CFD Modelling of Hydrate Formation in Oil-dominated Flows

Abstract

Abstract A three-dimensional CFD (Computational Fluid Dynamics) model for hydrate formation in oil-dominated flows is derived based on the Eulerian multiphase flow modelling approach. The model assumes that oil is the continuous fluid and it carries water droplets and methane gas bubbles as dispersed phases. The hydrate formation model therefore considers three fluid phases: continuous oil, dispersed water droplets and dispersed gas bubbles. As the water droplets are converted to hydrate, they become solid particles. The model calculates the transfer of methane from the gas bubbles into oil as mass transfer from the gas to the liquid phase. The dissolved methane in oil is therefore available for hydrate formation at the surface of the water droplets when the flow conditions are right, typically when the fluid temperature drops below the hydrate equilibrium temperature by the nucleation temperature of 6.5 °F. The consumption of the dissolved methane in the hydrate formation process is modelled as mass transfer from the oil to the water phase. The conversion of the water droplets into hydrate is also modelled as a reaction process in which a fraction of the water is converted into hydrate, the hydrate mass fraction. When the hydrate mass fraction equals zero the droplets are pure water and when the hydrate mass fraction equals one the droplets are fully converted into hydrate. The model solves the conservation equations for mass, momentum and energy for all three phases according to the standard Eulerian multiphase flow model. Momentum exchanges between the phases such as drag and buoyancy forces are included in the model. Heat and mass transfer processes involved in hydrate formation as described above are added to multiphase flow equations via source terms. A simple example is presented in this paper to illustrate the modelling of the hydrate formation process described above. Introduction Hydrate is often regarded as the " Number One?? problem in flow-assurance in offshore oil and gas operations. The financial cost of hydrate plugging a flow-line can be extremely high. As the industry moves into deeper water and using longer tiebacks the hydrate risk increases greatly. A better understanding of hydrate formation and reliable modeling and analysis tools are needed to enable flow-assurance engineers to make proper risk assessment say for a new flow-line and operating procedures with confidence. One-dimensional models for predicting hydrate-plug formation in flowlines are available and they have been applied successfully for subsea tiebacks (Davies et al., 2009). In this paper we consider extending the modeling approach to three-dimensional analsysis using CFD (Computational Fluid Dynamics) method. CFD Eulerian multiphase flow modelling approach. We consider an " oil-dominated?? flow in which the main flow is oil and it is carrying gas bubbles and water droplets/hydrate particles, as illustrated in Figure 1. We model the flow using 3 fluid phases in an Eulerian multiphase flow model (Lo, 2005). The 3 fluid phases are:Phase 1: Oil- continuous fluid.Phase 2: Gas- dispersed bubbles.Phase 3: Water/hydrate- dispersed droplets turn into hydrate particles. The proportion of water turned into hydrate is given by the mass fraction of solid in the water droplet. We will call this mass fraction the " hydrate fraction?? denoted by fH. When fH=0 we have pure water in the droplet and when fH=1 we have pure hydrate particle.