Open hole multilateral well completion pipe string and use method

Abstract

The invention relates to the field of oil field well completion engineering and oil production engineering, in particular to an open hole multilateral well completion pipe string and a using method.The open hole multilateral well completion pipe string comprises a temporary plugging pipe string and a well completion pipe string, the temporary plugging pipe string is arranged in an upper branch of a multilateral well, and the well completion pipe string is arranged in a lower branch of the multilateral well; the well completion pipe string is provided with a seating short section and a switchable sliding sleeve, the switchable sliding sleeve is arranged at the junction of the upper branch and the well completion pipe string, and the seating short section is arranged below the switchable sliding sleeve. Fracturing of the lower branch is completed firstly by locating the short section and the openable sliding sleeve, then the openable sliding sleeve is opened, the upper branch is dredged, fracturing of the upper branch is completed, plugging of the lower branch and the upper branch can be conducted, commingling and separate mining of the branches are achieved, cost is low, and operation is easy.

Description

Technical Field

[0001] The present invention relates to the fields of oilfield completion engineering and oil production engineering, and specifically relates to an open-hole multilateral well completion string and a usage method thereof. Background Art

[0002] With the in-depth development of oilfield exploitation, the development of new low-cost technologies and processes plays a crucial role in increasing oil production and stabilizing production in oilfields. The multilateral well technology is a new drilling and completion technology developed after directional wells, sidetracks, and horizontal well technologies. Drilling several branch wells in one wellbore can reuse the upper well section, reduce drilling costs, and increase the oil drainage area. It has obvious technical and economic advantages in solving the exploitation mode of special reservoirs, improving the oil production and recovery rate of new and old oilfield wells, etc.

[0003] The completion methods of multilateral wells are very different from those of vertical wells and horizontal wells, and the technical difficulty is relatively large. When selecting and designing the completion methods of multilateral wells, the isolation, control, and wellbore stability of the main wellbore and each branch wellbore section are mainly considered. According to the interface support, interface sealing, and re-entry function of the branch wells, the branch wells are divided into 6 levels. To achieve the requirements of combined production and separate production between different branches of the multilateral well, the technology is at least level 4 and above. However, the related drilling and completion technologies are complex and costly, and it is difficult to meet the low-cost requirements of oilfields. The cost of branch wells at level 3 and below is relatively low, but they do not have the function of separate production.

[0004] Currently, open-hole multilateral well completion mainly adopts open-hole completion or screen completion methods. Through natural production, it is impossible to perform independent fracturing of each branch wellbore, and only the combined production function can be achieved. In the case of water production from a certain branch, effective water shutoff operations cannot be carried out, and the application scope is limited.

[0005] Therefore, there is an urgent need to provide an open-hole multilateral well completion string and a usage method thereof, which can be applied to branch wells below level 3 and achieve effective water shutoff operations for each branch. Summary of the Invention

[0006] In order to overcome the problems existing in the prior art, the purpose of the present invention is to provide an open-hole multilateral well completion string and a usage method thereof.

[0007] To achieve the above purpose, the present invention provides the following technical solution: An open-hole multilateral well completion string includes a temporary plugging string and a completion string. The temporary plugging string is arranged in the upper branch of the multilateral well, and the completion string is arranged in the lower branch of the multilateral well;

[0008] A landing nipple and a switchable sliding sleeve are further arranged on the completion string. The switchable sliding sleeve is arranged at the intersection of the upper branch and the completion string, and the landing nipple is arranged below the switchable sliding sleeve.

[0009] The present invention is further configured such that: a slider is disposed inside the switchable sliding sleeve, a plurality of through holes are circumferentially disposed on the switchable sliding sleeve, and the slider slides inside the switchable sliding sleeve to open or close the through holes.

[0010] The present invention is further configured such that: a hollow moving cavity is disposed inside the slider, a moving cavity inlet is disposed above the moving cavity, a moving cavity outlet is disposed below the moving cavity, a ball seat with a funnel-shaped structure is disposed at the top of the slider, and the top diameter of the ball seat gradually contracts inward to the moving cavity inlet; the diameters of the moving cavity inlet and the moving cavity outlet are smaller than the diameter of the moving cavity.

[0011] The present invention is further configured such that: the opening and closing of the switchable sliding sleeve are controlled by any one of a soluble ball or a switching tool.

