Electrically controlled toe fracturing sliding sleeve

Abstract

The application discloses an electrically-controlled toe fracturing sliding sleeve and belongs to the technical field of oil and gas drilling and production equipment, which comprises joints, an inner cylinder, an outer cylinder, a sliding sleeve, a piston body, a support sieve plate, a water-absorbing expansion material cavity, an explosive cavity, a first isolation film, an electronic ignition device and a triggering mechanism, wherein the joints are two, the two ends of the inner and outer cylinders are respectively in sealed connection with the two joints, the sliding sleeve is located in the annular cavity between the inner and outer cylinders and is used for blocking the communication of the sand blasting holes on the inner and outer cylinders, and the first isolation film is used for separating the water-absorbing expansion material in the water-absorbing expansion material cavity from water. The explosive of the explosive substance is used to drive the piston body to move, the first isolation film is damaged after the piston moves, the water-absorbing expansion material is contacted with water and is expanded after absorbing water, the explosive substance required by the application is less, the impact on the pipe string is small, meanwhile, the expansion material of the application is used to absorb water and expand to drive the sliding sleeve to move, and the whole moving process is slower and more stable.

Description

Technical Field

[0001] This invention relates to the field of oil and gas drilling and production equipment technology, and in particular to fracturing equipment, specifically an electrically controlled toe fracturing sleeve. Background Technology

[0002] The toe-end fracturing sleeve is a key piece of equipment in downhole fracturing operations. Its structure mainly consists of an inner cylinder, an outer cylinder, and a sleeve located between the two. The main function of the sleeve is to seal the blasting holes on the inner and outer cylinders, thereby effectively blocking fluid flow between the two cylinders. Once the sleeve moves under external force, allowing the blasting holes to connect, the fracturing operation can proceed smoothly.

[0003] In existing technologies, toe-end fracturing sleeves typically employ a method where both ends of the sleeve are connected to fluids inside the inner cylinder and outside the outer cylinder, respectively. The increased pressure difference between the inner and outer cylinders drives the sleeve's movement. Furthermore, to prevent malfunctions during wellbore pressure testing, multiple shear pins are usually used to secure the sleeve to either the inner or outer cylinder. However, this shear pin-based fracturing often introduces an error margin of approximately 15%, and the deviation in opening pressure increases further with the number of pins, undoubtedly causing numerous inconveniences for cementing and gauging operations.

[0004] To address the aforementioned issues, patent CN107288602B discloses an electrically controlled toe-end fracturing sleeve, which precisely controls the opening and closing of the flow channel at the connection end between the inner cylinder and the sleeve via an electric mechanism. The electric mechanism only opens the flow channel when it is determined that the sleeve needs to be moved, thereby reducing the number of shear pins used and ensuring that the fluid pressure in the inner cylinder can easily push the sleeve to move after the flow channel is opened, thus reducing the deviation in opening pressure. However, this design adds an electric drive mechanism, making the overall structure more complex.

[0005] To simplify the structure, patent CN117345188A discloses a pre-programmed, electrically controlled toe-end fracturing sleeve. It uses electronic ignition to detonate the explosive, utilizing the high-pressure shock wave generated by the explosion to propel the sleeve, thus eliminating the need for an electric drive mechanism. However, it is worth noting that because the sleeve needs to withstand a significant pressure difference between the inner and outer cylinders during wellbore pressure testing, and because both the sleeve and the inner and outer cylinders are made of hard materials, even slight deformation of the inner or outer cylinders or the entry of impurities into the friction surface can cause a sharp increase in the sliding friction of the sleeve, severely affecting its successful opening. To ensure successful opening, a large amount of explosives must be used to generate an extremely strong shock wave, but this shock wave is destructive and could potentially damage the surrounding casing, such as causing deformation or breakage, thereby affecting normal oil and gas extraction. Summary of the Invention

[0006] To address at least one of the aforementioned problems, the present invention provides an electrically controlled toe-end fracturing sleeve, which has the advantages of high success rate of opening, smooth opening process, and minimal damage to the tubing.

