An experimental and thermodynamic study on CH4-CO2 replacement mechanism in various clathrate structures and depressurization-assisted replacement for effective CH4 production and CO2 sequestration

Abstract

Department of Urban and Environmental Engineering (Environmental Science and Engineering)Natural Gas Hydrates (NGHs) have been regarded as a future clean energy resource because of the enormous deposit of it in the earth. The development of NGHs could be a solution to fulfill the demands for natural gas that are constantly increasing to reduce CO2 emission. Among the known methods for the production of natural gas from NGHs, the replacement method has been suggested as a promising technology because it could achieve carbon-neutral energy production via swapping of CH4 in NGHs with CO2. Three different structures of NGHs (sI, sII, and sH) have been discovered through several explorations. Distinctive thermodynamic characteristics of each structure required the study for the precise replacement mechanism in various structures of NGHs, but previous studies for replacement were mainly focused on sI hydrate. This study not only investigated the influence of flue gas injection in various hydrate reservoirs to reveal replacement mechanisms but also developed the depressurization-assisted replacement for enhancing the economic feasibility of the replacement. To verify the replacement mechanism of the flue gas injection into various structures of gas hydrate, sII CH4 + C3H8 and sH CH4 + methylcyclopentane hydrate were used in this study. The influence of feed gas composition was investigated with three mixture gases (CO2 (20%) + N2 (80%), CO2 (40%) + N2 (60%), and CO2 (60%) and N2 (40%)) to enhance the replacement efficiency. The extent of replacement depending on the pressure and compositions of feed gas were measured to compare the efficiency and kinetics of guest exchange. The structure transition and cage-specific guest distribution before and after replacement were examined by using 13C nuclear magnetic resonance (13C NMR) and powder X-ray diffraction (PXRD). In contrast to sII hydrate - CO2 replacement, N2 inclusion occurred in sII hydrate-flue gas replacement resulted in iso-structural replacement and a lower replacement efficiency than expected. The competitive inclusion of CO2 and N2 occurred in small (512) cages of sII hydrate, resulted in lower efficiency than pure CO2 injection. Furthermore, the mechanism of lattice expansion in sII hydrate-flue gas replacement was revealed by using molecular dynamics simulation in this study. In sH hydrate-flue gas replacement, partial structure transition was observed depending on the composition and pressure of the feed gas, it not only accelerated the replacement but enhanced the efficiency. Although the higher CO2 composition in the feed gas enhanced the structure transition, it also reduced the inclusion of N2 by the decreased partial pressure of N2 in feed gas resulted in the low extent of replacement. In addition, the depressurization-assisted replacement was investigated to overcome the low productivity of replacement. As a preliminary study, an effective driving force for dissociation of gas hydrate was investigated to efficiently control initial depressurization. The modified chemical potential-based driving force was revealed to the most optimal driving force for controlling depressurization without relevance of the reservoir temperature via various production tests in one-dimensional sediment packing reactor. Although partial dissociation of gas hydrate occurred in the early period of depressurization-assisted replacement, instant re-formation of gas hydrate was observed after CO2 injection which could assure the geological safety of hydrate-bearing sediment. A remarkable enhancement of replacement efficiency was observed as increasing the dissociation ratio of initial gas hydrate. Furthermore, the result of this study demonstrated that a fast flow rate of CO2 could enhance the productivity of depressurization-assisted replacement by reducing CH4 re-enclathation. The overall experimental results not only provided valuable insights for a comprehensive understanding of replacement mechanism in various NGHs but also demonstrated the technical feasibility of depressurization-assisted replacement which could be a breakthrough for efficient CH4 recovery and CO2 sequestration in the replacement.ope