## 2 Optimization of Sand Control Methods in Hydrate Reservoirs  
### Analysis of Sand Production and Plugging Mechanisms  
Sand production is mainly affected by two factors. The first is its formation properties, including geologic formation lithological characteristics, sedimentary compaction, etc. The second is its production factors, as different testing conditions will have different effects on the formation of rocks and fluids, and sand control medium or wellbore plugging, which affect continuous production, can easily occur. The sand production mechanism is different under different reservoir geological conditions. The sand production of unconsolidated sandstone reservoirs--among which the sand production of weak consolidated or medium consolidated sandstone layers is most significant--is caused by the destruction of the rock near the bottom hole, mainly because of the poor consolidation and low strength of the rock, which mean the destruction of the bottom well and the formation of sand can easily occur under the high-pressure conditions of production [6; 7; 8]. In addition, different pressure changes affect hydrate fluid production, and the fluid flow will then influence formation sand production, as different pressure changes will lead to the formation of sand. The gas hydrate reservoirs in the South China Sea mainly produce argillaceous fine silt, and the reservoirs generally produce particles that are finer than those of general hydrate reservoirs; these sediment particles migrate more easily during the testing process. In the decomposition of hydrates, there are complex gas-liquid sand-carrying flows; the medium and mechanisms of sand control are not clear for argillaceous fine silt, and there are some problems, such as the difficulty of sand control. Sand production of argillaceous fine silt hydrate reservoirs is a complex dynamic process related to the migration of fluid and solid particles, so it is extremely difficult to reveal the sand extraction mechanisms, and to study the sand production law, to take sand control measures. [PERSON] et al. [9; 10; 11] proposed for the first time a mathematical model of sand detachment suitable for hydrate formation based on the sand extraction model established by [PERSON] and [PERSON]; the sand production mechanical behavior of hydrate reservoirs was described by the erosion mechanics criterion:  
\[dm_{ssi}=-m_{ssi}\omega_{2}M^{dtc}\times\left[\frac{s_{w}}{s_{w}+s_{g}}H\left( \frac{i_{w}}{i_{w}^{cri}}-1\right)+\frac{s_{g}}{s_{w}+s_{g}}H\left(\frac{i_{g}}{ i_{g}^{cri}}-1\right)\right]dt \tag{1}\]  
where \(m_{ssi}\) refers to the unit volume mass of solid sand, kg/m\({}^{3}\); \(\omega_{2}\) refers to the control parameter of sand detachment velocity, unit \(h^{-1}\); \(M^{dtc}\) refers to the potential energy of sand detachment, dimensionless; \(s_{w}\) and \(s_{g}\) refer to the saturation of water and gas fluids, respectively, dimensionless; \(i_{w}\) and \(i_{g}\) are the hydraulic gradients of water and gas fluids, respectively, dimensionless; \(i_{w}^{cri}\) and \(i_{g}^{cri}\) refer to the critical hydraulic gradient of sand detachment caused by water and gas, respectively, dimensionless; \(H(x)\) is the unit step function (\(H(x)=0\), when \(x\leq 0\), and \(H(x)=1\), when \(x>0\)). The critical hydraulic gradient was used to characterize the driving force of sand production for sand separation judgment, and the critical hydraulic gradient was correlated with the hydrate saturation. [PERSON] et al. [12; 13] systematically simulated and analyzed the influence laws of sensitive parameters such as the gas hydrate decomposition effect, differential production pressure, bottom-hole temperature, and the reservoir permeability of sand control production. It is indicated that the stress concentration caused by the wellbore effect and the formation strength reduction resulting from the hydrate decomposition effect were the fundamental reasons for sand production.