Double-oil-pipe cementing plug pipe column for leakage well

Abstract

The invention relates to the technical field of multi-string well sealing, in particular to a leakage well double-oil-pipe cementing plug string which comprises a large oil pipe, the large oil pipe is used for being arranged in a casing pipe and forming an oil sleeve annulus with the casing pipe, the bottom of the large oil pipe is connected with a check valve used for slowing down the flow speed of fluid, and a fluid inlet of the check valve faces upwards. The small oil pipe is fixed in the large oil pipe, the bottom end of the small oil pipe is close to the fluid inlet of the check valve, the large oil pipe and the small oil pipe form a large and small oil pipe annulus, and the large and small oil pipe annulus is provided with a valve for closing the large and small oil pipe annulus. The problems that the accuracy of the plugging position of a cement plug is reduced and even plugging fails due to turbulent flow caused by low liquid level in a pipe column and unobvious buffering effect between clear water and cement paste during grouting of a serious leakage well are solved.

Description

Technical Field

[0001] The invention relates to the technical field of multi-pipeline well sealing, and in particular to a double oil pipe cement plugging pipe column for a leakage well. Background Art

[0002] Most oil fields are multi-layer systems, and the water content and pressure of each layer are different. During the development process, the contradiction between layers becomes more and more prominent, and cement plugs are often needed to seal the lower layer sections that are temporarily unused or abandoned. The general process of existing cement plugs is: put the bakelite plug into the casing, and then lower the cement plug string into the casing. The cement plug string pushes the bakelite plug to the set position. Because there is a lot of accumulated water in the casing at this time, the accumulated water level is close to the wellhead. After the cement plug string is lowered into the casing, it is connected with the casing. Because of the connection of the pipeline, the liquid level in the cement plug string is flush with the liquid level between the annular space formed by the casing and the string. Then, clean water is used for positive circulation well washing. Clean water is pressurized and injected into the string through a surface pump. Clean water enters from the string and flows out from the annular space formed by the casing and the string, thereby completing the positive circulation well washing. After the positive well washing is completed, cement slurry is injected into the well through a surface pump from the cement plug string under pressure and replaced with clean water. At this time, the accumulated water in the string will buffer the cement slurry, slowing down the falling speed of the cement slurry and making it less likely for the cement slurry to mix with the accumulated water. After the cement slurry flows out of the string, it enters the annular space formed by the casing and the string, and finally stays at the set position. Then the cement plug string is lifted up to about 2 meters above the designed top surface of the cement plug, and liquid is injected into the annular space formed by the string and the casing for reverse circulation well washing. Clean water is injected from the annular space formed by the casing and the string, and excess cement slurry is washed out of the oil pipe. Then the string is lifted up 50-100 meters, the well is shut down and waited for solidification, and the cement slurry forms a cement plug after solidification underground.

[0003] During on-site construction, if there is serious leakage in the formation, there will be no return during positive circulation or reverse circulation well washing, and because of the serious leakage, there is little water in the well. After the pipe string is lowered into the casing, the liquid level in the pipe string is also very low. If the conventional cement plug injection method is used for construction, after the cement slurry is injected into the pipe string, the buffering effect of the cement slurry due to the accumulated water is not obvious, resulting in excessive flow rate and turbulent state of the cement slurry. When the cement slurry passes through the pipe string into the annular space formed by the casing and the pipe string, the cement slurry is in a turbulent state, resulting in mixing of the cement slurry and the accumulated water, which reduces the accuracy of the cement plugging position and may even lead to plugging failure. Summary of the invention

[0004] The purpose of the present invention is to provide a double oil pipe cement plug string for leaking wells to solve the problem that when injecting cement plugs into serious leaking wells, the low liquid level in the pipe string causes turbulence between clean water and cement slurry, resulting in serious mixing of the fluids and reducing the accuracy of the cement plug's sealing position or even failing to seal.

