Numerical analysis on gas production from silty hydrate reservoirs in the South China sea by depressurizing: The effect of permeability reduction caused by pore compression

Natural Gas Hydrate (NGH) is widely discovered in silty sediments with a high compressibility in the South China Sea. The permeability reduction in these reservoirs during depressurization is inevitable. Here, we innovatively take the effect of permeability reduction into account to improve the accuracy of production forecast and suggest the optimal production pressures for two different typical reservoirs (i.e., Class I: Site W19 and Class III: Site SH2) in this area. In this study, two typical reservoir-scale production models of NGH reservoir are built, and the effects of bottomhole pressure and pore compressibility and permeability correlation coefficients on gas and water extraction are quantitatively evaluated. The simulation results indicate that for silty NGH reservoirs, the effect of permeability reduction on gas and water recovery during depressurization cannot be overlooked. Pore compressibility and permeability correlation coefficients will significantly affect production, and with an increase in both parameters, the cumulative gas and water recovery will decrease. However, the total gas production will firstly increase and then decrease with a reduction in bottomhole pressure. In addition, the optimal production pressures of sites W19 and SH2 are 1.5 MPa and 5.5 MPa respectively. These findings can give some valuable suggestions for future field tests in this region.