Horizontal well oil extraction balance control pipe column, system and method

Abstract

The invention belongs to the technical field of oilfield development, and particularly relates to a horizontal well oil extraction balance control tubular column, system and method. The balance control pipe column is organically composed of a guide plug, a tail pipe, a balance inflow control tool module, a packer, a chamfering oil pipe and a releasing packer. The balance control pipe column, the releasing pipe column, the production pipe column and the oil pipe form a horizontal well balance oil production system. A horizontal well section perforation section is blocked and opened in a segmented mode, and each section corresponds to one balance inflow control tool module, so that the interference contradiction of wellbore flow resistance difference caused by the influence of factors such as a long horizontal well section, reservoir heterogeneity and a well track between section clusters in a layer is overcome, injected water breakthrough and bottom water coning are restrained, and the well flow resistance difference interference contradiction caused by the influence of the factors such as the long horizontal well section and the reservoir heterogeneity is restrained. And the productivity of each layer section is uniformly exerted. The integral assembly construction is simple and convenient, the operation is simple, the underground self-adaptive control can be realized, and the device can be effective for a long time without human intervention after being tripped into an oil well at one time.

Description

Technical Field

[0001] The present invention belongs to the technical field of oilfield development, and particularly relates to a horizontal well oil production balanced control string, system and method. Background Art

[0002] As an important means to improve the single well production and transform the development mode, horizontal wells have been applied in various oilfields for many years. At present, more than 80% of the proven reserves in PetroChina are low-permeability reservoirs. Horizontal wells are the main technology for effectively developing low-quality reservoirs. The annual production capacity deployment ratio of horizontal wells in the oilfield is 50% - 65%. The horizontal well section is long, the number of fractured sections is large, the interference contradiction between clusters in the layer is prominent, and the production difference is obvious. Therefore, it is urgent to achieve efficient and balanced oil production in each section of the horizontal section, inhibit the breakthrough of injected water and bottom water coning, uniformly exert the production capacity of each layer section, and improve the development effect of horizontal wells.

[0003] At present, for the balanced oil production technology, packers are used for sectional packoff. A one-way injection valve and an automatic gas and water control valve are installed in each section. Gas is injected from the ground into each reservoir for energy storage, driving crude oil to flow along the formation pores or fractures, into the perforation channels, and then into the wellbore. Oil, gas and water enter the integrated injection-production string through the downhole automatic gas and water control valve. Downhole crude oil is produced to the ground by means of natural flow or mechanical production. Or packers are used to separate the oil and water layer sections, and during the pumping process, the flow rates of the oil distribution nozzle and the water drainage nozzle are adjusted to adjust the pressure of oil and water in the reservoir, so that the oil and water in the reservoir always remain balanced, and the oil recovery rate is improved. Or a central pipe is run in the horizontal well section to overcome the wellbore pressure drop and friction, limit the inflow at the heel end, release the production capacity at the toe end, and achieve balanced oil production.

[0004] However, when using the above oil production technologies for production, there are problems such as complex process strings, the need for manual monitoring and adjustment on the ground, and low automation level. Summary of the Invention

[0005] The present invention provides a horizontal well oil production balanced control string, system and method. One of the purposes is to provide a device and method that can solve the problems of long horizontal well sections, large number of fractured sections, prominent interference contradiction between clusters in the layer, and obvious production differences. Another purpose is to provide a device and method with a short overall construction period, simple operation, and capable of effectively avoiding delaying the production capacity construction period.

[0006] To achieve the above purposes, the technical solution adopted by the present invention is: A horizontal well oil production balanced control string, comprising a guiding plug, a tail pipe, a balanced inflow control tool module, a packer, a chamfered tubing, and a releasing packer; a plurality of packers are provided, and the plurality of packers are used to block and separate intervals; a balanced inflow control tool module is connected between adjacent packers through a connecting tubing; the balanced inflow control tool module at the front end is connected to one end of the tail pipe, and the other end of the tail pipe is connected with a guiding plug; the balanced inflow control tool module at the end is connected to the releasing packer through the chamfered tubing.

[0007] The releasing packer used is an inserted pipe packer of model Y445.