[0005] The double oil pipe cement plug string for lost wells of the present invention adopts the following technical scheme:

[0006] A double oil pipe cementing plug string for a leaking well comprises a large oil pipe, which is used to be arranged inside a casing and form an oil-casing annulus with the casing. A check valve for slowing down the flow rate of a fluid is connected to the bottom of the large oil pipe, and a fluid inlet of the check valve faces upward. The large oil pipe also comprises a small oil pipe, which is arranged inside the large oil pipe, and the bottom end of the small oil pipe is close to the fluid inlet of the check valve. The large oil pipe and the small oil pipe form an annulus between the large and small oil pipes. The string also comprises a valve for opening and closing the annulus between the large and small oil pipes.

[0007] The present invention is an improvement on the original cementing plug string. The specific cementing plug string is a double-layer string composed of a large oil pipe and a small oil pipe. A single-flow valve that can slow down the flow rate of the fluid is connected to the bottom of the large oil pipe. The fluid inlet of the single-flow valve faces upward, and the small oil pipe is connected to the inside of the large oil pipe. The bottom of the small oil pipe is close to the fluid inlet of the single-flow valve. The large and small oil pipes form an annulus between the large and small oil pipes with the small oil pipe. In this way, the double-layer string is set up. When in use, the inside of the small oil pipe is isolated from the annulus between the large and small oil pipes and the annulus between the oil casing. When washing the well, clean water is injected from the small oil pipe to reach the annulus between the large and small oil pipes through the small oil pipe. At this time, the single-flow valve has not reached the opening pressure. The check valve will open, and clean water will return from the annulus of the large and small tubings. At this time, the clean water will fill the small tubing and the annulus of the large and small tubings. When the cement slurry is being squeezed in, because there is enough accumulated water in the small tubing to buffer the cement slurry, the cement slurry will not generate turbulence due to excessive speed during the downward process. When the cement slurry reaches the annulus of the large and small tubings, the annulus valves of the large and small tubings are closed. At this time, the cement slurry will not return along the annulus of the large and small tubings. Under the action of the ground pumping pressure, the check valve reaches the opening pressure at this time, the check valve opens, and the cement slurry enters the oil casing annulus. The check valve further decelerates the cement slurry to prevent the cement slurry from mixing with the accumulated water in the oil casing annulus. In order to prevent the clean water that replaces the cement slurry from entering the oil casing annulus to cause the annulus liquid level to rise, and increase the liquid column pressure on the leaking formation, the check valve is immediately closed after the ground pumping pressure is reduced to ensure the accuracy of the cement plugging position and the success rate of plugging.

[0008] Furthermore, the small oil pipe includes an upper small oil pipe and a lower small oil pipe, which are fixedly connected to the large oil pipe through a double pipe joint. The double pipe joint is arranged on the large oil pipe and is located at the upper end of the single-flow valve. The double pipe joint separates the annulus of the large and small oil pipes into an upper annulus of the large and small oil pipes and a lower annulus of the large and small oil pipes. The double pipe joint is provided with a first through hole for connecting the upper and lower small oil pipes and a second through hole for connecting the upper and lower annuli of the large and small oil pipes.

[0009] The double-pipe joint is connected to the large oil pipe and is located at the upper end of the check valve. The upper small oil pipe and the lower small oil pipe of the small oil pipe are fixed inside the large oil pipe through the double-pipe joint. The double-pipe joint divides the annulus of the large and small oil pipes into an upper annulus of the large and small oil pipes and a lower annulus of the large and small oil pipes. The double-pipe joint is provided with a first through hole for connecting the upper and lower small oil pipes and a second through hole for connecting the upper and lower annuli of the large and small oil pipes. In this way, the small oil pipe can be stably fixed inside the large oil pipe, and because of the first through hole and the second through hole, forward and reverse circulation well washing and forward replacement of cement slurry will not be hindered by the double-pipe joint provided inside the large oil pipe.

[0010] Furthermore, the double pipe joint includes a body having a channel inside which fluid can flow, and threads are respectively provided at both ends of the body for connecting with a large oil pipe. The channel includes a central through hole located at the center of the channel, and the central through hole constitutes the first through hole. The channel also includes a peripheral through hole, which is arranged around the central through hole, and the peripheral through holes constitute the second through hole. Threads are respectively provided at both ends of the central through hole for connecting with the upper small oil pipe and the lower small oil pipe.

