Feasibility of Hydrogen Storage in Depleted Hydrocarbon Chalk Reservoirs: Assessment of Biochemical and Chemical Effects

Hydrogen storage is one of the energy storage methods that can help realization of an emission free future by saving surplus renewable energy for energy deficit periods. Utilization of depleted hydrocarbon reservoirs for large-scale hydrogen storage may be associated with the risk of chemical/biochemical reactions. In the specific case of chalk reservoirs, the principal reactions are abiotic calcite dissolution, acetogenesis, methanogenesis and biological souring. Here, we use PHREEQC to evaluate the dynamics and the extent of hydrogen loss by each of these reactions in hydrogen storage scenarios for various Danish North Sea chalk hydrocarbon reservoirs. We find that: (i) Abiotic calcite dissolution does not occur in the temperature range of 40-180◦ C. (ii) If methanogens and acetogens grow as slow as the slowest growing methanogens and acetogens reported in the literature, methanogenesis and acetogenesis cannot cause a hydrogen loss more than 0.6% per year. However, (iii) if they proceed as fast as the fastest growing methanogens and acetogens reported in the literature, a complete loss of all injected hydrogen in less than five years is possible. (iv) Co-injection of CO2 can be employed to inhibit calcite dissolution and keep the produced methane due to methanogenesis carbon neutral. (v) Biological sulfate reduction does not cause significant hydrogen loss during 10 years, but it can lead to high hydrogen sulfide concentrations (1015 ppm). Biological sulfate reduction is expected to impact hydrogen storage only in early stages if the only source of sulfur substrates are the dissolved species in the brine and not rock minerals. Considering these findings, we suggest that depleted chalk reservoirs may not possess chemical/biochemical risks and be good candidates for large-scale underground hydrogen storage.