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

Abstract

Summary Modeling of hydrate reservoirs has revealed large time-scale discrepancies between the mechanisms involved. Compared with the fluid- and heat-flow terms, the time scale 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 timesteps to ensure convergence. Further investigations have shown that, for sharp decomposition cases in which dissociation occurs in a narrow region, nonphysical oscillation becomes a simulation issue, unless very small time-steps are chosen. A 3D numerical model incorporating heat and fluid flow with kinetics of decomposition and reformation of hydrates has been developed. In this paper, a methodology for the use of larger timesteps 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, allows selection of different timesteps for the 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 without operator splitting requires adjusting the overall timestep, such that the problem with the faster reaction requires very small timesteps. The application of operator splitting in this case allows for a stable solution with much larger timesteps. The contribution of this work is developing a methodology to improve the computational-time of large-scale simulations of gas-hydrate reservoirs without loss of accuracy.