[0011] Both ends of the double-tube joint body are provided with threads for connection with the large oil pipe. The threaded connection is relatively tight, and because the threads have their own sealing properties, they meet the needs of downhole cement plugging. The body has a channel for fluid to flow through, which is used to pass water and cement slurry. The central through hole is arranged in the center of the channel, and the two sides of the central through hole are connected to the upper and lower small oil pipes through threads. The peripheral through holes are arranged around the central through hole, so that the peripheral through holes also connect the upper and lower annuli of the large and small oil pipes. Because the oil pipes are connected together by threads, it is more convenient to connect the double-tube joint to the large oil pipe by threads, and the structure is simple.

[0012] Furthermore, the upper small oil pipe includes a sealing cylinder and an inserted sealing tube. The sealing cylinder is connected to one end of the central through hole by a thread. An inserted sealing tube is arranged in the sealing cylinder to achieve sealing of the sealing cylinder. The inserted sealing tube can move in the sealing cylinder. One end of the inserted sealing tube is provided with a thread connected to the upper small oil pipe. The position of the upper small oil pipe can be adjusted by moving the position of the inserted sealing tube.

[0013] The sealing cylinder is connected to one side of the central through hole by a thread, and an inserted sealing tube is arranged in the sealing cylinder. The inserted sealing tube completes the sealing of the sealing cylinder. The inserted sealing tube can also move in the sealing cylinder, and one end of the inserted sealing tube is connected to the upper small oil pipe by a thread. When in use, the position setting of the double pipe joint may not be so precise. By moving the inserted sealing tube in the sealing cylinder, the position of the connection thread with the upper small oil pipe can be adjusted, making the connection more convenient.

[0014] Furthermore, the inserted sealing tube is a segmented structure connected by thread, and an elastic seal sleeved on the tube segment is sandwiched between adjacent connecting segments. Tightening the adjacent tube segment squeezes the elastic seal sleeved on the tube segment, and the elastic seal radially deforms to achieve sealing with the sealing tube.

[0015] The inserted sealing tube is also a segmented structure connected together by threads. Elastic sealing members are sleeved on the pipe sections between adjacent connecting sections. When the inserted sealing tube is inserted into the sealing tube, the adjacent pipe sections can be rotated so that the elastic sealing members between the adjacent pipe sections are squeezed and radially deformed, thereby achieving sealing of the sealing tube.

[0016] Furthermore, the one-flow valve is an adjustable pressure one-flow valve with adjustable opening pressure.

[0017] Because the leakage occurs at different depths, the depth of the water level in the casing is also different. The one-way valve is set to an adjustable pressure one-way valve with adjustable opening pressure. If the water level is high, the liquid level in the pipe column is also high, and the buffering force of the one-way valve is small, so the opening pressure can be reduced. Conversely, if the water level is low, the liquid level in the pipe column is also low, and the buffering force of the one-way valve is large, so the opening pressure must be increased. Because the one-way valve is a one-way valve with adjustable opening pressure, it can adapt to different working conditions.

[0018] Furthermore, the adjustable pressure check valve includes a valve body having a channel inside the valve body for fluid to flow through, a fluid inlet and a fluid outlet are arranged at both ends of the valve body, a valve seat is arranged near the fluid inlet, and a base is arranged near the fluid outlet, a spring is arranged between the valve seat and the base to keep the valve ball pressed against the valve seat, and the other end of the spring is equipped with an adjusting bolt for adjusting the pressure, and the adjusting bolt is screwed onto the base.

[0019] The adjustable pressure check valve is different from the existing check valve in that the adjustable pressure check valve can adjust the opening pressure through the adjusting bolt assembled at one end of the spring. When the opening pressure needs to be increased, the adjusting screw screwed on the base is screwed in toward the fluid inlet, the spring is compressed, the preload force increases, and the valve ball and the valve seat fit more closely. When the opening pressure needs to be reduced, the adjusting screw screwed on the screw ring is screwed out toward the fluid outlet, the spring is restored, the preload force is reduced, and the spring is in contact with the valve ball and the adjusting bolt instead of being fixedly connected. This split setting does not require the entire valve to be replaced during maintenance if it is damaged, but only the damaged parts need to be replaced, saving maintenance costs and facilitating installation.

