Near Wellbore Formation Integrity in Sedimentary Geothermal Storage Systems: Insights from THM Models
Authors: Mutlu Ovunc Uno , Ghassemi Ahmad , Boitnott Gregory 
Published: September 2023

ABSTRACT
The Geothermal Battery Energy Storage (GBES) concept has been proposed as a large-scale renewable energy storage method beyond short-term durations. GBES has the potential to handle the variable nature of renewable solar and wind power by allowing their excess energy to heat brine at the surface which is then injected into a high porosity sedimentary reservoir. The subsurface reservoir forms a thermal battery and stores the heated brine. A subsurface water saturated high porosity sedimentary reservoir can be an ideal long-duration storage vessel, thereby providing nearly unlimited storage capacity compared to other geologic formation types. However, efficiency and safety of the GBES operations (i.e., flow and mechanical integrity of the formation) depend on the response of the wellbore and sedimentary formations to coupled Thermo-Hydro-Mechanical (THM) or Thermo-Hydro-Mechanical-Chemical (THMC) loads induced by the injection-production-reinjection cycles. If operational parameters of GBES systems are not optimized with data unique to sedimentary formations, near wellbore formation integrity issues can lead to reduced heat output. In this paper we review potential issues related to THM behavior of a GBES system and illustrate geomechanical concepts that can be adapted to provide an uplift to GBES operations.

INTRODUCTION
Solar and wind energy are key in our transition to a net-zero carbon future. While abundant and cost-competitive, they are variable, intermittent and their energy supply does not match all demand needs. Therefore, the key to unlocking our shift to 100% renewable energy generation is the ability to store energy beyond short-term durations such that power and heat are available on-demand at any day.
The Geothermal Battery Energy Storage (GBES) concept has been proposed as a large-scale renewable energy storage method beyond short-term durations. GBES has the potential to handle the variable nature of renewable solar and wind power by allowing their excess energy to heat brine at the surface which is then injected into a high porosity sedimentary reservoir. The subsurface reservoir in this context forms a geothermal battery and stores the heated brine. Stored brine can then be produced for power generation (e.g., Organic Rankine Cycle) when necessary and subsequently the same brine can be reinjected after a heating cycle at the surface. Figure 1 illustrates the GBES concept and the cyclic nature of the operations. Colder brine is drawn from a porous and permeable sedimentary reservoir via production well(s). Brine is then heated using excess energy from the renewable sources (e.g., solar or wind) and injected back into the same reservoir through a nearby injection well. When energy is needed, hot/cold brine is produced under sufficient pressure to remain in liquid phase and generate electricity via a geothermal power plant or used for direct heating. This cycle is repeated at various regular intervals to match the energy demand (see Green et al. 2021).