Impact of mass balance calculations on adsorption capacities in microporous shale gas reservoirs

Determination of the adsorbed reservoir capacity of gas shales by adsorption analyses as done routinely by mass balance maybe in significant error if the effects of pore-size dependent void volume (porosity) is not considered. It is shown here that with increasing pressure, helium, which is invariably used to measure void volume, can access pores that are not available for adsorption to gases with larger kinetic diameters as highlighted by experiments with zeolites of known pore-size distribution. Helium can diffuse and/or adsorb in restricted pores of the microporous samples, as indicated by a larger void volume with pressure. The error in adsorption calculations due to helium void volume calibrations for high pressure methane isotherms is most significant with low organic-carbon content, moisture-equilibrated shales and mudrocks in which the overall adsorptive capacity is low. In such samples negative adsorption can be calculated due to the void volume of helium used in the mass balance calculations exceeding the void volume to methane – a reflection of greater pore-space accessibility of the smaller helium molecule than methane. The amount of the error introduced by using helium void volume in mass balance calculations is pore-size and pore-size distribution dependent. Organic-rich shales and mudrocks or coals which do not show negative methane adsorption also maybe in error but the error is masked by their larger adsorbed gas capacities. Such findings underline the importance of analysis gas-type as kinetic diameter size will influence the penetrability/diffusion of the gas through the sample and hence the calculated adsorbed gas capacities.