Page 26 - Drilling Technology in Nontechnical Language

Chapter 1 – DRILLING GEOLOGY 17

Fig. 1–10. A blowout preventer positioned on the well

Geological Input to Drilling Wells

When a well is being planned, the geologists will create a prediction
showing what lithological sequence is expected as the well is drilled. The
drilling engineers will use this and other information to design the well.
Figure 1–11 is a lithologic column for a planned well. The thick line
with bent ends shown at 1,165 m designates a fault; the wavy line at 1,690
m shows an unconformity. The geologist will get information from various
sources, mainly other wells drilled in the area and seismic surveys, but
also using the known geological history of the area. For a well drilled
in an unknown area, the predicted geology will involve much intelligent
guesswork. For a known oil field with many other wells drilled close by,

the prediction should be pretty accurate. Drillers are always skeptical of
geological predictions, but they are a necessary part of planning a well.

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