Guest exchange dynamics and structural evolution in C3H8 hydrates upon CO2 injection: Implications for CO2 storage via guest replacement

Abstract

CH4-CO2 replacement is an innovative, carbon-neutral technology that injects CO2 into natural gas hydrates to simultaneously enable energy resources production and greenhouse gas storage. This study investigated the structural and compositional evolution of structure II (sII) C3H8 hydrates during guest replacement by CO2 under three different injection pressures. A combination of gas chromatography, powder X-ray diffraction, and in-situ Raman spectroscopy was used to examine guest behaviors, cage occupancy, and structural transformation in real time. At lower pressures (2.0 and 3.0 MPa), the sII framework remained stable while CO2 selectively participated in the empty small (512) cages of the initial sII C3H8 hydrates, resulting in gradual guest enclathrating behaviors. However, when the injection pressure exceeded 3.0 MPa, a structural transition from sII to sI hydrates occurred immediately following CO2 injection. Experimental results showed that at 3.5 MPa, the compositional evolution of the hydrate phase was primarily governed by changes in the weight fractions of each hydrate structure, rather than by variations in individual cage occupancies. Thermodynamic free energy (Delta Gtot) calculations indicated that CO2 occupancy in the small (512) cages of sII hydrates had an optimal threshold, beyond which further incorporation became unfavorable, thereby inducing a structural transition to sI hydrates. These findings offer valuable insights into guest exchange mechanisms in sII hydrate systems and support the development of sII hydrate-based applications, particularly for gas storage and transportation, as well as CO2 capture and storage.