Characterising Gas Shales by Laboraotry Adsorption and Field Desorption Analyses
Bustin, R. Marc
University of British Columbia, Vancouver, BC
Gas shales as typically defined, include significant reservoir gas in the sorbed state. Additionally almost all shales have intergranular and to A lesser extent fracture porosity commonly in the range of 1-10% that is available to free (non-sorbed). In order to better understand the gas storage capacity of shales and try to resolve some of the discrepancies in early published data, a field, laboratory and numerical study was undertaken of selected gas shales and potential gas shales.
In gas shales, mass flow of the free gas occurs at similar rates to diffusion because of the low permeability (micro-darcy range) and high tortusity. Thus assessing gas-in-place of gas shales using typical canister desorption techniques results in erroneous data due to co-mingling of free and sorbed gas. Additionally, escape of free gas from the shale retards diffusion of the sorbed gas. Typical in place analytical procedures treat all the gas captured during canister testing as sorbed gas and hence the sorbed gas component is markedly over estimated. Adding the free gas capacity calculated from porosity and water saturation measurements to the erroneous sorbed gas measurements for volumetric gas in place calculations results in over estimation of gas-in-place.
An additional problem replete in the early published data is the use of finely milled gas shale samples for adsorption capacity analyses by the adsorption isotherm technique. Our experimental results show that milling shale to micrometre sized particles prior to analyses results in an artificially high sorption capacities as measured by adsorption isotherms. Such results are readily attributable to the exponential increase in surface area with particle size reduction. This problem is not as evident with coal because of the higher internal surface area of coal. Our data show that most deeply buried gas shales hold more gas in the free gas state than sorbed state.