Reservoir simulation studies and techno-economic analysis of underground hydrogen storage in a depleted gas reservoir – A case study from northwestern Germany

To achieve a carbon-free economy in the medium term, hydrogen (H 2 ) has been proposed as a viable solution. This requires large-scale subsurface storage options, especially if green H 2 produced from fluctuating renewable energy sources like wind and solar energy is considered. While H 2 has already been stored successfully in salt caverns for decades, H 2 storage in porous media like hydrocarbon-depleted reservoirs and saline aquifers still requires further research. We use an almost depleted gas reservoir in northwestern Germany to test for given injection/withdrawal cycles various H 2 storage scenarios regarding different cushion gases (CG). The case study field represents a faulted reservoir in a highly fractured rock of Upper Permian (Zechstein) age, consisting mainly of dolomite as reservoir rock and anhydrite as cap rock. A dynamic reservoir model for the time span from 1959 to 2023 and history-matched using the comprehensive calibration data available for the production phase provides the basis for hypothetical seasonal H 2 storage, intending to store around 300 MMsm 3 using an isothermal compositional reservoir simulator (E300) with seven components. Nine prediction cases were simulated. Following the same injection/withdrawal cycles, the cases vary regarding the composition of the CG injected: N 2 +CH 4 , H 2 +N 2 , H 2 +CH 4 , H 2 +CO 2 , pure CH 4 , pure CO 2 , pure N 2 , and pure H 2, and the ninth scenario was done with the same conditions as one of the cases but with a further prediction of 16 years. The detailed parameter input modifications from each case will be depicted in Chapter 3. The first eight scenarios follow a 10-year prediction model (2 injection cycles and 8 withdrawals) based on the results of all eight simulations, at least on the first 4 cycles, less H 2 is recovered, except if pure H 2 is injected from the beginning as a buildup phase. Despite this, all simulations show a higher H 2 recovery for the last cycle (8th withdrawal), from 96% (pure N 2 as CG) to 99% (pure H 2 as CG). The techno-economic results, ignoring at the moment separation costs, show a variance on the Levelized Cost of Hydrogen Storage (LCHS) from 1.78 $/kg to 2.39 $/kg depending on the CG applied. CG plays a significant role in the Total Capitalized Cost (TCC). Thereon, we intend to search for the optimal CG to be applied and economically feasible. • H 2 storage in depleted carbonate reservoirs from the Zechstein in northwestern Germany with dual porosity and permeability. • Use of isothermal compositional reservoir simulator (E300) to test cushion gases (CG) like N 2 , CH 4 , and CO 2 with diffusion. • Levelized Cost of Hydrogen Storage (LCHS) varies from 1.78 $/kg to 2.39 $/kg based on CG choice. • Certain CGs and mixtures achieve both high H 2 recovery and heating values, suggesting cost-effective applications.