Casing annulus fluid injection apparatus and method

Abstract

The present application relates to natural gas development technical field, specifically to a kind of casing annulus fluid injection device and method, the device includes connecting cylinder, plunger and conveying assembly, connecting cylinder has the first passageway for plunger linear sliding, plunger is sealingly connected with the inner wall of first passageway, two ends of plunger are respectively provided with inlet and outlet, plunger is provided with second passageway, second passageway has arc-shaped corner, conveying assembly is sealingly connected with the inner wall of second passageway, one end of conveying assembly extends into second passageway from inlet and extends from outlet, to be injected in casing annulus in downlink and fluid.The present application can inject blocking agent at fixed point under the condition that casing annulus is under pressure and does not affect gas well production, improve casing annulus gas channeling plugging effect, realize simple, low-cost plugging operation of casing annulus.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of natural gas development, and more particularly to a casing annulus fluid injection device and method. BACKGROUND

[0002] In the process of natural gas development, with the increase of exploitation time, the problem of casing annulus pressure increases year by year, which causes many gas wells to have safety hazards.

[0003] At present, there are mainly two methods to solve the problem of casing annulus pressure: one is to use the method of workover, to pull out the production tubing, to punch the production casing, and to inject the plugging agent to block the gas channeling passage, which has high cost, low success rate, and affects the normal production of the gas well, especially for low pressure sensitive gas reservoirs, the workover will affect the productivity of the gas well, causing the loss of gas production. The other is to inject the plugging agent into the casing annulus or to inject the plugging agent into the casing annulus through the pipe insertion method to block the casing annulus gas channeling passage, wherein the general injection of the plugging agent has the problems of difficult injection, pollution of the plugging agent by the fluid in the annulus, difficulty in determining the blocking position, and low success rate; the pipe insertion method is difficult to be inserted into the required depth due to the 90° angle between the casing valve and the casing annulus, the small annular gap, and the existence of the casing coupling, which affects the blocking effect after the injection of the plugging agent and the subsequent construction. SUMMARY

[0004] In order to solve the problem of difficult gas channeling passage blocking in the prior art, the present application provides a casing annulus fluid injection device and method, which can inject the plugging agent at a fixed point under the condition of casing annulus pressure and without affecting the production of the gas well, improve the casing annulus gas channeling blocking effect, and realize simple and low-cost blocking operation of the casing annulus.

[0005] In order to achieve the above-mentioned purpose, the casing annulus fluid injection device of the technical solution comprises a connecting cylinder, a plunger and a conveying assembly, the connecting cylinder has a first channel for the linear sliding of the plunger, the plunger extends into the first channel from one end of the connecting cylinder and is sealingly connected with the inner wall of the first channel, the two ends of the plunger are respectively provided with mutually perpendicular inlet and outlet, the plunger is provided with a second channel which is in communication with the inlet and outlet respectively, the second channel has an arc-shaped corner, the conveying assembly is sealingly connected with the inner wall of the second channel, one end of the conveying assembly extends into the second channel from the inlet and extends out of the outlet, so as to descend in the casing annulus and inject the fluid.

[0006] In the technical solution, one end of the connecting cylinder is connected with the first casing annulus valve, the plunger can be pushed into the casing annulus after the first casing annulus valve is opened, at this time, the first channel and the second channel are in communication with the casing annulus, the outlet position of the plunger is adjusted to be aligned with the casing annulus, then the conveying assembly enters the casing annulus under the guidance of the second channel and goes down, in the process, the conveying assembly can realize 90° turning when passing the corner, in the process of going down, the pump machine is used to input the pressurized corrosion-proof liquid into the conveying assembly to clean the pipe wall of the casing annulus, when the conveying assembly goes down to the position of the gas channel, the blocking agent is conveyed to the position of the gas channel through the conveying assembly to block.

[0007] As a preferred solution, in order to reduce the friction between the conveying assembly and the second channel, the corner of the second channel is a smooth arc surface, a plurality of rolling elements are arranged along the arc direction of the corner, the rolling elements are rollers or balls, the rolling elements at the corner can convert the rigid friction of the conveying assembly into rolling friction, thereby reducing the friction resistance of the conveying assembly in going down and recovery, and also reducing the wear of the conveying assembly.

