Method of subterranean steam generation by in situ combustion of coal

This specification discloses a method of recovering the thermal energy of a subterranean coal formation that is penetrated by at least two wells. The subterranean coal formation is fractured by hydraulic fracturing techniquesto form at least two spaced apart fractures which extend through the coal formation and communicate with both of the wells. A combustion-supporting gas is injected into a first fracture and the coal is ignited to form a combustion front. Injection of the combustion-supporting gas is continued to propagate the combustion front through the coal formation along the first fracture. The heat or thermal energy generated by the combustion of the coal is conducted through the formation intermediate the fractures and into a second fracture. Water is injected via one well into the second fracture and flowed therethrough to the other well to absorb the thermal energy released by the combustion of the coal. The water, in absorbing the thermal energy, is at least partially converted into steam and the steam and associated hot water are produced via the other well to the surface of the earth.