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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. [Display omitted] • 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. [ABSTRACT FROM AUTHOR]

Natural gas hydrates have drawn worldwide attention as a significant clean energy source in the 21st century. Depressurization is known to be the most effective method for hydrate exploitation owing to its feasibility and economic benefits. Recently, stepwise depressurization has received extensive attention owing to its advantages of effective prevention of ice generation and hydrate reformation during gas recovery. In this work, we optimized the 2-D simulator to model the evolution of temperature, pressure, hydrate saturation, and gas production behavior in hydrate sediments during decomposition process via stepwise depressurization. The effect of various reservoir characteristics on gas production was analyzed by introducing the sensitivity factor. The results demonstrated that higher specific heat capacity of the reservoir would promote gas production in the early stage of the reaction; however, this effect is reversed in the later stage. A higher thermal conductivity of the hydrate cores can dramatically enhance the gas production rate; however, this advantage is weakened with increasing thermal conductivity. Reducing the gas relative permeability hinders the hydrate decomposition process, and the water relative permeability has minimal effect on gas production. Based on the sensitivity analysis, the thermal conductivity and gas relative permeability are the main factors affecting hydrate decomposition. • Numerical simulation of gas production by step-wise depressurization is presented. • Higher thermal conductivity enhanced gas production via stepwise depressurization. • Gas permeability and thermal conductivity are main factors of gas recovery. [ABSTRACT FROM AUTHOR]

Natural gas resources trapped in hydrate reserves worldwide are a new substitute for conventional energy. To safely and efficiently extract natural gas from hydrates, fields test and laboratory investigations were systematically conducted. Salinity is a significant factor impacting the exploitation of the marine hydrates. In this study, a two-dimensional cylindrical model was developed to determine the influences of salinity on gas recovery induced by depressurization and thermal stimulation. The results indicated that, with the same exploitation method, higher salinity accelerated the pressure-drop propagation rate and temperature increment spread, thereby promoting the hydrate dissociation rate and shortening the gas production process. Furthermore, thermal stimulation improved the average gas generation rate by almost five times that with depressurization at the same salinity. • Salinity accelerates the pressure and temperature propagation rates. • Salinity promotes the gas production rate and shortens the process. • Thermal stimulation improves the average gas generation rate more. [ABSTRACT FROM AUTHOR]