[0008] As a preferred solution, in order to realize long-distance conveying of fluid, the conveying assembly comprises a conveying pipe matched with the inner diameter of the second channel and a traction head, one end of the conveying pipe is connected with the traction head, the traction head is provided with a jet hole in communication with the conveying pipe, the corrosion-proof liquid is sprayed from the jet hole to the oblique back of the conveying assembly to push the conveying assembly to go down.

[0009] As a preferred solution, the jet hole is arranged at an angle α with the conveying pipe, the angle α is 40°-50°, in this angle arrangement, when the pressurized fluid is sprayed from the jet hole, the traction head can be pushed to drive the conveying pipe to go down in the casing annulus, and the pipe wall of the casing annulus can be cleaned at the same time.

[0010] As a preferred solution, in order to realize the sealed connection between the first channel and the plunger, the outer wall of the plunger is provided with a plurality of first grooves arranged along the axial direction, each first groove is provided with a first sealing ring matched with the first channel, when the plunger slides in the first channel, the first sealing ring can seal the device, so that the device can work under the condition of pressure in the casing annulus.

[0011] As a preferred solution, in order to realize the sealed connection between the second channel and the conveying assembly, the second channel is provided with a second groove, and a compression ring and a second sealing ring matched with the plunger are arranged in the second groove, and a top rod is screwed to one end of the plunger close to the inlet, one end of the top rod extends into the second groove and presses the compression ring and the second sealing ring, when the top rod is screwed into the second groove and presses the compression ring and the second sealing ring, the sealing ring is elastically deformed and tightly pressed on the outer wall of the conveying assembly to realize sealing.

[0012] As a preferred solution, in order to improve the sealing performance, the compression ring and the second sealing ring are respectively provided with a plurality of compression rings and second sealing rings, and the plurality of compression rings and second sealing rings are alternately arranged along the axial direction of the second channel, and the compression ring is used to extrude the second sealing ring, so that the plurality of sealing rings are deformed at the same time and tightly pressed on the outer wall of the conveying assembly.

[0013] In order to achieve the above purpose, the casing annulus fluid injection method of the technical solution uses the casing annulus fluid injection device, wherein the two sides of the casing annulus are respectively provided with the first casing valve and the second casing valve in communication, and the method comprises the following steps:

[0014] S1: connecting the closed discharge tank with the first casing valve, opening the first casing valve to release pressure into the casing annulus until the pressure in the casing annulus is stable;

[0015] S2: connecting one end of the connecting barrel with the second casing valve, and performing sealing detection on the conveying assembly;

[0016] S3: opening the second casing valve, pushing the plunger into the casing annulus through the second casing valve, adjusting the outlet of the plunger to correspond to the casing annulus, and pushing the conveying assembly out of the outlet along the second channel for at least 5m;

[0017] S4: injecting the corrosion-resistant liquid into the conveying assembly through the pump and slowly pressurizing to make the conveying assembly slowly descend to the position where the conveying assembly reaches the gas channeling passage of the casing annulus, and after injecting the plugging agent into the conveying assembly for 0.8m 3 ~1.2m 3 , continue to inject the corrosion-resistant liquid to push the plugging agent to the position of the gas channeling passage;

[0018] S5: after the plugging agent injection is completed, the conveying assembly is retracted into the second channel, and then the plunger is retracted into the first channel, and the first casing valve and the second casing valve are closed.

[0019] As a preferred solution, in order to ensure the smooth conveying of the fluid, in step S2, the sealing detection on the conveying assembly is that the pump is used to convey the corrosion-resistant liquid to the conveying device and pressurize to 1.5 times greater than the pressure in the casing annulus, and the pressure is stabilized for at least 10 minutes.

[0020] As a preferred solution, in step S4, the down speed of the conveying assembly is less than 10 m / min, and the winch is connected with one end of the conveying assembly and controls the down speed of the conveying assembly.

