Nuclear magnetic resonance properties of rocks at elevated temperatures

Although the NMR properties of fluid-saturated rocks have been studied for decades, the temperature dependence of these properties has never been systematically investigated. A hot-air-flow loop has been constructed that can elevate the temperature of a core sample. An algorithm to estimate the distribution of NMR relaxation times from Fast Inversion Recovery/CPMG data has been developed and used to analyze the effects of temperature. Proton magnetic resonance measurements have been made on an assortment of water-saturated sandstones and carbonates and an oil-saturated sandstone at temperatures ranging from 25 to 175°C. The relaxation times of all the water-saturated sandstones and most of the carbonates depend only weakly on temperature. This indicates that the water-saturated rocks studied are in the fast-diffusion limit and the intrinsic surface relaxation strength is independent of temperature. In all the rocks studied the magnitude of the NMR signal, which is proportional to porosity, decreases with increasing temperature in accordance with the Curie Law. Data from oil-saturated sandstone clarify the roles of molecular diffusion, material-specific surface interactions, and bulk fluid relaxation.