Study on coalbed methane accumulation characteristics and favorable areas in the Binchang area, southwestern Ordos Basin, China

The Binchang area of southwestern Ordos Basin is one of the most promising areas for low-rank coalbed methane (CBM) in China. This work investigates the Jurassic Yanan Formation coal and CBM geology and accumulation characteristics of CBM in the southwestern Ordos Basin based on data from 46 wells and laboratory measurements of 14 coal samples from 7 mines. The results show that coal rank in the Binchang area is mainly sub-bituminous A and high-volatile C bituminous (0.46%-0.73%R-o). Coals are dominated by inertinite (14.7-85.6%); less abundant are vitrinite (8.5-77.7%) and liptinite (1.5-15.2%). Minerals are found only in small amounts (0.4-8.3%). Permeability is between 0.04 and 25.3 mD, and porosity ranges from 2.4% to 20.1%. Most coal pores are less than 100 nm in diameter, making them favorable for gas adsorption but unfavorable for gas permeability. Pore morphology is represented mainly by micro- and mesopores with a well-connected and ink-bottle shaped (narrow throat and wide body) morphology. These coals are characterized by a high adsorption volume of more than 3.0 x 10(-3) ml/g. Methane isothermal adsorption measurements of 12 coal samples revealed that their maximum adsorption capacity (on a dry and ash-free basis) varies from 5.06 to 13.37 m(3)/t, depending on moisture content. However, under the influence of gas preservation conditions, the in-place gas content is generally 0.11-6.26 m(3)/t. Finally, based on a comprehensive analysis of coal thickness, gas content, hydrogeology conditions, roof, floor, and depth properties, this study indicated that the best prospective target areas for CBM production are forecasted to be the Tingnan and Dafosi areas, which are located in the syncline, central south part of the study area. (C) 2012 Elsevier B.V. All rights reserved.