Development of efficient multiscale multiphysics models accounting for reversible flow at various subsurface energy storage sites

Abstract

Energy storage is an essential component of future energy systems with a large share of renewable energy. Apart from pumped hydro storage, large scale energy storage is mainly provided by underground energy storage systems. In this thesis we focus on chemical subsurface storage, i.e., the storage of synthetic hydrogen or synthetic natural gas in porous formations. To improve understanding of the complex and coupled processes in the underground and enable planning and risk assessment of subsurface energy storage, efficient, consistent and adequate numerical models for multiphase flow and transport are required. Simulating underground energy storage requires large domains, including local features such as fault zones and a representation of the transient saline front, and simulation times spanning the whole time of plant operation and beyond. In addition, often a large number of simulation runs need to be conducted to quantify parameter uncertainty, and efficient models are needed for data assimilation as well. Therefore, a reduction of model complexity and thus computing effort is required. Numerous simplified models that require less computational resources have been developed. In this thesis we focus on a group of multiscale models which use vertically integrated equations and implicitly include fine-scale information along the vertical direction that is reconstructed assuming vertical equilibrium (VE). Classical VE models are restricted to situations where vertical equilibrium is valid in the whole domain during most of the simulated time. This may not be the case for underground energy storage, where simulated times may be too short and locally a high degree of accuracy and complexity may be required, e.g., around the area where gas is extracted for the purpose of energy production. The three core chapters of this thesis present solutions to adapt VE models for the simulation of underground energy storage, with increasing complexity.