PHASE FIELD THEORY MODELING OF PHASE TRANSITIONS INVOLVING HYDRATE MUHAMMAD QASIM, KHURAM BAIG, BJØRN KVAMME* Department of Physics and Technology University of Bergen Allégaten 55, 5007 Bergen NORWAY Bjorn.kvamme@ift.uib.no , http://www.uib.no Abstract: - Natural gas hydrates are thermodynamically unstable in reservoirs due to contacts with the undersaturated phases and these contacts may both lead to dissociation and formation. This phenomenon allows forming CO2 hydrate and producing CH4 through injection of CO2 into CH4 hydrate. A thin layer of water between CH4 hydrate and CO2 phase has to be considered in reservoir. The nucleation on water-CO2 interface is expected to be very slow, therefore to speed the CO2 hydrate formation a small region of CO2 hydrate is considered on the water-CO2 interface. The exchange of CO2 and CH4 is expected to become faster due to introduction of water layer between CH4 hydrate and CO2 phase. The kinetic rates of hydrate formation and dissociation towards these undersaturated phase are vital in the understanding of natural hydrates in sediments and the impact of contact with surrounding fluid phases and adsorbed phase (on mineral surfaces). Common to this exchange process and the dissociations toward undersaturated phases is that the kinetic rates will depend on how fast the processes happen. This effect comes into play due to the impicit implementation of hydrodynamics. A better picture of free energy is always important as it is the driving force to decide the faith of flow and hydrate formation or dissociation. This is achieved by implementing the non-thermodynamic model implicitely in the phase field simulation. This will allow specially giving a proper effect of free energy gradients. Key-Words: - Phase field theory, Natural gas hydrate, Hydrodynamics, Dissociation, Hydrate, Exchange 1 Introduction Gas hydrates are ice-like substances of water molecules encaging gas molecules (mostly methane) that form under high pressure and low temperature conditions within the upper hundred meters of the sub-seabed sediments [1]. These gas hydrates are widely distributed in sediments along continental margins, and harbor enormous amounts of energy. Massive hydrates that outcrop the sea floor have been reported in the Gulf of Mexico [2]. Hydrate accumulations have also been found in the upper sediment layers of Hydrate ridge, off the coast of Oregon and a fishing trawler off Vancouver Island recently recovered a bulk of hydrate of approximately 1000kg [3]. Håkon Mosby Mud Volcano of Bear Island in the Barents Sea with hydrates is openly exposed at the ocean floor [4]. These are only few examples of the worldwide evidences of unstable hydrate occurrences that leaks methane to the oceans and eventually may be a source of methane increase in the atmosphere. Hydrates of methane are not thermodynamically stable at mineral surfaces. From a thermodynamic point of view the reason is simply that water structure on hydrate surfaces are not able to obtain optimal interactions with surfaces of calcite, quarts and other reservoir minerals. The impact of this is that hydrates are separated from the mineral surfaces by fluid channels. The sizes of these fluid channels are not known and are basically not even unique in the sense that it depends on the local fluxes of all fluids in addition to the surface thermodynamics. Stability of natural gas hydrate reservoirs therefore depends on sealing or trapping mechanisms similar to ordinary oil and gas reservoirs. Many hydrate reservoirs are in a dynamic state where hydrate is leaking from top by contact with groundwater/seawater which is under saturated with respect to methane. Dissociating hydrate degasses as bubbles if dissociation rate is faster than dilution in surrounding fluids and/or surrounding fluid is supersaturated. The kinetic rate depends on mass transport dynamics as well as thermodynamic driving force. Phase field theory is a power full tool to quantify this balance and provide a theoretical Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology ISBN: 978-1-61804-065-7 222