Application of Operator Splitting Technique in Numerical Simulation of Gas Hydrate Reservoirs

Abstract

Abstract Modeling of hydrate reservoirs has revealed large timescale discrepancies between the involved mechanisms. Compared to the fluid and heat flow terms, the timescale of the kinetics term is orders of magnitude smaller, especially when the intrinsic reaction rate is large. Previous studies have shown that simulation of hydrates may require very small time steps, due to a convergence problem. Further investigations have shown that, for sharp decomposition cases where dissociation occurs in a narrow region, non-physical oscillation becomes a simulation issue, unless very small time steps are chosen. A three-dimensional numerical model incorporating heat/fluid flow, with kinetics of decomposition and (re)formation of hydrates has been developed. In this paper, a methodology for the use of larger time steps is proposed without loss of accuracy. The focus of this work is on the decoupling of the reaction and flow operators. The decoupling, or so-called operator splitting, helps to select different time steps for different mechanisms. The success of the splitting methodology in saving computational time is demonstrated for two cases. The first case shows oscillatory solutions, and the application of operator splitting allows for an oscillation free solution at a smaller run time. In the second case, a problem with a range of reaction constants is studied. Obtaining a stable solution requires adjusting the overall time step, such that the problem with the larger reaction rate requires very small time steps. The application of operator splitting in this case allows for a stable solution with much larger time steps. The contribution of this work is the computational timesaving in large-scale simulations of gas hydrate reservoirs without losing accuracy. Furthermore, using the splitting technique does not require a change of physically determined parameters, including the reaction constants.