Concept of the Minimum Cost Combination Casing String
The casing program of most oil wells represents the greatest single item of expense in well cost. It can be as much as 18% of the completed well cost. Therefore, even a small reduction in casing cost can save a considerable amount of money. This objective has traditionally been achieved by initially minimizing the number and length of strings and then by designing a combination casing string.
In vertical wells, optimumizing a combination casing string has been a challenge for casing designers. The optimization principle is based on considering the possibility of several combinations of grade, weight, thread and smallest allowable section length that satisfy some predetermined external load condition. Eventually, a combination casing string is selected that allows the minimum total cost. Insofar as there are a very large number of combinations, several stepwise procedures have been developed for casing grade, weight, and thread selection without explicit cost expressions. Generally it is observed, when following these procedures, that the casing price increases with increasing rasing grade, weight, and strength (burst, collapse, pipe body yield, and connection). Thus, the lowest grade and weight casing, with the lowest possible values of mechanical strength, should give the lowest cost. Unfortunately, this procedure does not always yield the minimum cost simply because the casing grade, weight and cost cannot be simultaneously minimized.