[0012] The present invention is further configured such that: a lock core groove with a funnel-shaped structure is disposed inside the landing nipple, and the top diameter of the lock core groove gradually contracts inward.

[0013] The present invention is further configured such that: the temporary plugging string includes a first round head guide shoe, a first perforated screen pipe, a POP valve, a first open hole packer, and a hydraulic release tool; one end of the temporary plugging string is installed with the first round head guide shoe, and from the first round head guide shoe, the first perforated screen pipe, the POP valve, the first soluble open hole packer, and the hydraulic release tool are sequentially installed on the corresponding tubing.

[0014] The present invention is further configured such that: the first open hole packer is a soluble open hole packer.

[0015] The present invention is further configured such that: a dead plug is disposed inside the POP valve, and the dead plug is made of a soluble material.

[0016] The above purpose is that, by using a soluble open hole packer, reliable sealing can be achieved during the drilling process of the lower branch. In the case where the upper branch cannot be dredged by pressure, the upper branch can be dredged by the self-dissolution of the packer.

[0017] Moreover, the dead plug of the POP valve is made of a soluble material. In the case where pressure dredging is ineffective, the upper branch can be dredged by self-dissolution.

[0018] The present invention is further configured such that: the completion string further includes a second round head guide shoe, a second perforated screen pipe, a float collar, an isolation valve, a differential pressure sliding sleeve, a second open hole packer, and an insert packer; one end of the completion string is installed with the second round head guide shoe, and from the second round head guide shoe, the second perforated screen pipe, the float collar, the isolation valve, the differential pressure sliding sleeve, the second open hole packer, and the insert packer are sequentially installed.

[0019] The present invention is further configured such that: the landing nipple and the switchable sliding sleeve are disposed between the second open hole packer and the insert packer.

[0020] The present invention also discloses a method for using a completion pipe string for an open hole branch well, which is applicable to the above-mentioned completion pipe string for an open hole branch well, and comprises the following steps:

[0021] S1: Send the drill pipe to lower the temporary plugging pipe string to the predetermined position of the upper branch;

[0022] S2: The pressure inside the temporary plugging pipe string is pressurized to P1, and the first open hole packer is set to achieve temporary plugging of the upper branch;

[0023] S3: Continue to pressurize the temporary plugging pipe string to a pressure of P2, so that the hydraulic release device is disengaged, and the temporary plugging pipe string is released, leaving the temporary plugging pipe string in the wellbore of the upper branch, wherein P1 < P2;

[0024] S4: injecting a chip blocking liquid into the upper part of the temporary plugging pipe string by inserting a drill pipe; preferably, a high temperature resistant self-dissolving gel is used;

[0025] S5: The completion pipe string is then sent into the open-hole branch well through the drill pipe. After it is lowered into the predetermined position of the lower branch, the isolation valve pressure-holding ball is inserted to block the isolation valve, and the pressure is increased to P3 to block the entire completion pipe string;

[0026] S6: Continue to pressurize the completion string. When the pressure reaches P4, expand and seal the second open hole packer. When the pressure reaches P5, use the hanging and sealing plug packer.

[0027] S7: Check the sealing and hanging of the intubation packer. After the sealing and hanging are successful, hold the pressure to P6 to open the differential pressure sleeve, dredge the lower branch, and wait for the fracturing of the lower branch.

[0028] S8: Pull out the drill pipe and lower the return pipe string. According to the actual needs on site, tools such as expansion joints and dilution sleeves can be installed on the return pipe string;

[0029] S9: By inserting a soluble ball or a switch tool, the switchable sleeve is controlled to be opened or closed to achieve fracturing of the upper branch.

[0030] The present invention is further configured as follows: in step S4, the chip blocking liquid is a high temperature resistant self-dissolving gel.

[0031] The present invention is further configured as follows: in step S9, the switchable sleeve is controlled by the soluble ball, specifically, the soluble ball is put into the sleeve to open the switchable sleeve, the pressure in the pipe of the return pipe string is increased to P7 to open the POP valve to clear the upper branch, and the upper branch is fracturing, wherein P1<P2<P7.

[0032] The present invention is further configured as follows: in step S9, the downhole switch tool for controlling the switchable sliding sleeve specifically means that a lock core is lowered into the lock core groove inside the seating nipple to block the lower branch, and then the switch tool is lowered to open the switchable sliding sleeve. After the switchable sliding sleeve is opened, the pipeline is pressured to pressure P7 to open the POP valve and dredge the upper branch for fracturing the upper branch, where P1 < P2 < P7.

