The material and type of casing pipe should be selected according to the drilling structure design and engineering needs. Deep-hole core drilling has a long construction period and a large casing depth. The drill pipe string will hit and wear the casing seriously during drilling. If there is a problem with the casing string in the hole, it will cause serious accidents in the hole, or even scrap Therefore, the quality of the drilled casing string must meet the following requirements.
1. Material requirements
The material of seamless steel pipe
is generally selected according to parameters such as casing depth and quality.
When choosing the casing material, the quality of the casing string should also be considered. When the casing quality is less than 10t, ordinary low carbon steel grade can be selected; for 10-20t, steel grade DZ40～DZ50 should be selected; for ≥20t, steel grade above DZ50 should be selected. In addition, the material of the casing joint should be one level higher than the steel grade requirement of the casing body.
2. Casing bending requirements
The curvature of the casing means that the casing is curved in the length direction, expressed by the measured arc height of the maximum bend of the casing, divided into local curvature (mm/m) and full length curvature (%). The general requirements for deep hole drilling are: casing diameter ≤ Φ89mm, partial curvature should be within the range of 1mm/1.5m; casing diameter> Φ89mm, local curvature should be within the range of 1mm/1m.
3. Requirement for casing ovality and wall thickness unevenness
The difference between the maximum outside diameter and the minimum outside diameter of the casing cross section is called the ellipticity (or out of roundness). Generally, it is stipulated that the ovality of the casing does not exceed 80% of the outside diameter tolerance. The diameter of the casing is less than or equal to Φ89mm, and the ovality is controlled within 0.5mm; the diameter of the casing is more than Φ89mm, and the ovality is controlled within 1.0mm.
The unequal wall thickness of the casing cross section and the longitudinal pipe body is called wall thickness unevenness. It is generally stipulated that no more than 80% of the pipe wall thickness tolerance. The diameter of the casing is less than or equal to Φ89mm, and the unevenness of the wall thickness is controlled within 0.3mm; the diameter of the casing is> Φ89mm, and the unevenness of the wall thickness is controlled within 0.5mm.
4. Casing connection method and processing quality requirements
1) There are three ways of casing connection: direct connection, flat joint connection and coupling connection.
Under normal circumstances, when the casing mass is less than 10t, the direct connection or flat joint connection is adopted; when the casing mass is more than 10t, the coupling connection mode is adopted.
2) Casing joints and coupling materials for deep holes should be heat treated to increase their strength.
3) The thread center lines at both ends of the casing joint should be coincident, and the coaxial deviation should not be greater than 0.25mm at either end.
4) Casing, joint, and coupling thread standards mainly include geological core drilling standards and petroleum drilling standards. Petroleum pipe thread standards are often implemented in deep hole and large diameter geological core drilling.
5) The machining accuracy of casing, joint, and coupling thread must be strictly in accordance with the corresponding specifications.
6) After the casing, joints, and couplings are processed and formed, the through-hole inspection shall be carried out with a standard inner diameter gauge.