Page 87 - Drilling Technology in Nontechnical Language

78 Drilling Technology in Nontechnical Language Second Edition

This is the part of the hole that everybody is really interested in. If
hydrocarbons are present, a lot of wireline logs are run over several days. If
the wireline logs indicate that hydrocarbons are likely to be in commercial
quantities and can be produced, a liner will be cemented in place. Holes
are blown through the side of the liner to allow the hydrocarbons to be
produced during a test.

A liner is essentially a string of casing that does not extend all the
way to the surface. A liner is suspended from a liner hanger, which uses
hardened steel teeth to dig into the last casing ID and so suspend the liner.
The advantages of running a liner include reduced cost (a much shorter
length of casing pipe, less cement needed). The disadvantages include
increased complexity because of the tools that must be manipulated while
a long way inside the well.
While drilling through the reservoir section, there are indications on
the surface of hydrocarbon presence. When drilling through a gas-bearing
zone, increased levels of gas dissolved in the drilling mud can be detected.
When drilling through oil-bearing rock, the rock cuttings will show the
presence of oil in the pore spaces.
At this point, it is very important to gain the maximum amount of
information from the reservoir. The best way to assure this is to take a
core sample while drilling through the reservoir. Often on an exploration

well, the coring equipment will stand by on the rig, and on the first good
indication of hydrocarbon presence, the drill bit will be pulled out and the
drilling BHA replaced with a coring assembly.

A core is taken by drilling with a special bit (called a core bit) that has
a hole in the middle. As the bit drills a doughnut-shaped hole, a column of
uncut rock will stick up inside the bit. Behind the bit is a special mechanism
for gripping this rock and holding it in a special container.
Coring is slow and expensive, but the value of the information usually
makes this worthwhile because it allows better decisions in the short term
(designing the well test) and in the long term (should the reservoir be
developed, and how?).
Once the reservoir is cored (which may take more than one core bit run
to achieve), the driller will run in with a normal drill bit, ream through the
cored section, continue drilling to total depth (TD), and complete the well
(fig. 3–23).

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