Modeling Two-Phase Flow Caused by Hydrate Dissociation in a Reservoir on the Concept of Global Pressure

Abstract

Summary The classic Darcy’s two-phase flow equation has a variety of mathematically equivalent formats, such as pressure/saturation (PS), two-phase pressure (PP), and global pressure/saturation (GP). Based on the concept of global pressure, we derive a new formulation for two-phase flow caused by hydrate dissociation in porous media, coupled with the mass and energy conservation equations, thereby offering a novel theoretical frame for gas hydrate extraction simulation. The new model provides in-depth insights into complex flow fields: The dissociation of hydrates under thermal stimulation may lead to two flow fields of gas and water with different directions in the reservoir. The simulation results are in good agreement with the recorded data set of Masuda’s and Chong’s experiments, which verifies the correctness and applicability of the new model. Based on numerical simulations of the same hydrate dissociation experiments, the efficiency of the GP method was compared with the PP method published in our previous work. The results show that the GP method has more than two times the timestep size of the PP method for unsteady flow and 1.5 times for steady flow. Furthermore, the relative residual of the GP method is even two orders of magnitude lower than that of the PP method for two-phase flows with complex variations of pressure gradient and fluid flux. Therefore, the GP method is significantly more efficient than the PP method for simulating gas hydrate development. The proposed GP method improves the computational efficiency of hydrate extraction simulation at the laboratory scale and understanding the mechanisms of two-phase flow in reservoirs caused by hydrate dissociation. It may have potential advantages for field-scale simulation of hydrate development, which requires further studies to demonstrate.