GB2163465A

GB2163465A - Drill rod for drilling boreholes

Info

Publication number: GB2163465A
Application number: GB08421195A
Authority: GB
Inventors: Timothy John Godfrey Francis
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-08-21
Filing date: 1984-08-21
Publication date: 1986-02-26
Also published as: GB8421195D0

Abstract

A drill rod for use in making up a drill string for drilling holes in the Earth's crust in the presence of drilling mud is formed with a buoyancy space (4) which can be filled with liquid, such as water, less dense than the drilling mud to reduce the weight of the rod when immersed in the mud. A vent hole (10) is provided at the bottom of the buoyancy space (4) to equalise the pressures in the buoyancy space and in the surrounding mud. A further closable vent (9) is provided at the top of the buoyancy space (4) to permit air to vent from the buoyancy space (4) when it is primed with the buoyancy liquid. <IMAGE>

Description

SPECIFICATION Drill rod for drilling boreholes This invention relates to drill rod lengths for use in drilling boreholes employing a flushing medium.

Drilling of deep holes or boreholes can be employed in prospecting for minerals and, where these are gaseous or liquid, for bringing them to the surface. A cutting bit is rotated to cut into the rock, the bit being supported at the end of a plurality of drill rod lengths comprising a drill string. Known drill rod lengths comprise a hollow tube to allow a flushing medium to be pumped down the borehole to the bit so that cut rock debris is washed back up to the surface between the drill string and the borehole wall.

The flushing medium also acts as a coolant for the drill bit and serves to consolidate the borehole wall to prevent its collapse. The flushing medium can also be employed to drive the rotation of the cutting bit.

One limitation on the depth of the borehole in these cases is the weight of the bit and drill string.

Ultimately, the combined weight can exceed the maximum tensile stress which the drill string can support, leading thereby to breaking of the drill string. For example, the weight W of a steel rod per unit length immersed in a fluid density p, is given by W=A(p,-p,) where p, density of steel and A = crosssectional area of the rod.

If the length of the steel rod is L and it is suspended from its upper end, the tensile stress at the upper w end is given by tensile stress = A = (Ps-Pf) L.

If, the maximum allowable stress for the rod is five tonnes per centimetre2, and P = 7.8 gm/cm2 and p, = 1.0 gm/cm3 then the maximum length of rod which can be suspended is 7.35 kilometres.

It is an object of the invention to provide a drill pipe length which can reduce the effects of the abovementioned limitation.

According to one aspect of the invention there is provided a drill rod length for borehole drilling comprising one or more channels through the length of the rod for conducting a flushing medium therethrough and having at least one buoyancy portion capable of containing a volume of buoyancy medium having a specific gravity less than the specific gravity of the flushing medium.

By providing a drill rod length having a buoyancy portion containing a buoyancy medium having a specific gravity less than the specific gravity of the flushing medium, supportive flotation is provided so that the weight per unit length of the drill rod length is reduced. Consequently, the maximum supportable drill string comprising a plurality of drill rod lengths can be increased. Thus, deeper boreholes can be drilled before the likelihood of breakage of the drill string. Alternatively, the reduced weight of the drill string provided by the drill rod length according to the invention allows the use of lower quality steel for the drill rod length and the use of drilling derricks having lower lifting capacities.Furthermore, it is possible to provide a larger diameter drill rod length than hitherto known whilst at the same time reducing the volume of flushing medium required during drilling. Consequently, large diameter boreholes can be drilled without excessive volumes of flushing medium being employed.

Preferably, the buoyancy portion includes a vent towards one end thereof and a vent towards the other end thereof to allow equalization of pressure between the buoyancy portion and the surround outside the drill rod length. By providing drill rod lengths with such vents it is possible to prime with buoyancy medium each new drill rod length that is added to the drill string. Conveniently, the first-mentioned vent is closable and in one aspect of the invention can be closable by connection together of the adjacent drill rod length.

Preferably, separation means is included in the buoyancy portion between the vents to reduce mixing of flushing medium from the surround with the buoyancy medium.

