Study of Vuggy Carbonates Using NMR and X-Ray CT Scanning

Summary Most existing nuclear magnetic resonance (NMR) permeability correlations for carbonates assume that vugs do not contribute to permeability. The objective of this work is to improve permeability estimation from NMR responses for carbonates by including vug connectivity. Six carbonate samples from a west Texas field were studied. NMR T2 measurement, mercury porosimetry, thin-section imaging [using optical microscopy and a scanning electron microscope (SEM)], computerized tomography (CT) scanning, and electrical measurements were conducted. The permeability of the samples studied is controlled by the channels that connect vugs. Capillary pressure and NMR T2 measurements show multimodal pore-throat and pore-body-size distributions. A modified Chang model, which includes the tortuosity factor, is proposed and proven to yield a better permeability prediction than the original Chang model. It is also shown that for the samples studied, the tortuosity can be estimated from NMR T2 distribution, hence allowing permeability prediction from the NMR T2 distribution alone.