Investigation of coalbed methane potential in low-rank coal reservoirs – Free and soluble gas contents

Abstract

In low-rank coal (lignite and subbituminous coal) reservoirs, it is difficult to investigate the potential of free and soluble gases at different burial depths because of the lack of measuring methods available in practice. In this work, Mariotte’s law was adopted to predict free gas content and methane solubility in coal seam water was studied to calculate soluble gas content. Coal samples were collected from Chinese typical low-rank coal-bearing basins. This study shows volume of pores occupied by free gas becomes smaller when moisture content and confining pressure are high. Methane dissolving tests in four coal seam water samples under set temperatures and pressures show that methane solubility increases with increasing pressure and temperature. Pressure seems to be a more effective influencing factor than temperature on methane solubility although temperature effect is enhanced at high temperature and pressure. A mathematical model of in situ methane content containing adsorbed, free and soluble gases, was established to evaluate the in situ gas content of low-rank coal reservoirs at burial depths from 600m to 1400m. While the in situ gas content of the studied coal reservoirs increases with burial depth, the percentage of the free and soluble gases in the in situ contents ranges from 8% to 34%, and hence have to be taken into account in the evaluation of coalbed methane (CBM) potential of low-rank coal reservoirs for CBM recovery.