GB2030632A

GB2030632A - Borehole Plug

Info

Publication number: GB2030632A
Application number: GB7838149A
Authority: GB
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1977-09-30
Filing date: 1978-09-26
Publication date: 1980-04-10

Abstract

A probe or plug comprises a cylindrical member, e.g., a bullet shaped head (1) and a cylindrical base (2), a flexible annular flange 10, a gas conduit (3) extending through the base (2) and head (1) ending in a series of inlets (4), a grout conduit (6) extending through the base (2) and connected to a series of outlets (7) which is separated from the inlets (4) by a rubber flange (10) which abuts the wall of the borehole (14) tightly. The probe is sealed into the borehole (14) by pumping grout through the grout conduit (6) the flange (10) preventing the grout from entering the inlets (4). Gas which accumulates around the head (1) is withdrawn through the gas conduit (3) and analysed. <IMAGE>

Description

SPECIFICATION Borehole Probe This invention relates to a borehole probe, and in particular, but not exclusively to a probe for use in the measurement of the methane content of a coal seam.

In most coal mines the coal seams, and, to a lesser extent, the surrounding rock strata contain methane gas, which can slowly leak out of the coal into the face areas and roadways. If the methane is allowed to accumulate it is possible for an explosion to occur, and this can have serious consequences, including death of miners and loss of a workable face. It is therefore desirable to be able to estimate the methane content of a coal seam so that adequate ventilation can be provided so as to prevent the accumulation of explosive pockets of methane gas.

This estimate may be achieved in one of two ways. The first method involves taking broken or pulverised coal and measuring the amount of methane emitted therefrom under set conditions.

This is usually carried out in a laboratory and can be very inaccurate, especially if the history of the sample, which will have come from a distance away, is not known to the tester.

The second method is more direct in that it can be used in situ but is indirect in that it measures the pressure of the gas as it accumulates in a sealed borehole, and does not measure the actual amount of gas in a sample. The result is obtained by drilling a borehole, usually, though not necessarily, horizontally into a coal seam and sealing a probe into the borehole, using hydraulic or pneumatic seals. The probe is connected to a pressure gauge. From a reading of the pressure gauge an estimate can be given of the gas content of the coal seam. However, the results given by this method are unreliable. It is thought that this is due to inefficient sealing of the probe in the borehole.

It is an object of the present invention to provide an improved probe for use in the above second method.

Therefore, according to a first aspect of the present invention, there is provided a probe comprising a generaily cylindrical member having a base, and a flexible annular flange on the member, which member has a gas conduit extending from the base to beyond the flange and a grout conduit extending from the base to before the flange, the flange being adapted, in use, to abut tightly the wall of a bore hole and thereby to prevent grout, which is supplied to the probe to seal it in the borehole, from blocking the gas conduit.

Conveniently, each conduit at its end remote from the base finishes in a series of inlets/outlets, the inlets/outlets of the grout conduit being on the opposite side of the flange to the inlets/outlets of the gas conduit.

Preferably, the end of the member remote from the base is profiled to a pointed or bullet-shaped head so that the probe may be inserted easily into the borehole.

The flange is preferably made of a tough flexible rubber, such as a neoprene rubber, but may also be made of any material which will distort against the borehole wall to give a seal past which the grout may not go.

Preferably further flanges, conveniently two or three, are provided on the probe between the first flange and the base, dividing the area to which grout is supplied into sections. Conveniently a cylindrical sleeve of resilient material, such as rubber, is fixed for instance by clamps or jubillee clips over each section, such that as grout is pumped into the probe, the sleeves expand and abut the wall of the borehole tightly, to increase the efficiency of the seal. Advantageously, a pin hole is made in the sleeve covering the section nearest the base through which hole air may pass, so that any air in the grout supply line will be removed before the grout is pumped in.

The probe may be made of any material.

However, if it is to be used in a coal mine it should be made of a material which passes the customary rigorous safety standards. In this case it is preferred that the probe is made either of a synthetic resin, such as nylon, or of wood, such as beechwood.