[0020] Furthermore, the valve body is detachably connected to the base.

[0021] The screw ring and the valve body are detachably connected, so that after the valve ball and the spring are installed at the specified position, the adjusting bolt can be screwed onto the base, and then the base and the valve body can be connected. This has a simple structure and is easy to install.

[0022] Furthermore, a threaded hole for connecting with an adjusting bolt is arranged in the center of the base, and through holes for fluid circulation are arranged around the base.

[0023] A threaded hole for connecting with an adjusting bolt is arranged in the center of the base, and through holes for fluid circulation are arranged around the base. This arrangement not only meets the installation requirements of the adjusting bolt, but also meets the requirements of fluid circulation.

[0024] Furthermore, an internal thread for connecting to a large oil pipe is provided at the fluid inlet position of the check valve.

[0025] The fluid inlet of the valve body is provided with an internal thread for connecting with a large oil pipe, so that after the valve body is connected with the large oil pipe, the cement slurry will not be blocked by the boss formed at the threaded connection when passing through the threaded connection, ensuring smooth flow of the cement slurry. In addition, because the valve body and the oil pipe are connected through internal threads, the cement slurry directly enters the valve body after passing through the oil pipe, and will not penetrate into the threaded connection, will not damage the thread, and ensures the stability of the connection. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A schematic diagram of the use process of the first embodiment of the double oil pipe cement plug string for a leaking well of the present invention (the sealing cylinder and the valve are not shown);

[0027] Figure 2 It is a cross-sectional view of an adjustable pressure check valve;

[0028] Figure 3 It is a schematic diagram of the connection between the double pipe joint and the sealing cylinder;

[0029] Figure 4 for Figure 3 AA view;

[0030] Figure 5 A cross-sectional view of the inserted sealing tube.

[0031] In the figure: 1-upper small oil pipe, 2-inserted sealing pipe, 3-double pipe joint, 4-large oil pipe, 5-casing annulus, 6-casing, 7-adjustable pressure check valve, 8-bakelite plug, 9-lower formation, 10-upper formation, 11-lower annulus of large and small oil pipes, 12-upper annulus of large and small oil pipes, 13-accumulated water, 14-cement slurry, 15-lower small oil pipe, 21-upper joint, 22-first sealing assembly, 23-second sealing assembly, 24-first "O"-shaped sealing ring, 25-intermediate body, 26-third sealing assembly, 27-fourth sealing assembly, 28-second "O"-ring, 29-lower joint, 31-sealing cylinder, 32-body, 33-first female thread, 34-second female thread, 35-sealing ring, 36-center through hole, 37-peripheral through hole, 38-third female thread, 39-first male thread, 71-valve body, 72-valve ball, 73-spring, 74-adjusting bolt, 75-base, 76-valve seat, 77-fluid inlet, 78-fluid outlet. DETAILED DESCRIPTION

[0032] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0033] The double oil pipe cement plugging string for leaking wells of the present embodiment is particularly suitable for the cement plugging operation process of high leaking wells.

[0034] Embodiment 1 of the double oil pipe cement plug string for lost well of the present invention: Figure 1 As shown, the double oil pipe cement plug string for the leaking well includes a large oil pipe 4 and a small oil pipe, the small oil pipe includes an upper small oil pipe 1 and a lower small oil pipe 15, the large oil pipe 4 is inserted into the casing 6 to form an oil-casing annulus 5, and the adjustable pressure check valve 7 is connected to the large oil pipe 4 by an internal thread set at its fluid inlet 77, the fluid inlet 77 faces upward, and the bottom end of the lower small oil pipe 15 is close to the fluid inlet 77 of the check valve.

