Print this pageImpact of Shale Properties from Pore Structure and Measurements of Gas Permeability and Diffusivity of Tight Reservoir Rocks: Different Approaches and Their Applications
Knowledge of the pore structure of gas shales and other micro-porous rocks is of critical importance in establishing the original gas in place and flow characteristics of the rock matrix. Methods of measuring pore volume, pore size distribution, sorptive capacity, and flow characteristics of shales, inherited from the coalbed methane and conventional reservoir rock analyses, although widely applied, are of limited value in characterising many shales and may yield erroneous results. The use of He as routinely applied to measure porosity, permeability, and diffusivity may result in non-systematic errors due to the molecular sieving effect of the fine pore structure to larger molecules such as reservoir gases. Utilizing gases with larger adsorption potentials than He, including nitrogen and all reservoir gases, to measure porosity or permeability of rocks with high surface area is a viable alternative, but requires correction for adsorption in the analyses. We expand on several approaches to measure permeability and diffusivity with consideration of gas adsorption, which has not been explicitly considered in previous studies. Our new models explicitly correct for adsorption during pulse decay measurements of core under reservoir conditions, as well as on crushed samples used to approximate permeability or diffusivity. We also present a method to determine permeability or diffusivity from data captured during core desorption as carried out during coal or shale desorption analyses for gas in place determinations. Our new approach utilizes late-time data from experimental pressure decay data, which we show to be more reliable and theoretically (and practically) accurate than the early-time approach commonly used to estimate gas transport properties.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009