A well-test model for gas hydrate dissociation considering a dynamic interface

Abstract

With the consumption of conventional energy, hydrate, as a convenient source of clean energy, is playing a more significant role in energy consumption in the world. Therefore, it is necessary to understand the properties of gas hydrate reservoirs. In this paper, we develop an analytical gas hydrate dissociation (GHD) model by incorporating a dynamic dissociation interface into the radial diffusivity equations.First, a cylindrical geometry represents the hydrate reservoir, and its in-situ hydrate dissociation is driven by depressurization. The process of dissociation of gas hydrate divides the reservoir into two regions by a dynamic interface, containing dissociated gas and undissociated hydrates. Based on that, a radially composite model with moving interface is proposed. Using the Laplace transform and the Stehfest numerical inversion algorithm, we obtain the analytical solutions of the bottom-hole pressure (BHP) and pressure transient behaviors of the proposed model for gas hydrate wells. Then, the flow stages of the analytical GHD model are divided, and seven regimes are identified according to the pressure transient behaviors, mainly including inner-zone radial flow stage, dissociating stage, transitional flow stage, improvement stage, and outer-zone radial flow stage. Furthermore, a sensitivity analysis is conducted to study the impacts of the mobility ratio, inner-zone radius, and dissociation factor (DF) on the pressure transient behaviors. Results show that the mobility ratio mainly influences the upward level of the transitional flow stage between dissociating stage and improvement stage. The inner zone radius correlates the initiation point of the dissociating stage. A field application is conducted to prove the practicability of the model presented in this work. The proposed model compares well with the actual well testing data obtained from an example gas hydrate reservoir. The properties from the actual gas hydrate reservoir are also estimated.