NMR techniques for quantifying gas adsorption in porous media: principles, applications, limitations, and future directions.

Gas adsorption plays a critical role in understanding fluid storage and transport within porous media, particularly in unconventional reservoirs, where it directly influences reservoir characterization and resource recovery. Moreover, it serves as one of the promising storage mechanisms for gases such as CO2, methane, and hydrogen, making its accurate estimation essential for both energy production and CO2 sequestration. Nuclear Magnetic Resonance (NMR) has emerged as a powerful tool in petrophysics, offering valuable insights into pore saturation, fluid typing, and pore size distribution. In addition, NMR can be used in situ to assess adsorption, a significant advantage over other laboratory techniques that cannot be applied directly in the field. Unlike conventional adsorption measurement tools that only provide total gas uptake, NMR offers the unique capability to quantify compositional gas adsorption, enabling differentiation between adsorption in different pore systems. This review provides a comprehensive analysis of the various approaches used to estimate gas adsorption in porous media and examines the role of NMR in petrophysical evaluations, highlighting key formulations and detailed methodologies. Advanced NMR techniques, including low-field spectroscopy, Magic Angle Spinning (MAS), and Magnetic Resonance Imaging (MRI), are discussed, including underlying theory, laboratory-scale applications, and inherent limitations. Although there have been no reported field applications of these methods for the quantification of gas adsorption, further research into this area will eventually make it possible to fully harness the potential of these methods in the future.