The Effect of Reclaimed Mine Lands and Shallow Coal from Soil Gas Chemistry as a Monitoring Tool in Sequestration Projects
Using soil gas chemistry to detect leakage from underground reservoirs (i.e. microseepage) requires that the natural range of soil gas flux and chemistry be fully characterized. To meet this need, soil gas flux (CO2, CH4) and the bulk (CO2, CH4) and isotopic chemistry (δ13C-CO2) of shallow soil gases (<1 m) were measured at 28 locations distributed among two active oil and gas fields, an active strip mine, and a relatively undisturbed research forest in eastern Kentucky. The measurements apportion the biologic, atmospheric, and geologic influences on soil gas composition under varying degrees of human surface disturbance.
The measurements also highlight potential challenges in using soil gas chemistry as a monitoring tool where the surface cover consists of reclaimed mine land or is underlain by shallow coals. For example, enrichment of δ13C-CO2 and high CH4 concentrations in soils have been historically used as indicators of microseepage, but in the reclaimed mine lands similar soil chemistry characteristics likely result from dissolution of carbonate cement in siliciclastic clasts having δ13C values close to 0‰ and degassing of coal fragments. The gases accumulate in the reclaimed mine land soils because intense compaction reduces soil permeability, thereby impeding equilibration with the atmosphere. Consequently, the reclaimed mine lands provide a false microseepage anomaly.
In addition, low permeability zones associated with shallow coals in undisturbed areas might impede upward gas migration. To investigate the effect of these materials on gas migration and composition, four 10 m deep monitoring wells were drilled in reclaimed mine material and in undisturbed soils with and without coals. The wells, configured with sampling zones at discrete intervals, show the persistence of some of the aforementioned anomalies at depth. Moreover, high CO2 concentrations associated with coals in the vadose zone suggest a strong affinity for adsorbing CO2. Overall, the low permeability of reclaimed mine lands and coals and CO2 adsorption by the latter is likely to reduce the ability of surface geochemistry tools to detect a microseepage signal.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009