Physical characteristic analysis of unconsolidated sediments containing gas hydrate recovered from the Shenhu Area of the South China sea

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.