Specific Surface and Fluid Transport in Sandstones Through NMR Studies

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Abstract
Summary: Recent nuclear magnetic resonance (NMR) studies in water-saturated porous media showed that magnetic resonance relaxation of 1H nuclei is a powerful tool for studying the interplay between geometry and fluid transport. Proper combinations of spin-lattice relaxation lifetime, T1, and porosity allow permeability to be predicted. T1, as defined here, provides a bridge between structural and transport properties because it can be viewed as a dynamically weighted (by diffusion) version of the specific surface. In this paper, we probe this role of T1 for a suite of clean sandstone samples in which, besides permeability and porosity, specific surface by mercury porosimetry and the formation resistivity factor (FRF) also have been measured. We studied the correlations among these properties and found that the ability of T1 to estimate permeability is a result of its linear dependence on the PV-to-surface ratio, Vp/S. For clean sandstone rocks, one may view the reciprocal electrical resistivity formation factor, 1/F, as representing the transport properties and the factor T12 as representing the distance scale, giving k∝T12/F. A suitable analysis of the spin-lattice relaxation curve can yield an estimate of an appropriate specific surface pertinent to permeability because it indirectly accounts for the connectivity of the pore space. This NMR approach constitutes a useful technique for a better reservoir characterization and for studying some elusive properties of natural porous media in a nondestructive manner. Theoretical and phenomenological correlations among permeability, porosity, FRF, and PV-to-surface ratio can be established.