[0008] The packer used is a non-slip-jaw fluid-encounter self-expanding packer.

[0009] The balanced inflow control tool module includes an upper joint, a pressure sensing valve, an induction cavity, a shape memory alloy spring, an adjusting member, a transmission member, a sliding valve, a lateral bridge inlet, a central flow passage, a housing, and a lower joint; the housing is a tubular structure with a central flow passage in the center; the upper and lower ends of the housing are respectively connected with an upper joint and a lower joint, and threads for connecting with the chamfered tubing, connecting tubing, or tail pipe are provided on the side walls of the upper joint and the lower joint; a pressure sensing valve is connected to the outer side wall of the housing near the upper joint end; an induction cavity is provided on the housing, and the pressure sensing valve is connected to the entrance of the induction cavity; a shape memory alloy spring is connected in the induction cavity, one end of the shape memory alloy spring is fixed on the inner side wall of the induction cavity, and the other end of the shape memory alloy spring is connected to the sliding valve through the adjusting member; the sliding valve is slidably connected between the housing and the lateral bridge inlet.

[0010] A horizontal well balanced oil production system at least includes a balanced control string, and also includes a releasing string, a production string, and a tubing; the releasing string is used for the lowering and pressure application and setting of the balanced control string, and is placed in the tubing when the balanced control string is lowered and pressure applied and set; the production string is used for oil production; during oil production, the production string is placed in the tubing and is hermetically connected to the releasing packer at the upper end of the balanced control string.

[0011] The production string includes an inserted pipe, a sucker rod pump, a sucker rod, and a pumping unit; the upper end of the balanced control string is connected to the tubing through the inserted pipe; a sucker rod pump is provided in the tubing, and the upper end of the sucker rod pump is connected to the pumping unit through the sucker rod.

[0012] A balanced oil production method for a horizontal well balanced oil production system includes the following steps Step 1: Process the wellbore; Ream the wellbore and wash the well with sand to the artificial bottom hole; Step 2: Assemble the horizontal well balanced production string; Step 3: Lower the assembled horizontal well balanced production string into the target interval in the horizontal well; Step 4: After confirming that the setting depth of the releasing packer in the balanced control string is correct, drop in a steel ball. After the ball reaches the designated position, apply pressure to set the releasing packer; Step 5: Apply pressure according to the preset pressure, and maintain the pressure for the preset time. Subsequently, apply pressure to the preset value and then suddenly reduce the pressure to the preset value to achieve releasing, and remove the releasing string; Step 6: Assemble the production string, and lower the production string into the wellbore and seal it with the releasing packer at the upper end of the balanced control string to form an oil production system; Step 7: Start the pumping unit. When the pressure in a certain section is abnormal, the pressure sensing valve on the balanced inflow control tool module in this section opens, and the well fluid enters the induction cavity. When the shape memory alloy spring encounters the high-temperature well fluid, the slide valve closes, restricting the fluid in this layer from entering. On the contrary, after the pressure returns to normal, the pressure sensing valve closes, the temperature drops, the shape memory alloy spring returns to its original state, the slide valve opens, and the produced fluid in this section enters the central flow-through channel again, realizing balanced oil production in the horizontal well section in real time.

[0013] In Step 5, pressure is applied at a preset pressure of 10 MPa, and then the pressure is maintained for 5 minutes; after applying pressure of 12 MPa, the pressure is suddenly reduced to 2 MPa to achieve releasing.

[0014] Beneficial effects: (1) By segmentally blocking and opening the perforated intervals in the horizontal well section of the present invention, each section corresponds to a balanced inflow control tool module, which overcomes the interference contradictions caused by the differences in wellbore flow resistance due to factors such as reservoir heterogeneity and wellbore trajectory in the intra-layer segment clusters, inhibits the breakthrough of injected water and bottom water coning, and evenly exerts the production capacity of each layer segment.

[0015] (2) The overall assembly and construction of the present invention are simple, the operation is easy, the downhole can adaptively control, and it can be effectively used for a long time after being lowered once without manual intervention.