The gas conduit may be connected to a pressure gauge by a nylon or rubber tube. The grout may be supplied to the grout conduit through a nylon, rubber or flexible metal pipe. The grout may either be cementitious or resinous, and should be of such a consistency that it is able to permeate the borehole wall to a certain extent, (if sleeves are not present) but should not be able to flow around the flange.

Preferably a valve is incorporated into the grout supply line, conveniently, in or near to the base, so that when the grout supply is disconnected or turned off the grout does not ooze out of the borehole. This is especially desirable where the probe includes resilient sleeves.

According to a second aspect of the invention, there is provided a method of measuring the gas content of a rock stratum, which method comprises drilling a blind borehole into the stratum, connecting a pressure gauge and a grout supply to the respective conduits of a probe as hereinbefore described, inserting the probe base last into the borehole so that the or each flange abuts the borehole tightly and the probe forms a closed space at the blind end of the borehole, supplying grout to the grout conduit so that the probe is sealed into the bore, allowing the grout to set, and measuring the pressure of the gas in the sealed blind end of the bore. It is envisaged that the above method will give more reliable results than the methods previousiy used.It is thought that this will be because, where there are no sleeves, the grout will penetrate and seal more efficiently an annular area of fractured ground around the borehole through which the gas in the stratum could otherwise escape. If sleeves are present it is thought that the resilient material will adapt its shape to the side of the borehole, thereby to effectively seal the borehole. To increase the sealing effect of the grout it is desirable that, after the grout has set, a further charge of grout should be pumped into the borehole behind the probe. The grout will have to be supplied through a fresh tube, since the original grout supply conduit will be blocked up by the original grout.The second grout charge should be supplied at higher pressure than the first charge to ensure adequate penetration of the fractured ground and should be allowed to set before any readings of the pressure gauge are taken.

Preferably the borehole is drilled in a stepwise fashion, having its blind end narrower than the open end. This enables the probe to be inserted more easily into the borehole. The borehole is conveniently made horizontal, although the orientation of the borehole has no effect on the use of the invention.

The pressure gauge may read directly in pressure, or may be arranged to give a digital output, either to a display device or to a control or warning circuit.

The gas conduit may also be connectible to a gas supply so that gas may be pumped into the borehole. Furthermore the gas conduit may be connectible to a gas sampling device which can be used to analyse the gas in the space. A series of probes may then be used to estimate the gas flow through a rock stratum. A probe connected to a gas supply can be sealed in one part of the stratum and a probe connected to a sampling device can be sealed in another part of the stratum. A tracer gas is pumped into the stratum via the first probe and the second probe and its attached sampling device can be used to detect the tracer gas when it arrives at the second probe.

The time taken for the tracer gas to appear in the space by the second probe is measured, and from these data it is possible to estimate the gas flow rate through the stratum.

The tracer gas should be inert and may be a radioactive gas, or an easily recognisable gas, such as sulphur hexafluoride.

Although this invention is envisaged as being especially applicable to use in coal mines, it is not so limited and may be used in any other mining or drilling operation wherein accumulations of gas are a problem.

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a diagrammatic sectional side view of a first probe according to the invention sealed into a borehole in a coal seam, and Figure 2 shows a diagrammatic section side view of a second probe according to the invention sealed into a borehole in a coal seam.

Referring to Figure 1, a first probe according to the invention comprises a bullet-shaped head 1 and a member 2 which are, in this case, screwfitted together and both of which are made of nylon. However the parts of the probe may be push-fitted and glued together and/or made of any other suitable material. A part of a gas conduit 3, including four gas inlets 4, is formed in the head 1. A nylon tube 5, which forms the rest of the gas conduit 3, is screw fitted onto the end of the part of the gas conduit 3 formed in the head 1. The nylon tube 5 passes through a grout conduit 6 formed in the member 2. The grout conduit 6 includes eight grout outlets 7, and terminates in a base 8 through which the nylon tube 5 passes. The base 8 includes a screw attachment 9 for connecting the grout conduit to a grout supply (not shown).The nylon tube 5 has on its end remote from the probe a connection for a pressure gauge (not shown) and a second connection for a sampling device (not shown).

