Real-Time Tracking of CO2 Injected into a Subsurface Coal Fire through High-Frequency Measurements of the 13CO2 Signature

CO₂ was injected into a coal fire burning at a depth of 15 m in the subsurface in southwestern Colorado, USA. Measurements were made of the 13CO₂ isotopic signature of gas exhaust from an observation well and two surface fissures. The goal of the test was to determine (1) whether CO₂ with a distinct isotopic signature could be used as a tracer to identify flow pathways and ravel times in a combustion setting where CO₂ was present in significant quantities in the gases being emitted from the coalbed fire, and (2) to confirm the existence of a self-propagating system of air-intake and combustion gas exhaust that has been previously proposed. CO₂ was injected in three separate periods. The 13CO₂ isotopic signature was measured at high frequency 0.5 Hz) before, during, and after the injection periods for gas flowing from fissures over the fire and from gas entering an observation well drilled into the formation just above the fire ut near the combustion zone. In two cases, a shift in the isotopic signature of outgassing CO₂ provided clear evidence that injected CO₂ had traveled from the injection well to the rervation point, while in a third case, no response was seen and the fissure could not be assumed to have a flowpath connected with the injection well. High-frequency measurements of the 13CO₂ signature of gas in observation wells is identified as a viable technique for tracking CO₂ injected into subsurface formations in real-time. In addition, a chimney-like coupled air-intake and exhaust outlet system feeding the combustion of the coal seam was confirmed. This can be used to further develop strategies for extinguishing the fire.