## 2 Optimization of Sand Control Methods in Hydrate Reservoirs
### Analysis of Sand Production and Plugging Mechanisms
\[\varepsilon_{p}=\sqrt{\frac{2}{3}(\varepsilon_{p1}^{2}+\varepsilon_{p2}^{2}+ \varepsilon_{p3}^{2})} \tag{2}\]  
where \(\varepsilon_{p}\) refers to the equivalent plastic strain, dimensionless; \(\varepsilon_{p1}^{2}\), \(\varepsilon_{p2}^{2}\), \(\varepsilon_{p3}^{2}\) refers to the plastic principal strain in three directions, dimensionless. When \(\varepsilon_{p}<1\%\), the reservoir does not produce sand; when \(1\%<\varepsilon_{p}<3\%\), the sand production in the reservoirs can be controlled; when \(\varepsilon_{p}>3\%\), there is destructive sand production of the reservoir. [PERSON] et al. [14] analyzed the stress of the formation sand at the decomposition front of the hydrate and the internal formation sand in the decomposition area, and established a calculation model for the critical flow velocity of the formation sand in the unconsolidated sediment. Initiation and migration conditions of sand at the front of hydrate decomposition are as follows:  
\[(F_{h}-F_{e})r_{s}\sin\theta\geq F_{g}r_{s}\cos\theta+2F_{f}r_{s} \tag{3}\]  
Conditions for the initiation and migration of the formation sand in the decomposition zone:  
\[(F_{d}-F_{1})r_{s}\geq\big{(}F_{g}+F_{e}\big{)}r_{s}+2F_{f}r_{s} \tag{4}\]  
where \(F_{g}\) is the floating weight of the sand, uniting in N; \(F_{h}\) is the interaction force of sand, unit in N; \(F_{h}\) is the driving force generated by the liquid flow, unit in N; \(F_{d}\) is the traction force of the water flow, unit in N; \(F_{e}\) is the cohesive force, unit in N; \(F_{f}\) is the friction between sand grains, unit in N; \(F_{l}\) is the lifting force of the fluid, unit in N; \(r_{s}\) is the radius of the formation sand, unit in m.  
[PERSON] et al. [15; 16] utilized the experimental device for comprehensive simulation and evaluation of the sand control effect of mechanical screens. According to the geological condition and exploitation situation of hydrate reservoirs, they observed the changes in the amount of sand passing, displacement differential pressure, flow rate and permeability under different screen and sand-blocking media installations. They also analyzed the sand plugging mechanisms and sand control feasibility of the different screens. The experiment observed that the plugging of the sand screen could be divided into three stages: the beginning of plugging, the intensification of plugging and the equilibrium of plugging, that is, the permeability begins to decrease slowly, decreases rapidly, then slows down gradually until it no longer changes to the stable stage. Based on the three stages, four plugging mechanisms were further revealed, including the gas-liquid sand-carrying flow in the reservoirs after hydrate decomposition, the internal bridge plugging of the sand-blocking medium of the screen, the sorting bridge plugging in the external space of the screen, and the formation of a high-permeability mud skin of the screen with high-argillaceous-content fine silt. Among them, for the argillaceous fine silt gas hydrate reservoirs, the formed high-permeability mud skin of the screen with high-argillaceous-content fine silt could play an auxiliary role in sand plugging. The fine silt with a high argillaceous content will form a stable mud skin layer on the surface of the sand screen. The mud skin layer could play an auxiliary role in preventing the formation sand from entering into the wellbore, thereby reducing the risk of sand plugging. On the other hand, due to the low seepage resistance characteristics of gas and water, the mud skin layer with a permeability of approximately 5 um\({}^{2}\) has a relatively low flow resistance under the conditions of gas and water flow, enabling it to flow relatively smoothly. This maintains the permeability of reservoirs, reduces the pressure loss of fluid flow, and improves the efficiency of hydrate production test.