[0016] The above description is only an overview of the technical solution of the present invention. In order to understand the technical means of the present invention more clearly and be able to implement it according to the content of the description, the following further details the preferred embodiments of the present invention in conjunction with the accompanying drawings. Brief description of the drawings

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0018] Figure 1It is a structural schematic diagram of the device of the present invention; Figure 2 It is a structural schematic diagram of the balanced inflow control tool module in the present invention.

[0019] In the figure: 1. guide wire plug; 2. tail pipe; 3. balanced inflow control tool module; 4. packer; 5. chamfered oil pipe; 6. drop packer; 7. cannula; 8. oil pump; 9. oil pipe; 10. sucker rod; 11. pumping unit; 12. upper joint; 13. pressure sensing valve; 14. induction chamber; 15. memory alloy spring; 16. adjusting part; 17. transmission part; 18. sliding valve; 19. lateral bridge inlet; 20. central flow channel; 21. shell; 22. lower joint. DETAILED DESCRIPTION

[0020] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0021] Embodiment 1: according to Figure 1 and Figure 2 A horizontal well oil production balanced control string shown in the figure includes a guide wire plug 1, a tail pipe 2, a balanced inflow control tool module 3, a packer 4, a chamfered oil pipe 5 and a release packer 6; a plurality of packers 4 are provided, and a plurality of packers 4 seal and separate the layer sections; a balanced inflow control tool module 3 is connected between adjacent packers 4 through a connecting oil pipe; the balanced inflow control tool module 3 located at the front end is connected to one end of the tail pipe 2, and the other end of the tail pipe 2 is connected to the guide wire plug 1; the balanced inflow control tool module 3 located at the end is connected to the release packer 6 through the chamfered oil pipe 5.

[0022] In actual use, the design of the guide wire plug 1 ensures the sealing of the entire pipeline.

[0023] In specific applications, multiple packers 4 and a drop packer 6 seal and separate the layer sections. The corresponding number of packers 4 is lowered according to the requirements of horizontal section end separation. The drop packer 6 is located above the inclination point to seal and anchor the entire balanced control string.

[0024] A balanced inflow control tool module 3 is correspondingly arranged between each pair of packers 4. When working, when the pressure exceeds the upper limit of the balanced inflow control tool module 3, the balanced inflow control tool module 3 is in a closed state, restricting the fluid in this section from entering the oil pipe and exerting the production capacity of other layers.

[0025] In the present invention, by segmentally blocking and opening the perforated sections of the horizontal well section, each section is correspondingly provided with a balanced inflow control tool module 3, which overcomes the interference contradiction caused by the differences in wellbore flow resistance within the formation due to factors such as long horizontal well sections, reservoir heterogeneity, and wellbore trajectory, suppresses the breakthrough of injected water and the coning of bottom water, and uniformly exerts the productivity of each section.

[0026] The overall assembly and construction of the horizontal well balanced oil production device of the present invention are simple, the operation is easy, it has downhole adaptive control, and it is effective for a long time after being lowered once without human intervention.

[0027] Embodiment 2: According to Figure 1 A horizontal well oil production balanced control string shown, which is different from Embodiment 1 in that: the releasing packer 6 used is an intubating packer of model Y445.

[0028] Furthermore, the packer 4 used is a non-slip-jaw fluid-expandable packer.

[0029] During actual use, the releasing packer 6 uses an intubating packer of model Y445. If the pump fails and needs to be replaced, during pump inspection, it is lifted together with the intubation 7 from the ground, and the components below the packer still remain in the wellbore. After replacing the integral barrel pump, the intubation 7 is inserted back into the packer to reconstitute the production string.

[0030] The packer 4 used is a non-slip-jaw fluid-expandable packer, which is convenient for effective sealing and later removal.

