Pore-size data in petrophysics: a perspective on the measurement of pore geometry

Mercury injection capillary pressure (MICP), backscattered electron images (BSEI) and nuclear magnetic resonance (NMR) provide pore-geometry parameters useful for understanding Variations in rock properties. MICP pore size is an area-equivalent diameter of the throats connecting the pore system. The MICP distribution contains a point (the turning point) that reveals where mercury first encounters the permeable network. This point identifies both critical pore size and connected porosity. BSEI measure pore area and diameter. Both parameters are unrelated to any specific morphological element. NMR pore size is a derivative of the pore-surface:volume ratio. Like BSEI, NMR pore size is unrelated to morphological elements. Like MICP, NMR provides a pore-size distribution that represents the entire sample volume. All three techniques provide a data distribution suitable for cross-correlation. Matching distributions determines the character of the permeable pore system. NMR and MICP distributions compare through the entire time and pressure spectra, matching best in the central part of the distributions. The match suggests pore-throats merge with the pore channels in the permeable part of the system. BSEI distributions match only with the low-pressure, stow-time parts of the other distributions. However, image porosity has a stronger correlation with permeability than any of the other parameters, suggesting that the BSEI distribution is near the turning point.