[0033] The present invention is further configured as follows: during the construction process, if water flows out from the lower branch, a lock core is inserted until the lock core groove inside the seating nipple blocks the lower branch; if water flows out from the upper branch, a switch tool is lowered to close the switchable sliding sleeve.

[0034] In summary, the beneficial effects of the above technical solutions of the present invention are as follows:

[0035] Through the seating nipple and the switchable sliding sleeve, the present invention first completes the fracturing of the lower branch, then opens the switchable sliding sleeve to dredge the upper branch, and completes the fracturing of the upper branch. It can block the lower branch and the upper branch, realizing combined production and separate production of the branches, and has the characteristics of low cost and simple operation. Description of the Drawings

[0036] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for describing the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention, and those of ordinary skill in the art can also obtain other drawings based on these drawings without creative efforts.

[0037] Figure 1 It is a schematic diagram of a temporary plugging string when there is a leak in the upper branch of the present invention.

[0038] Figure 2 It is a schematic diagram of the lower branch string and the back-in string in the open-hole dual-branch completion of the present invention.

[0039] Figure 3 It is a schematic diagram of an open-hole dual-branch completion string (fracturing).

[0040] Figure 4 It is a schematic diagram of the opening and closing of the switchable sliding sleeve.

[0041] Figure 5 It is a schematic diagram of the structure after the seating nipple is blocked.

[0042] Figure 6 It is a flowchart of the usage method of an open-hole branch well completion string according to Embodiment 2 of the present invention.

[0043] In the drawings, the list of components represented by each reference numeral is as follows:

[0044] 1 - Upper casing, 2 - Upper branch, 3 - Temporary plugging string, 301 - First round - head guide shoe, 302 - First perforated screen pipe, 303 - POP valve, 304 - First open - hole packer, 305 - Hydraulic release tool, 306 - Drill pipe, 4 - Lower branch, 5 - Completion string, 501 - Second round - head guide shoe, 502 - Second perforated screen pipe, 503 - Float collar, 504 - Isolation valve, 5041 - Isolation valve pressure - holding ball, 505 - Differential pressure sliding sleeve, 506 - Second open - hole packer, 507 - Landing nipple, 5071 - Lock core, 508 - Switchable sliding sleeve, 5081 - Switching - sleeve pressure - holding ball, 5082 - Switching tool, 5083 - Ball seat, 5084 - Moving cavity, 509 - Insertion packer, 6 - Return - insertion string, 601 - Expansion joint. Detailed implementation mode

[0045] In order to enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. Based on the embodiments of the present invention, other similar embodiments obtained by those of ordinary skill in the art without creative efforts shall all fall within the scope of protection of the present invention. In addition, the directional terms mentioned in the following embodiments, such as "upper", "lower", "left", "right", etc., are only with reference to the directions of the drawings. Therefore, the directional terms used are for illustration rather than limiting the present invention.

[0046] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this application belongs. The terms used in the description of this application in this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0047] It should be noted that when an element is considered to be "connected" to another element, it can be directly connected to the other element or connected to the other element through an intermediate element. In the following embodiments, "connection", if there is an electrical signal or data transfer between the connected circuits, modules, units, etc., should be understood as "electrical connection", "communication connection", etc.

[0048] The present invention will be further described below in conjunction with the drawings and preferred embodiments.

[0049] Embodiment:

[0050] As Figures 1 - 5 shown, it is a preferred embodiment of the present invention. A completion string for an open - hole multilateral well includes a temporary plugging string 3 and a completion string 5. An upper casing 1 is also provided outside the completion string for the open - hole multilateral well. The temporary plugging string 3 is arranged in the upper branch 2 of the multilateral well, and the completion string is arranged in the lower branch 4 of the multilateral well.

[0051] AsFigure 1 As shown, the temporary plugging string includes a first round head guide shoe 301, a first perforated screen pipe 302, a POP valve 303, a first open hole packer 304 and a hydraulic release tool 305; one end of the temporary plugging string 3 is installed with the first round head guide shoe 301, and from the first round head guide shoe 301, the first perforated screen pipe 302, the POP valve 303, the first soluble open hole packer 304 and the hydraulic release tool 305 are successively installed on the corresponding tubing. The first open hole packer 304 is a soluble open hole packer. A dead plug is arranged in the POP valve 303, and the dead plug is made of a soluble material.