[0031] Embodiment 3: According to Figure 1 and Figure 2 A horizontal well oil production balanced control string shown, which is different from Embodiment 1 in that: the balanced inflow control tool module 3 includes an upper joint 12, a pressure sensing valve 13, an induction cavity 14, a shape memory alloy spring 15, an adjusting part 16, a transmission part 17, a sliding valve 18, a lateral bridge-type inlet 19, a central flow-through channel 20, a housing 21 and a lower joint 22; the housing 21 is a tubular structure with a central flow-through channel 20 in the center; the upper and lower ends of the housing 21 are respectively connected with an upper joint 12 and a lower joint 22, and threads for connecting with the chamfered tubing 5, connecting tubing or tail pipe 2 are arranged on the side walls of the upper joint 12 and the lower joint 22; a pressure sensing valve 13 is connected to the outer side wall of the housing 21 near the upper joint 12 end; an induction cavity 14 is arranged on the housing 21, and the pressure sensing valve 13 is connected to the inlet of the induction cavity 14; a shape memory alloy spring 15 is connected in the induction cavity 14, one end of the shape memory alloy spring 15 is fixed on the inner side wall of the induction cavity 14, and the other end of the shape memory alloy spring 15 is connected with the sliding valve 18 through the adjusting part 16; the sliding valve 18 is slidably connected between the housing 21 and the lateral bridge-type inlet 19.

[0032] During actual use, when the pressure of a certain section is abnormal, the pressure sensing valve 13 opens, and well fluid enters the sensing cavity 14. When the shape memory alloy spring 15 encounters the high-temperature well fluid, the sliding valve 18 closes to restrict the entry of the fluid of this layer. On the contrary, after the pressure returns to normal, the pressure sensing valve 13 closes, the temperature drops, the shape memory alloy spring 15 returns to its original state, the sliding valve 18 opens, and the produced fluid of this section enters the central flow-through channel 20 again through the lateral bridge-type inlet 19 to achieve balanced oil production in the real-time horizontal well section.

[0033] Example 4: Refer to Figure 1 and Figure 2 A horizontal well balanced oil production system includes at least a balanced control string, and also includes a releasing string, a production string and a tubing 9. The releasing string is used for the lowering and pressure application for setting of the balanced control string, and is placed inside the tubing 9 during the lowering and pressure application for setting of the balanced control string. The production string is used for oil production. During oil production, the production string is placed inside the tubing 9 and is hermetically connected to the releasing packer 6 at the upper end of the balanced control string.

[0034] Further, the production string includes an inserted pipe 7, a submersible pump 8, a sucker rod 10 and a pumping unit 11. The upper end of the balanced control string is connected to the tubing 9 through the inserted pipe 7. A submersible pump 8 is arranged inside the tubing 9, and the upper end of the submersible pump 8 is connected to the pumping unit 11 through the sucker rod 10.

[0035] During actual use, first, the wellbore is circulated and sand flushed until the artificial bottom hole. Then, the horizontal well balanced production string is assembled. Next, the assembled horizontal well balanced production string is lowered into the target interval in the horizontal well. After confirming that the setting depth of the releasing packer 6 in the balanced control string is correct, a steel ball is dropped. After the ball reaches the designated position, pressure is applied to set the releasing packer 6. Pressure is applied according to the preset pressure of 10 MPa, and then the pressure is stabilized for 5 minutes. Then, after applying a pressure of 12 MPa and suddenly dropping to 2 MPa, releasing is achieved and the releasing string is removed. Then, the production string is assembled and lowered into the wellbore and hermetically connected to the releasing packer 6 at the upper end of the balanced control string to form an oil production system. Then, the pumping unit 11 is started. When the pressure of a certain section is abnormal, the pressure sensing valve 13 on the balanced inflow control tool module 3 in this section opens, and well fluid enters the sensing cavity 14. When the shape memory alloy spring 15 encounters the high-temperature well fluid, the sliding valve 18 closes to restrict the entry of the fluid of this layer. On the contrary, after the pressure returns to normal, the pressure sensing valve 13 closes, the temperature drops, the shape memory alloy spring 15 returns to its original state, the sliding valve 18 opens, and the produced fluid of this section enters the central flow-through channel 20 again to achieve balanced oil production in the real-time horizontal well section.

[0036] In the present invention, the perforated sections of the horizontal well section are segmented and blocked and opened, and each section corresponds to a balanced inflow control tool module, which overcomes the interference contradiction of the wellbore flow resistance difference caused by factors such as the long horizontal well section, reservoir heterogeneity, and wellbore trajectory within the layer segment cluster, suppresses the breakthrough of injected water and bottom water coning, and evenly exerts the productivity of each layer segment.

