Comprehensive characterization of petrophysical properties in shale by solvent extraction experiments and 2D NMR

Abstract

Shale oil is one of the important unconventional fossil fuels resources. Understanding the petrophysical properties of shale contributes to the exploration and clean production of shale oil resources. 1H nuclear magnetic resonance (NMR) is a potential technique that can provide for shale petrophysical properties. However, current NMR-based technique for shale petrophysical characterization, due to the complexity of shale, usually requires some auxiliary geochemical experiments. Since a multi-step solvent extraction (MSSE) experiment has a merit in separating hydrocarbons and water in shale, in this work, two-dimensional (2D) NMR measurements combined with MSSE experiments were proposed for the first time to study the petrophysical properties of shale. A mixture of dichloromethane and methanol in a ration of 9:1 was prepared as a solvent for the extraction. Sufficient natural air-drying time allows for the removal of dichloromethane while retaining methanol. The comprehensive experiments show that the signal change induced by residual methanol from T1/T2-T2 map, before and after extraction and natural air-drying, can be used to identify pore-fluid and evaluate the mobility of hydrocarbon. For insight into the essence of experimental phenomena, quasi-1D/2D translational diffusion model were employed to analyze the T1/T2 and T2 responses of pore fluids. Wettability characteristics and detailed hydrocarbon mobility of shale were obtained from the theoretical analyses based on the translational diffusion models and T1/T2-T2 map. Combined the experimental results with the theoretical analysis, a comprehensive interpretation scheme was established for shale based on the T1/T2-T2 map, including fluid identification, wettability evaluation, and hydrocarbon mobility characterization.