[0007] The specific solution of the present invention is as follows:

[0008] An electrically controlled toe-end fracturing sleeve, comprising:

[0009] Two spaced-apart connectors;

[0010] The inner cylinder is connected to two connectors at each end;

[0011] The outer cylinder is connected to two connectors at both ends, and the inner cylinder is located inside the outer cylinder and the two are arranged at intervals. Both the inner cylinder and the outer cylinder have radially penetrating sandblasting holes on their side walls.

[0012] The sliding sleeve, which is slidably and sealingly connected to the outer wall of the inner cylinder and the inner wall of the outer cylinder, is used to block the connection channel between the sandblasting holes on the inner and outer cylinders; the sliding sleeve divides the annular space between the inner and outer cylinders into upper and lower parts.

[0013] A ring-shaped support sieve plate located in the annular cavity between the inner cylinder and the outer cylinder, and fixedly connected to the inner cylinder or the outer cylinder. The support sieve plate is provided with sieve holes that penetrate the support sieve plate body along the axial direction of the inner cylinder.

[0014] The water-absorbing and expanding material cavity is located in the annular cavity between the inner and outer cylinders and between the supporting sieve plate and the sliding sleeve. It is used to store the water-absorbing and expanding material. The size of the water-absorbing and expanding material is larger than the size of the sieve holes on the supporting sieve plate.

[0015] A first isolation membrane used to separate water-absorbing and swelling materials from water;

[0016] An explosive chamber located in the annular cavity between the inner and outer cylinders, used to store explosive materials;

[0017] The piston body is slidably and sealingly connected to the outer wall of the inner cylinder and the inner wall of the outer cylinder. One end of the explosive chamber is sealed and fixedly connected to the inner or outer cylinder, and the other end is connected to one end of the piston body, so that the movement of the piston body is linked to the volume of the explosive chamber. When the volume of the explosive chamber increases, the piston body moves, thereby breaking the first isolation membrane and allowing the water-absorbing and swelling material to come into contact with water. The piston body is made of elastic material.

[0018] An electronic ignition device used to detonate explosive substances in a blast chamber;

[0019] A triggering mechanism used to activate an electronic ignition device.

[0020] In one specific embodiment of the present invention, the first isolation membrane is annular and is sealed to the inner and outer cylinders, and the annular cavity between the first isolation membrane and the piston body is a water storage chamber for storing water.

[0021] Furthermore, the piston body is provided with a membrane-breaking mechanism at one end facing the first isolation membrane, which is used to puncture the first isolation membrane, and the water storage chamber has a gas phase space.

[0022] As a specific embodiment of the present invention, the inner cylinder or the outer cylinder is provided with a first fluid channel that radially penetrates the body. The first fluid channel is connected to the water-absorbing and expanding material cavity and is used to introduce water into the water-absorbing and expanding material cavity. The first isolation membrane is located in the water-absorbing and expanding material cavity. The piston body is provided with a membrane-breaking mechanism at one end facing the first isolation membrane. The membrane-breaking mechanism can pass through the through hole on the sieve plate, thereby piercing the first isolation membrane.

[0023] Furthermore, the inner or outer cylinder is provided with a balance hole that radially penetrates the body. The balance hole is connected to the explosive chamber and is used to introduce water into the explosive chamber to balance the pressure at the upper and lower ends of the sliding sleeve. The explosive chamber is provided with a second isolation membrane to separate the water from the explosive.

[0024] Furthermore, the second separator is made of an elastic material.

[0025] As a specific embodiment of the present invention, the sliding sleeve is provided with a through hole corresponding to the sandblasting hole. When the through hole is aligned with the sandblasting hole, the sandblasting holes on the inner cylinder and the outer cylinder are centered and connected. When the through hole is far away from the sandblasting hole, the sandblasting holes on the inner cylinder and the outer cylinder are closed.

[0026] Furthermore, a limiting component is fixedly provided on the outer wall of the inner cylinder or the inner wall of the outer cylinder, so that the sliding sleeve will no longer move after the sandblasting holes on the inner cylinder and the outer cylinder are aligned with the through holes on the sliding sleeve.

