Numerical investigation on fluid migration effects during depressurization production of natural gas hydrate reservoir with underlying gas

Natural gas hydrate reservoir with underlying gas layer (UGL) has broad prospects for commercial exploitation. In order to have a clearer understanding of the effects of underlying fluid migration on the evolution of physical and mechanical characteristics during depressurization production, this paper adopts a multi-field coupled model to investigate the fluid migration effects during 360 days depressurization production with a horizontal well. Due to low pore pressures propagation through sediment with heterogeneous distribution of permeability and water saturation, fluid migration and production rate change with time. Pore water migration shows accelerated behavior leading to massive water production under the influence of low pore pressure propagation from low to the high water saturation layer. Upward migration of the underlying warm fluid drives the hydrate decomposition leading edge downward, which is accompanied by hydrate generation under conditions of stress permeability evolution and low temperature environment. No significant hydrate generation occurs in the reservoir when there is no underlying gas migration. The fluid migration behavior from the UGL contributes to the uplift phenomenon at reservoir bottom during depressurization production. Permeability enhancement measures for UGL can promote the upward migration of warm fluids from UGL. When the permeability of UGL is increased, the gas production rate will be enhanced under long-term depressurization production condition.