Conveniently the drill rod length comprises two concentrically aligned cylinders, the outer cylinder including thread means for joining two adjacent drill rod lengths and the inner cylinder includes seal means to seal to the inner cylinder of adjacent drill rod lengths.

According to another aspect of the invention there is provided a drilling apparatus having drill rod lengths as claimed in any one of Claims 1 to 7.

According to a further aspect of the invention there is provided a method of borehole drilling employing drill rod lengths according to any one of Claims 1 to 7 and comprising the steps of:-(a) rotating a drill bit to cut a borehole; (b) providing a plurality of said drill rod lengths to form a drill string between the drill bit and the entrance of the borehole to communicate rotative power and to supply a flushing medium to the drill bit; (c) feeding said drill rod lengths through a body of buoyancy medium to fill the buoyancy portion with buoyancy medium and to isolate the buoyancy portion from adjacent drill rod lengths, prior to the buoyancy portion entering the borehole.

In this way, a simple and easy method is provided for priming the buoyancy portion of each new drill rod length with buoyancy medium.

Examples of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates a cross-sectional view of a borehole containing a drill rod length embodying the present invention.

Figure 2 illustrates a longitudinal section through the drill rod length shown in Figure 1.

Figure 3 illustrates a longitudinal section through a further drill rod length embodying the invention.

Figure 4 illustrates a method of priming the buoyancy portion of the drill rod length with buoyancy medium prior to entering the borehole.

Referring to Figure 1, a borehole 1 drilled in rock contains a drill rod length having an outer cylindrical wall 3. The space surrounding the outer wall 3 filling the gap between the drill rod length and the borehole wall contains a flushing medium, for example, drilling mud. As can be seen from Figure 2, the drill rod length includes an inner cylinder 5 concentrically aligned with the outer cylinder 3. The inner cylinder encloses a channel 6 through which drilling mud can be pumped from the entrance to the borehole down towards the drill bit at the lower end of the drill string. This drilling mud subsequently returns to the surface via the gap 2.

The annular space 4 is filled with a buoyancy medium having a specific gravity less than the drilling mud, for example fresh or salt water can be employed. Consequently, referring to the radii shown in Figure 1, the approximate effective density of the drill rod length (neglecting end fittings) is given by p, = p, (r2# - r,) + p,, (r2 - r2) + p, (r4 - r23) (r4- - r12) where p, = density of the cylinder walls p, = density of water Thus, assuming the drilling mud is fluid and neglecting viscous effects the weight of the drill rod length per unit length is given by w 10 (r43 - r31) (P, - pm) tonnes W 10 km where p, = density of drilling mud (in gm/cm3) p, is in gm/cm3 r1, r4 are in cm.

An example is given below to illustrate the effective density of the drill rod length embodying to the present invention and for comparison a known drill pipe with dimensions r1, r2.

Example Let r1 = 5.239 cm = 2 1116", r2 = 6.350 cm = 2 1/2" r3 = 14.240 cm = 5.606", r4 = 15.240 cm = 6" p, = 7.8 gm/cm3, p, = 1.0 gm/cm3, p, = 2.2 gm/cm3 then (i) Effective density p, = 2.41 gm/cm3 (ii) Weight in hole = 13.5 tonnes/km (iii) For comparison, a conventional drill pipe with dimensions rl, r2 would weigh 22.7 tonnes/km The weight of the drill rod length in the borehole can be increased or decreased by altering the radii of the inner and outer cylinders 5 and 3, alternatively mud of different specific gravity could be employed.

Indeed, if the drilling mud density is increased, the weight per unit length of the drill rod length in the string can be reduced towards zero.

Figures 2 and 3 illustrate alternative ends for the drill rod length. it is important that the annular portion 4, which comprises the buoyancy portion for the drill rod length is isolated between drill rod lengths.