[0027] Compared with existing technologies, it has the following advantages:

[0028] This invention utilizes the explosion of an explosive substance to propel the piston body. After the piston moves, it breaks the first isolation membrane, allowing the water-absorbing and expanding material to absorb water and expand upon contact with it. Since the explosive substance is only used to propel the piston body and not the heavier sliding sleeve, less explosive substance is needed, resulting in less impact on the tubing. At the same time, this invention uses the expansion material to propel the sliding sleeve, making the entire movement process slower and further reducing the impact on the tubing. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of a specific embodiment of the electrically controlled toe-end fracturing sliding sleeve of the present invention;

[0030] Figure 2 yes Figure 1 Front view of the middle sliding sleeve;

[0031] Figure 3 This is a schematic diagram of another specific embodiment of the electrically controlled toe-end fracturing sleeve of the present invention;

[0032] In the figure, there are: connector 100; inner cylinder 210; outer cylinder 220; sliding sleeve 310; piston body 320; support sieve plate 510; water-absorbing and expanding material cavity 610; explosive cavity 620; first isolation membrane 710; second isolation membrane 720; electronic ignition device 810; triggering mechanism 820; upper connector 110; lower connector 120; sandblasting hole 201; sieve hole 501; membrane breaking mechanism 321; limiting member 202; water storage cavity 630; first fluid channel 203; balance hole 204; and connecting hole 205. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0034] Example

[0035] Please refer to Figures 1-3This document illustrates the overall structure of two specific embodiments of the electrically controlled toe-end fracturing sleeve 310 of the present invention. The electrically controlled toe-end fracturing sleeve includes a connector 100, an inner cylinder 210, an outer cylinder 220, a sleeve 310, a piston body 320, a supporting sieve plate 510, a water-absorbing and expanding material cavity 610, an explosive cavity 620, a first isolation membrane 710, an electronic ignition device 810, and a triggering mechanism 820. There are two connectors 100, an upper connector 110 and a lower connector 120, spaced apart by a certain distance. Both ends of the inner cylinder 210 are sealed to the two connectors 100, and both ends of the outer cylinder 220 are also sealed to the two connectors 100. The inner cylinder 210 is located inside the outer cylinder 220, and the two are arranged at a certain distance apart. An annular cavity is formed between the two. Both the inner cylinder 210 and the outer cylinder 220 have radially penetrating sandblasting holes 201 on their side walls, which are arranged opposite to each other. The sliding sleeve 310 is located in the annular cavity between the inner cylinder 210 and the outer cylinder 220 and is slidably sealed to the outer wall of the inner cylinder 210 and the inner wall of the outer cylinder 220. It is used to block the connection channel between the sandblasting holes 201 on the inner cylinder 210 and the outer cylinder 220, thereby separating the fluid inside the inner cylinder 210 and the outside of the outer cylinder 220. The sliding sleeve 310 divides the annular space between the inner cylinder 210 and the outer cylinder 220 into upper and lower parts. When the sliding sleeve 310 moves, the sandblasting holes 201 on the inner cylinder 210 and the outer cylinder 220 can be aligned and connected, so that sandblasting fracturing operations can be carried out. The supporting sieve plate 510 is annular and located in the annular cavity between the inner cylinder 210 and the outer cylinder 220. It is fixedly connected to the inner cylinder 210 or the outer cylinder 220 and is used to support the water-absorbing and expanding material. The supporting sieve plate 510 has sieve holes 501 that penetrate the body of the supporting sieve plate 510 along the axial direction of the inner cylinder 210. The water-absorbing and expanding material cavity 610 is located in the annular cavity between the inner cylinder 210 and the outer cylinder 220, and is located between the supporting sieve plate 510 and the sliding sleeve 310. It is used to store the water-absorbing and expanding material. The size of the water-absorbing and expanding material is larger than the size of the sieve holes 501 on the supporting sieve plate 510 to prevent the water-absorbing and expanding material from flowing out of the sieve holes 501. The first separating membrane 710 is used to isolate the water-absorbing and expanding material from water. The explosive cavity 620 is located between the inner cylinder 210 and the outer cylinder 220. The annular cavity between the two is used to store explosive substances; the piston body 320 is slidably and sealed to the outer wall of the inner cylinder 210 and the inner wall of the outer cylinder 220; one end of the explosive chamber 620 is sealed and fixedly connected to the inner cylinder 210 or the outer cylinder 320, and the other end is connected to one end of the piston body 320, so that the movement of the piston body 320 is linked to the volume of the explosive chamber 620. When the volume of the explosive chamber 620 increases, the piston body 320 moves, thereby destroying the first isolation membrane 710, which in turn causes the water-absorbing and swelling material to absorb water and expand after contact with water, pushing the sliding sleeve 310 to move; the electronic ignition device 810 is used to detonate the explosive substances in the explosive chamber 620, thereby changing the volume of the explosive chamber 620, which in turn pushes the piston body 320 to move, thereby destroying the first isolation membrane 710;The triggering mechanism 820 is used to trigger the electronic ignition device 810 to detonate the explosive material in the explosive chamber 620.

