FACTORS AFFECTING THE GAS CONTENT OF POCAHONTAS BASIN COAL BEDS, SW VIRGINIA: IMPLICATIONS FOR COAL-BASED CARBON CAPTURE/STORAGE AND ENHANCED COAL BED METHANE PRODUCTION

The USDOE estimates that Pocahontas Basin ( SW Virginia and southern WV) coal beds can store approximately 398 million tons of CO₂, and potentially enhance the recovery of >2.5 TCF of enhanced coal bed methane (ECBM). Twenty-six Lower Pennsylvanian coal samples from a test well in SW Virginia were analyzed geochemically and petrographically in order to determine if there were any salient characteristics that might influence gas content. The site and stratigraphic storage interval was chosen because it contained methane-producing coal beds that are thin (avg. thickness 1.3 ft), fairly deep (468 to 2146 ft), and as such, have little potential for future mining. Collectively, these characteristics make them attractive targets for carbon capture and storage (CCS) and enhanced gas recovery (EGR), via CO₂ injection. Petrographically, most of the samples are dominated by vitrinite group macerals. This is significant because vitrinite is more microporous than liptinite and inertinite, and as such, should be capable of adsorbing large volumes of both CH₄ and CO₂. In addition, all of the samples have random reflectance values between 0.91 and 1.15, indicating sufficient thermal maturity for economic gas generation.

Coal depth, coal rank, and coal composition were all found to in influence gas content. First, samples with >1000 ft of cover had average gas contents of 328.8 scf/ton (raw), and an average Ro of 1.04, whereas samples with <1000 ft of cover had an average gas content of 127.6 scf/ton (raw), and an average Ro of 0.93. Second, samples with >15 % ash (dry) had average gas contents of 203.1 scf/ton (raw), whereas samples with <15 % ash had average gas contents of 306.1 scf/ton (raw). Third, samples with >15 % detrovitrinite + gelovitrinite had average gas contents of 220.7 scf/ton (raw), whereas samples with <15 % detrovitrinite + gelovitrinite had average gas contents of 315.4 scf/ton (raw). Although both groups had essentially identical total vitrinite contents, samples with higher proportions of telovitrinite were found to produce the most gas.

As a considerable volume of gas is left unrecovered in many CBM operations, CO₂ injection may be one way to exploit the unrecovered gas, while at the same time permanently storing significant amounts of CO₂, an important greenhouse gas.