Modeling of hydrate formation and dissociation in porous media

Gas hydrates are ice-like compounds formed by inclusion of gas molecules in water clathrates under high pressure and low temperature. The large amount of hydrocarbon gas trapped by hydrates provides an important potential energy resource for the future. Gas recovery from hydrate deposits involves the formation and dissociation of hydrates in porous media.; In this work, two types of methodology (kinetic simulation and equilibrium simulation) are developed to model the formation and dissociation of SI methane hydrate in porous media. In kinetic simulation, the hydrate transition is considered as a kinetic process with the Kim-Bishnoi model, and water freezing (ice melting) is assumed to be an equilibrium phase transition. In equilibrium simulation, all the phase transitions are assumed to be in equilibrium. In both the kinetic and the equilibrium simulation, the primary variable switch method (PVSM) is used to track the phase transitions.; The kinetic simulation is used in laboratory-scale modeling. Good match of the steady state in the formation system of gas and aqueous phase (G+A) is obtained between modeling results and experimental data, but clear discrepancy of the dynamic pressure history is observed. In the formation system of gas and ice (G+I), it is difficult to have uniform hydrate generation in the core in the absence of salt. If salt is uniformly present in the G+I system, hydrate can be formed uniformly. In hydrate dissociation, the simulated gas recovery matches that of an experiment. Sensitivity analyses indicate the temperature, phase saturation, salt concentration, depressurization pressure, and overburden heat are important factors during hydrate dissociation. Two regimes, kinetics-controlled regime and flow-controlled regime, are identified during the depressurization of hydrate samples.; Equilibrium phase transition is a good assumption for field-scale modeling as indicated by the comparison of the modeling results between kinetic and equilibrium simulations. Hydrate accumulations in offshore sediments can be predicted accounting for geological structures. For gas production, if the hydrate layer is underlain by free gas layer, gas can be recovered simply by depressurization; if there is an aquifer layer immediately below the hydrate layer, gas recovery can be achieved by thermal stimulation combined with depressurization.