Page 117 - Petroleum Geology

CHAPTER 5

DRILLING

INTRODUCTION
Those who use and interpret data must know how the data were acquired.
This is particularly true for petroleum geologists because drilling operations
can affect the nature of the data acquired from them. We shall begin with a
short review of cable-tool drilling because it illustrates some of the principles,
but it is unlikely that the reader will ever be concerned with data from wells
drilled by cable tool.

CABLE-TOOL DRILLING

The young oil industry did not have to invent drilling: it simply adopted
and adapted the equipment and techniques of the water-well drillers. The
petroleum industry owes even more than that to the water-well drillers be-
cause it was the accidental occurrence of oil and gas in water and brine wells
that encouraged the early attempts at drilling specifically for oil. Oil had
been found in Pennsylvania, U.S.A., in a borehole drilled for salt, 40 years
before Drake’s famous well of 1859.
Holes were drilled in the ground by the cable tool, or percussion, method.
The essential mechanism is shown in Fig. 5-1. The bottom of the hole is struck
by a bit suspended on a cable. The cyclic motion at the surface is converted
to vertical reciprocal motion, and the repeated blows cut and break the rock.
By feeding the cable from the drum by means of the brake, the hole is deepen-
ed; and the drum can be connected to the motor for pulling the bit for dress-
ing (sharpening, and restoring it to its proper diameter).
To drill a straight and round hole, it is essential that the bit rotates. This is
accomplished by the lay of the rope, and a rope socket connection to the
tools that allows the rope to rotate freely when slack. When lifting the tools
off bottom, the weight stretches the rope, imparting a torque through the
lay. This turns the bit slightly. On impact with the bottom of the hole, the
rope is momentarily slack, and twists back to its natural lay, removing the
torque. Lifting again imparts a slight rotation to the bit, so that on impact
it is not quite in the same orientation as before - and so on. Modem ropes
are of flexible steel wire with a left-hand lay, which imparts a clockwise rota-
tion to the bit when viewed from above. Drilling not only dulls the bit, but
also wears the shoulders so that it becomes under-gauge and drills a slightly