Dehydration Effect on the Pore Size, Porosity, and Fractal Parameters of Shale Rocks: Ultrasmall-Angle X‑ray Scattering Study

The characterization of pore networks is extremely important in understanding transport and storage phenomena in unconventional gas and oil reservoir rocks. An ultrasmall-angle X-ray scattering (USAXS) measurement has been performed on Silurian black shales from the Baltic Basin, Poland, from a wide range of depths along a burial diagenetic sequence. This study provides insight into the nature of the pore structure, including the pore size distribution, total porosity, and fractal dimensions of the rocks. Samples were measured in both their air-dried state, equilibrated at ∼50% relative humidity, and prior to dehydration by drying at 200 °C to make a comprehensive comparison of the pore structure changes induced by dehydration. Two trends were observed: porosity values decreased with depth as expected from the models of porosity evolution with burial and increased upon sample dehydration. The USAXS-measured porosity values show very good correspondence with the measurements by immersion porosity methods. The increase in porosity upon dehydration was found to be dominated by a volume increase in the pores of 100–1000 nm diameter; the pores were filled by capillary water and clay-bound water in the air-dry state and liberated during drying. The geometric irregularities of pore–shale rock interfaces have been quantified by fractal dimension. The removal of water from the sample also serves to increase the fractal dimension suggesting that the removal of water molecules increases the surface or mass irregularity. Implications to shale porosity measurement and shale gas models are discussed.