The in-seam hole has been very successful compared with surface degassing operations because of the distance that boreholes can be drilled in-seam. Given the 0.1-5 millidarcy initial permeability ranges for many mined seams, the alternative techniques of drainage utilising cavity completion or hydrofracture have not been successful. The benefits of in-seam drilling are enhanced by the ability to control the direction of boreholes so as to take advantage of directional permeability and to fit it in with mine geometry.
During this 20-year period the length of borehole has increased from 250m up to a practical maximum of about 1500m. The end distance is currently limited by deviations from a smooth trajectory and the borehole stability. It is theoretically possible to drill about a 4km length of straight horizontal borehole with 70mm OD, 60mm ID drill rods in a 95mm diameter borehole before friction causes the drill rods to helically buckle and lock up in the borehole. Drilling down dip increases the distance that can be reached as does drilling with a stiffer/lighter drill pipe.
Given a typical longwall block length of 2.5km it is desirable to drill this distance. Limitations still exist if this drilling is conducted from the workings. If drilling could be conducted remote from the workings then significant advantages could be realised including drilling operations being carried out separately from the workings thus not hindering production. In addition, if in-seam gas drainage holes could be drilled separately from the mine then greater drainage lead times may be obtained. Longer lead time means boreholes may be drilled at wider spacings thus reducing the number of holes required. This represents a significant cost saving.
The oil industry has for some time drilled long directional boreholes from surface. The longest of these have measured laterally for over 10km. They are drilled vertically down and then turned with a large radius bend (R>350m) to drill on, invariably downslope. The process is aided by large diameter drill pipe, good fluid control and a very smooth hole trajectory. Medium radius drilling (120m
Various shorter curvature systems exist, some of which use coiled tubing. Coiled tubing may be forced to turn a sufficiently tight bend that it yields. Because of this the drill string has a limited life. There is even a drilling system that utilises hydraulic hose as the drill string and can be turned at right angles within a vertical borehole.
Generally the tighter the radius turned by the drill string, the shorter the distance that can be reached and the higher the risk of retrieval. Having a good solid drill pipe to twist and pull on to get out of a stuck in-hole situation has huge advantages, particularly as large volumes of fluid can be pumped down the pipe too assist the process.
In light of the depth of most coal mines and the cost constraints on drilling equipment, the medium radius option is the most cost effective way to get down to a coal seam from surface and to drill on in-seam. The critical factors that need be considered are:
* Getting to seam cheaply.
* Staying in-seam.
* Coping with faults.
* Getting water out of the borehole (this is essential in most instances for gas flow to occur).
In the solution proposed getting water out of the hole and getting to seam cheaply are concurrently achieved. Drilling from surface to the seam may be accomplished at reasonable cost if the borehole diameter is not large. Installing running water pumps in large diameter deviated boreholes is prohibitively expensive. The solution is to drill the horizontal to in-seam well and to drill a vertical dewatering well to intersect it. The intersection process is executed by placing a transmitter into the vertical well and drilling the in-seam borehole to intersect that vertical well. Sigra Pty Ltd has developed the transmitter receiver technology that makes this possible. The receiver is part of Sigra's geosteering tool designed specifically for coal.
The geosteering tool is a borehole survey tool that incorporates geophysical sensors to guide the driller through rolls, faults and other geological irregularities. Keeping a hole open through faulted zones is eased during drilling from surface by the mud pressure in the borehole. In cases where faults are intersected, screening may be required to keep the borehole open when the hole produces.
An illustration of the proposed system is shown. This system is technically possible now.
An estimate for a central Queensland mine indicates a cost reduction in degassing from current in-seam drilling practices of 90 cents per tonne to 30cents/tonne utilising the system shown in the figure. At this cost the system can be made profitable by the use of on-site electricity generation. — Ian Gray is a Sigra director.
