GB2245916A - "Borehole plug" - Google Patents

Abstract

A plug for locating in a borehole comprises two aligned cylindrical members 10, 11 with aligned axial bores 13 and a cylindrical hollow resilient sleeve 12 between them. The members 10, 11 are of a synthetic polymeric material and are threaded together so that they may move together axially and thereby compress the resilient sleeve 12. One or more brass dogs 17 are pivoted at the forward end of the plug and urged outwardly into a rearward-facing position by springs A. A rotational drive is imparted to the rearward end of the plug via a shear pin 23, designed to break when the rotational force applied exceeds a predetermined value. <IMAGE>

Description

Borehole Plug The present invention is a plug which is devised for use in stopping up boreholes.

When a borehole is being drilled, for example in investigating the maRerup of successive layers of rock or other materials, it is frequently desired to seal the borehole completely at some point in its depth. Typically it is desired to form a cement seal to close off the borehole at that point. For this purpose, it has been proposed to locate a metal plug just above that depth and then to inject a suitable grout through the plug, which grout then sets to form the desired seal.

One available form of plug comprises two axially aligned, cylindrical steel members which can be screwed closer together in situ to expand a rubber sleeve located between them, to cause the rubber sleeve to seal against the wall of the borehole and thereby hold the plug in place during subsequent injection of grout. In order to lock one of the steel members against rotation while the other member is screwed on to it, that first member is provided with a pair of resilient steel fingers, which extend -forwardly with respect to the plug (that is, further into the borehole) and are urged radially outwardly by their resilience into contact with the wall of the borehole.

This prior available plug suffers fran at least two important disadvantages in use. Firstly, when drilling in mineral-bearing formations such as coal, it is not unusual to encounter flammable and/or explosive gases and it is therefore important to avoid using materials, such as steel, iich can give rise to sparks. Secondly, the plug offers only a limited resistance to axial movement in a rearward direction and may therefore became displaced under pressure from the grout or other material in the borehole.

It is an object of the present invention to provide an improved form of borehole plug, in which the foregoing disadvantages of the existing plug are reduced or eliminated.

The borehole plug according to the present invention comprises two axially aligned, generally cylindrical members having aligned axial bores therein, a generally cylindrical hollow sleeve of a resilient material disposed between those members and in mutual axial alignment with them, the generally cylindrical members being formed of a synthetic polymeric material and being in threaded mutual engagement to permit their relative axial movement, whereby to apply an axially compressive force to the hollow sleeve, one or more brass dogs, pivoted within the region of a first end (the "forward" end) of the plug and urged in a generally radially outward and rearwardly-facing direction by spring means, rotational drive means adapted to engage the second end (the "rearward" end) of the plug, and a shear pin, located to provide a rigid connection between the drive means and the plug and adapted to break when the rotational force upon the plug exceeds a predetermined value.

In the following description, the words "forward" and "rearward" refer to the positions and movement of the plug relative to the borehole, in that the forward end of the plug is disposed further into the borehole than the rearward end.

The aligned generally cylindrical members are made of a synthetic polymeric material and therefore are not prone to generating sparks like the steel hitherto used for existing such plugs. The particular material used is selected according to the desired properties but in general is preferably a hard and rigid material. Various nylons are suitable for this purpose, among which may specifically be mentioned nylon 66 and also the material sold by the firm Polypenco Limited under the trade name Nylatron GSM. The latter is a partially cross-linked nylon 6 material modified by the addition of mDlybdenum sulphide.The preferred materials are laminated phenolic resins, of which the most preferred is the fabric-laminated phenolic resin sold by Tufnol Ltd. as their "Whale" brand, modified by coating with graphite particles to enhance its antistatic properties.

The generally cylindrical hollow sleeve is located between the two generally cylindrical numbers such that these members may nsve relative to each other to squeeze the sleeve between them and thereby expand it in a radially outward direction. To this end, the members are threaded together so that when they are rotated relative to each other they more together in an axial direction to apply compression to the ends of the sleeve. The sleeve is formed of a resilient material such as a natural or synthetic rubber.

Adjacent to the forward end of the plug, upon the adjacent generally cylindrical member, one or more brass dogs are pivoted.

Preferably at least two brass dogs are provided, preferably symmetrically disposed around the circumference of the member.

In a particularly preferred form of the invention there are four such brass dogs. By virtue of their pivoting, and under pressure frcm associated spring means, the dogs are urged generally radially outwardly relative to the plug such that their outer ends are directed towards the rearward end of the plug. In this position they perform two functions. Firstly, by engaging the wall of the borehole, they hold the forward member against rotation and thereby allow relative rotation of the two members.

Secondly, the fact that they are rearwardly directed means that they assist the sleeve, once expanded, in resisting pressures within the borehole tending to eject the plug fram the borehole.

The spring means provided to urge the brass 'dogs outwardly may conveniently be rubber springs located, for example in sockets, between the dogs and the adjacent generally cylindrical member.

Rotation of the rearward member is effected by a rotational drive means which engages the rearward end of the plug and is connected to the plug by a shear pin.

The drive means conveniently takes the form of an adaptor, driven in turn by the drilling line. The adaptor may, for example, include a short cylindrical section which enters the bore in the rearward member and is held there by a radially-directed shear pin.

The shear pin transmits the drive to the rearward member of the plug and is designed to break when the sleeve has been expanded to such an extent that further relative rotation of the two rembers would, without the shear pin, damage the members, for example by breaking the threads thereon. The shear pin may be of brass.

