Shale rock core analysis using NMR: Effect of bitumen and water content

Abstract

Hydrocarbon production from unconventional shale reservoirs has gained increasing worldwide focus, spurred largely by improvements in production techniques. Nuclear magnetic resonance (NMR) is increasingly being explored as a means of characterizing such shale rock. Compared to conventional sandstone and carbonate rocks, shales present a much lower porosity and permeability with NMR signal arising from both producible hydrocarbon fluid content as well as semi-solid organic solids and bitumen. Interpretation of the resultant NMR signal from shales is consequently comparatively more complex. To this end we apply combined NMR free induction decay (FID) and Carr-Purcell-Meilboom-Gill (CPMG) measurements and interpret the spliced data using a combined Gaussian and exponential inversion method. This was applied to a range of shale samples with variable moisture content in an attempt to assess the accuracy of such an approach to delineate the inherent predominantly bitumen signal from that of the added water. The analysis was performed at a range of frequencies (20, 40 and 60 MHz), which are applicable to more widely accessible bench-top NMR spectrometers. Consistently, the Gaussian component of the resultant T2*|T2 distribution was found to be independent of moisture content, scaling rather with the total organic content of the respective shale cores. In contrast, the exponential component of the resultant T2*|T2 distribution was found to scale linearly with the moisture content of the cores.