SELECTION OF CASING AND BIT SIZESThe design of the casing sizes used in drilling onshore and offshore wells is performed from the bottom to the top, starting with the production tubing.
The tubing size is designed to maintain a specific wellhead flowing pressure or allow the well to produce at a specified flowrate. The size of the production casing or liner is based on the size of the tubing. The production casing should have an inside diameter such that there is adequate radial clearance between the tubing and casing to allow for fishing the tubing during workover operations.
To enable the production casing to be placed in the well, the bit size used to drill the last interval of the well must be 0.375”-0.5” or preferably 0.75” larger than the OD of the production casing, see Table below.
|Casing and Bit Sizes table|
strings generally can be placed safely without getting casing stuck problems.
The selected drilling bits sizes should provide sufficient clearance between the borehole of the drilled well and the casing to allow for drilling mud cake on the borehole and for installing casing centralizers and scratchers.
Sufficient clearance is also necessary to prevent premature dehydration of the cement and the formation of cement bridges during cementing. The bit used to drill the hole for the production casing must fit inside the casing string above, see below Table.
This, in turn, determines the minimum size of the second deepest casing string. With similar considerations, the drilling bit size and casing size of successively shallower oil/gas well segments are selected.
Above Table shows casing IDs and drift diameters for various standard casing sizes and wall thicknesses. The pipe manufacturers assures that a bit smaller than the drift diameter will pass through every joint of casing bought. In most instances, bits larger than the drift diameter but smaller than the ID will also pass, but this is not good practice.
The most commonly used bit sizes are highlighted in above Tables. Selection of casing sizes that permit the use of commonly used bits is advantageous because the bit manufacturers make readily available a much larger variety of drilling bit types and features in these common sizes.