An Improved Technique for Deriving Drainage Capillary Pressure from Nmr T2 Distributions

Nuclear Magnetic Resonance (NMR) T2 measurements can be used directly to estimate petrophysical properties of reservoir rocks such as porosity and irreducible water saturation. They are also sensitive to pore-size distribution enabling the estimation of permeability and potentially capillary pressure. Several methods have been proposed to derive synthetic drainage capillary pressure information directly from NMR data [1-5]. However empirical correction factors are usually introduced in the scaling factor to improve the capillary pressure prediction at low wetting phase saturation. This paper presents an improved method to derive primary drainage capillary pressure from T2 distributions of fully saturated rocks. The modification is based upon experimental observations that T2 signal amplitude increases at shorter times when comparing data from partially saturated cores with that from fully saturated cores. This is usually assumed to be due to water (or wetting fluid) remaining in crevices in such partially saturated cores. Such water has a faster relaxation rate as it has a higher surface area to volume ratio. We propose the use of a simple model of triangular pores to estimate the irreducible water saturation and the T2 distribution of the drained core. The drainage capillary pressure is then derived from an effective cumulative T2 distribution, obtained by combining the measured T2 distribution from the fully saturated core with the distribution calculated at drained conditions from the triangular capillary model. The method has been applied successfully to a number of sandstone core samples with a wide range of permeabilities. The triangular pore model predicts reasonably well the connate water saturation at various capillary pressures. The NMR derived and primary drainage capillary pressure curves show a very good agreement. The improvement in prediction mainly takes place at high capillary pressures and is automatically calculated as a function of the T2 distribution and the pressure applied. INTRODUCTION The determination of representative capillary pressure curves is of great importance for reservoir characterization and the evaluation of fluid distribution within the reservoir. The connate water saturation and its distribution are directly related to capillary pressure. There are various standard measurement techniques used to determine core plug capillary