Variable gas saturation in coalbed methane reservoirs of the Black Warrior Basin: Implications for exploration and production

Abstract

Gas saturation is highly variable in Carboniferous coalbed methane reservoirs of the Black Warrior Basin (USA), and diverse geologic data derived from more than three decades of exploration and development provides insight into the origin of this variability and the consequences for production performance. Coalbed methane is produced from numerous thin (∼ 0.3 to < 4 m) coal seams that are distributed through 700 to 1200 m of stratigraphic section. Gas content tends to increase with depth, and gas saturation typically ranges from < 10 to > 95%. Importantly, this range of variation is commonly developed among closely spaced seams within the same well. Exploration has to date been successful where coal seams are in the thermogenic gas window. Geochemical evidence indicates that thermogenic gases have been altered by subsurface water movement and may have been augmented with late-stage biogenic gases, particularly in areas affected by fresh-water recharge. Adsorption isotherms of coal samples in the Black Warrior Basin typically have Langmuir volume between 15 and 30 cm3/g and Langmuir pressure between 2 and 6 MPa. Both Langmuir pressure and Langmuir volume correlate significantly with coal rank. In deep, normally pressured coal beds where reservoir pressure is substantially above Langmuir pressure, even a small degree of undersaturation can necessitate prolonged dewatering before a large reservoir volume can reach the critical desorption pressure. Permeability, moreover, decreases exponentially with depth, which can make dewatering coal seams deeper than 800 m difficult. Where reservoir pressure is relatively low, however, isotherm geometry indicates that significantly undersaturated reservoirs can be near the critical desorption pressure. Consequently, areas of underpressure favor high gas production along with minimal water production.