Measurement of adsorption phase densities with respect to different pressure: Potential application for determination of free and adsorbed methane in coalbed methane reservoir

Abstract

Evaluation of free and adsorption gas in the in-situ reservoir is extremely significant for the exploration and production of coalbed methane. The adsorbed gas content is commonly estimated based on isothermal adsorption experiments from the volumetric or gravimetric methods that are essentially measurements of the excess adsorption amount, but not the absolute adsorption amount. To evaluate the in-situ gas content, the excess adsorption amount must be calibrated as the absolute adsorption amount through mathematical correction. However, the theoretical base for these mathematical assumptions is either unreasonable or questionable, which limited the application of these methods. We conducted the excess isothermal adsorption experiments on a coal core plug sample, and meanwhile used NMR to quantify the free and adsorbed methane, and to obtain absolute adsorption isotherms, and then to obtain the accurate values of the in-situ densities and volumes of adsorbed phase methane during the gas adsorption process. Results show that the adsorbed phase methane density increased with pressure, showing a Langmuir-type function, while the density decreased with temperature. The adsorbed phase methane volume is independent of gas pressure and temperature, based on which as well as the adsorption potential theory, the absolute adsorption isotherms at different temperatures were predicted from one set of absolute adsorption experiments using NMR. The method for calculations of the absolute adsorption amount and adsorption phase volume from NMR is applied to estimate the free and adsorbed methane in the in-situ reservoir. The adsorption gas dominates in shallow formations, whereas the free gas dominates in deep strata. The excess adsorption can result in evident overestimation of free gas and underestimation of adsorption gas in the reservoir.