Sarah Gelman

Natural, or geologic, hydrogen may be a future low-carbon subsurface energy resource. To assess the potential for geologic hydrogen throughout the conterminous United States, the U.S. Geological Survey has developed a methodology to model subsurface hydrogen systems. Following a similar method developed for petroleum systems, Chance of Success (COS) is defined here as a fractional probability that a viable accumulation of natural hydrogen can exist in the subsurface. Several components make up the geologic hydrogen system model and are required for viable accumulations: 1) a source of natural hydrogen, 2) a reservoir to store hydrogen, and 3) a competent seal to retain hydrogen and prevent mechanical or capillary leakage to the Earth’s surface. The geologic characterization of these components is described in a companion abstract (Hearon et al., this conference), while the motivation and broad-scale results are discussed in another companion abstract (Ellis et al., this conference). This contribution will focus on the quantitative methods developed to model hydrogen systems, its components, sub-components, and COS analysis, ultimately generating the first edition of a natural hydrogen prospectivity map of the conterminous United States. Lateral migration of hydrogen in the subsurface is presumed to be possible, analogous to the migration of other buoyant subsurface fluids (i.e., oil and gas). While migration can occur in both sedimentary and non-sedimentary settings, migration pathways likely follow stratigraphic boundaries in sedimentary settings. In contrast, fractures likely dictate flow in crystalline rocks and would be difficult to predict on a large scale. Here, we limit our initial focus to only sedimentary migration pathways. Using a continental-scale map of total sedimentary thickness to generate an elevation map of the top of basement (or the floor of sedimentary rocks), we calculate fluid flow paths. When potential sources of geologic hydrogen occur under or within sedimentary basins, hydrogen is presumed to migrate laterally and up-dip following the contours of basement topography, allowing an area of broad spatial extent to have exposure to migrating hydrogen. Our methodology further considers the potential for a reduction in hydrogen access as distance increases away from possible sources due to consumption by biotic and abiotic processes. The geologic hydrogen system COS is calculated by multiplying the individual component COS values (source including migration, reservoir, and seal) which were generated from a series of 21 geologic sub-components (see Hearon et al., this conference) and their individual COS values. Since sub-components may overlap spatially (for example, a source of hydrogen due to serpentinization, and a source due to radiolysis), the sub-components are combined by multiplying chances of failure, rather than chances of success, ultimately increasing the component COS due to this overlap. Finally, uncertainty is quantified by assigning low, mid, and high cases to create distributions of sub-component COS values, which are then propagated in a Monte Carlo method to generate stochastic outcomes where the final results are presented as P90, P50, and P10 maps for geologic hydrogen prospectivity.

Co-authors: Jane S. Hearon, Scott A. Kinney, Robert F. Miller, Christopher C. Skinner, and Geoffrey S. Ellis