The plug rray be used in order to enable a cement seal to be formed in the borehole or may, in appropriate circumstances, itself form the seal. For the first purpose, it is necessary to inject a suitable grout via the axial bore extending through the plug and a non-return valve is then provided at the forward end of the plug. In a preferred form, the valve is a simple flap, for example of brass, pivoted so as to close the forward end of the axial bore. Preferably, the pivoting of the flap is limited to ensure that it will always return under rearward pressure fram the grout. For example, it may be restricted to a maximum angular rrvanent of the order of 30 degrees between its closed and its fully open positions.

Alternatively, the plug may be formed closed at its forward end, in which case it may be used for itself sealing off a borehole without the use of cement.

As underground conditions change or the requirements of a borehole are modified, it may be desired to reopen a sealed borehole. For this purpose, the flap or closed end of the plug may readily be drilled through or the whole plug may be drilled out and these operations are rendered much easier by virtue of the specified materials of construction of the plug.

The invention will now be further described with reference to the acoompanying drawing which illustrates, by way of example only, one preferred embodiment of the borehole plug according to the present invention, shown in axial cross-section.

The illustrated plug comprises a forward member 10 formed, for example, of grachite-coated Tufnol "Whale" brand fabric-laminated phenolic resin and in threaded engagement with a rearward member 11 of the same material. Aligned with these two members and surrounding the threaded sections of the members is a neoprene rubber sleeve 12.

The sleeve 12 is graded with harder rubber in its end sections and a softer rubber in its central section. An axial bore 13, which extends through the two aligned members 10 and 11, is closed off at the forward end of the plug (at the left of the plug as illustrated) by a brass flap 14, pivoted to the member 10 upon a brass pivot 15.

A stop 16 limits the opening movement of the flap 14 to a maximum angle of 30 degrees.

At the forward end of the member 10, four brass dogs 17 brass are pivoted upon/pivots 18, which are symmetrically located around that end. Rubber springs 19, housed in sockets 20 in the member 10, urge the dogs 17 radially outwardly so that, when they are free to do so, they point in the direction of the rear of the plug.

An adaptor 21, shown in broken lines in the drawing, has a short cylindrical extension 22 which is introduced into the member 11 and retained there by a brass shear pin 23. The adaptor 21 and shear pin 23 allow a rotary drive to be imparted to the member 11.

When the illustrated plug is to be used to allow a concrete seal to be formed in a borehole, the plug is fitted to the adaptor 21 as illustrated and introduced by the drill string to a position in the borehole just short of the desired position of the seal. The dogs 18 readily retract to allow movement of the plug into the borehole but, under pressure fram the rubber springs 19, they are able to engage the wall of the borehole and thereby prevent rotation of the member 10. Thus, when the nwriber 11 is now rotated via the adaptor 21, this member advances along the threads of the member 10 and squeezes the rubber sleeve 12, thus causing the sleeve to expand outwardly into engagement with the wall of the borehole. When the resistance to further rotation of the member 11 reaches a predetermined value, selected to prevent damage to the threads on the two members, the shear pin 23 breaks, thereby disconnecting the adaptor 21 fran the member 11.

A suitable grout is now introduced, via the drill string and the adaptor 21, into and through the plug and beyond the flap 14 into the borehole. The flap 14 prevents backflow of the grout fram the hole. When a desired pressure has been established in the grout in the borehole, the drill string and adaptor 21 are withdrawn for cleaning and re-use.

Claims (12)

1. A plug for a borehole, which plug comprises two axially aligned, generally cylindrical members having aligned axial bores therein, a generally cylindrical hollow sleeve of a resilient material disposed between those members and in mutual axial alignment with them, the generally cylindrical members being formed of a synthetic polymeric material and being in threaded mutual engagement to permit their relative axial movement, whereby to apply an axially compressive force to the hollow sleeve, one or more brass dogs, pivoted within the region of a first end (the "forward" end) of the plug and urged in a generally radially outward and rearwardlyfacing direction by spring means, rotational drive means adapted to engage the second end (the "rearward" end) of the plug, and a shear pin, located to provide a rigid connection between the drive means and the plug and adapted to break when the rotational force upon the plug exceeds a predetermined value.

2. A borehole plug as claimed in claim 1, wherein the synthetic polymeric material is a nylon.

3. A borehole plug as claimed in claim 1, wherein the synthetic polymeric material is a laminated phenolic resin.

4. A borehole plug as claimed in any of the preceding claims, wherein the resilient material is a natural or synthetic rubber.

5. A borehole plug as claimed in any of the preceding claims, comprising at least two said brass dogs.

6. A borehole plug as claimed in any of the preceding claims, wherein the spring means comprises one or more rubber springs located between the brass dogs and the adjacent generally cylindrical member.

7. A borehole plug as claimed in any of the preceding claims, wherein the rotational drive means comprises an adaptor, driven in turn by the drilling line.

8. A borehole plug as claimed in claim 7, wherein the adaptor includes a short cylindrical section which is adapted to enter the bore in the rearward generally cylindrical manner and to be held there by a radiallydirected shear pin.

9. A borehole plug as claimed in any of the preceding claims, wherein the shear pin is of brass.

10. A borehole plug as claimed in any of the preceding claims, having a non-return valve at its forward end.

11. A borehole plug as claimed in claim 10, wherein the non-return valve is a pivoted flap.

12. A plug for a borehole, substantially as hereinbefore described with reference to, and as illustrated in, the accanpanying drawings.