Simulation of Hydrate Dynamics in Reservoirs

Gas hydrates in reservoirs are generally not in thermodynamic equilibrium, and there may be several competing phase transitions involving hydrate. Formation of carbon dioxide (CO2) hydrates during aquifer storage of CO2 involves roughly 10 vol% increase compared with groundwater. Dissociation of hydrate toward undersaturated fluid phases involves the same level of contraction. Hydrate phase transitions are generally fast (scales of seconds) compared with mineral dissolution and precipitation, and it is unlikely that a time-shifted explicit coupling to geomechanical analysis will be able to capture the appropriate dynamic couplings between flow and changes in stress. The need for geomechanical integrity of the storage site therefore requires a reservoir simulator with an implicit solution of mass flow, heat flow, and geomechanics. And because CO2 involved in hydrate is also involved in different geochemical reactions, we propose a scheme where all possible hydrate formation (on water/CO2 interface, from water solution, and from CO2 adsorbed on mineral surfaces) and all different possible dissociations are treated as pseudoreactions, but with kinetics derived from advanced theoretical modeling. The main tools for generating these models have been phase-field-theory (PFT) simulations, with thermodynamic properties derived from molecular modeling. The detailed results from these types of simulations provide information on the relative impact of mass transport, heat transport, and thermodynamics of the phase transition, which enable qualified simplifications for implementation into RetrasoCodeBright (RCB) (Saaltink et al. 2004). The primary step was to study the effect of hydrate growth or dissociation with a certain kinetic rate on the mechanical properties of the reservoir. Details of the simulator and numerical algorithms are discussed, and relevant examples are shown.