Study on the mechanism of gravel layer blockage in sand control for natural gas hydrate reservoirs

Based on the formation sand diameter parameters in the Shenhu sea area of the South China Sea, the Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) coupling is used to simulate the blockage of the gravel layer under the complex conditions of gas-liquid-solid three-phase, and the feasibility of the numerical simulation is verified by the sand control simulation test device. The sand control process of gravel filling layer is analyzed from the microscopic point of view, and the influence of different underground physical properties on the blockage of gravel layer is analyzed. The results show that: (1) The blockage of gravel layer can be divided into three stages: initial stage, sand bridging accumulation stage and equilibrium stage. The large particle size sand plays an important role in the surface blockage and flow field flow of the gravel layer, and the small particle size sand mainly affects the internal blockage of the gravel layer. (2) Sand particles larger than the median particle size of the reservoir are easy to form blockage in the area near the surface of the gravel layer. Sand particles smaller than the median particle size of the reservoir will invade the interior of the gravel layer and cause sand invasion, forming a sand production risk. According to the median grain size, it is necessary to select appropriate gravel to reduce the risk of sand control. (3) The porosity and permeability in the blocked area are positively correlated. The closer to the surface of the gravel layer, the weaker the permeability is until it reaches stability. The closer to the surface of the gravel layer, the faster the gravel layer forms a stable blockage. (4) Because the sand carrying capacity of gas phase is weaker than that of liquid phase, the higher the gas-liquid ratio of reservoir, the slower the formation of stable blockage and the lower the risk of sand production.There is a critical value of the production flow rate. When the flow rate is less than the critical value, the time to form a stable blockage is slower and the risk of sand production increases. When the flow rate is higher than the critical flow rate, the difficulty of sand bridging increases and the risk of sand production increases. The greater the flow rate, the smaller the gas phase permeability ratio after blockage balance; the greater the reservoir heterogeneity coefficient, the higher the gas phase permeability ratio, and the slower the sand particles form a stable blockage. However, when the reservoir heterogeneity coefficient is greater than 7, the risk of sand production is extremely high. It should be considered to enhance the sand retaining performance of the gravel layer or to select the sand control screen with high-precision sand control design principles. This study provides a strong basis for sand control in field gravel filling layer completion.