"The theory and practice of recovering fluids from underground reservoirs with wells has long been the subject of study. Water and petroleum products drawn by wells have long been vital parts of our social and economic system. However recent interest in underground combustion and the accompanying flow processes has been stimulated by the incurring energy shortage. Investigation is underway in various countries, for example Canada, the Soviet Union, and the U. S .A., on in situ combustion of coal, oil shale and tar sands. These in situ processes often require the drilling of wells for injection of air into the material being processed and the recovery of the resulting combustion products."