NMR Desaturation and Surface Relaxivity Measurements on Clastic Rocks

Abstract

Abstract Archie1 recognized the importance of information obtained over multiple observational scales. As confidence grows in properties measured by vastly improved logging capabilities, we tend to ignore Archie's original insight. Nuclear Magnetic Resonance2, NMR, yields rock independent estimates of porosity as well as estimates of bound and movable water and the distribution of pore bodies. These measures are combined with empirical core studies to infer permeabilities and to predict capillary pressure from NMR data. Our experimental study is designed to evaluate these empirical relationships and to understand the variability in T2 response due to the intrinsic properties of the porous network. We have analyzed 90 cores with mixed orientation from five different clastic formations. The analysis included high pressure mercury injection, NMR desaturation, permeability, porosity and mineralogical measurements. NMR measurements were performed with a 2 MHz spectrometer. The porosity of the cores ranged from 4% to 23% while measured air permeabilities ranged from 0.01 to 900 md. The T2 cutoff i.e., the boundary between free and bound water, for the complete set of samples ranges from 6 to 100 msec which represents significant departures from the typically assumed 33 ms cutoff for clastics. Mineralogical dependence is observed in the behavior of T2 cutoff. In general, the permeability estimation based on a weighted geometric mean of the T2 time performed better than the model based on the ratio of free fluid index to bound volume index. Mapping of pore bodies as measured with NMR to pore throat derived from Hg injection resulted in estimation of surface relaxivities which ranged from 6 to 50 μm/sec. These relaxivities were used to generate pseudo-capillary pressure curves from NMR which map well with the measured capillary pressures. Comparison between the cumulative NMR and mercury data yields insight into differences in the pore structure between samples. Mineralogical composition of the matrix influences the surface relaxivity especially if paramagnetic ions (e.g. Fe2+) are present3. On an average there is a general decrease in surface relaxivity with increase in quartz content.