Neutron scattering: A subsurface application review

Geomaterials and filling fluids properties that are pertinent to a geologic porous media can be characterized using a range of methods, such as nuclear magnetic resonance, X-rays, infrared spectroscopy, and neutron scattering (NS). In this context, NS features as an important tool elucidate key properties of a porous medium, which has recently gained significant attention. Key rock properties that can be measured by NS include: rock texture (i.e. crystallographic preferred orientation), mechanical properties (i.e. stress and strain) as well as porous medium properties (pore porosity, pore size and connectivity). In addition, NS imaging can help elucidate the phase behaviour of confined reservoir fluids in rock matrix under prevailing pressures and temperatures. Thus, a precise characterization of these properties (amongst other multiphase flow attributes) is critical for several applications in varied fields such as hydrocarbon reservoirs, geothermal systems, crystallography, geomechanics and geochemistry. Low neutron attenuation by most substances (deep sample penetration) and strong neutron attenuation by hydrogen are essential features of neutrons that allow NS to collect high-quality data across a wide variety of subsurface conditions. These features enable NS to be ideally suited to some applications as compared to other techniques such as X-rays and magnetic resonance imaging (MRI). For example, X-rays may not have sufficient resolutions for examining nanopore structures and confined fluids. Contrastingly, MRI is limited by the visualization of a range of pore sizes. However, NS can capture angstrom-to-micron-scale information of atomic to meso-to-macro-scale structures of rocks and fluids (i.e. hydrogen-rich fluids) inside a porous medium. These insights are vital for predictive reservoir models, where meaningful reservoir-scale (hectometre-scale) predictions can be performed. However, when compared to X-rays, neutrons have weak sources and/or low signals; therefore, experimental time can be quite long and samples need to be relatively large. Other limitations of NS (some may be also true of other techniques) include problems like accessing neutron sources (e.g. complicated nuclear processes for neutron production and small number of available instruments when compared to X-rays), high costs, and the strong absorption of neutron signals by some elements [e.g. cadmium (Cd), boron (B), and gadolinium (Gd)]. Despite the potential of NS, a review that considers key NS subsurface applications, limitations, and outlooks is currently lacking. Thus, in this review, we describe the basic concepts, experiments, methods, requirements, restrictions, and applications of NS for rock and fluid characterization. This study finds that despite its overall challenges, NS is a promising technique for characterizing subsurface rock and fluid systems, opening diverse avenues for future technological and scientific research within this area.