid flows sequentially from the return outlet 28 through the return pump 24, the return pipe 23, and the return pipe 41, finally entering the gas chamber 42 of the guide pipe 10, thus achieving the purpose of internal and external circulation of drilling fluid.

[0032] like Figure 3 , Figure 4As shown, when drilling reaches the shallow gas deposition layer 5, if the high-pressure shallow gas 44 overflows into the wellbore 9, it will cause an increase in the pressure of the drilling fluid chamber 43. At this time, the control system receives a signal that the rotation speed of the return pump 24 has increased, and controls the return pump 24 to shut down and the injection pump 18 to start, injecting drilling fluid into the drilling fluid chamber 43 through the supply pipe 38. After injection, the drilling fluid level in the conduit 10 rises, which can generate greater pressure, so that the drilling fluid pressure and the formation pore pressure reach a new equilibrium state, avoiding the continuous overflow of shallow gas 44 and its accumulation in the conduit 10, which could lead to safety risks.

[0033] More importantly, the gas chamber 42 at the top of the guide tube 10 provides space for the drilling fluid chamber 43 to raise the fluid level, allowing drilling fluid to be smoothly injected into the guide tube 10 to generate greater pressure, ensuring that the bottom hole pressure and formation pore pressure are in balance. Furthermore, the top of the guide tube 10 can seal the gas chamber 42, preventing drilling fluid from being directly ejected from the guide tube 10 when high-pressure shallow gas 44 overflows into the wellbore 9, thus avoiding potential leakage of drilling fluid into the external seawater 2 and ensuring the stability of the guide tube 10 and the drilling platform 1. In addition, the sealed gas chamber 42 at the top generates auxiliary pressure, which works in conjunction with the drilling fluid level pressure to effectively cope with sudden changes in bottom hole pressure, thereby improving the safety of drilling operations.

[0034] In this embodiment, as Figure 2 As shown, the conduit 10 is also equipped with an injection port 27, which is spaced apart above the return outlet 28. The drilling system for controllable mud level in the surface section also includes an inert gas device 21, and an injection pipe 22 is connected between the inert gas device 21 and the injection port 27. Furthermore, the injection pipe 22 is equipped with an injection control valve 30, and the control system is electrically connected to the injection control valve 30 to regulate the amount of inert gas injected into the gas chamber 42.

[0035] Inert gas is injected into the gas chamber 42 of the conduit 10 through the inert gas device 21, the injection pipe 22, and the injection port 27, eliminating the risk of accidental combustion and explosion in the top space of the conduit 10. It should be noted that inert gas does not refer to rare gases in professional chemistry, but rather to gases that are less likely to combust or explode than natural gas, such as air, nitrogen, carbon dioxide, and other non-flammable gases.

[0036] As a further preferred embodiment, a return control valve 29 is provided on the side of the return pipe 23 near the return outlet 28. The control system is electrically connected to the return control valve 29. The control system is used to control the return control valve 29 to close when it receives a signal that the rotational speed of the return pump 24 increases. When the rotational speed of the return pump 24 increases, the return control valve 29 can quickly close the return pipe 23, with a fast response speed and more reliable on / off control, thereby effectively dealing with the situation where shallow gas 44 overflows into the wellbore 9.

[0037] Furthermore, a flow meter 32 is installed on the side of the return pipe 23 away from the return outlet 28. The control system is electrically connected to the flow meter 32 and is used to receive the drilling fluid return flow signal detected by the flow meter 32. The flow meter 32 can comprehensively and accurately monitor the circulation status of the drilling fluid. In addition, a return control valve 31 is installed on the side of the return pipe 41 near the conduit 10, and a supply control valve 34 is installed on the side of the supply pipe 38 near the conduit 10. The control system is electrically connected to the return control valve 31, the supply control valve 34, and the switching control valve 33, respectively, which can flexibly control the return and circulation path of the drilling fluid and meet the requirements of normal drilling fluid circulation and replenishment pressurization.

[0038] In some embodiments of this application, the conduit 10 is further provided with a level sensor 25, which is used to detect the drilling fluid level inside the conduit 10. The control system is electrically connected to the level sensor 25. Specifically, the level sensor 25 includes a first probe and a second probe. The first probe is inserted into the drilling fluid chamber 43 to detect the pressure of the drilling fluid, and the second probe is located on the outside of the conduit 10 to detect the pressure of the seawater 2. The level sensor 25 is used to calculate the drilling fluid level inside the conduit 10 based on the pressure difference between the drilling fluid and the seawater 2.

[0039] The pressure difference between the drilling fluid inside the conduit 10 and the external seawater 2 is collected by the level sensor 25. The level sensor 25 calculates the drilling fluid level inside the conduit 10 based on the pressure difference. After the drilling fluid level signal is transmitted to the control system, the real-time data of the drilling fluid level, drilling fluid supply and return flow are integrated and calculated. Based on this, the return pump 24 is adjusted to the required speed to ensure that the drilling fluid level 45 is stably maintained at the height set by the driller on the drilling platform 1 at any time.