[0037] The overall assembly and construction of the present invention are simple, the operation is easy, it can be adaptively controlled downhole, and after being lowered once, it can be effective for a long time without human intervention.

[0038] Example Five: Refer to Figure 1 and Figure 2 , an oil production method for a horizontal well balanced oil production system, including the following steps: Step 1: Process the wellbore; Run a drift and wash the wellbore with sand to the artificial bottom hole; Step 2: Assemble the horizontal well balanced production string; Step 3: Lower the assembled horizontal well balanced production string into the target interval in the horizontal well; Step 4: After confirming that the setting depth of the release packer 6 in the balanced control string is correct, drop in a steel ball. After the ball reaches the designated position, apply pressure to set the release packer 6; Step 5: Apply pressure according to the preset pressure, and keep the pressure stable for the preset time. Then, after applying pressure to the preset value, suddenly reduce the pressure to the preset value pressure to achieve release, and remove the release string; Step 6: Assemble the production string, and lower the production string into the wellbore and seal it with the release packer 6 at the upper end of the balanced control string to form an oil production system; Step 7: Start the pumping unit 11. When the pressure of a certain section is abnormal, the pressure sensing valve 13 on the balanced inflow control tool module 3 in this section opens, and the well fluid enters the induction cavity 14. When the shape memory alloy spring 15 encounters the high-temperature well fluid, the slide valve 18 closes to restrict the fluid of this layer from entering. On the contrary, after the pressure returns to normal, the pressure sensing valve 13 closes, the temperature drops, the shape memory alloy spring 15 returns to its original state, the slide valve 18 opens, and the produced fluid of this section enters the central flow channel 20 again to realize real-time balanced oil production in the horizontal well section.

[0039] Further, in Step 5, the pressure is applied at a preset pressure of 10 MPa, and then the pressure is kept stable for 5 minutes; after applying a pressure of 12 MPa, the pressure is suddenly reduced to 2 MPa to achieve release.

[0040] In actual use, the present invention segments and seals the perforated sections of the horizontal well section, and each section corresponds to a balanced inflow control tool module, overcoming the interference contradiction of the wellbore flow resistance difference caused by factors such as reservoir heterogeneity and wellbore trajectory in the long horizontal well section between intra-layer clusters, suppressing the breakthrough of injected water and bottom water coning, and evenly exerting the productivity of each section.

[0041] The overall assembly and construction of the present invention are simple, the operation is easy, it can adaptively control downhole. After being lowered once, it can be effective for a long time without human intervention.

[0042] Without conflict, those skilled in the art can combine the relevant technical features in the above examples according to the actual situation to achieve the corresponding technical effects. The specific combinations are not elaborated here one by one.

[0043] It should be noted that all directional indications (such as up, down, left, right, front, back...) in the embodiments of the present invention are only used to explain the relative position relationship and movement conditions between components in a specific posture (as shown in the drawings). If the specific posture changes, the directional indications will also change accordingly.

[0044] In addition, the descriptions involving "first", "second", etc. in the present invention are only for descriptive purposes, and cannot be understood as indicating or implying their relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one of such features.

[0045] As mentioned above, these are only the preferred embodiments of the present invention. The present invention will not be limited to these embodiments shown in this article, but will conform to the broadest scope consistent with the principles and novel features disclosed herein. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A horizontal well oil production balanced control string, characterized in that: It includes a guiding plug (1), a liner (2), a balanced inflow control tool module (3), a packer (4), a chamfered tubing (5) and a releasing packer (6); multiple packers (4) are provided, and the multiple packers (4) clamp and separate the intervals; a balanced inflow control tool module (3) is connected between adjacent packers (4) through a connecting tubing; the balanced inflow control tool module (3) at the forefront is connected to one end of the liner (2), and the other end of the liner (2) is connected with a guiding plug (1); the balanced inflow control tool module (3) at the rearmost end is connected to the releasing packer (6) through the chamfered tubing (5).

2. The equalizing control string for horizontal well oil production according to claim 1, characterized in that: The releasing packer (6) used is an intubating packer with the model Y445.

