ALBERT

Description: Highlights • Thermodynamic and kinetic influences of NaCl on HFC-125a hydrate were investigated. • NaCl enrichment in the unconverted solution resulted in a lower conversion. • The presence of NaCl had little effect on the ΔH of HFC-125a hydrate. • The hydrate dissociation was retarded due to the formation of NaCl⋅2H2O. In this study, HFC-125a was selected as a hydrate-forming guest for gas hydrate-based desalination. The thermodynamic and kinetic effects of NaCl on HFC-125a hydrates were investigated with a primary focus on phase equilibria, gas uptake, dissociation enthalpy, and dissociation behavior. The equilibrium curve of HFC-125a hydrate shifted to higher pressure regions at any given temperature depending on the concentration of NaCl. The presence of NaCl also reduced the gas uptake and conversion to hydrates, because of the enrichment of NaCl in the solution during gas-hydrate formation. Even though NaCl did not affect the dissociation enthalpy of the HFC-125a hydrate, the thermograms obtained using a high-pressure micro-differential scanning calorimeter (HP μ-DSC) demonstrated that HFC-125a + NaCl hydrates started to dissociate at lower temperatures due to NaCl in unconverted solutions. Rietveld refinement of powder X-ray diffraction (PXRD) patterns indicated that the HFC-125a hydrate (sII) was transformed into Ih as it dissociated. The dissociation of HFC-125a + NaCl hydrates was retarded and completely ended at higher temperatures compared to the pure HFC-125a hydrate by the sodium chloride dihydrate (NaCl⋅2H2O). Overall, these results could facilitate a better understanding of HFC-125a hydrates in the presence of NaCl; further, they might also be useful in the design and operation of hydrate-based desalination plants using HFC-125a.

Description: Highlights • The effects of the combined method on HBS geomechanical properties were examined. • Mechanical behavior depended on dissociation ratios and GH saturations. • Mechanical strength of the replaced HBSs was significantly recovered. • The combination of depressurization and replacement increased total CH4 recovery. • Optimum replacement occurred at a dissociation ratio of 20% with CO2 injection. Abstract This study analyzed the potential effects of gas hydrate (GH) exploitation on the geomechanical properties of hydrate-bearing sediment (HBS) by examining the combined effects of depressurization and CO2 injection using triaxial compression tests. The stress-strain behavior of the initial CH4 HBS showed strong hardening-softening characteristics and high peak strength, whereas milder hardening-softening behavior and reduced peak strength were observed after partial (20, 40, 60, and 80%) or complete GH dissociation (100%), indicating that the mechanical behavior clearly depended on dissociation ratios and GH saturations. In response to CO2 injection in partially dissociated HBS, subsequent CH4–CO2 hydrate exchange, and secondary CO2 hydrate formation, the mechanical strength of the replaced HBS recovered significantly, and stress-strain characteristics were similar to that of the 20% dissociated CH4 HBS. Although total CH4 recovery was increased by the combination of depressurization and replacement, optimum recovery was found at a dissociation ratio of 20% followed by replacement because production by replacement decreased as the dissociation ratio increased. These results contribute to the understanding of how depressurization and CO2 injection schemes may be combined to optimize energy recovery and CO2 sequestration. In particular, this research demonstrates that CH4–CO2 hydrate exchange and secondary GH formation are suitable methods for controlling and maintaining the mechanical stability of HBSs.