[0035] A double pipe joint 3 is connected to the large oil pipe 4 at the upper part of the adjustable pressure check valve 7. The structure of the double pipe joint 3 is as follows: Figure 3 As shown, the double pipe joint 3 includes a body 32, which is connected to the large oil pipe 4 through threads arranged at both ends. The body 32 has a channel for fluid to flow through, and the channel includes a central through hole 36 located at the center of the channel, and the central through hole 36 constitutes the first through hole. The channel also includes a peripheral through hole 37, which is arranged around the central through hole 36, and the peripheral through hole 37 constitutes the second through hole. The central through hole 36 has threads at both ends for connecting the upper small oil pipe 1 and the lower small oil pipe 15. The structure of the central through hole 36 and the peripheral through hole 37 is as shown in FIG. Figure 4As shown, the large oil pipe 4 and the small oil pipe form a large and small oil pipe annulus, and the pipe string also includes a valve for opening and closing the large and small oil pipe annuli, and the double pipe joint 3 separates the large and small oil pipe annuli into an upper annulus 12 and a lower annulus 11 of the large and small oil pipes, and the peripheral through hole 37 is used to connect the upper annulus 12 and the lower annulus 11 of the large and small oil pipes.

[0036] The upper small oil pipe 1 includes a sealing cylinder 31 and an inserted sealing tube 2. The sealing cylinder 31 is connected to the double pipe joint 3 through the central through hole 36 and the second female thread 34 located on one side of the first female thread 33. The male thread of the sealing cylinder 31 used to connect with the second female thread 34 is not a tapered thread with a good self-sealing effect. Therefore, a sealing ring 35 is arranged at the junction of the second female thread 34 and the central through hole 36 to enhance the sealing effect. The connection between the sealing cylinder 31 and the double pipe joint 3 is as shown in FIG. Figure 3 shown.

[0037] The sealing tube 31 is provided with an insert sealing tube 2, and the structure of the insert sealing tube 2 is as follows: Figure 5As shown, the inserted sealing tube 2 includes an upper joint 21 , and the elastic sealing component includes a first sealing component 22 , a second sealing component 23 , a third sealing component 26 , and a fourth sealing component 27 . The upper joint 21 includes an upper joint large diameter section and an upper joint small diameter section. The diameter of the upper joint large diameter section is larger than that of the upper joint small diameter section, so a boss is formed at the junction. The upper joint large diameter section is provided with a thread connected to the upper small oil pipe 1, and the upper joint small diameter section is sleeved with a first sealing component 22 and a second sealing component 23, and the upper joint small diameter section is provided with a male thread for connecting to the intermediate body 25; the intermediate body large diameter section at the upper part of the intermediate body 25 is provided with a female thread connected to the upper joint 21, and the boss formed by the upper joint large diameter section and the upper joint small diameter section and the boss formed at the threaded connection between the upper joint 21 and the intermediate body 25 press the first and second sealing components, and the intermediate body 25 is provided with an intermediate body small diameter section at the lower end. The diameter of the intermediate body large diameter section is larger than that of the intermediate body small diameter section, so a boss is formed at the junction, and the intermediate body small diameter section is provided with a male thread connected to the lower joint 29, and the intermediate body small diameter section is sleeved with a third sealing group 26 and a fourth sealing component 27. The upper end of the lower joint 29 has a female thread connected to the intermediate body 25. The boss formed by the intermediate body large diameter section and the intermediate body small diameter section and the boss formed at the threaded connection between the intermediate body 25 and the lower joint 29 press the third and fourth sealing components. When the upper joint 21 is rotated, the first and second sealing components and the third and fourth sealing components will be squeezed and deformed, thereby achieving the sealing of the sealing cylinder 31. The first "O"-shaped sealing ring 24 is provided at the threaded connection between the upper joint 21 and the intermediate body 25, and the second "O"-shaped sealing ring 28 is provided at the threaded connection between the intermediate body 25 and the lower joint 29. The first and second "O"-shaped sealing rings are used to seal the threaded connection. A horseshoe-shaped guide shoe is provided at the lower end of the lower joint 29 to facilitate the insertion of the sealing tube 2 into the sealing cylinder 31. Because during construction, the fixed position of the double-tube joint 3 will not be very precise, the inserted sealing tube 2 is set in the sealing tube 31 and can slide up and down along the inner cavity of the sealing tube 31, and one end of the inserted sealing tube 2 is connected to the upper small oil pipe 1. When the position of the double-tube joint 3 is inaccurate, the threaded position connected to the upper small oil pipe 1 can be properly adjusted by inserting the sealing tube 2, which is convenient for connecting with the upper small oil pipe 1.

