A comprehensive review on shale studies with emphasis on nuclear magnetic resonance (NMR) technique

Multi-scale shale studies put a significant emphasis on high-resolution investigations from nanometer to decametre scales. Despite that multiple advanced techniques have been used in shale studies, they are mostly limited to the detection scopes and have restricted capacity for high-resolution characterization of shale nanopores with substantial heterogeneity. Therefore, it remains a challenge for accurate resource estimation in unconventional shales. The nuclear magnetic resonance (NMR) is an advanced technique enabling non-destructive and fast measurements, and has the advantage of high-resolution evaluation of shale formations and nanopore structure. Petrophysical studies using NMR have made breakthroughs in shale studies. However, multi-scale shale investigations with emphasis on NMR technique have not been fully reviewed. This paper thus provides an overview of the capabilities of NMR in multidisciplinary shale studies to largely improve accuracy in unconventional resource estimations. We proposed a multi-scale and quantitative NMR detection method for accurate characterization of the nanopore structure and fast relaxation fluids. The laboratory NMR core analysis and NMR well logging can be applied for the detection from nanometer to decametre scales, respectively, and precisely measure shale reservoir properties, including total/effective porosities, clay-bound water (CBW) contents, pore size distribution, surface relaxivity, absolute permeability, wettability, and fluid types. Importantly, with NMR application, new research areas such as the integrated supercritical CO₂ enhanced shale gas recovery (scCO₂-ESGR) and carbon geo-sequestration, and the advanced underground hydrogen storage (UHS) in shales can be developed to achieve the target of long-term energy supply and net-zero carbon emission. New techniques such as in-situ kerogen pyrolysis are also improved by using NMR dynamic monitoring.