[0021] Compared with the prior art, the present application has the following advantages:

[0022] 1. The device of the present application can realize 90° turning of the conveying assembly through the arc-shaped corner and the rolling member arranged at the corner, and the rolling member can convert the rigid friction of the conveying assembly into rolling friction, thereby reducing the frictional resistance of the conveying assembly during the down and recovery processes, and reducing the wear of the conveying assembly.

[0023] 2. The first channel and the second channel are sealed through the first sealing member and the second sealing member, so that the device can inject the plugging agent at a fixed point under the condition that the casing annulus is under pressure and the gas well production is not affected, improve the casing annulus gas channeling plugging effect, and realize simple and low-cost plugging operation of the casing annulus.

[0024] 3. The pressurized fluid is injected into the conveying pipe, and the fluid is sprayed out from the inclined spray hole arranged in the pulling head to push it downward, and the pipe wall of the casing annulus can be cleaned during the downward process. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a structural schematic view of the casing annulus fluid injection device of the present application;

[0026] Figure 2 is a structural schematic view of the conveying assembly;

[0027] Figure 3 is an installation schematic view of the casing annulus fluid injection device of the present application;

[0028] Figure 4 is a working schematic view of the casing annulus fluid injection device of the present application;

[0029] Figure 5 is a flowchart of the casing annulus fluid injection method of the present application.

[0030] In the figure: connecting barrel 1; first channel 11; plunger 2; second channel 21; corner 211; rolling member 2111; inlet 22; outlet 23; first groove 24; first sealing ring 241; second groove 25; second sealing ring 251; pressure ring 252; jacking rod 253; conveying assembly 3; conveying pipe 31; pulling head 32; joint 33; screw rod 4; casing annulus 5; first casing valve 51; second casing valve 52; closed discharge tank 6; winch 7. DETAILED DESCRIPTION

[0031] The accompanying drawings are for illustrative purposes only and should not be construed as limiting this patent. To better illustrate this embodiment, some components in the drawings may be omitted, enlarged, or reduced, and do not represent the actual dimensions of the product. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings. The positional relationships described in the drawings are for illustrative purposes only and should not be construed as limiting this patent.

[0032] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "long," and "short" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present patent. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0033] The technical solution of the present invention will be further described in detail below through specific embodiments and with reference to the accompanying drawings:

[0034] Example 1:

[0035] like Figures 1 to 4 As shown, a sleeve annular fluid injection device is provided, including a connecting cylinder 1, a plunger 2, and a delivery assembly 3. The connecting cylinder 1 has a first channel 11 for the plunger 2 to slide linearly. The plunger 2 extends from one end of the connecting cylinder 1 into the first channel 11 and is sealed to the inner wall of the first channel 11. One side of the plunger 2 and one side of the connecting cylinder 1 are respectively provided with mating flanges. The flanges of the plunger 2 and the flanges of the connecting cylinder 1 are connected by screws 4 to adjust the position of the plunger 2 in the first channel 11. The two ends of the plunger 2... The plunger 2 is provided with an inlet 22 and an outlet 23 that are perpendicular to each other. The plunger 2 is provided with a second channel 21 that communicates with the inlet 22 and the outlet 23 respectively. The corner 211 of the second channel 21 is a smooth arc surface. The second channel 21 has an arc-shaped corner 211. One end of the conveying assembly 3 is connected to an inclined injection hole 321 to push the conveying assembly 3 to move. One end of the conveying assembly 3 extends from the inlet 22 into the second channel 21 and extends out from the outlet 23. The conveying assembly 3 is sealed to the inner wall of the second channel 21.

[0036] In this embodiment, the device can operate under the condition of pressurized casing annulus without affecting gas well production. If the gas well is shut down, there is no need to consider the issue of pressurized casing annulus. The plunger 2 can be directly connected to the second casing valve 52 through its flange, and pressurized fluid can be introduced into the conveying component 3 to provide it with downward power.

[0037] Specifically, such as Figure 2 As shown, the conveying assembly 3 includes a conveying pipe 31 adapted to the inner diameter of the second channel 21 and a traction head 32. One end of the conveying pipe 31 is connected to the traction head 32, and the injection hole 321 is provided on the traction head 32 and communicates with the conveying pipe 31.

