16-28 Sources and Sinks of Geologic Hydrogen: Towards a Radiolytic Hydrogen Yield Model for Mudstones

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

Hydrogen has a gravimetric energy density about two and half times that of petroleum fuels and represents a low carbon energy resource. Currently, most hydrogen used in the USA is produced by steam reforming of natural gas yielding CO₂ as a byproduct. Geologic hydrogen seeps have been encountered in nearly every environment from soils to mid-ocean ridges. Based on empirical data collected from soils and other samples, as well as theoretical calculations of subsurface hydrogen generation, the calculated quantity of geologic hydrogen is approximately 1.4 x 10¹⁶ MJ. Every new study seemingly increases the estimate of total geologic hydrogen by an order of magnitude or more.

Despite the large potential, microorganism communities utilize geologic hydrogen as an energy source, representing a major sink in the shallow subsurface. Unsaturated hydrocarbons and other abiotic chemical processes also consume hydrogen. The highly diffusive nature of hydrogen compared to petroleum limits trapping to regions with very tight seals. However, numerous petroleum wells around the world have encountered significant hydrogen resources indicating that even though losses are large, sources must be significant. For example, one well in Mali has sustained the energy demand for a nearby village for several years. Recent developments including the first prospectivity map for geologic hydrogen in the conterminous United States and active exploration drilling targeting the Midcontinent Rift in Kansas and Cambrian reservoirs in Saskatchewan underscore the growing interest and the emerging viability of this largely untapped geofuel.

Radiolysis, the splitting of water molecules by ionizing radiation, produces molecular hydrogen continuously as long as radioactive isotopes and water are co-located. Organic-rich mudstone formations may be particularly well suited for this process because they commonly contain elevated concentrations of radioactive K, U, and Th, host water-saturated pore space, and are laterally extensive. Published radiolytic hydrogen yield models are explored here using organic-rich mudstone parameters. The results highlight model sensitivities and uncertainties. Future work will include adding variables such as water saturation, stopping power of common mudstone lithologies, and grain size that were either not parameterized or simplified in published models. The model is intended to be scalable and readily applicable to a wide range of mudstone formations as a screening tool for radiolytic hydrogen exploration in sedimentary basins.

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