| Desorption tests measure the amount of gas present in coal seams, and when conducted before development drilling can be used to predict early production rates of coalbed methane wells. Data from 136 desorption tests from 30 core holes are compared to subsequent production histories of surrounding wells. Coals are from the Carboniferous Pottsville Formation in the Black Warrior Basin of Alabama. The best predictor for gas production is "gas yield at 120 days, divided by sample depth". This quotient has a higher correlation with gas production than other variables, including original gas in-place. Although the gas content of coal tends to increase with depth, the permeability, hence the recovery factor, tends to decrease with depth. Thus, coalbed methane exploration should seek the optimal balance between gas yield and depth. Successful wells are characterized by coals with high gas yields at shallow depths.; Desorption tests of slow-desorbing coals should be run a minimum of 120 days, or else a 120-day gas yield should be extrapolated if tests are terminated sooner. A useful equation for extrapolating gas yields is {dollar}y = atsp{lcub}b{rcub}/(tsp{lcub}b{rcub} + c),{dollar} where {dollar}y{dollar} is gas yield, {dollar}t{dollar} is time in days, and {dollar}a, b,{dollar} and {dollar}c{dollar} are empirically-derived constants determined from early desorption data for each test.; The upper part of the Pottsville Formation thickens in the southern part of the coalbed methane producing district. This is partly due to units within the formation becoming thicker, and partly from addition of younger strata at the top of the formation. Besides the Black Creek, Mary Lee, Pratt, Cobb, Gwin, Utley, Brookwood, and Sipsey coal groups, a younger coal group, the Moundville, is here defined for the first time. |
