Appraisal of Native Hydrogen Accumulation in Geological Formations under Uncertainty

A key issue related to potential accumulation of native hydrogen (H2) in geological formations is the risk of hydrogen loss due to conversion to other chemical species such as methane (CH4). Our study tries to quantify how uncertainties linked to carbonate-clay reactions (CCR) reflect in evaluation of the geogenic methane generation and the associated losses of native H2. We rely on a modeling workflow developed by Ceriotti et al. (2017) for evaluating geogenic carbon dioxide, CO2, generation through CCR in sedimentary basins. As a showcase, we consider a one-dimensional (vertical) model patterned after a typical sedimentary compaction setting. Such a model provides the dynamics of porosity, temperature, and pressure along the vertical direction. Outputs of the compaction model are viewed as deterministic quantities. We then consider a given mineral composition and focus on the quantification of the parametric uncertainties associated with CCR. This is reflected in the uncertainty related to the values of thermodynamic equilibrium constants of the species involved in CCR and is then propagated onto the ensuing estimated CO2 release. Underground trapping of native H2 is conceptualized upon considering the subsurface as a natural chemical reactor that consumes a mixture of H2 (generated from serpentinization of ultramafic rocks) and CO2 (from CCR) yielding a mixture of H2/ CO2/ CH4. Our analysis considers that (a) complete mixing of the chemical species is attained and (b) geochemical reactions can be evaluated under thermodynamic equilibrium conditions. We then perform a modelling study framed in a stochastic context and relying on a numerical Monte Carlo framework. The latter is aimed at quantifying uncertainty associated with methane production following geogenic hydrogen and carbon dioxide generation. Our results are tied to (i) shallow, (ii) intermediate-depth, and (ii) deep reservoirs. Due to its preliminary nature, the study considers uncertainty solely in the CCR process as well as accumulation reservoir depth/pressure/temperature conditions. Our results suggest that accumulation of H2 in geological formations entails the risk of hydrogen loss due to conversion to CH4 by methanogenesis. They also suggest that deep geological formations (characterized by high temperature and pressure conditions) tend to limit hydrogen loss due to methanogenesis reactions. Thus, exploration of native H2 accumulations could target geological formations where the residing gas has low CO2 concentrations and where the mineralogical composition of reservoir rocks contains low amounts of carbon-bearing minerals. We provide a quantification of native hydrogen losses with the explicit inclusion of a stochastic assessment of some uncertainties linked to the geogenic generation of CO2.