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drilling 10, 20 or more wells from a single platform. The attainable precision
of deviated drilling is illustrated by its other important application: the con-
trol of a well that has blown out and destroyed the rig or platform, or required
the rig to be removed. A deviated hole is then drilled from a safe position,
to penetrate the troublesome formation as close to the original well as pos-
sible. Water, mud and cement are then pumped from the one to the other.
The principles of deviated drilling are best understood by comparison with
those of drilling vertical holes. To drill a vertical hole, the number of drill
collars made up in the assembly is such that their weight in mud is greater
than the weight to be put on the bit. Thus, when drilling normally, the lower
part of the drill collar assembly is in compression, for which the collars are
designed, while the upper part and the drill pipe are in tension. The neutral
point is within the drill collars, and it moves down when the weight on the
bit is reduced, and up when it is increased. A crooked hole is straightened by
reducing the weight on the bit, gravity then helping as with the taut line in
cable tool drilling.
A deviated hole is started vertically, and drilled to the “kick-off” point*.
The bit is then pulled, and a whipstock run. The whipstock is essentially a
wedge that forces the bit to deviate about so from the line of the hole.
This initial deflexion is oriented in the desired direction. In areas of soft sedi-
ments, the whipstock has been replaced by a bit with one eccentric nozzle
through which the mud jet blasts a hole slightly to one side, giving the same
effect.
The whipstock is now pulled, and an ordinary drilling bit is run on a short
assembly, or even on drill pipe without drill collars. Applying a light weight,
the assembly bends, and the hole is deviated beyond the so of initial de-
flexion. The rate of deviation, or build-up, is controlled by controlling the
weight on the bit, and so controlling the bending. The assembly is progres-
sively lengthened to the normal length by the addition of drill collars (one of
which will be of non-magnetic monel metal for deviation surveys using a
compass) and, at intervals between the collars, stabilizers or reamers are in-
serted to hold the assembly centrally in the hole. The angle of deviation is
then controlled by the stabilizer spacing at the bottom of the assembly, the
length of the short drill collar inserted between the lowest stabilizer and the
bit (sometimes called a “stinger”), and the weight on the bit. The course is
controlled to some extent by the speed of rotation, but may also be controll-
ed if necessary by a primary deflexion tool - the whipstock, or the bit with
an eccentric nozzle.

* Belco Petroleum Corporation designed a “tilt rig” for developing shallow offshore reser-
voirs that could not otherwise be developed because the conventional method would have
a kick-off point within or below the reservoirs. Grames and Reyner (1967) report on the
use of these rigs for developing a shallow Peruvian offshore reservoir that lies at depths be-
tween 150 and 400 m.