[0038] The structure of the adjustable pressure check valve 7 is as follows: Figure 2As shown, when the opening pressure of the adjustable pressure check valve 7 is set, if the opening pressure needs to be increased, the adjusting bolt 74 on the base 75 can be screwed into the fluid inlet 77, and the base 75 is detachably connected to the valve body 71, and the base 75 is arranged near the fluid outlet 78 in the valve body 71, and a threaded hole connected to the adjusting bolt 74 is provided in the middle of the base 75, and through holes for fluid to flow through are provided around it, and a spring 73 for keeping the valve ball 71 pressed against the valve seat 76 is provided between the valve seat 76 and the base 75, and the other end of the spring 73 is equipped with an adjusting bolt 74 for adjusting the pressure. This arrangement has a simple structure and is easy to install. In addition, when parts are damaged, there is no need to replace the entire part, but only the damaged parts need to be replaced, thereby saving maintenance costs. At this time, because the adjustable bolt 74 is screwed in, the spring 73 is compressed, and the clamping force increases, so that the valve ball 72 in contact with the other end of the spring 73 fits more closely with the valve seat 76 near the fluid inlet 77. The valve ball 72 is located on the side of the valve seat 76 facing the fluid outlet 78. The valve ball 72 and the valve seat 76 are arranged in this way to ensure that the fluid can only enter from the fluid inlet 77 and flow out from the fluid outlet 78, but cannot flow in the opposite direction. To reduce the opening pressure, the adjusting bolt 74 is screwed out toward the fluid outlet 78. At this time, the spring 73 is restored, the clamping force is reduced, and the opening pressure is reduced accordingly.

[0039] An internal thread for connection with the large oil pipe 4 is provided at the fluid inlet 77 of the valve body 71. The fluid inlet 77 faces the direction of the liquid outlet of the large oil pipe 4. The adjustable pressure check valve 7 is installed on the outside of the large oil pipe 4. Compared with being installed inside the large oil pipe 4, it can ensure that the fluid enters the valve body 71 smoothly without being blocked by the boss formed at the threaded connection, and no cement slurry will penetrate into the threaded connection to cause damage to the thread. In this way, different opening pressures can be adjusted according to different working conditions, so that the flow rate of each fluid is slowed down when passing through, turbulence is eliminated, and then the mixed phase phenomenon is eliminated, thereby ensuring the success rate of cement plug sealing and the accuracy of the sealing position.

[0040] When the double oil pipe cementing plug string for the leaking well is used, it is necessary to seal the lower formation 9, adjust the opening pressure of the adjustable pressure check valve 7 according to the liquid depth of the accumulated water 13 in the casing 6, connect the double pipe joint 3 to the designed position of the large oil pipe 4, connect the sealing tube 31 with the double pipe joint 3, connect the lower small oil pipe 15 through the third female thread 38 arranged on the side of the first male thread 39 set in the central through hole 36, and the large oil pipe 4 pushes the bakelite plug 8 to between the lower formation 9 and the upper formation 10. Insert the sealing tube 2 and the upper small oil pipe 1, and insert the inserted sealing tube 2 into the sealing tube 31. At this time, pressurized clean water is injected from the upper small oil pipe 1, and the clean water enters the inserted sealing tube 2 set in the sealing tube 31 through the upper small oil pipe 1, and then flows downward after passing through the inserted sealing tube 2, passes through the central through hole 36 located in the center of the double pipe joint 3, and then flows through the lower small oil pipe 15 threadedly connected with the double pipe joint 3 to reach the annulus 11 under the large and small oil pipes. At this time, the adjustable pressure check valve 7 will not open because the opening pressure has not been reached, and the clean water will go up along the annulus 11 under the large and small oil pipes. When it reaches the double pipe joint 3, because the annular through hole 37 arranged around the central through hole 36 connects the annulus 11 under the large and small oil pipes with the annulus 12 on the large and small oil pipes, the clean water can continue to go up to the annulus 12 on the large and small oil pipes through the annular through hole 37, so that the positive circulation well washing is completed. At this time, the annulus 12 on the large and small oil pipes, the annulus 11 under the large and small oil pipes and the small oil pipes are all filled with clean water. At this time, the cement slurry and the replacement clean water are positively replaced into the upper small oil pipe 1. Because the entire pipe string is filled with clean water, the cement slurry 14 provides sufficient buffering during the downward process, eliminating turbulence, and thus eliminating the occurrence of mixed phase phenomenon. When the cement slurry 14 reaches the annulus 11 under the large and small oil pipes, the gates of the annulus of the large and small oil pipes are closed. At this time, due to the pressurizing effect of the cement slurry, the pressure reaches the opening pressure of the adjustable pressure check valve 7. The adjustable pressure check valve 7 opens, and the cement slurry will not continue to ascend along the annulus of the large and small oil pipes, but will be replaced into the oil casing annulus 5 above the bakelite plug 8 through the adjustable pressure check valve 7.

