Numerical studies of hydrate dissociation and gas production behavior in porous media during depressurization process

Abstract

In this study, a numerical model is developed to investigate the hydrate dissociation and gas production in porous media by depressurization. A series of simulation runs are conducted to study the impacts of permeability characteristics, including permeability reduction exponent, absolute permeability, hydrate accumulation habits and hydrate saturation, sand average grain size and irreducible water saturation. The effects of the distribution of hydrate in porous media are examined by adapting conceptual models of hydrate accumulation habits into simulations to govern the evolution of permeability with hydrate decomposition, which is also compared with the conventional reservoir permeability model, i.e. Corey model. The simulations show that the hydrate dissociation rate increases with the decrease of permeability reduction exponent, hydrate saturation and the sand average grain size. Compared with the conceptual models of hydrate accumulation habits, our simulations indicate that Corey model overpredicts the gas production and the performance of hydrate coating models is superior to that of hydrate filling models in gas production, which behavior does follow by the order of capillary coating>pore coating>pore filling>capillary filling. From the analysis of t_1/2, some interesting results are suggested as follows: (1) there is a “switch” value (the “switch” absolute permeability) for laboratory-scale hydrate dissociation in porous media, the absolute permeability has almost no influence on the gas production behavior when the permeability exceeds the “switch” value. In this study, the “switch” value of absolute permeability can be estimated to be between 10 and 50 md. (2) An optimum value of initial effective water saturation S_w,e exists where hydrate dissociation rate reaches the maximum and the optimum value largely coincides with the value of irreducible water saturation S_wr,e. For the case of S_w,e<S_wr,e, or S_w,e>S_wr,e, there are different control mechanisms dominating the process of hydrate dissociation and gas production.