RESERVOIR PRESSURE AND TEMPERATURE IN THE BLACK WARRIOR BASIN: IMPLICATIONS FOR CARBON SEQUESTRATION AND ENHANCED COALBED METHANE RECOVERY

Sorption of gas onto coal is sensitive to pressure and temperature, and carbon dioxide can potentially become a supercritical fluid within coalbed methane reservoirs. More than 4,000 wells have been drilled in the coalbed methane fields of the Black Warrior basin in west-central Alabama, and the hydrologic and geothermic information from geophysical well logs provides a robust database that can be used to assess the potential for carbon sequestration in coal-bearing strata.

Hydrostatic pressure gradients in the coalbed methane fields range from normal (0.433 psi/ft) to extremely underpressured (<0.050 psi/ft). Pressure-depth plots establish a bimodal regime in which 70 percent of the wells have pressure gradients greater than 0.300 psi/ft, and 20 percent have pressure gradients lower than 0.100 psi/ft. Pockets of extreme underpressure are developed around deep longwall coal mines and in areas far removed from underground mining. Reservoir temperature within the coalbed methane target zone generally ranges from 80 to 125°F, and geothermal gradient ranges from 6.00 to 15.00°F/1,000 ft. Geothermal gradient data have a strong central tendency about a mean of 9.33°F/1,000 ft.

Geothermal gradients within the coalbed methane fields are high enough that reservoirs never cross the gas-liquid condensation line for carbon dioxide. However, reservoirs have potential for supercritical fluid conditions where reservoir pressure exceeds 1,074 psi, which is equivalent to a depth of 2,480 feet under normally pressured conditions. All target coal beds are subcritically pressured in the northeastern half of the coalbed methane exploration fairway, whereas those same beds had supercritical pressure in the southwesternmost gas fields prior to gas production. Coal can contain carbon dioxide under supercritical conditions, which may redevelop as reservoirs equilibrate after abandonment. But supercritical isotherms indicate non-Langmiur conditions under which some carbon dioxide may remain mobile in coal or may react with formation fluid or minerals. Hence, carbon sequestration and enhanced coalbed methane recovery show great promise in subcritical reservoirs, and additional research is required to assess the behavior of carbon dioxide in coal under supercritical conditions where additional sequestration capacity may exist.