Anatomy of subsurface coal fires [electronic resource] : a case study of a coal fire on the Southern Ute Indian Reservation
Keywords: Energy Resources Engineering.

Underground coal fires pose environmental, social, health, and economic hazards. They can be significant sources of CO2 emission, toxic fumes such as aromatics, carbon monoxide, and hydrogen sulfide, and cause surface subsidence as coal in the ground is consumed and the overburden collapses. These fires are world-wide occurrences, and they have been documented in the U.S., India, China, Indonesia, Australia, and South Africa, among other coal-bearing countries. Many coal fires burn persistently over time scales of many years to decades, despite efforts to control them. In this project, results of a field-based research at a subsurface coal fire known as the North Coalbed Fire (NCF) near Durango, CO, on the Southern Ute Indian Reservation are used to delineate the life cycle of a natural coal fire. Field measurements included mapping of surface fissures that could acts as oxygen influx points, delineation of combustion zone boundaries (subsurface temperature, surface snow melt, surface deformation, and magnetometer data), and measuring properties of the gases being emitted at the site (gas composition, d13C isotope signatures). One of the phenomena observed at the NCF---which is also commonly observed at other coal fires around the world---was the formation of fissures above the burning and previously burned areas. Numerical simulations were formulated and employed to describe the relationship between fissure locations and apertures and the magnitude of the collapse in the subsurface. Based on these measurements and simulations, detailed conceptual pictures of the NCF were created, and a CO2 pilot injection test was designed and conducted at this site to determine whether the combustion zone could be flooded by injecting CO2 through wells that were drilled near the combustion zone. Results of this pilot project demonstrated that (1) air required to support continued combustion flows through a zone of fractured rocks that forms where coal has burned previously, (2) the primary driving force for air flow is the density-driven flow of hot combustion product gases through surface fissures that form as a result of subsidence as the coal burns, and (3) CO2injected into the air flow zone is drawn by the density-driven flow into the combustion zone, replacing a portion of the air flow that sustains combustion. The pilot test results suggest that a CO2 injection scheme can be designed to control these fires if sufficient CO2 (or other inert gas) is available to reduce the oxygen flow to the combustion zone below levels required to support continued combustion, and if the combustion region can be cooled sufficiently to prevent resumption of the air flow when injection ceases. Three independent methods provided consistent estimates of the rates of coal consumption, CO2 emission, and the amount of air required to sustain subsurface combustion reactions. These estimates in turn, helped determine how much inert gas must be injected to overwhelm the combustion zone with the injected gas. The mass flow rate of inert gas required was approximately 100 kg/min. The duration of the injection period was estimated to be on the order of a year in order to cool the rocks so that the potential for reignition can be minimized. This result was derived using first-order calculations and a flow simulation model.