A first annular flange 10 of neoprene rubber is fixed around the screw connection between the head 1 and the member 2 of the probe. Two further annular flanges 11, 12 are fitted in spaced apart relation onto the member 2.

The probe is used in the following way. A borehole 14 is drilled into a coal seam 15. The diameter of the borehole 14 is just less than the outer diameter of the annular flanges 10 to 12.

The drilling also causes the formation of an area of fractured ground 1 6 around the borehole 14.

The nylon tube 5 of the probe is connected to a pressure gauge (not shown) and a sampling device (not shown) via their respective couplings (not shown). The screw attachment 9 is connected to a grout supply (not shown) via a second nylon tube (not shown). The two nylon tubes are taped together and held between a pair of wooden dowels (not shown).

The probe is inserted into the borehole 14. This is facilitated by the bullet shaped head 1 of the probe and by the stiffening effect of the dowels on the nylon tube 5 and grout supply tube. The probe is inserted until it is close to, but not touching, the blind end of the borehole 14. The annular flanges 10 to 12 abut tightly the borehole 14, and a closed space 1 7 is formed by the probe and the blind end of the borehole 14.

A cementitious grout 1 8 is then supplied under pressure from the grout supply through the grout conduit 6 and outlets 7 into the space between the member 2 and the borehole 14. The flanges 10 to 12 limit the movement of the grout 18 which is constrained to fill the gap between the member 2 and the borehole 14 and to a limited extent to permeate the fractured ground 1 6 around the base 2 of the probe. The grout 1 8 is allowed to set, and then a further charge of grout is supplied under greater pressure to the borehole 14, from a second grout supply tube (not shown).

The second charge of grout cannot pass through the grout conduit 6 which is blocked up by the first grout charge. The second charge of grout fills up the space behind the probe and permeates to a greater extent the fractured ground 1 6 thereby to seal the probe in the seam 1 5. The presence of the flanges 10 to 12 and the set first charge of grout ensures both that the second charge of grout does not penetrate the space 1 7 and that it does penetrate the fractured ground 1 6. The second charge of grout is allowed to set, and when it is set the probe is tightly sealed into the coal seam 15, and no gas is able to pass around the probe through the fractured ground 1 6.

The efficiency of a probe may be checked in the following way. A series of further holes may be drilled around the hole in which the probe is located. These further holes should be slightly shorter than the original hole. Grout is then pumped into these holes. If the grout in the original hole has efficiently sealed the probe into place, the pumping of grout into the further holes will alter the pressure recorded on the pressure gauge. If the seal is inefficient little or no change in the reading of the pressure gauge will be observed.

While the probe is being sealed into the seam, and after the seal is complete, gas, normally methane, begins to accumulate in the space 17 causing the pressure in this space 1 7 to increase.

The pressure increase is transmitted via the gas inlets 4, gas conduit 3, and nylon tube 5 to the pressure gauge. The pressure eventually achieves an equilibrium value, and, from this pressure value an estimate of the gas content of the seam 1 5 can be determined. The composition of the gas which accumulates in the space 1 7 may be determined using the sampling device.

A measure of the rate of gas flow through the seam 15 or strata may be obtained in the following way. A sample of a tracer gas, for instance sulphur hexafluoride is injected into the seam 1 5 at a point remote from the sealed in probe. It is preferably injected in from a second probe which is sealed into the seam 1 5 or strata remote from the first probe and is attached via nylon tube 5 to a gas supply. The amount of sulphur hexafluoride which accumulates in the space 1 7 is measured using the sampling device at reguiar time intervals, and from these measurements it is possible to estimate the rate of gas flow through the seam 1 5.

Since all the component parts of the probe are made of nylon or similar low strength material(s) it is not necessary to recover the probe before the seam is mined. Since nylon is at least as easy to cut as is coal the probe will present no obstacle to the efficient mining of the seam 15. After mining the shredded bits of the probe will be easily separable from the mined coal in a conventional coal preparation plant.