[0036] When this device is lowered into the well, its upper connector 110 is connected to the upper tubing string, and its lower connector 120 is connected to the oil pipe with a plug, so that the fluid inside the inner cylinder 210 and outside the outer cylinder 220 are separated to ensure the pressure test. After the pressure test is completed, the triggering mechanism 820 is triggered by external force to meet the triggering conditions, thereby triggering the electronic ignition device 810 to activate and detonate the explosive material in the explosive chamber 620. The pressure in the explosive chamber 620 increases rapidly, thereby pushing the piston body 320 to move axially, destroying the first isolation membrane 710. The water-absorbing and expanding material gradually expands upon contact with water, thereby pushing the sliding sleeve 310 to move, so that the sandblasting holes 201 on the inner cylinder 210 and the outer cylinder 220 are aligned and connected to each other, so that fluid can be transported from the inner cylinder 210 to the outside of the outer cylinder 220 to realize sandblasting fracturing operation. In this invention, the piston body 320 does not bear radial high pressure differential, therefore, it can be made of an elastic material, such as elastic rubber. Thus, its movement resistance is significantly lower than that of the rigid sliding sleeve 310, especially when the inner cylinder 210 and outer cylinder 220 are deformed, the difference in movement resistance between the two becomes more significant. In this invention, the shock wave from the explosion of an explosive substance provides thrust to the piston body 320. Since the piston body 320 requires less thrust, the smaller the amount of explosive substance required, the less impact the shock wave from the explosion has on the tubing. At the same time, compared to opening the sliding sleeve 310 with an explosive shock wave, this invention moves the sliding sleeve 310 by absorbing water and expanding the material, resulting in a slower movement speed and a smoother opening process.

[0037] In some embodiments, such as Figure 1As shown, the end of the piston body 320 facing away from the first isolation membrane 710 is the explosive chamber 620. The first isolation membrane 710 is annular and sealed to the inner cylinder 210 and the outer cylinder 220. The annular cavity between the first isolation membrane 710 and the piston body 320 is the water storage chamber 630, used to store water. When the volume of the explosive chamber increases, the piston body 320 squeezes the water storage chamber 630, increasing the pressure difference on both sides of the first isolation membrane 710, causing it to rupture. This allows the water in the water storage chamber 630 to enter the water-absorbing and expanding material chamber 610 through the sieve holes 501 on the support sieve plate 510 and come into contact with the water-absorbing and expanding material. In addition, to prevent the piston body 320 from moving accidentally (moving before the explosive substance detonates), an anti-movement structure can be provided for the piston body 320. For example, the piston body 320 can be fixed to the inner cylinder 210 or the outer cylinder 220 by shearable pins (not shown in the figure), or a blocking component can be provided to increase its sliding friction or movement resistance. In some embodiments, to enable the piston body 320 to break the first isolation membrane 710 with less force when moving (this can further reduce the amount of explosive and reduce the impact of the explosion on the tubing), the piston body 320 is provided with a membrane-breaking mechanism 321, such as a conical needle, at one end facing the first isolation membrane 710 for piercing the first isolation membrane 710. A portion of the gas phase space is retained in the water storage chamber 630, for example, by filling it with air or nitrogen. Thus, the gas in the water storage chamber 630 has a pressure-buffering function, making the pressure in the water storage chamber 630 lower than the pressure when the piston body 320 is fully filled with water when moving. The first isolation membrane 710 is pierced by the tip of the membrane-breaking mechanism 321, rather than by the pressure of the water. Furthermore, in some embodiments, the sliding sleeve 310, such as... Figure 2 As shown, the sliding sleeve 310 is provided with a through hole 311 corresponding to the sandblasting hole 201. When the through hole 311 is aligned with the sandblasting hole 201, the sandblasting holes 201 on the inner cylinder 210 and the outer cylinder 220 are aligned and connected. In order to prevent the sliding sleeve 310 from moving excessively, a limiting member 202, such as a limiting ring, can be fixedly installed on the outer wall of the inner cylinder 210 or the inner wall of the outer cylinder 220 so that the sliding sleeve 310 will no longer move after the sandblasting holes 201 on the inner cylinder 210 and the outer cylinder 220 are aligned with the through hole 311 on the sliding sleeve 310.

