EFFECTS OF SHALLOW COAL AND RECLAIMED MINE LAND ON MONITORING, MITIGATION, AND VERIFICATION STRATEGIES

Soil gas flux and shallow soil gas chemistry (<1 m) was 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. They also provide a heretofore absent geochemical baseline critical for recognizing reservoir leakage (i.e., microseepage) that might result from CO₂ injection in carbon sequestration and enhanced oil recovery projects in and near the study sites.

The measurements also highlight potential challenges in using soil gas chemistry as a MMV tool where the surface cover consists of reclaimed mine land or is underlain by shallow coals. As compared to undisturbed upland soils (average summer 100 cm d¹³C-CO₂= -20.5 ± 3.8‰), reclaimed mine land soil gases often show enriched d¹³C-CO₂ (-3.2 ± 2.3‰) values and CH₄ concentrations (average 100 cm= 107 ± 406 ppmv) in excess of atmospheric (1.9 ± 0.1 ppmv). Such anomalies have been used as indicators of microseepage in conventional oil and gas fields. For reclaimed mine lands, however, the anomalies likely result from reactions involving coal fragments and siliciclastic clasts cemented with marine carbonate in the mine spoil. Soil gas anomalies associated with reclaimed mine lands thus provide a false indication of microseepage, which suggests different geochemical criteria might be needed to recognize microseepage where the surface cover is reclaimed mine material.

Further challenges might arise from low permeability zones associated with compacted soils in reclaimed mine lands and shallow coals in undisturbed areas that might impede upward gas migration. Moreover, organic matter in coals has been demonstrated to have a strong affinity for adsorbing CO₂. Overall, this impedance could reduce the ability of surface geochemistry tools to detect a microseepage signal. To investigate the effect of these materials on gas migration, four monitoring wells were drilled in reclaimed mine material or through shallow coals to a depth of approximately 10 m. The wells were configured with discrete sampling zones and injection tubes that will provide the opportunity to analyze gas transport times over specified intervals.