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• The long-term joint production from hydrate and shallow gas layer provides a 4.1 times enhancement of the cumulative gas yield. • The regulated layer-dependent scheme effectively alleviated the interbedded pressure difference by ∼70%, avoiding the interlayer instability. • An energy return on investment (EROI) analysis was carried out to evaluated the optimized joint production method. Recent field tests to recover natural gas from marine gas hydrate reservoirs in Japan and China have exhibited worldwide attention to this strategic energy form; significant challenges still remain in improving the accumulative gas yield for a better economic efficiency. According to the geological survey in marine hydrate reservoirs, there exists a concomitant free gas layer underlying the gas hydrate reservoir. Consequently, here we propose a new scheme to jointly produce the gases from hydrate layer and its underlying shallow gas layer. It was found that a dual horizontal well deployment respectively in the corresponding two layers could contribute to a 4.1 times cumulative gas yield comparing that of a single vertical well scenario. This implies a remarkable enhancement of gas productivity via making full use of the gases in the shallow gas layer. Notably, a potential risk of interlayer failure could occur upon the great interbedded pressure difference (maximum 10.7 MPa) arising from the varying behavior of pressure propagation in the layers. This can be effectively alleviated by separately controlling the depressurization scheme in different layers; a mild step-wise depressurization was suggested in the more permeable shallow gas layer while the hydrate layer with a lower permeability should experience a sharper pressure drop. This was found beneficial reducing the interbedded pressure difference by about 70% without intervening the gas production. An energy return on investment (EROI) analysis showed a promising positive energy harvest of our optimized layer-dependent pressure scheme. It could be therefore a potential method to be applied in the field tests in the South China Sea to improve the economic efficiency without disturbing the reservoir stability. [ABSTRACT FROM AUTHOR]

Gas hydrate exploitation may trigger the destruction of the overburden structure and settlement of the overburden layer on the gas hydrate-bearing sediments. It is extremely urgent to explore the basic laws governing the physical and mechanical properties of the overburden layers before performing commercial exploitations of hydrates. However, relatively few studies have focused on the physical and mechanical properties of the overburden layer (uppermost ~10 m) on the gas hydrate-bearing sediments of the South China Sea. In this research, we studied the physical and mechanical properties of the overburden sediments from this area, including the particle size distribution, the specific surface area, the pore size distribution, the water content, the liquid/plastic limits, and the mineral and chemical compositions of the particles, as well as the strength and deformation characteristics. According to the results, we found that the overburden sediments of the Shenhu area are characterized as well-graded clayey silts and considered to be internally stable and self-filtering. The isothermal adsorption-desorption curve is a Type H3 hysteretic loop, indicating that the shape of the particles occurs primarily in a plate form, and most of the pores are slit-shaped. The plasticity index in this area is relatively lower than that in other areas, which suggests that there is a risk of overburden collapse during the production process of hydrates. The mineral composition of the overburden sediment particles is primarily quartz, plagioclase, calcite and some accessory clay minerals, which can provide guidance for remolding the overburden sediments in the laboratory. The sediments show strain hardening and shear contraction behavior upon shearing; in addition, the established strength criterion will contribute to the formulation of constitutive equations for the overburden sediments, which can be used as essential input parameters in gas hydrate production simulations. • The Shenhu overburden sediments are in a plate form and well-graded clayey silts. • The sediments have a medium plasticity and medium compressibility. • The sediments show strain hardening and shear contraction behavior. • The strength, modulus and volumetric strain increase with the confining pressure. [ABSTRACT FROM AUTHOR]

Laboratory research based on the physical characteristics of unconsolidated sediments containing gas hydrate was carried out on hydrate-bearing sediment samples recovered from China National Offshore Oil Corporation (CNOOC) drilling expeditions in 2015 and 2017 at the station of LW3 in the northern part of the South China Sea. We herein have measured the gas composition, index properties, surface characteristics, thermal properties, and mechanical properties of these sediment cores drilled at different seafloor depths. Based on the measurement results, more than 99% of the gas composition is methane gas. The hydrate-bearing sediments recovered from Shenhu Area are non-diagenetic rock, the fraction of grain size which less than 40 μm was more than 90%. The results could provide useful guidance for potential sand-control system operations. Such small and consolidated particles indicated that the application of a novel solid fluidization mining method is viable and promising. The fine-grained layers were found to be mostly self-filtering and internally stable for the seepage flow during production. Gas hydrate mostly occurs in vein-type, and bioclastics consisting of marine planktonic diatoms and foraminifera were also found. The mineral components of the sediments were mostly quartz, clay minerals like illite and some feldspar like albite. The thermal conductivities changed from 1.4 to 1.77 W/(mK), which suggests that the isolated depressurization methods could exacerbate the problem of gas hydrate reformation or ice generation; thus, heat injection in wellbores should be combined for gas hydrate production strategies. The low liquid limit (38%) and plastic limit (24%) imply that stability of the production platform should be taken into account for avoiding submarine landslide impacts. The results of mechanical property analyses indicated that hydrate-bearing sediments at deeper sites have a relatively higher strength at hydrate layer of 120–133 mbsf in the Shenhu Area of the South China Sea. • Physical properties of HBS recovered from the Shenhu area in the South China Sea were analyzed using different methods. • The comparison of characteristics of gas hydrate samples with other various hydrate regions was summarized. • The guidance of this hydrate sediments properties for future gas hydrate exploitation was discussed. [ABSTRACT FROM AUTHOR]