[0040] It should be noted that the drilling fluid pressure acting at the bottom of the well consists of two parts: hydrostatic pressure and circulating pressure. When calculating the reasonable height of the drilling fluid level 45, the control system needs to simultaneously estimate this circulating pressure. Furthermore, when the drilling fluid supply remains stable, the rotational speed of the return pump 24 decreases, causing surface drilling fluid to leak into the seabed sediment 4 or shallow gas deposits 5. In this case, the rotational speed of the return pump 24 is controlled to reduce the return flow rate, thereby maintaining the stability of the drilling fluid level 45.

[0041] In this embodiment, the drilling system for controllable mud level in the surface well section also includes a drilling string 16 and a fluid supply pump 17. The drilling string 16 is located on the guide tube 10 and passes through the gas chamber 42 and the drilling fluid chamber 43. The top of the drilling string 16 protrudes from the guide tube 10. A fluid supply pipe 37 connects the drilling string 16 and the drilling fluid tank 19. The fluid supply pump 17 is located on the fluid supply pipe 37, and the control system is electrically connected to the fluid supply pump 17. Furthermore, the drilling system for controllable mud level in the surface well section also includes a drilling platform 1 and a drilling rig 7. The drilling platform 1 is located on the seabed, and the drilling rig 7 is located on the drilling platform 1 and is vertically opposite the guide tube 10. The drilling rig 7 is also equipped with a lifting device 6, which is connected to the drilling string 16. According to the drilling operation requirements, the drilling string 16 and its drill bit 8 can be smoothly raised or lowered using the lifting device 6.

[0042] It should be noted that the drilling system with controllable mud level in the surface section also includes a drilling fluid purifier 20. The drilling fluid purifier 20 is connected to the return pipe 23 and the drilling fluid tank 19, and the return pipe 41 is located on the side of the drilling fluid purifier 20 away from the drilling fluid tank 19. The drilling fluid purifier 20 is a shale vibrating screen that filters out drill cuttings from the returned drilling fluid to ensure the cleanliness of the drilling fluid entering the drilling fluid tank 19. The purified drilling fluid is pumped into the drilling string 16 through the supply pipe 37 and the supply pump 17, and finally delivered to the drill bit 8 for circulation.

[0043] As a further preferred embodiment, the guide tube 10 has a segmented structure, comprising a first tube section 11, a connector sleeve 12, and a second tube section 13 connected sequentially from bottom to top. The first tube section 11 is inserted into the wellbore 9 on the seabed, and the connector sleeve 12 is located on the upper side of the seabed. The return outlet 28, the gas injection port 27, and the liquid level sensor 25 are all located on the connector sleeve 12. The guide tube 10 adopts a segmented design, with the return outlet 28, the gas injection port 27, and the liquid level sensor 25 all integrated into the connector sleeve 12, enabling docking operations with existing guide tubes and making assembly more convenient.

[0044] Additionally, the top of the conduit 10 is equipped with a rotary control head 14 and a flow divider 15. The flow divider 15 is connected to the gas chamber 42. The rotary control head 14 is used to seal the top of the gas chamber 42, and the flow divider 15 is used to guide the gas in the gas chamber 42 outward. The control system is electrically connected to the rotary control head 14 and the flow divider 15. The top of the conduit 10 is also connected to an exhaust pipe, which is connected to the gas chamber 42. The exhaust pipe is equipped with an overboard exhaust valve 26.

[0045] When shallow gas 44 is detected to overflow into the wellbore 9, the control system quickly shuts off the rotary control head 14 or the diverter 15, and simultaneously closes the return control valve 29. Then, drilling fluid is injected into the guide pipe 10 through the injection pump 18 to raise the drilling fluid level 45 to a new height, thereby establishing and stabilizing sufficient bottom hole pressure increment to prevent shallow gas 44 from further invading the wellbore 9.

[0046] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A drilling system with controllable mud level in the surface well section, characterized in that, include: The guide tube (10) is placed in the wellbore (9) on the seabed; The conduit (10) is provided with a gas chamber (42) and a drilling fluid chamber (43) that are connected vertically. The drilling fluid chamber (43) is connected to the wellbore (9). The top of the conduit (10) can seal the gas chamber (42). The conduit (10) is also provided with a return outlet (28) that is connected to the drilling fluid chamber (43). A drilling fluid tank (19) is located outside the conduit (10), and a return pipe (23) is connected between the drilling fluid tank (19) and the return outlet (28); a supply pipe (38) is connected between the drilling fluid tank (19) and the gas chamber (42), and the supply pipe (38) is equipped with an injection pump (18). A return pump (24) is provided on the return pipe (23). A return pipe (41) is connected to the side of the return pipe (23) away from the return outlet (28), and the return pipe (41) is connected to the gas chamber (42). The control system is electrically connected to the return pump (24) and the injection pump (18), respectively; The control system is configured to receive a signal that the rotational speed of the return pump (24) increases when shallow gas (44) overflows into the wellbore (9), and control the return pump (24) to close and the injection pump (18) to open, so as to inject drilling fluid into the drilling fluid chamber (43).