[0038] Figure 1 In one embodiment, the electrically controlled toe-end fracturing sleeve 310 carries water itself. In other embodiments, the water in the inner cylinder 210 and the outer cylinder 220 can also be used to expand the water-absorbing and swelling material, such as... Figure 3As shown, the inner cylinder 210 or the outer cylinder 220 is provided with a first fluid channel 203 that radially penetrates the body. The first fluid channel 203 communicates with the water-absorbing and expanding material cavity 610 and is used to introduce water into the water-absorbing and expanding material cavity 610. The first isolation membrane 710 is located in the water-absorbing and expanding material cavity 610 and wraps around the water-absorbing and expanding material. The piston body 320 is provided with a membrane-breaking mechanism 321 at one end facing the first isolation membrane 710. The membrane-breaking mechanism 321 can pass through the sieve hole 501 on the supporting sieve plate 510, thereby piercing the first isolation membrane 710 and allowing water to enter the other end of the cavity. In some embodiments, the inner cylinder 210 or the outer cylinder 220 is provided with a radially penetrating balance hole 204 that communicates with the explosive chamber 620. This balance hole 204 is used to introduce water into the explosive chamber 620, thereby balancing the pressure at both ends of the piston body 320 and reducing the thrust of the moving piston body 320. A second isolation membrane 720 is provided in the explosive chamber 620 to separate the water from the explosive material. Note that the smaller the size of this balance hole 204, the better, to prevent excessive leakage of explosive gas from this hole, which would excessively reduce the shock wave on the piston body 320. In some embodiments, a connecting hole 205 communicating with the outer cylinder 220 or the inner cylinder 210 is also provided below the piston body 320 to better balance the pressure at the upper and lower ends of the piston body 320.

[0039] In this invention, the electronic ignition device 810 and the triggering mechanism 820 are both commonly used devices in the prior art, and there are various specific forms. For example, the setting disclosed in CN117345188A is adopted, in which a circuit controller and a pressure sensor are used as the triggering mechanism 820. The pressure sensor is placed on the inner wall of the inner cylinder 210, and the circuit controller is placed in the annular cavity between the inner cylinder 210 and the outer cylinder 220. The circuit controller is electrically connected to the electronic ignition device 810 and the pressure sensor respectively, and is used to receive the data from the pressure sensor and send a command to the electronic ignition device 810 to whether to ignite. When the pressure inside the inner cylinder 210 reaches a predetermined value, the circuit controller sends an ignition command to the electronic ignition device 810.

[0040] The explosive material in this invention refers to the explosive used in well bottom operations, which is a commonly used material in oil and gas well blasting. Its specific type can be selected according to the operating temperature and other factors, which will not be described in detail here.

[0041] The first and second isolation membranes in this invention are both used to separate substances. Various materials in the prior art are suitable. In some embodiments, the second isolation membrane is made of an elastic material, such as elastic rubber. This can prevent gas from leaking from the balance holes. The number of balance holes can be appropriately increased to avoid them becoming blocked and affecting water entering the explosive chamber.

[0042] The water-absorbing and swelling material in this invention is a commonly used material. Conventional water-absorbing and swelling resins can meet the requirements. Different water-absorbing and swelling materials have different expansion ratios and expansion times. For example, the composite water-absorbing and swelling bag disclosed in CN104695378B uses high molecular weight polyethylene water-absorbing resin particles, which can generally expand completely in 2 to 4 minutes, while the expansion time of ordinary water-absorbing and swelling bags is about 10 minutes. Of course, some other water-absorbing and swelling materials have a slower expansion rate. When using it, water-absorbing and swelling materials with different expansion times can be selected as needed to control the opening time of the sliding sleeve 310.

