METHANE DIFFUSION AND DESORPTION IN COAL (NEW MEXICO)

The diffusion and desorption of methane through coal has been considered. The commonly used "unipore" diffusion model is found to be inadequate for describing methane desorption from coal particles. A bidisperse pore diffusion model is presented which describes observed desorption rates for the entire time-scale of desorption. A pulse tracer experimental system has been developed which allows the rapid determination of the bidisperse pore model's diffusion parameters. The validity of this technique is demonstrated for two test systems and for three different coal samples.;Using the knowledge gained about diffusion in coal, a new technique for measuring the in-situ methane content of coal has been developed. This technique uses drill cuttings obtained from conventional drilling operations. This eliminates the expensive and time-consuming coring requirement of past techniques. This new technique was tested at 14 wells in the San Juan Basin of New Mexico. The methane content of 30 samples was found to range from 1 to 6 cm('3)/g at overburden depths of 300 to 1050 m.;Steady state diffusion mechanisms of methane, ethane and propane transport through coal have been determined using a constant pressure, counter-diffusion experiment. Gas phase transport was found to be a combination of bulk and Knudson diffusion. The relative magnitude of each of these two mechanisms was dependent upon pressure, hydrocarbon gas and coal type. Surface transport of adsorbed hydrocarbons was observed for most coal/hydrocarbon combinations. The determination of the magnitude of surface transport is complicated by molecular sieving of helium/hydrocarbon in the counter diffusion experiment.