Surface‐to‐volume ratio, charge density, nuclear magnetic relaxation, and permeability in clay‐bearing sandstones

Abstract

Based on measurements on some 100 sandstone core samples, mainly from oil fields from various parts of the world, we found the following regressions between volume‐to‐surface ratio [Formula: see text], permeability to fluid flow k, exchange cation molarity [Formula: see text], and proton NMR decay constant [Formula: see text] in water‐saturated rocks (see Figure 1): [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text]. Here R is the regression coefficient, ϕ is the porosity and m the conductivity exponent; [Formula: see text] in normality (meq/ml), k in millidarcies, and [Formula: see text] in milliseconds, [Formula: see text] in μm. Including the tortuosity factor [Formula: see text] in conjunction with a pore‐size parameter as represented by [Formula: see text], [Formula: see text], or [Formula: see text] improves the correlation with permeability and reduces the residual error. The best predictor for log k is log [Formula: see text]. The exponents in the above correlations agree reasonably with those expected from simple models. These correlations provide a numerical basis for assessing how well some of these quantities can be estimated from others in log interpretation. They also provide a basis for assessing the importance of the factors that interfere with and thereby weaken the correlations.