A second probe according to the present invention is shown in Figure 2, to which reference is now also made. The second probe is exactly the same as the first probe except that two rubber sleeves 19, 20 are fitted onto the probe, one between the flanges 10 and 11 and one between the flanges 11 and 12. The sleeves are clamped on by jubillee clips 21. Also the second probe has a one way valve 22 in the grout supply line. This is only shown diagrammatically and comprises a spring biased ball bearing adapted to locate in a part cylindrical seating.

The second probe is used as in the same way as the first probe, except that when the first charge of grout is supplied the rubber sleeves are expanded so that they tightly abut the fractured ground 16, thereby sealing the borehole. When the grout supply is turned off the valve 22 operates to stop the grout flowing back down the line. A second charge of grout is supplied to seal the probe in the borehole, and measurements etc.

are taken as described above.

This the probe according to the present invention provides a useful addition to the complement of tools used to improve the safety and efficiency of coal mining operations.

Claims

1. A probe comprising a generally cylindrical member having -a base, and a flexible annular flange on the member, which member has a gas conduit extending from the base to beyond the flange and a grout conduit extending from the base to before the flange, the flange being adapted, in use, to abut tightly the wall of a borehole and thereby to prevent grout, which is supplied to the probe to seal it in the borehole, from blocking the gas conduit.

2. A probe according to claim 1, in which each of the conduits at its end remote from the base finishes in a series of inlets/outlets.

3. A probe according to claim 1 or 2, in which the member is pointed or bullet-shaped at its end remote from the base.

4. A probe according to any one of the preceding claims, in which the flange is made of a tough flexible rubber.

5. A probe according to any one of the preceding claims, and including further flanges on the member, between the first flange and the base..

6. A probe according to claim 5, and including a resilient sleeve fixed onto the member between each adjacent pair of flanges.

7. A probe according to claim 6, and including a pin-hole in the sleeve nearest the base.

8. A probe according to any one of the preceding claims, which is made of nylon or beechwood.

9. A probe according to any one of the preceding claims, and including a valve in the grout supply line.

10. A probe substantially as hereinbefore described with reference to the accompanying drawings.

11. A method of measuring the gas content of a rock stratum, comprising drilling a blind borehole into the stratum, connecting a pressure gauge and a grout supply to the respective conduits of a probe according to any one of the preceding claims, inserting the probe base - last into the borehole so that the or each flange abuts the borehole tightly and the probe forms a closed space at the blind end of the borehole, supplying grout to the grout conduit so that the probe is sealed into the bore, allowing the grout to set, and measuring the pressure of the gas in the sealed blind end of the bore.

12. A method according to claim 1 and including the step of pumping into the borehole behind the probe a further charge of grout after the first charge of grout has set.

13. A method according to claim 12, in which the second charge of grout is supplied at a higher pressure than the first charge.

14. A method according to any one of claims 11 to 13, in which the borehole is drilled in stepwise fashion.

1 5. A method of measuring the gas content of a rock stratum, substantially as hereinbefore described with reference to the accompanying drawings.

1 6. A method of measuring the rate of gas flow through a rock stratum, comprising drilling a pair of spaced boreholes into the stratum, connecting a gas supply and a grout supply to the respective conduits of a first probe according to any one of claims 1 to 10, connecting a gas sampling device and a grout supply to the respective conduits of a second probe according to any one of claims 1 to 10, inserting one probe into each of the boreholes base - last so that the or each flange on each probe abuts its borehole tightly and each probe forms a closed space at the blind end of its borehole, supplying grout to the grout conduits so that each probe is sealed into its borehole, allowing the grout to set, pumping a gas into the first probe and measuring the time elapsed between the gas being pumped into the first probe and its being detected by the sampling device connected to the second probe.

17. A method according to claim 16, in which the gas is sulphur hexafluoride.

18. A method according to claim 16, in which the gas is radio-active.

1 9. A method of measuring the rate of gas flow through a rock stratum substantially as hereinbefore described with reference to the accompanying drawings.