[0041] After the cement slurry 14 is replaced into the casing annulus 5 and reaches the set position, the valves of the annuli of the large and small oil pipes are opened, and the well is backwashed with clean water. The clean water is replaced from the annuli of the large and small oil pipes, and first enters the upper annulus 12 of the large and small oil pipes. When the clean water descends to the double pipe joint 3, it enters the lower annulus 11 of the large and small oil pipes through the annular through hole 37. Because the opening pressure of the adjustable pressure check valve 7 is not reached, the clean water enters the lower small oil pipe 15. When it ascends along the lower small oil pipe 15 to the double pipe joint 3, it enters the sealing cylinder 31 through the central through hole 36 and finally returns. In this way, the excess cement slurry 14 in the large and small oil pipes above the adjustable pressure check valve 7 is cleaned.

[0042] The second embodiment of the double oil pipe cement plug column for leaking wells of the present invention is different from the first embodiment in that the spring is fixedly connected with the adjusting bolt, the valve body is detachably connected with the base, and the center of the base is provided with a threaded hole that matches the adjusting bolt. During installation, the adjusting bolt is first screwed into the base through the threaded hole, and when the adjusting bolt passes through the base, the adjusting bolt is fixedly connected with the spring, and finally the base is connected with the valve body.

[0043] The third embodiment of the double oil pipe cement plug column for lost wells of the present invention is different from the first embodiment in that the thread of the sealing sleeve used to connect with the double pipe joint is set as a conical thread with good self-sealing property, and there is no need to set a sealing ring at the connection between the sealing sleeve and the double pipe joint.

[0044] Embodiment 4 of the double oil pipe cementing plug column for leaking wells of the present invention is different from embodiment 1 in that a double pipe joint is no longer provided, but an inner flange is provided on the inner wall of the large oil pipe, a small oil pipe through hole with the same diameter as the small oil pipe is opened in the center of the inner flange, threaded holes are provided around the small oil pipe through hole, a large oil pipe connecting hole is provided between adjacent threaded holes, an outer flange is provided on the outer wall of the small oil pipe, an outer flange through hole matching with the inner flange threaded hole is provided on the outer flange, and an outer flange connecting hole matching with the large oil pipe connecting hole for connecting the upper and lower annuli of the large and small oil pipes is also provided on the outer flange, and the large and small oil pipes are fixed by bolts inserted through the inner flange threaded holes and the outer flange through holes.

[0045] The fifth embodiment of the double oil pipe cement plug column for a leaking well of the present invention is different from the first embodiment in that the inserted sealing pipe is an integrated structure, and a groove for sleeve-mounting an elastic sealing member is provided on the inserted sealing pipe.

[0046] Embodiment 6 of the double oil pipe cement plug column for leaking wells of the present invention is different from embodiment 1 in that the upper small oil pipe is an ordinary oil pipe, and the sealing cylinder and the inserted sealing pipe are no longer connected. The position of the oil pipe is connected to the upper small oil pipe by connecting oil pipes of different lengths on the double pipe joint.

[0047] The seventh embodiment of the double oil pipe cement plug column for leaking wells of the present invention is different from the first embodiment in that the adjustable check valve is a non-pressure-adjustable check valve.