[0052] As Figure 2 , Figure 3 shown, the completion string 4 includes a second round head guide shoe 501, a second perforated screen pipe 502, a float collar 503, an isolation valve 504, a differential pressure sliding sleeve 505, a second open hole packer 506 and an insert packer 509; one end of the completion string 4 is installed with the second round head guide shoe 501, and from the second round head guide shoe 501, the second perforated screen pipe 502, the float collar 503, the isolation valve 504, the differential pressure sliding sleeve 505, the second open hole packer 506 and the insert packer 509 are successively installed.

[0053] Combined with Figure 4 shown, a landing nipple 507 and a switchable sliding sleeve 508 are further arranged on the completion string 4, and the landing nipple 507 and the switchable sliding sleeve 508 are arranged between the second open hole packer 506 and the insert packer 509. The switchable sliding sleeve 508 is arranged at the intersection of the upper branch 2 and the completion string 5, and the landing nipple 507 is arranged below the switchable sliding sleeve 508.

[0054] A slider is arranged inside the switchable sliding sleeve 508, a plurality of through holes are arranged in the circumferential direction of the switchable sliding sleeve 508, and the slider slides inside the switchable sliding sleeve 508 to realize the opening or closing of the through holes.

[0055] A hollow moving cavity 5084 is arranged inside the slider, an inlet of the moving cavity 5084 is arranged above the moving cavity 5084, an outlet of the moving cavity 5084 is arranged below the moving cavity 5084, a ball seat 5083 with a funnel-shaped structure is arranged at the top of the slider, and the top diameter of the ball seat 5083 gradually contracts inwards to the inlet of the moving cavity 5084; the diameters of the inlet and outlet of the moving cavity 5084 are smaller than the diameter of the moving cavity 5084.

[0056] The opening or closing of the switchable sliding sleeve 508 is controlled by any one of a soluble ball or a switching tool 5082.

[0057] As Figure 5As shown, a lock core groove with a funnel-shaped structure is provided inside the setting nipple 507. The diameter of the top of the lock core groove gradually contracts inward, and the shape of the lock core matching the setting nipple 507 is adapted to the shape of the lock core groove.

[0058] Embodiment 2:

[0059] As Figure 6 shown, a method for using a completion string for an open-hole multilateral well is applicable to the above-mentioned completion string for an open-hole multilateral well, and includes the following steps:

[0060] S1: Lower the temporary plugging string 3 to the predetermined position of the upper branch 2 through the drill pipe 306.

[0061] S2: Apply pressure inside the temporary plugging string 3 to the pressure P1 to set the first open-hole packer 304 and achieve temporary plugging of the upper branch 2.

[0062] S3: Continue to apply pressure inside the temporary plugging string 3 to the pressure P2 to disengage the hydraulic release tool 305 and achieve the release of the temporary plugging string 3, leaving the temporary plugging string 3 in the wellbore of the upper branch 2, where P1 < P2.

[0063] S4: Inject a chip-blocking liquid through the drill pipe into the upper part of the temporary plugging string 3; preferably, a high-temperature self-dissolving gel is used.

[0064] The reliable pressure-bearing time of the high-temperature self-dissolving gel is greater than the drilling time of the lower branch. Through the chip-blocking liquid, prevent cuttings from blocking the upper branch during the drilling of the lower branch.

[0065] S5: Then lower the completion string 5 through the drill pipe 306. After lowering it to the predetermined position of the lower branch 4, insert the isolation valve pressure-holding ball 5041 to block the isolation valve 504 and apply pressure to the pressure P3 to seal the entire completion string 5.

[0066] S6: Continue to pressurize inside the completion string 5. When the pressure is held to the pressure P4, expand and seal the second open-hole packer 506; when the pressure is held to the pressure P5, set and hang the setting and hanging plug packer 509.

[0067] S7: Conduct seal and hanging verification on the plug packer 509. After successful seal and hanging verification, hold the pressure to the pressure P6 to open the differential pressure sliding sleeve 505 and dredge the lower branch 4, waiting for the fracturing of the lower branch 4.

[0068] S8: Pull out the drill pipe 306 and lower the back-insertion string 6. Tools such as an expansion joint 601 and a dilution sliding sleeve can be installed on the back-insertion string 6 according to actual on-site requirements.

