Coupled multiphase flow and pore compression computational model for extraction of offshore gas hydrates

During gas hydrate extraction multiphase flow occurs with transitioning gas hydrate phase followed by pore compression. Effective theoretical and numerical models with the ability to account gas hydrate are key to evaluate the feasibility of gas hydrate extraction and understand the physical mechanisms behind it. In this manuscript, a coupled multiphase flow computational model is developed to simulate the gas hydrate extraction where phase transition and pore compression occurs. The compressibility of each phase is coupled with the variation of gas hydrate saturation and porosity. An efficient decoupled implicit iteration method is proposed to solve the highly non-linear equations. Differing from the known implicit iteration, an algorithm with dynamic time step was used to consider gas hydrate mass in each grid and to maintain the stability of simulation. The proposed model is validated using Masuda's core sample experimental data. Then a conceptual offshore gas hydrate chimney is simulated, and the impacts of intrinsic permeability, lithology and gas hydrate saturation heterogeneity, and reservoir compression modulus on gas hydrate extraction efficiency are investigated using the model. The model shows the ability to predict the long-term gas hydrate production and correlated seabed stability problems.