# Sand Production Characteristics of Hydrate Reservoirs in the South China Sea
## 2 Research Status Worldwide
### Experimental Study of Natural Gas Hydrate Sand Production in the Sea Area  
Due to the problems of high cost, long periods and many uncontrollable factors in hydrate field tests, indoor experiments are the main means to study the mechanism of hydrate mining and obtain the related key parameters. Hydrate mining is accompanied by complex phase and temperature changes, and the deterioration of reservoir skeleton due to hydrate decomposition will further affect sand production. Therefore, the influence of hydrate decomposition on sand production should be considered in the relevant experimental studies. Early experiments on the sand emergence process of hydrate mining explored the effects of different temperatures, pressures, pressure drop amplitudes, pressure drop rates and other factors on the sand emergence process of hydrate mining. It proposed that the sand emergence mainly occurs during the pressure drop process and that the driving force for the sand emergence is the flow of water in the pore space rather than the decomposition of the produced gas. In addition, the flow rate of the water has been considered a key factor affecting the strength of the sand emergence [30]. A large-scale simulation system (72 L) was then used to study the effect of microparticles (mainly clay particles) in the pore space on the gas output of hydrate mining. It was pointed out that the direction of movement of microparticles in the pore space of the sediment is mainly controlled by the direction of the fluid flow, and whether or not the microparticles cause clogging of the flow channel is controlled by the geometrical characteristics. When the diameter of the deposit particles (D) is more than 2.4-6.4 times the diameter of the flowing microparticles (d), a single microparticle can flow freely in the pore space, while when the number of the flowing microparticles increases, clogging may occur even if D/d is close to 20 [31]. Through the construction of a visual hydrate mining experimental platform, mud and sand were directly observed in the formation, but no sand production was observed during the hydrate mining process. The amount of sand produced was only 0.012% of the total amount of sand in the experiment [32]. In the same one-dimensional filled gravel column permeability experiments, no large-scale sand production was observed, but fine sand intrusion into the gravel layer was observed after increasing the flow rate [33]. The sand emergence experiments under loose sand and dense sand conditions showed that loose sand mainly moved as a whole under the action of homogeneous flow, but the sand structure was relatively stable, while dense sand was more localised and prone to form large cavities [34; 35]. With the successful implementation of hydrate test mining in the Shenhu Sea area of the South China Sea, scholars have also carried out experimental studies on the sand release process of hydrate mining and explored the sand release characteristics of the muddy reservoir on the basis of the previous researchers. Among them, the study on the sand-blocking mechanism of a sieve tube and sand-blocking medium in the mud reservoir shows that the blocking process can be divided into three stages: the beginning of blocking, intensification and equilibrium [36]. The study of the sand release mechanism, sand release pattern and its influencing factors shows that hydrate decomposition will make the reservoir strength decrease greatly, and the leading edge of sand release in the production process is very close to the leading edge of hydrate decomposition, and the lower the saturation degree of hydrate, the lower the influence of its decomposition on the overall cohesive strength of the reservoir, and the more unlikely to produce sand [37]. The experiments of sand release under hydrate mining conditions were carried out for sandy and muddy non-metamorphic reservoirs, these experiments showed that the gas production rate and gas production pattern will affect the water-carrying capacity of the gas, thus affecting the amount of sand.