[0069] S9: Control the opening or closing of the switchable sliding sleeve 508 by inserting a soluble ball or lowering a switching tool 5082 to achieve the fracturing of the upper branch 2.

[0070] After the fracturing of the lower branch 4 is completed, the fracturing or water plugging of the upper branch can be achieved through the following two methods:

[0071] One is to control the switchable sliding sleeve 508 through a soluble ball. First, the soluble ball, that is, the pressure holding ball 5081 of the switchable sliding sleeve, is put in to open the switchable sliding sleeve 508, as shown in Figure a of Figure 4 . Pressurize the pipeline of the back-in string 6 to the pressure P7 to open the POP valve 303, dredge the upper branch 2, and perform the fracturing of the upper branch 2, where P1 < P2 < P7. If the upper branch 2 cannot be dredged by pressure, after the first open-hole packer 304 and the dead plug of the POP valve 304 are dissolved by themselves, (the switchable sliding sleeve has been opened and there is no need to put the ball again), perform the fracturing of the upper branch. During the production process, if the lower branch produces water, put in the lock core 5071 to block the lower branch 4; if the upper branch 2 produces water, lower the switching tool 5082 to close the switchable sliding sleeve 508 to block the upper branch 2. As shown in Figure c of Figure 4 , it is a schematic diagram of using the switching tool 5082 to close the switchable sliding sleeve 508. The top of the switching tool 5082 is an elastic claw. After the elastic claw contracts and extends into the moving cavity 5084 of the internal slider of the switchable sliding sleeve 508, the elastic claw opens, catches the entrance of the moving cavity 5084, drives the slider to rise, closes the through hole on the side of the switchable sliding sleeve 508, then contracts the elastic claw to disengage from the slider, and withdraws the switching tool.

[0072] The other is to lower the lock core 5071 to block the lower branch, and then lower the switching tool 5082 to open the switchable sliding sleeve 508, as shown in Figure b of Figure 4 . After the switchable sliding sleeve 508 is opened, pressurize to P7 to open the POP valve 303, dredge the upper branch, and perform the fracturing of the upper branch, where P1 < P2 < P7. If the upper branch 2 cannot be dredged by pressure, after the first open-hole packer 304 and the dead plug of the POP valve 304 are dissolved by themselves, perform the fracturing of the upper branch. Lower the fishing tool to take out the lock core 5071 for dual-branch combined production. During the combined production process, if the lower branch produces water, put in the lock core 5071 to block the lower branch 4; if the upper branch 2 produces water, lower the switching tool 5082 to close the switchable sliding sleeve 508.

[0073] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than a limitation on the protection scope of the present invention. Any simple modification or equivalent replacement of the technical solution of the present invention by those of ordinary skill in the art shall not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A completions string for a bare-eye multilateral well, characterized in that, It includes a temporary plugging pipe string and a completion pipe string, wherein the temporary plugging pipe string is arranged in the upper branch of the branch well, and the completion pipe string is arranged in the lower branch of the branch well; The completion pipe string is provided with a seating nipple and a switchable sliding sleeve, the switchable sliding sleeve is provided at the intersection of the upper branch and the completion pipe string, and the seating nipple is provided below the switchable sliding sleeve.

2. The openhole multilateral well completion string according to claim 1, wherein A slider is arranged inside the switchable sliding sleeve, and a plurality of through holes are arranged around the switchable sliding sleeve. The slider slides along the inside of the switchable sliding sleeve to open or close the through holes.

3. The open-hole multilateral well completion string according to claim 2, wherein A hollow moving cavity is arranged inside the slider, a moving cavity inlet is arranged above the moving cavity, a moving cavity outlet is arranged below the moving cavity, a ball seat with a funnel-shaped structure is arranged on the top of the slider, and the top diameter of the ball seat gradually shrinks inward to the moving cavity inlet; the diameters of the moving cavity inlet and the moving cavity outlet are smaller than the moving cavity diameter.

4. The open-hole multilateral well completion string according to claim 3, characterized in that, The switchable sliding sleeve is controlled to be opened or closed by any one of the structures of a soluble ball and a switch tool.

5. The openhole multilateral well completion string according to claim 1, wherein A lock core groove with a funnel-shaped structure is arranged inside the seating short section, and the top diameter of the lock core groove gradually shrinks inwards.

