PERMEABILITY OF COALS AND CHARACTERISTICS OF DESORPTION TESTS - IMPLICATIONS FOR COALBED METHANE PRODUCTION

Desorption tests, conducted before development drilling, can predict e arly production rates of coalbed methane wells. In this study, results from 136 desorption tests from 30 coreholes are compared to subsequen t production histories of surrounding wells. Coals studied are from th e Carboniferous Pottsville Formation in the Black Warrior Basin of Ala bama, USA. The investigation indicates that the best predictor for gas production is ''gas yield at 120 days, divided by sample depth''. Thi s quotient has a higher correlation with actual gas production than ot her variables examined, including original reserves in-place. Gas cont ent of coal generally increases with depth. However, coal permeability , and hence the recovery factor, tends to decrease with depth. Therefo re, coalbed methane exploration should focus on finding the optimal ba lance between gas yield and depth. Commercially successful wells are c haracterized by coals with high gas yields at shallow depths. Desorpti on tests, especially of slowly-desorbing coals, should be run for a mi nimum of 120 days, or else should be extrapolated to that time if test s are terminated earlier. (C) 1998 Elsevier Science B.V.