Anomalous dissociation behavior of structure II (CH4+C3H8) hydrate during depressurization for hydrocarbon recovery

Abstract

Natural gas hydrates (NGHs) are considered a promising future energy resource, with depressurization recognized as the most feasible and cost-effective production method. However, for structure II (sII) NGHs, there is insufficient research on the dissociation behavior under depressurization. Therefore, this study investigated the dissociation behavior of sII (CH4 + C3H8) hydrate during depressurization, with a particular focus on the formation and role of a C3H8-rich hydrate shell under varying thermodynamic conditions. Experiments were conducted at 274.2 K and 280.2 K using a one-dimensional (1-D) reactor and a high-pressure (HP) autoclave reactor to simulate natural sediment environments and to accurately capture dissociation dynamics. Uniform sII hydrate formation along the 1-D reactor prior to depressurization was confirmed via powder X-ray diffraction and 13C NMR spectroscopy. The dissociation behavior was strongly temperature-dependent: at 274.2 K, gas release was suppressed until the system pressure decreased to 0.2 MPa, close to the equilibrium pressure of pure C3H8 hydrate, whereas at 280.2 K, dissociation was triggered at around 0.6 MPa, just below the equilibrium pressure of sII (CH4 + C3H8) hydrate. These trends were consistent across reactor types and depressurization methods. Complementary analyses using in-situ Raman spectroscopy and high-pressure micro differential scanning calorimetry confirmed that a C3H8-rich hydrate shell acted as a kinetic barrier at 274.2 K, delaying internal hydrate decomposition-a phenomenon not observed at 280.2 K. These findings suggest that effective gas recovery from sII NGH reservoir via depressurization is more viable in geological settings with elevated temperatures, where such kinetic constraints are diminished.