Underground gasification of coal: The filling of dipping underground cavities

Abstract

In deep underground coal gasification the all-important parameter is the volume of coal that can be gasified between two boreholes. This volume depends to a large extent on the maximum attainable linking distance. Spectacular advances in recent years in deviated drilling techniques suggest that the linking of deep boreholes can more easily be achieved by drilling than by conventional methods such as fracturing. Gasification methods could then be developed in dipping coal seams in which the combustion front is, from time to time, driven updip by backward filling of the cavity with a filler such as sand. The results of filling tests in a scaled model are described. It is shown that dipping underground cavities can be completely filled by a process of sedimentation, with the exception of an updip channel along the coal face. Through this channel combustion can be re-initiated after removing the excess water with high-pressure gas. In this way a number of roughly parallel strips of coal could be gasified between two boreholes; serious subsidence at the surface would then be avoided. The tests also show that localized caving-in of the roof of the cavity does not seriously disturb the filling process or the development of the updip channel.