[0043] The above description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the embodiments of the present invention should be included within the protection scope of the present invention.

Claims

1. An electrically controlled toe-end fracturing sliding sleeve, characterized in that, Also includes: Two spaced-apart connectors; The inner cylinder is connected to two connectors at each end; The outer cylinder is connected to two connectors at both ends, and the inner cylinder is located inside the outer cylinder and the two are arranged at intervals. Both the inner cylinder and the outer cylinder have radially penetrating sandblasting holes on their side walls. The sliding sleeve, which is slidably and sealingly connected to the outer wall of the inner cylinder and the inner wall of the outer cylinder, is used to block the connection channel between the sandblasting holes on the inner and outer cylinders; the sliding sleeve divides the annular space between the inner and outer cylinders into upper and lower parts. A ring-shaped support sieve plate located in the annular cavity between the inner cylinder and the outer cylinder, and fixedly connected to the inner cylinder or the outer cylinder. The support sieve plate is provided with sieve holes that penetrate the support sieve plate body along the axial direction of the inner cylinder. The water-absorbing and expanding material cavity is located in the annular cavity between the inner and outer cylinders and between the supporting sieve plate and the sliding sleeve. It is used to store the water-absorbing and expanding material. The size of the water-absorbing and expanding material is larger than the size of the sieve holes on the supporting sieve plate. A first isolation membrane used to separate water-absorbing and swelling materials from water; The explosive chamber, located in the annular cavity between the inner and outer cylinders, is used to store explosive materials; The piston body is slidably and sealingly connected to the outer wall of the inner cylinder and the inner wall of the outer cylinder. One end of the explosive chamber is sealed and fixedly connected to the inner cylinder or the outer cylinder, and the other end is connected to one end of the piston body. When the volume of the explosive chamber increases, the piston body moves, thereby breaking the first isolation membrane and allowing the water-absorbing and swelling material to come into contact with water. The piston body is made of elastic material. An electronic ignition device used to detonate explosive substances in a blast chamber; A triggering mechanism used to activate an electronic ignition device.

2. The electrically controlled toe-end fracturing sleeve according to claim 1, characterized in that, The first isolation membrane is annular and is sealed to the inner and outer cylinders. The annular cavity between the first isolation membrane and the piston body is a water storage chamber for storing water.

3. The electrically controlled toe-end fracturing sleeve according to claim 2, characterized in that, The piston body is provided with a membrane-breaking mechanism at one end facing the first isolation membrane, which is used to puncture the first isolation membrane, and the water storage chamber has a gas phase space.

4. The electrically controlled toe-end fracturing sleeve according to claim 1, characterized in that, The inner or outer cylinder is provided with a first fluid channel that radially penetrates the body. The first fluid channel is connected to the water-absorbing and expanding material cavity. The first isolation membrane is located in the water-absorbing and expanding material cavity. The piston body is provided with a membrane-breaking mechanism at one end facing the first isolation membrane. The membrane-breaking mechanism can pass through the through hole on the sieve plate and puncture the first isolation membrane.

5. The electrically controlled toe-end fracturing sleeve according to claim 4, characterized in that, The inner or outer cylinder is provided with a balance hole that penetrates the body radially. The balance hole is connected to the explosive chamber and is used to introduce water into the explosive chamber to balance the pressure at the upper and lower ends of the sliding sleeve. The explosive chamber is provided with a second isolation membrane to separate the water from the explosive.

6. The electrically controlled toe-end fracturing sleeve according to claim 5, characterized in that, The second separator is made of an elastic material.

7. The electrically controlled toe-end fracturing sleeve according to claim 1, characterized in that, The sliding sleeve is provided with a through hole corresponding to the sandblasting hole. When the through hole is aligned with the sandblasting hole, the sandblasting holes on the inner and outer cylinders are connected. When the through hole is far away from the sandblasting hole, the sandblasting holes on the inner and outer cylinders are closed.

8. The electrically controlled toe-end fracturing sleeve according to claim 7, characterized in that, A limiting component is fixedly installed on the outer wall of the inner cylinder or the inner wall of the outer cylinder, so that the sliding sleeve will no longer move after the sandblasting holes on the inner cylinder and the outer cylinder are aligned with the through holes on the sliding sleeve.

Images