[0038] In this embodiment, the traction head 32 is connected to the delivery pipe 31 via a connector 33, wherein the delivery pipe 31 is a flexible hose.

[0039] Specifically, the injection hole 321 and the delivery pipe 31 are arranged at an angle α, where the angle α is 40° to 50°.

[0040] In this embodiment, the included angle α is preferably 45°. At this angle, the jet hole 321 provides downward power by jetting fluid backward and can also clean the inner wall of the sleeve annulus 5.

[0041] Specifically, the outer wall of the plunger 2 is provided with a plurality of first grooves 24 arranged at intervals along its axial direction, and each first groove 24 is provided with a first sealing ring 241 that cooperates with the first channel 11.

[0042] In this embodiment, the outer wall of the plunger 2 is provided with 6 annular first grooves 24, and the outer diameter of the first sealing ring 241 is slightly larger than the inner diameter of the first channel 11.

[0043] Specifically, the second channel 21 is provided with a second groove 25, and the second groove 25 is provided with a pressure ring 252 and a second sealing ring 251 that cooperate with the plunger 2. A push rod 253 is screwed to one end of the plunger 2 near the inlet 22. One end of the push rod 253 extends into the second groove 25 and presses the pressure ring 252 and the second sealing ring 251.

[0044] In this embodiment, the pressure ring 252 has high hardness and is not easily deformed, which can effectively compress the second sealing ring 251 to deform it, thereby enhancing the sealing performance.

[0045] Specifically, multiple pressure rings 252 and multiple sealing rings 251 are provided, and the multiple pressure rings 252 and multiple sealing rings 251 are arranged alternately along the axial direction of the second channel 21.

[0046] In the embodiment, the compression ring 252 is a hollow structure, the outer wall of the compression ring 252 is provided with external threads, the inner wall of the second groove 25 is provided with internal threads matched with the external threads, and the compression ring 252 is threadedly connected with the second groove 25.

[0047] Embodiment 2:

[0048] The embodiment is similar to the embodiment 1, and the difference is that, in the embodiment, as shown in Figure 1 、 3 , 4, the corner 211 is provided with a plurality of rolling members 2111 which are arranged at intervals along the arc direction of the corner 211, and the rolling members 2111 are rollers or balls.

[0049] In the embodiment, in order to facilitate processing, the rolling member 2111 is preferably a roller, a plurality of rotating shafts are respectively rotatably connected to the upper and lower sidewalls of the corner 211, and the plurality of rollers are respectively sleeved on the rotating shafts. Under the guidance of the rollers, the 90° turning of the conveying assembly 3 is realized, and the friction between the conveying assembly 3 and the corner 211 during the downward movement of the conveying assembly 3 is reduced.

[0050] Embodiment 3:

[0051] As shown in Figure 4 、 5 , the embodiment provides a casing annulus fluid injection method using the casing annulus fluid injection device, wherein the two sides of the casing annulus 5 are respectively provided with the first casing valve 51 and the second casing valve 52 which are in communication, and the method comprises the following steps:

[0052] S1: connecting the closed drain tank 6 with the first casing valve 51, and opening the first casing valve 51 to release pressure into the casing annulus 5 until the pressure in the casing annulus 5 is stable;

[0053] S2: connecting one end of the connecting barrel 1 with the second casing valve 52, and performing sealing detection on the conveying assembly 3;

[0054] S3: opening the second casing valve 52, pushing the plunger 2 into the casing annulus 5 through the second casing valve 52, adjusting the outlet 23 of the plunger 2 to correspond to the casing annulus 5, and pushing the conveying assembly 3 out of the outlet 23 for at least 5 m along the second channel 21;

[0055] S4: injecting the corrosion-proof liquid into the conveying assembly 3 through the pump and slowly pressurizing to slowly lower the conveying assembly 3 to the position where the conveying assembly 3 reaches the gas channeling passage of the casing annulus 5, and after injecting the plugging agent into the conveying assembly 3 for 0.8 m 3 ~1.2 m 3 , continuing to inject the corrosion-proof liquid to push the plugging agent to the position of the gas channeling passage;

[0056] S5: After the plugging agent injection is completed, the delivery assembly 3 is retracted into the second channel 21, and then the plunger 2 is retracted into the first channel 11, and the first casing valve 51 and the second casing valve 52 are closed.

