Numerical study on gas production via a horizontal well from hydrate reservoirs with different slope angles in the South China Sea

It is important to study the effect of hydrate production on the physical and mechanical properties of low-permeability clayey-silty reservoirs for the large-scale exploitation of hydrate reservoirs in the South China Sea. In this study, a multiphysical-field coupling model, combined with actual exploration drilling data and the mechanical experimental data of hydrate cores in the laboratory, was established to investigate the physical and mechanical properties of low-permeability reservoirs with different slope angles during 5-year hydrate production by the depressurization method via a horizontal well. The result shows that the permeability of reservoirs severely affects gas production rate, and the maximum gas production amount of a 20-m-long horizontal well can reach 186.8 m3/day during the 5-year hydrate production. Reservoirs with smaller slope angles show higher gas production rates. The depressurization propagation and hydrate dissociation mainly develop along the direction parallel to the slope. Besides, the mean effective stress of reservoirs is concentrated in the near-wellbore area with the on-going hydrate production, and gradually decreases with the increase of the slope angle. Different from the effective stress distribution law, the total reservoir settlement amount first decreases and then increases with the increase of the slope angle. The maximum settlement of reservoirs with a 0 degrees slope angle is up to 3.4 m, and the displacement in the near-wellbore area is as high as 2.2 m after 5 years of hydrate production. It is concluded that the pore pressure drop region of low-permeability reservoirs in the South China Sea is limited, and various slope angles further lead to differences in effective stress and strain of reservoirs during hydrate production, resulting in severe uneven settlement of reservoirs. To analyze the evolution law of physical and mechanical properties of a reservoir during hydrate production, a numerical simulation of long-term gas hydrate production by the depressurization method via a horizontal well was carried out based on the drilling data of the GMGS3-W19 site, the mechanical experimental data in the laboratory, and the Multiphysics-Coupling model. The characteristics of gas production, and the spatio-temporal evolution law of settlement and stress distribution of reservoirs with different slope angles during hydrate production by depressurization in 5 years were obtained and analyzed. It is concluded that the low permeability of reservoirs in the South China Sea limits the propagation range of pore-pressure drop and reduces the gas production rate and amount, and continuous hydrate dissociation will cause nonhomogeneous severe settlement of the hydrate reservoirs. image The response of hydrate reservoirs with various angle slopes to gas production is predicted. Low permeability would limit the long-term gas recovery rates of clayey reservoirs. Reservoirs without slope angle show the highest gas production and seabed settlement. There is a risk of potential damage of the production wellbore during long-term gas production.