The selection of casing size is determined from the inside outward and it starts from the bottom hole.

It’s based on the following steps:

1, Sizing of tubing is determined by inflow performance analysis.

2, Equipment is planned to install with tubing string. The biggest outer diameter will effect production casing.

3, Bit size for drilling the production section.

4, Casing size must be smaller than bit size and its must be bigger than the biggest component in completion string.

5, Once you get the last casing string, the upper string is selected by repeating bit selection and casing selection similar to step3 and step4.

In addition, you may need to use the following table to determine bit size and casing size.