[0057] Specifically, in step S2, the sealing detection of the delivery assembly 3 is that the pump machine delivers the corrosion-proof liquid to the delivery device and pressurizes to 1.5 times of the pressure in the casing annulus 5, and is stable at the pressure for at least 10 min.

[0058] Specifically, in step S4, the winch 7 is connected with one end of the delivery assembly 3 and controls the descending speed of the delivery assembly 3 to be less than 10 m / min.

[0059] The working process of the casing annulus fluid injection method is as follows: in the closed state of the first casing valve 51, the pipe line is connected with the closed discharge tank 6, then the first casing valve 51 is opened, the annulus pressure is released to 0.9 MPa, the delivery pipe 31 is connected with the towing head 32 through the joint 33, the delivery pipe 31 is inserted into the second channel 21 from the inlet 22, the rotating rod 253 is rotated to realize the sealing between the delivery pipe 31 and the second channel 21, the plunger 2 is inserted into the first channel 11 and connected with the connecting cylinder 1 through the screw rod 4, the connecting cylinder 1 is connected with the second casing valve 52 at the end away from the screw rod 4 in the closed state of the second casing valve 52, after the sealing detection of the device is completed, the second casing valve 52 is opened, the screw rod 4 is rotated to make one end of the plunger 2 extend into the casing annulus 5 and make the outlet 23 vertically downward, the delivery pipe 31 is manually inserted into the casing annulus 5 more than 5 m, the pump machine injects 2 MPa of corrosion-proof liquid into the delivery pipe 3, the corrosion-proof liquid is sprayed from the spray hole 321 to the rear to provide a towing force for the downward movement of the delivery pipe 31, the winch 7 controls the downward speed of the delivery pipe 31 to be 5 m / min, if the downward movement is blocked and cannot be moved downward, the delivery pipe 31 is stopped at the position and the injection of 2 MPa of corrosion-proof liquid is maintained, the pump is stopped after 2 min, the delivery pipe 31 is lifted by 15 m by the winch 7, the pump machine is slowly pressurized to 2.5 MPa, the delivery pipe 31 is controlled to be slowly lowered at a speed of 3 m / min, after reaching the specified depth, 1 m 3 of the plugging agent is injected into the delivery pipe 31, then the pump injection of 2 MPa of corrosion-proof liquid is continued to push the plugging agent to the specified depth, after the plugging agent is injected, the hose is slowly recovered, the towing head 32 is retracted into the second channel 21, the plunger 2 is retracted into the first channel 11, and the first casing valve 51 and the second casing valve 52 are closed.

[0060] Although the embodiments of the present application have been shown and described above, it should be understood that the above-described embodiments are exemplary and should not be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above-described embodiments within the scope of the present application.

[0061] Obviously, the above embodiments of the present application are only examples for clearly explaining the present application, and are not intended to limit the embodiments of the present application. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, it is not necessary and impossible to exhaust all the embodiments. Any modification, equivalent replacement and improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the claims of the present application.

Claims

1. An annulus fluid injection device, comprising: The utility model relates to a kind of injection devices, including connecting cylinder (1), plunger (2) and delivery assembly (3), the connecting cylinder (1) has the first passage (11) for the linear sliding of the plunger (2), the plunger (2) is inserted into the first passage (11) from one end of the connecting cylinder (1) and is sealedly connected with the inner wall of the first passage (11), two ends of the plunger (2) are respectively equipped with mutually perpendicular inlet (22) and outlet (23), the plunger (2) is equipped with the second passage (21) that is communicated with the inlet (22) and outlet (23) respectively, the second passage (21) has arc corner (211), the delivery assembly (3) is sealedly connected with the inner wall of the second passage (21), one end of the delivery assembly (3) is inserted into the second passage (21) from the inlet (22) and is exported from the outlet (23), to downlink in casing annulus (5) and inject fluid, the corner (211) of the second passage (21) is smooth arc surface, the corner (211) is equipped with a plurality of rolling elements (2111) that are arranged along its arc direction, the rolling element (2111) is cylinder or ball, the delivery assembly (3) includes delivery pipe (31) and traction head (32) that are adapted to the inner diameter of the second passage (21), one end of the delivery pipe (31) is connected with the traction head (32), the traction head (32) is equipped with injection hole (321) that is communicated with the delivery pipe (31), and corrosion-resistant liquid is sprayed from injection hole (321) to the oblique back of delivery assembly (3) to push delivery assembly (3) downlink.

