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Deviated drilling complicates the geologists’ work by distorting the logs,
compared with those of vertical holes, and by requiring correction of depths
measured along the borehole to true vertical depths. In the geological planning
of deviated boreholes, the execution of the plan should be borne in mind.
The tolerances of position -the “tunnel” within which the borehole should
be confined - should be as large as is compatible with the objectives of the
borehole. Course corrections are expensive, and an unnecessarily restricted
deviated hole will take longer to drill and cost more than a less restricted
one.
The measurement of deviation in a vertical hole is not normally a matter
that concerns the geologist greatly during drilling. The course of the well (for
it is most unlikely to be strictly vertical because that too costs money through
being too restrictive) will be known with sufficient precision on completion
from the directional survey data acquired during drilling and logging (from
the dipmeter, for instance). But in a deviated hole, the positions at different
depths along the borehole vary considerably, and may vary considerably
from those planned. Ultimately, the positions of significant parts of the bore-
hole must be known as precisely as the data allows.
It is important for the petroleum geologist to be familiar with the drilling
operations from which his basic data are acquired, and to learn the jargon of
those who drill the boreholes. He must understand the problems of getting
the data because he must make his own assessment of the value of the infor-
mation he seeks and the risks involved in getting it. He must also clearly
understand the limitations imposed on the data by the operations themselves.
A microfossil found in a sample bag labelled 2175 m, for example, is most
unlikely to have come from that depth because sample bags are usually labelled
with the drilling depth at the time the sample was taken from the shale shaker.
If it is the first occurrence of a species in that well, its probable depth depends
on drilling rate and mud-circulation rate in the annulus.
Above all, it must be remembered that drilling and the acquisition of geo-
logical data from the boreholes are but a part of a larger operation, and there
are limits on the time and money that can be spent on them.

REFERENCES

British Petroleum Company Limited, 1977. Our industry petroleum (5th ed.). The British
Petroleum Company Ltd., London, 600 pp.
Grames, L.R. and Reyner, R.R., 1967. Unique tilt rig drills shallow offshore wells. World
Oil, 165 (Nov.): 86-93.