3. The balanced control string for oil production in horizontal wells according to claim 1, characterized in that: The packer (4) used is a non-slip-jaw fluid-encounter self-expanding packer.

4. The balanced control string for horizontal well oil production according to claim 1, characterized in that: The balanced inflow control tool module (3) includes an upper sub (12), a pressure sensing valve (13), an induction cavity (14), a shape memory alloy spring (15), an adjusting part (16), a transmission part (17), a sliding valve (18), a lateral bridge-type inlet (19), a central flow-through channel (20), a housing (21) and a lower sub (22); the housing (21) is a tubular structure with a central flow-through channel (20) in the center; the upper and lower ends of the housing (21) are respectively connected with an upper sub (12) and a lower sub (22), and threads for connecting with the chamfered tubing (5), the connecting tubing or the liner (2) are provided on the side walls of the upper sub (12) and the lower sub (22); a pressure sensing valve (13) is connected to the outer side wall of the housing (21) near the upper sub (12) end; an induction cavity (14) is provided on the housing (21), and the pressure sensing valve (13) is connected to the inlet of the induction cavity (14); a shape memory alloy spring (15) is connected in the induction cavity (14), one end of the shape memory alloy spring (15) is fixed on the inner side wall of the induction cavity (14), and the other end of the shape memory alloy spring (15) is connected with the sliding valve (18) through the adjusting part (16); the sliding valve (18) is slidably connected between the housing (21) and the lateral bridge-type inlet (19).

5. A horizontal well balanced oil production system, characterized in that: It includes at least the balanced control string according to any one of claims 1-4, and also includes a releasing string, a production string and a tubing (9); the releasing string is used for the downhole operation and pressure application for setting of the balanced control string, and is placed in the tubing (9) during the downhole operation and pressure application for setting of the balanced control string; the production string is used for oil production; during oil production, the production string is placed in the tubing (9) and is hermetically connected to the releasing packer (6) at the upper end of the balanced control string.

6. The horizontal well balanced oil production system according to claim 5, characterized in that: The production string includes an intubation (7), a sucker rod pump (8), a sucker rod (10) and a pumping unit (11); the upper end of the balanced control string is connected to the tubing (9) through the intubation (7); a sucker rod pump (8) is arranged in the tubing (9), and the upper end of the sucker rod pump (8) is connected to the pumping unit (11) through the sucker rod (10).

7. The balanced oil production method of a horizontal well balanced oil production system according to any one of claims 5 or 6, characterized in that: It includes the following steps Step 1: Treat the wellbore; Ream the wellbore and wash the well with sand to the artificial bottom hole; Step 2: Assemble the horizontal well balanced production string Step 3: Lower the assembled horizontal well balanced production string into the target interval in the horizontal well; Step 4: After confirming that the setting depth of the releasing packer (6) in the balanced control string is correct, drop in a steel ball. After the ball reaches the designated position, apply pressure to set the releasing packer (6); Step 5: Apply pressure according to the preset pressure and maintain the pressure for the preset time. Then, apply pressure to the preset value and suddenly reduce the pressure to the preset value to achieve releasing, and remove the releasing string; Step 6: Assemble the production string and lower the production string into the wellbore to be sealingly connected to the releasing packer (6) at the upper end of the balanced control string to form an oil production system; Step 7: Start the pumping unit (11). When the pressure in a certain section is abnormal, the pressure sensing valve (13) on the balanced inflow control tool module (3) in this section opens, and the well fluid enters the induction cavity (14). When the shape memory alloy spring (15) encounters the high-temperature well fluid, the slide valve (18) closes to restrict the fluid in this layer from entering. On the contrary, after the pressure returns to normal, the pressure sensing valve (13) closes, the temperature drops, the shape memory alloy spring (15) returns to its original state, the slide valve (18) opens, and the produced fluid in this section enters the central flow-through channel (20) again to achieve real-time balanced oil production in the horizontal well section.

8. The balanced oil production method of a horizontal well balanced oil production system according to claim 7, characterized in that: In Step 5, the pressure is applied at 10 MPa according to the preset pressure, and then the pressure is maintained for 5 minutes; after applying pressure to 12 MPa, the pressure is suddenly reduced to 2 MPa to achieve releasing.

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