The effect of analytical particle size on gas adsorption porosimetry of shale

Abstract

Gas storage capacity and gas producibility of shale gas reservoirs are critically limited by shale porosity. In spite of its importance, porosimetric characterization of shale remains challenging due to highly heterogeneous structures, small average pore sizes, and wide pore size distributions. This study utilizes low-pressure N2 and CO2 gas adsorption porosimetry to investigate the evolution of micro- and mesopores in a suite of 11 New Albany Shale samples across a wide range of thermal maturity corresponding to vitrinite reflection Ro values from 0.35 to 1.41%. Mesopore volumes follow a nonlinear evolutionary path starting with a maximum in immature shale (sample 472-1). Subsequent intermittent minima in mesopore volumes during early and late maturity are consistent with the transformation of kerogen during the early mature stage (sample 554-2) and secondary cracking of bitumen/oil at the late mature stage (sample IL2), respectively. Micropore volumes display a varying trend throughout thermal maturation, and are significantly controlled by total organic carbon contents. Both meso- and micropore volumes are positively correlated with clay content and tend to decrease with an increase in feldspar content.A reduction in grain size of shale samples for gas adsorption porosimetry prominently enhances mesopore volumes, whereas the effects on micropore volumes are variable. These findings may be associated with the fact that smaller particles are able to attain complete adsorption equilibrium quickly, which in turn reduces experimental artefacts during gas adsorption porosimetry. Crushing of shale not only alters the shape of gas adsorption hysteresis loops, but also tends to tighten the openings of hysteresis loops by enhancing the connectivity of pores and reducing the likelihood of gas being trapped during desorption.