2. An annulus fluid injection device according to claim 1, wherein, The injection hole (321) and the delivery pipe (31) are arranged at an included angle α, and the included angle α is 40°-50°.

3. An annulus fluid injection apparatus according to claim 1, wherein, The outer wall of the plunger (2) is provided with a plurality of first grooves (24) arranged along the axial direction, and each first groove (24) is provided with a first sealing ring (241) matched with the first passage (11).

4. An annulus fluid injection apparatus according to claim 1, wherein, The second passage (21) is provided with a second groove (25), and the second groove (25) is provided with a compression ring (252) and a second sealing ring (251) matched with the plunger (2). One end of the plunger (2) close to the inlet (22) is screwed with a jack (253), one end of the jack (253) is inserted into the second groove (25) and presses the compression ring (252) and the second sealing ring (251).

5. An annulus fluid injection apparatus according to claim 4, wherein, The compression ring (252) and the second sealing ring (251) are respectively provided with a plurality of compression rings (252) and second sealing rings (251), and the plurality of compression rings (252) and second sealing rings (251) are alternately arranged along the axial direction of the second passage (21).

6. An annulus fluid injection method using the casing annulus fluid injection device according to any one of claims 1 to 5, wherein, The casing annulus (5) has a first casing valve (51) and a second casing valve (52) communicated on both sides, and the utility model is characterized by comprising the following steps: S1: connect the closed discharge tank (6) with the first casing valve (51), open the first casing valve (51) to release pressure into the casing annulus (5) until the pressure is stable; S2: connect one end of the connecting cylinder (1) with the second casing valve (52), and seal the detection of the delivery assembly (3); S3: open the second casing valve (52), push the plunger (2) into the casing annulus (5) through the second casing valve (52), adjust the outlet (23) of the plunger (2) to correspond to the casing annulus (5), and push the delivery assembly (3) out of the outlet (23) along the second channel (21) for at least 5 m; S4: inject the delivery assembly (3) with corrosion inhibitor and slowly pressurize to slowly lower the delivery assembly (3) to the gas channeling access location of the delivery assembly (3) to the casing annulus (5) and inject 0.8 m 3 of the blocking agent and continue to inject corrosion inhibitor to push the blocking agent to the gas channeling access location; 3 S4: inject the delivery assembly (3) with corrosion inhibitor and slowly pressurize to slowly lower the delivery assembly (3) to the gas channeling access location of the delivery assembly (3) to the casing annulus (5) and inject 0.8 m 3 of the blocking agent and continue to inject corrosion inhibitor to push the blocking agent to the gas channeling access location; 3 S4: inject the delivery assembly (3) with corrosion inhibitor and slowly pressurize to slowly lower the delivery assembly (3) to the gas channeling access location of the delivery assembly (3) to the casing annulus (5) and inject 0.8 m 3 of the blocking agent and continue to inject corrosion inhibitor to push the blocking agent to the gas S5: after the plugging agent injection is completed, the delivery assembly (3) is retracted into the second channel (21), and then the plunger (2) is retracted into the first channel (11), and the first casing valve (51) and the second casing valve (52) are closed.

7. The method of claim 6, wherein, In step S2, the sealing detection of the delivery assembly (3) is: the pump machine delivers the corrosion-proof liquid to the delivery assembly (3) and pressurizes it to be greater than 1.5 times the pressure in the casing annulus (5), and is stable at this pressure for at least 10 min.

8. The method of claim 6, wherein, In step S4, the winch (7) is connected to one end of the delivery assembly (3) and controls the downward speed of the delivery assembly (3) to be less than 10 m / min.

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