Wettability and fluid characterization in shale based on T1/T2 variations in solvent extraction experiments

Rock wettability and pore fluid properties are critical parameters for evaluating shale reservoirs, enhancing oil recovery, sequestering waste, and storing energy. Multiple petrophysical and geochemical experiments are routinely used to characterize these properties of shales. Two-dimensional nuclear magnetic resonance (NMR)based shale evaluation methods usually rely on T1/T2 difference in various fluids. However, T1/T2 value is inadequate for petrophysical characterization due to the similar T1/T2 value of different hydrogen types in shales. A brand-new characteristic parameter, T1/T2 variation during a solvent extraction experiment, is proposed for the characterization of shale petrophysical properties. The theoretical ranges of T1/T2 variation for different hydrogen components were determined from the type and content changes of hydrogen during extraction based on surface relaxation model. Joint experiments of multi-step solvent extraction and NMR T1-T2 were designed to validate the theoretical results. Both theoretical and experimental results show that the heterogeneity of shale is responsible for the different hydrogen components with a similar T1/T2 value. The results of joint experiment (1) show that the T1/T2 variation ranges for different hydrogen components are different, and (2) indicate that T1/T2 variations can be used to quantitatively evaluate the hydrogen type and saturation. The experimentally determined ranges and patterns of T1/T2 variation are consistent with the theoretical results. The quantitative relationship between T1/T2 variation and saturation, mobility, or wettability was further determined based on the theoretical variation of correlation time. Finally, a new interpretation scheme was established for the quantitative evaluation of wettability and fluid properties based on T1/T2 variations.