Experimental and computational investigation of guest encapsulation and structural transformation behaviors in C3H8 hydrate-CO2 replacement for energy recovery and CO2 sequestration

Guest exchange in natural gas hydrates via the injection of CO2 is a promising method for simultaneously achieving energy production and CO2 sequestration. In this study, the guest encapsulation and structural transformation behaviors involved in the structure II (sII) C3H8 hydrate - CO2 replacement were closely investigated through thermodynamic, compositional, and structural analyses. Guest compositions in the hydrate phase after replacement were measured under varying CO2 injection pressures (2.0-3.5 MPa). The CO2 compositions of the replaced hydrates remained nearly constant until the critical pressure point of 3.0 MPa, after which they increased sharply. To clarify this behavior and its distinction from that of sII (CH4 + C3H8) hydrate - CO2 replacement, a structural analysis and molecular dynamics were employed. Powder X-ray diffraction analysis, coupled with the Rietveld refinement, was conducted to determine the weight fractions of each phase and the cage occupancies of guest molecules after replacement. Unlike the system involving CH4, the initial sII hydrates in the C3H8 hydrate - CO2 system transformed into structure I (sI) hydrates only beyond the critical pressure point. CO2 was captured only in the small (512) cages, while C3H8 occupied the large (51264) cages of the sII hydrates. Free energy calculations revealed that the presence of CH4 in the small (512) cages enhanced the thermodynamic stability of sII hydrates with CO2 in the large (51264) cages. These experimental and computational findings provide valuable insights into the role of CH4 and the guest exchange mechanism in sII hydrate - CO2 replacement.