Directional drilling services company REI Drilling, based in Salt Lake City, has successfully applied HDD to effectively determine the lateral extent of burn and identify active oxidation.
Initially performed for development of in-seam boreholes for methane drainage, HDD is now routinely applied for geologic exploration, exploration of abandoned mine workings, and for the development of targeted boreholes into surrounding mine workings.
In a coal seam, burn characteristics include oxidized, or burned coal, which affects roof stability in proximate entries and condemns reserve areas. In some instances, oxidation is still active. From a mine planning perspective, burn is sporadic and unpredictable.
HDD is used to identify the contact between virgin coal and burn. Drill thrust, downhole water pressures, return volumes and cuttings are monitored while drilling. Clear indications of burn are reduced downhole drilling pressures, circulation loss, and a significant reduction in drilling thrust (weight on bit necessary to advance).
“The key to quantifying the extent of burn is planning for placement of strategic exploratory side-tracks during drilling of the initial borehole,” REI registered geologist Mike Bohan said.
“Well placed side-tracks can map the lateral extent of burn and provide mine operators with the information necessary to define mine plans that will maximize coal recovery from reserves near burn.”
In the figure the application of HDD to map coal burn between projected gate-road development and coal outcrop is shown. The outcrop near the reserve had extensive coal burn, but the lateral extent of burn toward the interior of the reserve was unknown.
The mine operator was concerned burn encroached on the reserve and used HDD to map the burn prior to committing to development. Vertical drilling was out of the question as it required extensive permitting and had environmental implications.
The approach was to develop an exploration borehole along the azimuth of the mine’s projected workings and flank the anticipated burn. During drilling, REI planned for placement of side-tracks to explore for the burn adjacent to the main borehole. No coal burn was intercepted during drilling of the main borehole.
From the main borehole, REI explored tangentially, toward outcrop, with numerous side-tracks. This effort identified the contact between virgin coal and burn along the length of the initial borehole. Side-tracking efforts were performed from inbye to outbye and required placement of mechanical packers after each interception with burn.
REI completed the effort and was able to provide the mine operator with the location of competent coal along the azimuth of projected developments, and the extent of burn relative to this line. This information was used by the mine operator to plan the development of this reserve.
REI offers a range of directional drilling services to the coal mining industry and operates a fleet of seven underground longhole drilling units.
