SURFACE TO IN-SEAM DRILLING FOR METHANE DRAINAGE AND EXPLORATION

Published in Australia's Longwalls, March 2000 as

IN-SEAM DRILLING FEASIBLE FROM SURFACE SAYS EXPERT

In-seam drilling from underground for methane drainage and exploration has served Australian mines extremely well for the past two decades. The in-seam hole has been extremely successful compared to surface degassing operations because of the distance that boreholes can be drilled in-seam. Given the 0.1 to 5 millidarcy initial permeability ranges for many of our mined seams, the alternative techniques of drainage utilising cavity completion or hydrofracture have not been successful. The advantage of in-seam drilling is compounded by the ability to control the direction of borehole so as to take advantage of directional permeability and to fit it in with mine geometry.

During this twenty year period the length of borehole has increased from 250 m up to a practical maximum of about 1500 m. The end distance is currently limited by deviations from a smooth trajectory and the borehole stability. It is theoretically possible to drill about a 4 km length of straight horizontal borehole using 70 mm OD, 60 mm ID drill rods in a 95 mm 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 2.5 km 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 can be incurred. These advantages come from the fact that the drilling operations can be accomplished separately from the workings thus not hindering production. In addition if in-seam gas drainage holes can be drilled separately from the mine then greater drainage lead times may be obtained. Increased lead time means that 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 reach laterally for over 10 km. They are achieved by drilling vertically down and then turn with a large radius bend (R > 350 m) 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 (120 m< R <350 m) is in the range of curvature currently used in the coal industry. 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.

Given 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:

1. Getting to seam cheaply
2. Staying in-seam
3. Coping with faults
4. 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 and 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 de-watering well to intersect it. The intersection process is achieved by placing a transmitter into the vertical well and drilling the in-seam borehole to intersect that vertical well. Sigra have developed the transmitter receiver technology that make 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.

D24

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 $0.90 cents per tonne to $0.30 cents per 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. Several options exist for this.