Numerical simulation of dissociation front of shenhu hydrate reservoirs by depressurization

Abstract

Natural gas hydrates (NGH), widely distributed in the deep ocean and the permafrost, are considered as a potential energy resource. Depressurization is a promising method to exploit NGH reservoirs. There is a dissociation front between dissociated zone and dissociating hydrate zone during the production of NGH reservoirs by depressurization. The study of the movement of the dissociation front is very essential to understand the formation stability and predict the gas production performance of hydrate reservoirs. Based on the geological conditions of hydrate reservoirs in the Shenhu area, numerical simulation methods are used to investigate the shape and movement of the dissociation front during the depressurization production, and then the sensitive factors affecting the movement of dissociation front are analyzed. The results show that the dissociation front is not a definitely interface between the dissociated and dissociating zone but a transition zone, and the shape of the dissociation front is deemed to a non-piston model when depressurization is conducted. The movement rate of dissociation front 1 (the position where the hydrate starts to dissociate) is almost approximately linear, but the movement rate of dissociation front 2 (the hydrate saturation is reduced to 0) increases as approximately an exponential pattern. The greater movement rate of dissociation front could be observed clearly under the condition of the lower production pressure, the greater intrinsic permeability, the smaller initial hydrate saturation and the higher initial temperature condition, respectively. Compared to the one-step depressurization, the dissociation front 1 and dissociation front 2 are both delayed when the multi-steps depressurization is conducted. The formation temperature will decrease because of the endothermic effect of hydrate dissociation. If the ice point is reached, the extra latent heat generated from ice will contribute to the increase of the hydrate dissociation rate. Therefore, when the depressurization method is used to exploit NGH reservoirs, the pressure can be reduced to the vicinity of the phase equilibrium pressure corresponding to the freezing point.