Nuclear Logging in Geological Probing for a Low-Carbon Energy Future – A New Frontier?

This paper examines the potential of nuclear logging techniques, ubiquitous in the petroleum industry, to extract geological information needed to transition to the low-carbon energy future being envisioned and explores the technological advances needed. Monte Carlo modeling and assessment of available measurements are utilized to examine four likely areas of application. Monitoring injected CO2 for CCS to mitigate climate change: PNC Sigma from a slim, generator-based dual-detector tool was able to track injected CO2 gas plume in a high-salinity formation. The tool’s simulation-derived spectral-fitted C/O ratio showed promise of directly identifying CO2 in aquifers, especially with a breakthrough. C/O ratios would be problematic in depleted gas reservoirs where injected CO2 would displace methane. The inelastic/capture ratios at the farthest detector of a three-detector tool appeared promising at CO2 saturations below 70%. Diffusion/transport effects, not generally accounted for, would have to be incorporated if capture data are used at high CO2 saturations. Assessing sites to bury high-level waste (HLW) from nuclear plants: Conventional reservoir characterization using density/PE, neutron porosity, and source-based n-gamma spectroscopy have shown promise. A D-T-based tool providing thermal and epithermal neutron porosity and mineralogy simultaneously would be a better option. The low-energy X-ray density tool being tested would provide a better lithology indicator. Monitoring buried nuclear waste: A moisture content map, in conjunction with a map of radioisotope movement, would indicate the presence of water in the subsurface and allow monitoring of pathways for contaminants migrating into water tables. Despite successful tests of commercial tools at the Hanford site for this application, they are not utilized, primarily due to a lack of calibration for fission product isotopes, and, in high radiation environments, these isotopes pose for scintillators. Additional calibration, radioactivity-free scintillators, and radiation hardening of devices are a must. Downhole quantification of minerals for electric vehicles and photovoltaics: Projected demands for these minerals would exceed currently available volumes, manifold, potentially requiring subsurface access to them vs. current near-surface open-pit mining. D-T-based mineralogy tools could locate them, but at high concentrations and if nuclear interaction probabilities are high. Generators emitting 109 n/s or higher and a low capture correction for inelastic would be preferred. Our simulation results indicate complex capture correction can be avoided if inelastic gamma rays, emitted below 100 nanoseconds, are utilized. This would require scintillators capable of recording gamma rays in tens of nanoseconds instead of microseconds. Nuclear, with its unique ability to characterize and monitor the subsurface, is well placed for the new frontier of geological probing for low-carbon energy generation, especially if the technology gaps identified are closed. Two areas need further attention: simulation technology and tool concept. Codes with dynamic visualization and nuclear data libraries with a full suite of elements of interest in petrophysics to design, calibrate, and assess tools, especially to provide a priori space-time profiles of attendant multiple radiation types, are needed. Secondly, the industry should consider switching from its current dual tracks for nuclear tools—radioactive sources for characterization and D-T generators for monitoring—to composite, advanced accelerator-based, multiple-parameter tools incorporating AI-guided PHM systems to minimize generator failure.