Page 112 - Drilling Technology in Nontechnical Language

Chapter 4 – PLANNING AND DRILLING A DEVELOPMENT WELL OFFSHORE 103

distance from this formation to one of interest is accurately known. There
are sometimes more immediate signs; a sudden change in the speed that
the bit drills indicates a change of formation drillability. (Drillability refers
to how easy a formation is to drill and is closely related to the compressive
strength of the formation. A lower rock compressive strength gives a higher
drillability. However, there are exceptions. A PDC bit might go from a
high-strength formation where it drills fairly fast because the formation is
suitable for PDC drilling to a formation of lower compressive strength that
does not favor PDC drilling.) In this case, drilling speed might decrease
rather than increase, which could help identify the next formation in the
geological sequence. The change is recognized straight away and may
then be confirmed by stopping to circulate bottoms up for a sample of

the formation.
Once the casing point is reached, drilling stops, but circulation
continues until the cuttings remaining in the annulus are out and the
wellbore is clean. The time to circulate the well clean is greater than the
time for bottoms up, because the cuttings in the annulus will fall through
the rising column of mud so that they rise slower than the mud itself.
Circulating until all the cuttings are out is called circulating clean.

Logging

On most hole sections of most wells (except in development projects
where very many wells have already been drilled), logs will be run.
Different specialists need information from downhole logs. The geologists

want to confirm the properties of the formations penetrated and any fluids
within the formations. The drillers want to measure the diameter of the
hole, using a tool (called a four-arm caliper) that measures using two pairs

of arms so that the hole size is measured in two places (fig. 4–14). This
allows accurate calculations of cement volumes. It also indicates how
stable the wellbore is and if the wellbore is more stable in one direction
than the other.
The drilling engineers also want to improve drilling performance on
the next well, and it is very useful to measure rock mechanical properties,
such as compressive strength. Compressive strength can be calculated by
sending a sound wave into a formation and placing a microphone some
distance away along the wellbore. The properties of the sound wave are

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