Simulation of Hydrate Dynamics in Reservoirs
Authors: Vafaei Mohammad Taghi , Kvamme Bjorn , Chejara Ashok , Jemai Khaled 
Published: May 2016

Abstract
Gas hydrates in reservoirs are generally not in thermodynamic equilibrium and there may be several competing phase transitions involving hydrate. Formation of carbon dioxide hydrates during aquifer storage of carbon dioxide involves roughly 10% volume increase compared to groundwater. Dissociation of hydrate towards under saturated fluid phases involves the same level of contraction. Hydrate phase transitions are generally fast (scales of seconds) compared to mineral dissolution and precipitation and it is unlikely that a time shifted explicit coupling to geo mechanical analysis will be able to capture the appropriate dynamic couplings between flow and changes in stress. The need for geo mechanical integrity of the storage site therefore requires a reservoir simulator with an implicit solution of mass flow, heat flow and geo mechanics. And since carbon dioxide involved in hydrate is also involved in different geochemical reactions we propose a scheme where all possible hydrate formation (on water/carbon dioxide interface, from water solution and from carbon dioxide adsorbed on mineral surfaces) as well as all different possible dissociations are treated as pseudo reactions but with kinetics derived from advanced theoretical modeling. The main tools for generating these models have been phase field theory simulations, with thermodynamic properties derived from molecular modeling. The detailed results from these types of simulations provides information on the relative impact of mass transport, heat transport and thermodynamics of the phase transition which enable qualified simplifications for implementation into RCB. 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.

Introduction
Natural gas hydrate in the reservoir is continuously attracting the attention of more researchers around the world and the reason is its importance from different aspects ranging from a potential energy resource to environmental threat. Hydrate can occur in sediments below the oceanic floor or in the permafrost wherever the thermodynamic conditions are suitable and water and guest molecules are available. Investigations show that there are huge resources of natural gas hydrate in the earth which due to the high volumetric concentration of methane gas per hydrate volume is considered as a substantial energy resource. Besides, methane combustion releases less CO2 per unit energy release compared to both coal and oil which means a cleaner fuel from environmental point of view. On the other hand methane can be over twenty times more aggressive than CO2 in trapping the heat in the atmosphere and in case of leakage from sediments it can affect the marine life and the climate substantially.
There are several scenarios for methane production from natural gas hydrate reservoirs. Depressurization method in which hydrate stability condition is disturbed by pressure reduction according to the water-gas-hydrate equilibrium curve of figure 1 resulting in hydrate dissociation and release of methane. It is currently considered as the most feasible process considering expenses and production rate and has been investigated by many research groups through simulation studies. Thermal stimulation is another method which is based on moving out from stability region by temperature increase. It is considered to be costly due to huge amount of energy waste to the surroundings. The third method is to use inhibitors such as methanol or brine to shift the equilibrium curve and dissociate hydrate according to figure 2 which is also costly. The final method is injection of CO2 into the methane hydrate reservoirs. CO2-hydrate is more stable than Ch4-hydrate. Therefore CO2-hydrate formation will provide the necessary heat to dissociate methane hydrate and it can be considered both as a natural gas production method and a CO2 sequestration process (Graue et al., 2008).