## 3 Design of the New Sand Control Screen
### Design Plan of the New Sand Screen  
According to the design theory of the base pipe and the protective pipe, combined with the distribution of gravel diameter and the plugging mechanisms of sand control in argillaceous fine silt hydrate reservoirs, the performance characteristics of different screens are analyzed comprehensively, and two new types of screens have been innovatively designed. Class A screens are screens with metal filter structures, and Class B screens are prefilled screens. The schematic diagram of the design structure of the Class A screen pipe is shown in Figure 2. It mainly comprises a base pipe, a metal precision filtration structure, a protection structure, etc. The overall structure has no welds, has relatively highstrength, and no interlayers, which can prevent sand plugging. The specific characteristics are as follows:  
1. Base pipe: The standard oil casing is selected as the central base pipe, with a number of multiple through-holes distributed on base pipes.
2. Metal filtration structure: The metal fiber structure of a preferred material between the inside and outside can be optimized and adjusted according to the characteristics of the reservoirs to form a variety of \"hole and crack\" structures, which could meet the accuracy requirements of sand control. It can also be adjusted for plug removal under the different pressures. The curved structural units are embedded on the surface of the central base pipe to increase the overall flow areas. There is a special protective sleeve on the outside to ensure that the screen has a certain deformation resistance ability and is not easily damaged.
3. Protection structure: With a novel protection structure, there are filter sand gaps on both sides of the arch bridge, which are axially staggered with the filter structure, reducing the direct contact between the precision filter structure and the argillaceous fine silt. Meanwhile, it also has the function of pressure deformation protection; thus, it can play a role in protecting the screen, enhancing the wear resistance of the screen and improving the screen service lifetime.  
As shown in Figure 3, the structure diagram of the Class B screen pipe mainly comprises a base pipe, a filtration structure and a special protection structure. By adopting a high-precision filling material, a new type of filtration structure and a special protection structure, a special porosity and permeability structure, it can provide a special pore permeability structure for the fine silt sand, and the double-layer filtration can realize the control of fine silt sand. It has the following characteristics:  
1. Base pipe: The selection of standard oil casing, with a number of multiple through-holes distributed on them.
2. Filtration structure: The internal and external double-layer bridge lateral slit structure can change the fluid flow direction of fluid. The outer layer, with the help of the arched protection structure, can prevent and control the formation of sand with large particle size, achieving the first level of sand control; the inner filter medium can prevent and control the formation of small sand particle, realizing the secondary level of sand control. The double-layer sand control is achieved for application to the hydrate reservoirs.
3. Protection structure: A specially designed protection structure. There are filter sand gaps on both sides, which can support the fixed filling. And an annular space gap is reserved between the base pipe and the protection structure to realize the internal annular flow of the screen and prevent the screen from being plugged.  
Figure 2: Schematic diagram of the design structure of Class A sand screen.  
After the main structure of the screen is determined, the detailed structures of the screen will be gradually studied, mainly including the structure, size, pore structure, parameters of the protection structure, etc. Finally, the experimental performance evaluation of the new type of screen will be carried out.