Dual liquid porosimetry: A porosity measurement technique for oil- and gas-bearing shales

Abstract Measuring and differentiating effective and ineffective porosity in oil- and gas-shale remains a challenging step in the calculation of hydrocarbon reserves. Shale composition, typically dominated by clay minerals and organic matter (OM), forms a complex pore system, making the proportion between effective and ineffective porosity variable and difficult to determine in the course of porosity measurement. In this study, total porosity was measured from a set of rock chips using both light kerosene (kerosene immersion porosimetry-KIP) and deionized (DI) water (water immersion porosimetry-WIP) as saturation-immersion fluids. Both KIP and WIP measurements were performed on samples equilibrated at 40% and 80% relative humidity (RH) and combined with water adsorption isotherms to estimate the range of clay-bound water (CBW). A combination of these procedures, and the use of kerosene and water as the saturation-immersion fluids, is hereafter called dual-liquid porosimetry (DLP). To test the method, two different burial diagenetic sequences of mudstone with different mineralogy and OM content were used. The results demonstrate that all the porosity values show a consistent declining trend with depth regardless of sample origin and type of saturation-immersion fluid used in the measurement process. Samples saturated with DI water were susceptible to swelling and the swelling decreased with sample depth, maturation and cementation, and increased with illite-smectite mineral content and degree of expandability. A consistently lower calculated grain density for KIP samples compared with WIP samples reflected incomplete pore saturation by kerosene. Kerosene saturation can be restricted by the presence of residual water that blocks kerosene pathways to small pores. This limitation, however, reflects geologic formation conditions making KIP measurement a good approximation for maximum liquid hydrocarbon-available porosity. Equilibrating the rock chips at 40% and 80% RH provides a rapid evaluation of the range of CBW that may represent ineffective porosity under various formation conditions. CBW min (40% RH) constitutes roughly 1W–2W smectite equivalent layers and gives an estimation of bound water close to high hydrocarbon saturation. CBW max (80% RH) represents the maximum possible bound water content in formations with higher water saturation. For samples, with low porosity and a significant content of expandable clay minerals and cation exchange capacity, CBW can constitute up to 100% of rock porosity. In order to avoid the limitations and assumptions of WIP (swelling) and KIP (incomplete saturation of nanopores), a combination of grain density and bulk density measured by WIP and KIP, respectively, was proposed. The paper further tests and discusses the pitfalls of saturation-immersion techniques.