Association between multiphase seepage and exploitation of natural gas hydrate based on the Shenhu area of South China Sea

Abstract

Multiphase seepage characteristics are vital factors for improving the recovery efficiency of natural gas hydrate (NGH) via stepwise depressurization. However, the difficulties to experimentally construct real and natural hydrate-bearing sediments in a laboratory hinder the study of the association between multiphase seepage and the development of NGH. In this study, an optimized simulation model based on the features of fine shaly silt sediment in the Shenhu area of the South China Sea is proposed to analyze the influence of seepage characteristics on hydrate exploitation induced by stepwise depressurization. Multiple perspectives, including the intrinsic permeability, gas and water relative permeability, porosity, and permeability anisotropy of the reservoir are regarded as sensitive parameters of the hydrate production. The exploitation economy of fine hydrate-bearing sediment was evaluated by introducing the Ste number, gas-water ratio (GWR), and sensitivity analysis. The results demonstrate that the contribution of sensible heat to hydrate exploitation in reservoirs with poor seepage characteristics is limited; the effects of the abovementioned parameters on the GWR are different; porosity and gas relative permeability are the most important factors that affect hydrate production. Thus, detailed geological examination should be performed to ensure the exploitation economy. This study provides a profound theoretical basis for the stepwise depressurization strategy design of NGH exploitation in the field. • Numerical simulation of gas production by stepwise depressurization is presented. •Reservoir seepage on hydrate exploitation is analyzed from multiple perspectives. •The exploitation potential and economy of different reservoirs are analyzed. •Porosity, intrinsic and gas relative permeability are key factors of gas recovery.