To achieve this, the rod length can be sealed at the lower end by a flange 7 as shown in Figure 3 and the upper end of the annular portion 4 sealed with a flange 8 which includes a vent valve 9. This vent valve can be closed after the buoyancy portion 4 is filled with the buoyancy medium. Alternatively, as shown in Figure 2 the upper end can again be sealed by a flange 8 having a vent valve 9, however the lower end of the buoyancy portion 4 is only sealed when the drill rod length is joined to the top of an adjacent drill rod length.

Similarly, the lower end of the buoyancy portion could be sealed by a flange and the upper end of the buoyancy portion 4 left unsealed or the upper end of the buoyancy portion could be sealed by a flange and the lower end of the buoyancy portion 4 left unsealed. In this way, the buoyancy portion 4 would be sealed either when a further drill rod length is attached to the upper end thereof or when the drill rod length is attached to the upper rod length of the drill string. Clearly, it is important that the central channel 6 is a continuous channel to provide a flow path for the flushing mud.

As shown in both Figures 2 and 3, a vent 10 is provided towards the lower end of the buoyancy portion so that the drill rod length can be immersed in a tank of water so that the buoyancy portion is filled up. This vent also allows equalization of pressure in the borehole between the bottom end of each buoyancy portion and the surround 2. The surround 2 is filled with the drilling mud returning to the surface.

To maintain the buoyancy of the drill rod length, it is important that mixing of the water in the buoyancy portion 4 with the drilling mud in the surround 2 is minimized. This can be achieved either by making the vent 10 sufficiently small or by including a filter 11 arranged to be of such porosity and permeability to allow easy passage of water in either direction, but has the ability of filter out mud solids. The filter may be inserted into the vent 10.

Each drill rod length includes a thread 12 at the upper end of the wall 3 and a thread 13 at the lower end of the wall 3, thus allowing easy fitting together of adjacent drill rod lengths. Some form of seal means 14 is provided to seal together the inner wall 5 of adjacent drill rod lengths to prevent thereby leakage of drilling mud from the channel 6 into the buoyancy portion 4.

Figure 4 illustrates a simple and convenient method of filling the buoyancy portions 4 during the drilling operation. The next drill rod length to be fitted to a drill string 15 is fed through a tank 16, filled with water, which is located at the entrance to the borehole 1. As can be seen from Figure 4 this allows the water to enter the buoyancy portion 4 via the vent 10 with air venting from the top of the buoyancy portion. The air may vent either through a valve or can simply vent from the buoyancy portion if the buoyancy portion is sealed by the lower end of a subsequently attached drill rod length. In this way, the buoyancy portion of each new drill rod length is primed with buoyancy medium prior to entering the borehole.

The embodiments illustrated show two concentric cylinders comprising the drill rod length. It will be apparent that more than one channel for the drilling mud could be provided and furthermore more than one buoyancy portion could be provided. Thus, the invention is not restricted to the geometric arrangement illustrated in Figure 1.

Claims

CLAIMS 1. A drill rod length for borehole drilling comprising one or more channels through the length of the rod for conducting a flushing medium therethrough and having at least one buoyancy portion capable of containing a volume of buoyancy medium having a specific gravity less than the specific gravity of the flushing medium. 2. A drill rod length as claimed in Claim 1 wherein said buoyancy portion includes a vent towards one end thereof and a vent towards the other end thereof to allow equalization of pressure between the buoyancy portion and the surround outside the drill rod length. 3. A drill rod length as claimed in Claim 2 wherein the first-mentioned vent is closable when the buoyancy portion is filled with said buoyancy medium. 4. A drill rod length as claimed in Claim 3 wherein the first mentioned vent is closed by connection to an adjacent drill rod length. 5. A drill rod length as claimed in any one of Claims 2 to 4 wherein separation means are included in the buoyancy portion between the two said vents to reduce thereby mixing of flushing medium with the buoyancy medium. 6. A drill rod length as claimed in any preceding claim wherein the drill rod length comprises two concentrically aligned cylinders. 7. A drill rod length as claimed in Claim 6 wherein the outer cylinder includes thread means for joining together adjacent drill rod lengths and the inner cylinder includes seal means for joining to the inner cylinder of adjacent drill rod lengths. 8. Drilling apparatus having drill rod lengths as claimed in any preceding claim. 9. A method of borehole drilling employing drill rod lengths according to any preceding claim and comprising the steps of:-(a) rotating a drill bit to cut a borehole; (b) providing a plurality of said drill rod lengths to form a drill string between the drill bit and the entrance of the borehole to communicate rotative power and to supply a flushing medium to the drill bit; (c) feeding said drill rod lengths through a body of buoyancy medium to fill the buoyancy portion with buoyancy medium and to isolate the buoyancy portion from adjacent drill rod lengths, prior to the buoyancy portion entering the borehole. 10. A drill rod length substantially as herein described with reference to the accompanying drawings. 11. A method of borehole drilling substantially as herein described with reference to the accompanying drawings. New claims or amendments to claims filed on 27 Jun 85 Superseded claims 1 to 11 New or amended claims:

1. A drill rod length for use in drilling holes in the Earth's crust, the drill rod length having one or more channels through the length thereof for conducting a flushing medium therethrough and having at least one buoyancy space capable of containing a volume of buoyancy liquid having a specific gravity less than the specific gravity of the flushing medium.

2. A drill rod length as claimed in Claim 1 wherein said buoyancy space is provided with a equaliza tion vent to allow equalization of pressure between buoyancy liquid in the buoyancy space and flushing medium surrounding the drill rod length.

3. A drill rod length as claimed in Claim 2 wherein the equalization vent is disposed at or adjacent the lower end of the buoyancy space.

4. A drill rod length as claimed in Claim 2, wherein the buoyancy space is provided with a further vent at or adjacent the upper end of the buoyancy space to permit air to vent from the buoyancy space when the space is primed with buoyancy liquid through the other vent.

5. A drill rod length as claimed in any of Claims 2 to 4 wherein the upper vent is closable.

6. A drill rod length as claimed in Claim 5 wherein the closable vent is adapted to closed by an adjacent drill rod length upon connection of the drill rod length to that adjacent drill rod length.

7. A drill rod length as claimed in any one of Claim 2 to 6 and further including separation means adjacent the equalization vent to reduce mixing of flushing medium with the buoyancy liquid.

8. A drill rod length as claimed in any of Claims 2 to 7 wherein the equalization vent is sufficiently small to prevent substantial mixing of the buoyancy liquid with the flushing medium.

9. A drill rod length as claimed in any preceding claim wherein the drill rod length comprises two coaxial cylinders, the inner cylinder providing a channel for flushing medium, and the buoyancy space being formed between the two cylinders.

10. A drill rod length as claimed in Claim 9 wherein the outer cylinder is screw threaded for connecting the drill rod length to adjacent drill rod lengths and the inner cylinder includes seal means for sealing engaging the inner cylinders of adjacent drill rod lengths.

11. A drill rod length substantially as herein described with reference to the accompanying drawings.

12. Drilling apparatus having a drill string provided by a plurality of drill rod lengths as claimed in any preceding claim.

13. A method of drilling a hole in the Earth's crust using a drilling apparatus as claimed in Claim 12 in which a drilling bit is rotated at the lower end of the drill string and the drill string is advanced down the hole to cut the hole, and in which drill rod lengths are added to the drill string at the top of the hole to increase the length of the drill string as the string advances down the hole, and in which flushing medium is fed to the drill bit by the drill string, characterised by the step, when adding a drill rod length to the drill string, of filling the buoyancy space of the drill rod length with a buoyancy liquid which is less dense than the flushing medium.

14. A method as claimed in Claim 13 wherein the filling step includes the steps of passing the drill rod length through a supply of the buoyancy liquid, permitting the buoyancy liquid to enter the buoyancy space, and venting air from the buoyancy space.

15. A method of borehole drilling substantially as herein described with reference to the accompanying drawings.