Page 49 - Drilling Technology in Nontechnical Language

40 Drilling Technology in Nontechnical Language Second Edition

line would break, pressure would drop to zero, and the valve would close.
During the Gulf War in 1991, the Iraqi army used explosives to blow
wellheads off wells in the desert. As these wells were not equipped with
SSSVs, they then flowed oil to the surface uncontrollably until specialist

“wild well control” experts were able to stop them weeks or months later.
In an exploration well, the completion is normally quite a simple

configuration used to test the performance of the well with a production
test or well test. This type of completion is called a test string.

Casing design

Casings in a well are designed from the bottom up. Starting at the total

depth of the well, the drilling engineer first has to decide what size the
hole needs to be at that depth. This will be determined by the completion
design. For instance, if the completion or test string design has a maximum
outside diameter of 5" (127 mm), the smallest standard-sized pipe that this
can run inside is a 7" (178 mm) outside diameter casing. Normally, a hole
of 8½" (216 mm) diameter would be drilled to accommodate a 7" (178 mm)
casing. Thus, the size of the hole to be drilled at the bottom of the well is
determined by the requirements of the completion tubing.

Given the fluid pressures in the rocks at the bottom of the well and
considering the strengths of the rocks higher up, it is determined where the
next highest casing string has to be cemented in place in order to contain
pressures from drilling the well. For drilling an 8½" (216 mm) diameter
hole, the next standard size of casing that this size of drill bit will fit inside

is 9⅝" (244 mm) outside diameter. This would normally be placed inside a
12¼" (311 mm) diameter hole.
Similarly, the depths and sizes of all of the casings in the well are
designed by the drilling engineers.
Casings and their connections must withstand the forces imposed by
the weight of the casing. Also additional forces are imposed by the act of
lowering the pipe into the hole (such as bending the pipe through bends
in the well path). It is also necessary to account for internal pressures and
high temperatures.

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