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UNIT 4
DRILLING OF OIL AND GAS WELLS

There are three main types of conventional wells. The most
common well is an oil well with associated gas. Natural gas wells
are wells drilled specifically for natural gas, and contain little or no
oil. Condensate wells are wells that contain natural gas, as well as
a liquid condensate. This condensate is a liquid hydrocarbon
mixture that is often separated from the natural gas either at the
wellhead, or during the processing of the natural gas. Depending
on the type of well that is being drilled, completion may differ
slightly. It is important to remember that natural gas, being lighter
than air, will naturally rise to the surface of a well. Because of this,
in many natural gas and condensate wells, lifting equipment and
well treatment are not necessary, while for oil wells many types of
artificial lift might be installed, particularly as the reservoir
pressure declines during years of production.

Drilling
Now that we have found a promising geological structure
and acquired the petroleum rights, it is time to go ahead and drill.
Regardless of all the sophisticated geological and geophysical
mapping that has been done, the only way to find out if there is
any oil or gas present under your land is to drill a well (Figure
4.1).
Any well that does not find petroleum in commercial
quantities is called a dry hole; even if the well does, as in this case,
find water. The well that we are about to drill is called a wildcat
well. This means that we will be drilling into a ge ologic structure
in which no oil or gas has yet been discovered. We will be
attempting to discover a new oilfield.
If a discovery is made on this geologic structure, we will
need to drill more wells (delineation wells) to determine the size of
our oilfield. Drilling wells is an expensive and risky business. The
costs can run into millions of dollars and the chance of success for
wildcats is only about one in ten.

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