Determination of pore size distribution in sedimentary rocks by proton nuclear magnetic resonance

Abstract

The distribution of pore sizes in sedimentary rocks can be determined from laboratory measurements of proton nuclear magnetic resonance (NMR) longitudinal relaxation ( T 1) in water-saturated rocks. The details of the longitudinal relaxation curve are converted into a T 1 distribution curve by approximating the data with a sum of single exponential decays. The non-linear least-squares minimization procedure includes a regularization term that reduces excursions in the computed density function, thus producing smooth and robust results. The T 1 distribution curve is scaled to pore size distributions by a phenomenological parameter that describes the strength of the proton interaction between the fluid and the pore wall. This parameter is relatively constant in sandstones, but is an order of magnitude less with greater variability in carbonates. There is excellent qualitative and quantitative agreement between pore size distributions determined by NMR and independently determined information on pore sizes for a variety of sedimentary rocks. The laboratory derived T 1 distributions are used to estimate the irreducible water saturation in sandstones by identifying the fraction of pores less than 4 μm in diameter.