A thermo-hydro-chemo-mechanical coupled model for hydrate reservoirs using OpenGeoSys: model development and simulation of the first offshore hydrate production test in the South China Sea

Natural gas hydrates are increasingly recognized as a promising alternative to traditional energy. The intricate thermo-hydro-chemo-mechanical (THMC) coupled process during in-situ hydrate exploitation poses substantial challenges for numerical modeling. This paper develops a THMC model to investigate the reservoir behavior during the hydrate dissociation based on the open-source FEM code OpenGeoSys, characterized by robust convergence. Locally, a nonlinear complementary problem (NCP) approach is employed to handle the strong nonlinearity associated with phase (dis)appearance, and a fourth-order Runge-Kutta (RK4) method is utilized to calculate the hydrate saturation. Globally, persistent primary variables are introduced to uniformly describe single-/two-phase flow. The THMC coupled process of the first offshore hydrate production test in the South China Sea has been investigated using this model. Results show that a local temperature rise appears in the free gas layer for heat convection from the underburden layer and the dissociation front exhibits a triangular distribution due to heat convection and liquid-gas migration. Moreover, inhomogeneous reservoir deformation and low gas production rate/gas-water ratio occur, where the gas production can be notably enhanced by decreasing permeability in the underburden layer. These findings provide valuable insights into optimizing the efficiency and safety of gas hydrate exploitation.