USING STABLE CARBON ISOTOPES TO MONITOR CO2 STORAGE IN COALBED METHANE RECOVERY SYSTEMS: A CASE STUDY FROM MARSHALL COUNTY, WEST VIRGINIA

The injection of carbon dioxide into depleted gas formations (such as unmineable coal seams) can aid in coalbed methane recovery. The purpose of this study is to test the applicability of stable carbon isotopes to track and monitor the fate of injected CO₂ within an enhanced coal bed methane recovery (ECBM) system. The site that is being monitored is the CONSOL Energy Inc. CO₂ sequestration Pilot Test Site in Marshall County, West Virginia. The injection and producing wells at the site make up a modified five spot pattern with two injection wells and two producing gas wells. At this site, CO₂ has been intermittently injected into the Upper Freeport coal seam at a depth of approximately 1200 ft since 2009. Water and gas samples are being collected from two Upper Freeport coal producing wells, three Pittsburgh coal producing wells, eight soil vadose wells, three shallow groundwater wells and surface water samples from nearby Fish creek. Sampling for isotopic analysis has been intermittent since summer of 2012. The injected CO₂ has δ¹³C value ranging between -11‰ to -11.99‰ V-PDB, which is within the range of a C4 plant based ethanol source. The minor carbon dioxide components of the produced gas collected from wells producing from Upper Freeport and Pittsburgh coal seams have average δ¹³CO₂ values of -17.3 and +22.7 ‰ V-PDB, respectively. In contrast, the average δ¹³C value of the soil CO₂ collected from monitoring wells installed in the soil vadose zone is -25.5‰ V-PDB. The distinct carbon isotope signature of the injected CO₂, soil CO₂ , and CO₂ in the coalbeds enables us to use δ¹³C as natural tracer to monitor the migration of CO₂ in the injected coal seam, and detect any potential leakage into overlying aquifers and shallow soil.