Anatomy of a Coalbed Fire: From Characterization to CO2 Pilot Injection Design at a Coal Fire Near Durango, CO

Abstract

Abstract Underground coal fires pose environmental, social, health, and economic hazards. Many coal fires burn persistently over time scales of many years to decades, despite efforts to control them. Results of research performed at a subsurface coal fire near Durango, CO, are used to delineate the life cycle of a natural coal fire. Methods to identify oxygen influx points (mapping of surface fissures), how these fissures are created (numerical simulations), combustion zone boundaries (subsurface temperature, surface snowmelt, and magnetometer data), and coal consumption rates (combustion product gas flux estimates) are described. Based on the locations of the mapped subsurface combustion zone, a CO2 pilot injection test was designed and conducted at this site. Results of this pilot project demonstrate 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) CO2 injected 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.