[0048] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. The patent protection scope of the present invention shall be based on the claims. All equivalent structural changes made using the contents of the description and drawings of the present invention should also be included in the protection scope of the present invention.

Claims

1. A double oil pipe cementing plug string for a lost well, comprising a large oil pipe, which is arranged inside a casing and forms an oil-casing annulus with the casing, characterized in that: A check valve for slowing down the fluid flow rate is connected to the bottom of the large oil pipe, and the fluid inlet of the check valve faces upward. The large oil pipe also includes a small oil pipe, which is arranged in the large oil pipe. The bottom end of the small oil pipe is close to the fluid inlet of the check valve. The large oil pipe and the small oil pipe form an annulus between the large and small oil pipes. The tubing string also includes a valve for opening and closing the annulus between the large and small oil pipes.

2. The double oil pipe cement plug string for leaking wells according to claim 1, characterized in that: The small oil pipe includes an upper small oil pipe and a lower small oil pipe, which are fixedly connected to the large oil pipe through a double pipe joint. The double pipe joint is arranged on the large oil pipe and is located at the upper end of the single flow valve. The double pipe joint separates the annulus of the large and small oil pipes into an upper annulus of the large and small oil pipes and a lower annulus of the large and small oil pipes. The double pipe joint is provided with a first through hole for connecting the upper and lower small oil pipes and a second through hole for connecting the upper and lower annuli of the large and small oil pipes.

3. The double oil pipe cement plug string for leaking wells according to claim 2, characterized in that: The double pipe joint includes a body, which has a channel inside for fluid to flow through, and threads are respectively arranged at both ends of the body for connecting with a large oil pipe. The channel includes a central through hole located at the center of the channel, and the central through hole constitutes the first through hole. The channel also includes a peripheral through hole, which is arranged around the central through hole, and the peripheral through holes constitute the second through hole. Threads are respectively arranged at both ends of the central through hole for connecting with an upper small oil pipe and a lower small oil pipe.

4. The double oil pipe cement plug string for leaking wells according to claim 3, characterized in that: The upper small oil pipe includes a sealing cylinder and an inserted sealing tube. The sealing cylinder is connected to one end of the central through hole by a thread. An inserted sealing tube is arranged in the sealing cylinder to achieve sealing of the sealing cylinder. The inserted sealing tube can move in the sealing cylinder. One end of the inserted sealing tube is provided with a thread connected to the upper small oil pipe. The position of the upper small oil pipe can be adjusted by moving the position of the inserted sealing tube.

5. The double oil pipe cement plug string for leaking wells according to claim 4, characterized in that: The inserted sealing tube is a segmented structure with threaded connection, and elastic seals sleeved on the tube sections are clamped between adjacent connecting sections. Tightening the adjacent tube sections squeezes the elastic seals sleeved on the tube sections, and the elastic seals radially deform to achieve sealing with the sealing tube.

6. The double oil pipe cement plug string for leaking wells according to claim 4, characterized in that: The one-flow valve is an adjustable pressure one-flow valve with adjustable opening pressure.

7. The double oil pipe cement plug string for leaking wells according to claim 6, characterized in that: The adjustable pressure check valve includes a valve body, the interior of the valve body has a channel for fluid to flow through, a fluid inlet and a fluid outlet are arranged at both ends of the valve body, a valve seat is arranged near the fluid inlet, and a base is arranged near the fluid outlet, a spring is arranged between the valve seat and the base to keep the valve ball pressed against the valve seat, and the other end of the spring is equipped with an adjusting bolt for adjusting the pressure, and the adjusting bolt is screwed onto the base.

8. The double oil pipe cement plug string for leaking wells according to claim 7, characterized in that: The valve body is detachably connected to the base.

9. The double oil pipe cement plug string for leaking wells according to claim 7 or 8, characterized in that: A threaded hole for connecting with an adjusting bolt is arranged in the center of the base, and through holes for fluid circulation are arranged around the base.

10. The double oil pipe cement plug string for leaking wells according to claim 6, characterized in that: An internal thread for connecting with a large oil pipe is arranged at the fluid inlet position of the check valve.

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