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The sediments in the South China sea are mainly composed of clayey silt, characterized by weak cementation, low strength, and poor permeability. These characteristics lead to slow gas and water transport and low gas production efficiency in the production process, which is not conducive to reservoir stability. Therefore, this paper studied the influence of different factors on the displacement and permeability of hydrate-bearing sediments by using remolded cores from the South China Sea. It was found that, when the depressurization method was used for hydrate decomposition, the displacement change in sediments could be divided into three stages: depressurization stage, decomposition stage, and creep stage. During the decompression stage, sediment deformation was rapid and displacement was small. During the decomposition process of hydrates, sediment deformation was slow and displacement was maximum. The creep stage had the slowest deformation and the smallest displacement. The displacement increased with the increase in initial porosity, hydrate saturation, effective pressure, and depressurization amplitude. The permeability of the sediments was lower than that of the original sediments after hydrate decomposition. This permeability damage increased with the increase in the sediment porosity, hydrate saturation, depressurization range and effective pressure. Furthermore, the displacement of sediments was positively correlated with the permeability damage. [ABSTRACT FROM AUTHOR]

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. • Effect of permeability reduction on gas recovery of NGH reservoirs was studied. • A related sensitive analysis affecting the permeability reduction was conducted. • Optimal production pressures of site W19 and site SH2 were suggested respectively. [ABSTRACT FROM AUTHOR]

The absence of impermeable overburden and underburden layers makes it difficult to develop the unconfined hydrate deposits. Using numerical simulation method, the hydrate dissociation and gas production behaviors are fully investigated during depressurization in Shenhu Area based on the available geological data. Results show that depressurization cannot result in significant pressure drop due to the presence of permeable boundaries, which leads to low gas production rate but excessive water production. To overcome these disadvantages, the method of forming artificial impermeable barriers by injecting gels in the permeable overburden and underburden is proposed in this work. Moreover, the effect of the radius of artificial impermeable barriers on gas production is analyzed numerically. Simulation results show that the artificial impermeable barriers prevent large amounts of seawater from entering the production well during the depressurization process, which significantly enhances the depressurization effect, and thus promotes hydrate dissociation and gas production. By forming the 90-m artificial impermeable barriers, the hydrate dissociation percent is greatly enhanced from 8.9% to 45.4%, the cumulative CH 4 production volume is increased from 4.46 × 106 ST m3 to 1.06 × 107 ST m3, while the average water production rate is remarkably decreased from 1412 m3/d to 360 m3/d. • Gas production from unconfined hydrate deposits is investigated with numerical simulation method. • The method of forming artificial impermeable barriers by injecting gels is proposed. • Depressurization is significantly enhanced after forming artificial impermeable barriers. • Hydrate dissociation percent is greatly enhanced from 8.9% to 45.4%. [ABSTRACT FROM AUTHOR]

In 2017, China successfully conducted its first offshore gas hydrate production test by using vertical well in the Shenhu area, achieving a total gas production of 3.09 × 105 m3. However, there is still a long way to commercial production. In this study, to evaluate and improve the gas production potential of China's first hydrate production site, depressurization, and the combination of depressurization and hot water injection, were applied to natural gas hydrate (NGH) reservoir in China's first offshore hydrate production site to analyze 10 years' hydrate production features. The results indicated that the decomposition front of hydrate when using depressurization in two horizontal production wells had obvious non-uniform characteristics, and there were rapid decomposition regions and hydrate reformation regions. Among them, the hydrate reformation region may have adverse effects on hydrate decomposition. The combination of depressurization and hot water injection could improve total gas production and alleviate hydrate reformation. However, when the temperature of hot water injected increased from 35 °C to 80 °C, the energy efficiency decreased from 10.22 to 1.53, as a lot of energy was consumed to heat the sediments in the reservoir. When the hot water injection time was reduced from 3650 days to 1825 days, the energy efficiency could be effectively increased from 10.22 to 21.78 without significant reduction of the gas production. This work contributed to the present understanding of production behavior of depressurization and the combination of depressurization and hot water injection, and helped to evaluate the influence of hot water injected on gas production from NGH reservoir. • Rapid decomposition region and hydrate reformation region coexist near the decomposition front. • Increasing hot water injection temperature 35 °C–80 °C will reduce energy efficiency from 10.22 to 1.53 • The reduction of hot water injection time from 10 years to 5 years will not significantly reduce gas production. [ABSTRACT FROM AUTHOR]