Synthesis of CO₂ hydrate capsules in partially water-saturated sediment as vessels for underground mechanical energy storage: Promoting effect of tetrahydrofuran and cyclopentane

The intermittency of renewable energy sources and the increase in renewable energy shares require energy storage capability to sustain the green transition. One promising solution for underground energy storage is the use of subsurface CO₂ hydrate capsules, which act as impermeable vessels that store compressed fluids and discharge them upon need. However, as part of the proof-of-concept study of this technology, the feasibility of constructing such vessels, e.g. CO₂ hydrate capsules, has yet to be demonstrated. This work employed CO₂ gas injection assisted by chemical solution into partially water-saturated sand sediments to synthesize CO₂ hydrate capsules. The concentrations of chemical solutions (THF, CP and DIOX) were screened out in the gas/liquid/sand system in terms of kinetic promotion, using a rocking cell. Separately, experiments were carried out with a new high-pressure chamber set-up that quantified the effects of water saturation and chemical promoters on the efficiencies of CO₂ hydrate synthesis in confined partially water-saturated sand. Results on the gas/liquid/sand system showed that 0.025 water/CP weight ratio CP solution and 1.3mol% THF solution induced the largest pressure drops of 18.4 ± 0.2 bar and 16.7 ± 0.1 bar, respectively, indicating the most promoted CO₂ hydrate formation kinetics. Results with the pressure chamber showed three stages during slow CO₂ gas injection: (1) initial pressure “build-up stage”; subsequent (2) CO₂ gas “uptake stage”; and (3) CO₂ injection “closing stage”. CO₂ hydrate formation kinetics of CO₂ hydrate retention percentage (S_CO2) and CO₂ hydrate density (ρ_CO2) were directly proportional to the initial water saturation (6.0–76.7%). Injection of THF or CP solutions increased S_CO2 by 16.5% or 18.5%, and ρ_CO2 by 74.5% or 128.3% compared to injection of water. The best performances were obtained at a fluid pressure of 27.0 bar, with a 0.025 water/CP solution-assisting CO₂ gas injection, in sediment with an initial water saturation of 32.6% and porosity of 44.6%, giving rise to S_CO2 of 67.2% and ρ_CO2 of 80.6%. These findings demonstrated that the injection of THF or CP solution with CO₂ gas facilitated the possibility of the formation and stabilization of subsurface CO₂ hydrate capsules.