2. The drilling system for controllable mud level in the surface well section according to claim 1, characterized in that, The conduit (10) is also provided with an injection port (27), which is spaced above the return outlet (28); the drilling system for controllable mud level in the surface section also includes an inert gas device (21), and an injection pipe (22) is connected between the inert gas device (21) and the injection port (27).

3. The drilling system for controllable mud level in the surface well section according to claim 2, characterized in that, The gas injection pipe (22) is equipped with a gas injection control valve (30), and the control system is electrically connected to the gas injection control valve (30) to regulate the amount of inert gas injected into the gas chamber (42).

4. The drilling system for controllable mud level in the surface well section according to any one of claims 1 to 3, characterized in that, The return pipe (23) is provided with a return control valve (29) on the side near the return outlet (28). The control system is electrically connected to the return control valve (29). The control system is used to control the return control valve (29) to close when it receives a signal that the rotation speed of the return pump (24) increases.

5. The drilling system for controllable mud level in the surface well section according to claim 4, characterized in that, A flow meter (32) is also provided on the side of the return pipe (23) away from the return outlet (28). The control system is electrically connected to the flow meter (32) and is used to receive the drilling fluid return flow signal detected by the flow meter (32).

6. The drilling system for controllable mud level in the surface well section according to claim 5, characterized in that, The reflux pipe (41) is provided with a reflux control valve (31) on the side near the conduit (10), and / or the supply pipe (38) is provided with a supply control valve (34) on the side near the conduit (10). The control system is electrically connected to the reflux control valve (31) and the supply control valve (34).

7. The drilling system for controllable mud level in the surface well section according to claim 2 or 3, characterized in that, The conduit (10) is also equipped with a liquid level sensor (25), which is used to detect the drilling fluid level in the conduit (10), and the control system is electrically connected to the liquid level sensor (25).

8. The drilling system for controllable mud level in the surface well section according to claim 7, characterized in that, The liquid level sensor (25) includes a first probe and a second probe. The first probe is inserted into the drilling fluid chamber (43) to detect the pressure of the drilling fluid. The second probe is located on the outside of the conduit (10) to detect the pressure of the seawater (2). The liquid level sensor (25) is used to calculate the drilling fluid level in the conduit (10) based on the pressure difference between the drilling fluid and the seawater (2).

9. The drilling system for controllable mud level in the surface well section according to any one of claims 1 to 3, characterized in that, The drilling system for controllable mud level in the surface well section also includes a drilling string (16) and a fluid supply pump (17). The drilling string (16) is located on the guide tube (10) and passes through the gas chamber (42) and the drilling fluid chamber (43). The top of the drilling string (16) protrudes from the guide tube (10). A fluid supply pipe (37) is connected between the drilling string (16) and the drilling fluid tank (19). The fluid supply pump (17) is located on the fluid supply pipe (37). The control system is electrically connected to the fluid supply pump (17).

10. The drilling system for controllable mud level in the surface well section according to claim 9, characterized in that, The drilling system for controllable mud level in the surface section also includes a drilling platform (1) and a drilling rig (7). The drilling platform (1) is located on the seabed, and the drilling rig (7) is located on the drilling platform (1) and is vertically opposite to the guide pipe (10). The drilling rig (7) is also equipped with a lifting device (6), which is connected to the drilling string (16).

11. The drilling system for controllable mud level in the surface well section according to any one of claims 1 to 3, characterized in that, The drilling system for controllable mud level in the surface well section also includes a drilling fluid purifier (20), which is connected to the return pipe (23) and the drilling fluid tank (19). The return pipe (41) is located on the side of the drilling fluid purifier (20) away from the drilling fluid tank (19).

12. The drilling system for controllable mud level in the surface well section according to claim 7, characterized in that, The conduit (10) has a segmented structure, which includes a first pipe section (11), a connector sleeve (12), and a second pipe section (13) connected sequentially from bottom to top. The first pipe section (11) is inserted into the wellbore (9) of the seabed. The connector sleeve (12) is located on the upper side of the seabed. The return outlet (28), the gas injection port (27), and the liquid level sensor (25) are all located on the connector sleeve (12).

13. The drilling system for controllable mud level in the surface well section according to any one of claims 1 to 3, characterized in that, The top of the conduit (10) is provided with a rotary control head (14) and a flow divider (15). The flow divider (15) is connected to the gas chamber (42). The rotary control head (14) is used to close the top of the gas chamber (42). The flow divider (15) is used to guide the gas in the gas chamber (42) outward. The control system is electrically connected to the rotary control head (14) and the flow divider (15).

14. The drilling system for controllable mud level in the surface well section according to any one of claims 1 to 3, characterized in that, The top of the conduit (10) is also connected to an exhaust pipe, which is connected to the gas chamber (42) and is equipped with an outboard exhaust valve (26).

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