Page 125 - Petroleum Geology

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in areas without experience for guidance. The protective string must be set
well into consolidated sediments, preferably landed with the shoe in an im-
permeable bed, such as a mudstone. Boreholes are logged before running each
string of casing (except the surface string, usually), and the precise depth for
the casing shoe is chosen after examination of the log. Geological considera-
tions are not the only ones. Drilling engineering aspects, such as the amount
of open hole (uncased) that it is considered safe to carry, are also important.
And in many areas, government regulations determine the depths of strings
other than the production string.
Depth measurements are made from the level of the rotary table, the height
of which above the surface of the ground and the survey datum level is deter-
mined. All depths for geological use are converted to depths below datum.
On completion of the well, the elevation of the rotary table (or derrick floor,
D.F., or kelly bushing, K.B.) is recorded with reference to the top of a casing
flange, so that depth measurements in service and work-over operations can
be related to the driller’s depths, recorded in the drilling reports.

Deviated or directional drilling

The efficient development of a petroleum reservoir sometimes requires
drainage points that are vertically beneath sites that are impossible to drill
from, or unpractical due to expense. Such points are reached by drilling bore-
holes that are intentionally deviated from the vertical below a practical drill-
ing site (Fig. 5-5). These skills, which have grown with experience since the
1930s, are now such that it is normal practice to develop offshore fields by

Fig. 5-5. Deviated boreholes drilled from a marine platform. T.D. = total depth, measured
along the borehole from the rotary table or derrick floor. More than 30 wells can be drilled
from a platform.