Page 111 - Drilling Technology in Nontechnical Language

102 Drilling Technology in Nontechnical Language Second Edition

Fig. 4–13. Calculating depths in a deviated well

Locating the casing point

With the exploration well described in the previous chapter, the casing
point was planned to be roughly at a certain depth. The casing was set
when a suitable formation was drilled into. In the development well, the
formations are reasonably well known and described. The decision as to
where the casing will be set can be made in the well design.

As drilling continues, the sequence of formations encountered
(identified by rock cuttings sampled at the surface) is compared to the

expected sequence and so the geological position of the bit is known. If
drilling is fast, it may be necessary to stop drilling and circulate bottoms
up for a sample. (Bottoms up is an expression referring to the time or
volume to circulate in order to bring mud at the bottom of the well up to
the surface. If circulating at 600 gal/min, and the annular volume is 24,000
gal, it will take 40 min to circulate bottoms up.) Otherwise a formation
might be completely drilled through before the cuttings can reach the
surface. Often a higher formation can be used as a kind of marker, if the

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