Abstract
The storage of hydrogen in underground reservoirs comprises a potential solution for balancing the fluctuating energy production from wind and solar power plants. In this concept, electrolysers are used to transform excessively produced electrical energy into chemical energy in the form of hydrogen. The resulting large volumes of hydrogen are temporarily stored in subsurface formations purely or in mixture with other gases. In times of high energy demand, the chemical energy is transformed back into electricity by fuel cells or engine generators. Key aspects in the development period and the subsequent cyclic operations of such a storage are the hydrodynamic behavior of hydrogen and its interaction with residual fluids in the reservoir. Mathematically, the behavior can be described by a compositional two-phase flow model with water and gas as phases and all relevant chemical species as components (H2, H2O, CH4, CO2, N2, H2S, etc.). The spatial variation of the gas phase composition between injected and initial gas leads to density and viscosity contrasts which influence the displacement process. The mixing of gases with different compositions is governed by molecular diffusion or mechanical dispersion dependent on the flow velocity. In the present paper, a numerical case study in a depleted gas reservoir was performed. The storage was charged with hydrogen for 5 years. Subsequently, 5 years of seasonal cyclic operation were simulated to predict injection and production rates, pressure response and composition of the produced gas stream .
Highlights
While conventional energy sources are capable to produce electricity continuously, the power generation of renewable energy is strongly fluctuating as a result of environmental influences
One promising answer is the conversion of electricity into hydrogen as an energy carrier, which is stored in underground formations, referred to as underground hydrogen storage (UHS) (Crotogino et al 2010)
Hydrogen is thereby generated by electrolysis, which is considered as a clean procedure, because it is produced from renewable energy sources
Summary
While conventional energy sources are capable to produce electricity continuously, the power generation of renewable energy is strongly fluctuating as a result of environmental influences. The hydrodynamic and substance specific behavior of hydrogen represents still a large uncertainty in porous underground storage applications In this context, especially the very small density and viscosity of hydrogen are suspected to complicate an effective displacement of a native reservoir fluid. The first operation period of porous underground storage schedules the sufficient concentration incline of the target storage gas in the drainage area of the production wells The efficiency of this displacement process depends on the physical properties of the displacing gas and the native reservoir fluid. Besides the physical behavior of the fluid density and viscosity, the difference of a defined bottom-hole flowing pressure Pwf and the actual reservoir pressure P in the well grid cell adjusts the amount of the injected or extracted mass to the arising reservoir response. A cross section through the reservoir is displayed in Fig. 3 where the figure is vertically stretched by the factor 7
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