Shale mineralogy alteration associated with subsurface H2 storage

Hydrogen is a promising energy carrier in pursuit of net-zero emission energy production. However, such a transition relies on our ability to properly generate, transport and store H2. For the latter purpose, subsurface reservoirs offer a realistic possibility to compensate the limitation of surface storage as they are suitable to accommodate large volumes at sufficient pressure. Among the variety of geologic formations that could serve this purpose, shale formations in depleted oil/gas reservoirs remain relatively unexplored. In the present study, we investigate the potential influence of injected H2 on shale reservoirs through the expansion of a 1-D core-scale reactive transport model previously designed for hydraulic fracturing systems. This multi-phase model was amended to be both relevant to H2 solubility and to microbially mediated reactions including sulfate reducing bacteria (SRB) and methanogens. A sensitivity analysis was performed to study how the introduction of H2 could affect shale mineralogy and ultimately the efficacy of storage. Through this approach, we show that the activity of methanogens and subsequent production of CH4 is a persistent pathway for hydrogen loss. In contrast, rapid SRB activity quickly consumes available SO4 in the system, thus regulating hydrogen loss and contamination. However, when additional sources of sulfate (e.g., oxidative pyrite dissolution) are available, the activity of SRB outcompetes the methanogens, becoming the primary pathway of H2 loss. When organic carbon sources are considered bioavailable, H2 loss due to SRB activity is diminished as more SO4 consumption is diverted to this competing pathway, however H2S production continues to pose a contamination issue. Finally, we use the model to consider the ways in which introduction of can indirectly affect the reactivity of the shale reservoir mineralogy. We show that the activity of microorganisms can impact the dissolution rate of carbonates minerals through their influence on the pH of the system. The implications of this study provide pathways for future investigation and experimental development to better understand shale systems as a storage option for H2.