Page 99 - Drilling Technology in Nontechnical Language

90 Drilling Technology in Nontechnical Language Second Edition

Hole and Casing Sizes

With a development well, the contingency to run an extra string of
casing if unexpected hole problems are encountered is less important
than on an exploration well. If a well can be drilled one size smaller (for
instance, 20" conductor instead of 30"), generally it will be cheaper to
drill the well by around 15%. There is a strong incentive to avoid building
in unnecessary contingencies against remote possibilities when the cost
penalty is so large.

The hole size of the final section is dictated by the required size of
the completion tubing and also by the type of completion. In a traditional
completion, the completion tubing extends all the way down to the
reservoir, inside the production casing or liner. A completion can also be
designed using a liner as part of the completion tubing. One advantage is
that the well can be drilled using smaller hole sizes and thus save a lot of
money. If the production of the well requires 5" diameter tubing, the well

could be finished by drilling a 6" hole for a 5" liner, rather than by drilling

8½" hole for a 7" liner. This is illustrated in figure 4–5.

Fig. 4–5. Traditional vs. monobore completion

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