Utility of NMR T2 distributions, connection with capillary pressure, clay effect, and determination of the surface relaxivity parameter ρ2

Nuclear magnetic resonance magnetization decays are now used routinely to determine reservoir properties of economic significance to the oil and gas industry. Although individual features of relaxation time distributions are subject to degradation in the presence of measurement noise, integrated and average measures of those distributions are remarkably stable. Properties such as porosity, producible porosity, hydraulic permeability, and the capillary pressure curve can be measured rapidly and continuously. Laboratory comparisons of NMR T2 distributions with two different capillary pressure measurements enable the surface relaxivity parameter ρ2 to be determined for sandstones. Comparison with relaxivities of synthetic materials having well-defined surface areas strongly suggests that the NMR-effective internal surface area of rocks is closely connected with the hydraulic radius, and, surprisingly, is not greatly affected by the presence of clay, which dominates the pore space surface area as measured by gas adsorption.