Sand production is the process in which formation sand and gravel would migrate into the wellbore by the flow of reservoir fluids. This is a problem that endangers the safety of hydrate exploitation. The aim of this study is to understand sand production in during hydrate exploitation. A novel experimental apparatus was constructed to examine sand production in the hydrate-bearing sediments by applying the depressurization method. The maximum pressure of apparatus is 30 MPa, the temperature could be controlled from -253.15 to 313.15 K. The pore pressure, crustal stress, production pressure, gas flow rate, water production volume and subsidence rates could be measured. The vertical wellbore comprised liner 1 (ø32 mm × 135 mm, 3 mm holes, arrangement: 36°, holes row gap: 10 mm) and liner 3 (ø24 mm × 135 mm, 2.5 mm holes, arrangement: 36°, hole row gap: 10 mm) with a sand screen. Besides gas in from top, the sediment chamber (ø158 mm × 120 mm) had a middle mesh screen that allowed gas and water enter or vacuumize the chamber circumferentially around the sediments (ø158 mm × 100 mm). Sand production situation was recorded by HD camera through the visual window on the gas-solid-liquid separator. Hydrate production was divided into three periods: water, gas with water drops, and gas. We found fine sand production in the first period and sand grains production in the second. The sand production behaviours through the different gas production rates in two production method - depletion gas production curve and stable gas production curve were discussed. The grain sizes of sand production were increasing in the production period with hydrate saturation decreasing. The temperature related characteristics of the hydrate layers differed during different stages of hydrate production. The unique sputtering occurring owing to the decomposition of the hydrate might have provided the driving force for sand migration, and water gas bubbles or gaseous water drops from the decomposed hydrate might have enhanced sand carrying capacity. The subsidence of hydrate-bearing sediments was influenced by sand production, whereas the maintenance of crustal stress possibly influenced the rate and magnitude of subsidence. The stimulation at late production in low hydrate saturation might conduct the subsidence and sand production increased dramatically.

The Shenhu hydrate reservoirs of northern South China Sea are speculated to form and accumulate under the free gas-rich environment, and accord with the flow-transportation-reaction process. During this process, the area has also experienced simultaneous alternations of the local geological structures and ambient environmental conditions. First-order reaction kinetics, which controls the hydrate reaction, is coupled with the seafloor sedimentation to quest the dynamic formation and aggregation mechanism of the Shenhu hydrate layers. Firstly, based on the indications from Peclet number, the hydrate reaction shall play more active roles during the natural sedimentation in this process. Secondly, when the average seafloor sedimentation rate and the initial seafloor were 5 cm/ka and 988 m, respectively, three moments (50 ka, 3 Ma, and 5 Ma) are chosen to investigate the evolution process of the hydrate reservoirs and exhibit the change of the local pressures, temperatures, dissolved methane and salt, phase saturations, stratum permeability, and pore capillary pressure. The results show that after 5 Ma the occurrence and distribution of hydrate-bearing sediments proceed to be nearly consistent with the current status. Finally, the effects of five factors, including methane flux, kinetic coefficient, initial fluid position, permeability and seafloor sedimentation rate which simultaneously control the accumulation together in this zone, are quantitatively discussed. It verifies that the reaction-sedimentation mechanism is applicable for researching the Shenhu hydrate system. Under this model it should have a small methane flux and relatively small reaction coefficient here.