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Geologic Heterogeneity in Coalbed Methane Reservoirs of the Black Warrior Basin: Implications for Gas Production and CO2 Sequestration

Jack C. Pashin, Geological Survey of Alabama, Tuscaloosa, AL

Coal is classified as a continuous-type unconventional gas reservoir, yet geologic heterogeneity has a profound effect on the performance of coalbed methane wells and the CO2 sequestration potential of coal. Heterogeneity in Pennsylvanian-age coalbed methane reservoirs of the Black Warrior basin arises from interplay among depositional, tectonic, hydrologic, and petrologic variables, and each of these variables poses different challenges for gas production and sequestration.

Reservoir coal beds in the Black Warrior basin of Alabama are distributed through 1000-2000 m of section and are thus ideally suited for multiple-seam well-completion techniques. Coal beds are within fluvial-deltaic allocycles of glacial-eustatic origin, and bed geometry and coal quality are variable. Although coal thickness is a fundamental control on resource distribution, it is a subordinate control on production performance. Thin-skinned folds and faults of extensional and compressional origin are abundant in the Black Warrior coalbed methane fields. Normal faults partition coalbed methane reservoirs, and productivity sweet spots exist along compressional fold hinges.

Hydrologic parameters affecting production and sequestration potential include water chemistry and pressure-temperature conditions. Meteoric recharge of coal beds exposed in fold limbs has resulted in freshwater plumes that extend deep into the basin. These plumes promote late-stage bacterial methanogenesis, and hence exceptionally high gas content in coal. The salinity of formation water influences how produced water can be disposed and is a legal constraint on where CO2 can be injected.

Coal rank and ash content affect the capacity of coal to hold gas. The gas sorption capacity of coal, moreover, correlates negatively with temperature and positively with pressure. Virgin pressure is heterogeneous, indicating compartmentalization, and pressure has been depleted to less than 100 psi in mature reservoirs. Injection of CO2 into pressure-depleted reservoirs has strong potential to not only reduce greenhouse gas emissions, but also to enhance coalbed methane recovery, thereby forming a basis for an 18% expansion of coalbed methane reserves in the Black Warrior basin.

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