Casing types of geological core drilling can be divided into steel pipe, cast iron pipe and plastic fiber pipe according to materials; According to the forming process, it can be divided into seamless steel pipe, seam welded steel pipe and spiral welded steel pipe. According to the function, it can be divided into: wall protection pipe, technical pipe, production pipe (oil pipe, filter pipe) and so on. The casing type is shown in Figure 3- 1.

(2) Selection of casing string

There are many types of casing used in drilling, which should be selected in consideration of function, drilling safety and casing throwing cost. See table 3- 1 for the selection and application scope of different types of casing.

Figure 3- 1 Common casing types

(a) cast iron pipes; Plastic fiber tube; Cement pipe; Seamless steel pipe; (e) Welded pipes with joints; Spiral tube; (g) filter tube

Table 3- 1 Recommended list of application scope of casing strings of different types and materials

Note: "*" in the table is mainly used for tubing for hydrological wells, "√" means applicable tubing, and "△" means high-strength seamless steel pipe.

In drilling construction, the principle of selecting casing type and material is:

1) Plastic fiber pipes (or PVC pipes), cast iron pipes and straight welded pipes have low strength, which are mostly used in shallow holes and water wells, and are not suitable for drilling wall casing.

2) Spiral welded pipes are generally used for deep hole drilling orifice pipes and 1000m shallow wall-protecting casing, and cement should be injected to reinforce the periphery of the casing.

3) Low-grade seamless steel pipes (20 # ~ 40 #) should be selected for drilling holes with hole depth 1000 ~ 2000m, and high-grade seamless steel pipes (general steel grade is above DZ40) should be selected for drilling holes with depth of 2000m.

4) When running multi-layer casing, in order to save casing cost, different strength types of casing can be selected according to different casing running depths. Low-grade steel pipes are selected for shallow holes, medium-grade steel pipes are selected for medium-deep holes, and high-grade steel pipes are selected for deep holes.

5) Casing (including movable casing), process casing and tail pipe running casing shall be made of seamless steel pipe with higher grade.

6) High-grade seamless steel pipes should be selected for special engineering drilling and long-term monitoring holes.