6. The openhole multilateral well completion string according to claim 1, characterized in that, The temporary plugging pipe string includes a first round head guide shoe, a first perforated screen, a POP valve, a first open hole packer and a hydraulic release device; the first round head guide shoe is installed at one end of the temporary plugging pipe string, and the first perforated screen, the POP valve, the first soluble open hole packer and the hydraulic release device are installed on the corresponding oil pipe in sequence from the first round head guide shoe.

7. The open-hole multilateral well completion string according to claim 6, characterized in that The first open hole packer is a soluble open hole packer.

8. The openhole multilateral well completion string according to claim 6, characterized in that, A dead plug is arranged in the POP valve, and the dead plug is made of soluble material.

9. The open-hole multilateral well completion string according to claim 1, characterized in that, The completion pipe string also includes a second round head guide shoe, a second perforated screen, a floating collar, an isolation valve, a pressure differential sleeve, a second open hole packer and a tubing packer; a second round head guide shoe is installed at one end of the completion pipe string, and a second perforated screen, a floating collar, an isolation valve, a pressure differential sleeve, a second open hole packer and a tubing packer are installed in sequence from the second round head guide shoe.

10. The open-hole multilateral well completion string according to claim 9, characterized in that, The landing nipple and the switchable sliding sleeve are arranged between the second open hole packer and the intubation packer.

11. A method for using a completion string in an open-hole multilateral well, applicable to the completion string in an open-hole multilateral well described in any one of claims 1-10, characterized in that, The following steps are involved: S1: Send the drill pipe to lower the temporary plugging pipe string to the predetermined position of the upper branch; S2: The pressure inside the temporary plugging pipe string is pressurized to P1, and the first open hole packer is set to achieve temporary plugging of the upper branch; S3: Continue to pressurize the temporary plugging pipe string to a pressure of P2, so that the hydraulic release device is disengaged, and the temporary plugging pipe string is released, leaving the temporary plugging pipe string in the wellbore of the upper branch, wherein P1 < P2; S4: injecting chip blocking liquid into the upper part of the temporary plugging pipe string by inserting the drill pipe; S5: The completion pipe string is then sent into the open-hole branch well through the drill pipe. After it is lowered into the predetermined position of the lower branch, the isolation valve pressure-holding ball is inserted to block the isolation valve, and the pressure is increased to P3 to block the entire completion pipe string; S6: Continue to pressurize the completion string. When the pressure reaches P4, expand and seal the second open hole packer. When the pressure reaches P5, use the hanging and sealing plug packer. S7: Check the sealing and hanging of the intubation packer. After the sealing and hanging are successful, hold the pressure to P6 to open the differential pressure sleeve, dredge the lower branch, and wait for the fracturing of the lower branch. S8: Pull out the drill pipe and lower the return pipe string; S9: Control the opening or closing of the switchable sliding sleeve by injecting soluble balls or lowering a switching tool, and implement the fracturing of the upper branch.

12. A method for using a completion string for a bare-eye multilateral well according to claim 11, characterized in that, In step S4, the chip-blocking liquid is a high-temperature resistant self-dissolving gel.

13. A method for using a completion string for a bare-eye multilateral well according to claim 11, characterized in that, In step S9, controlling the switchable sliding sleeve by the soluble ball specifically means injecting soluble balls to open the switchable sliding sleeve, pressurizing in the pipeline of the back-in string to pressure P7 to open the POP valve to dredge the upper branch, and implementing the fracturing of the upper branch, where P1 < P2 < P7.

14. A method for using a completion string for a bare-eye multilateral well according to claim 11, characterized in that, In step S9, controlling the switchable sliding sleeve by lowering the switching tool specifically means lowering the lock core into the lock core groove inside the landing nipple to block the lower branch, then lowering the switching tool to open the switchable sliding sleeve. After the switchable sliding sleeve is opened, pressurize in the pipeline to pressure P7 to open the POP valve to dredge the upper branch, and implement the fracturing of the upper branch, where P1 < P2 < P7.

15. A method for using a completion string for a naked-eye multilateral well according to claim 11, characterized in that, During the construction process, if water comes out from the lower branch, lower the lock core into the lock core groove inside the landing nipple to block the lower branch; if water comes out from the upper branch, lower the switching tool to close the switchable sliding sleeve.

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