Page 121 - Petroleum Geology

could accelerate the flow by reducing the mean density of the fluids in the
borehole. Gas without oil, unless contained by a column of mud, would tend
to pass through the mud and, by expansion of the bubbles, tend to empty the
hole of liquids. “Gushers” may have been the delight of drillers, but they
were wasteful both of natural resources and of reservoir energy.
Cable-tool rigs are still used extensively in the world for water drilling; and
they remained competitive in some areas for oil drilling, notably in Pennsyl-
vania (where it all started) until the 1950s. But for petroleum drilling, they
have been replaced by the rotary rigs.

ROTARY DRILLING

With the introduction of rotary drilling early in this century, much of the
terminology and jargon of cable-tool drilling - such as crown block, drilling
line, sand line, cuttings, fishing, striking oil or gas - were passed on, and
many of the principles were consciously or unconsciously adopted. While the
overall performance of rotary drilling exceeded that of cable tool, the advan-
tages were not in every respect. The quality of direct geological data tended
to diminish, and the petroleum geologist tended to lose touch with the rocks
themselves. These losses were to be compensated for to an important extent
by the introduction and development of electrical logging.
The process of rotary drilling is too well known to need much description
here. The drilling string is made up of an assembly of a bit, drill collars (with
reamers or stabilizers), and then drill pipe to the surface. Into the drill pipe
below the drill floor is screwed the kelly - a pipe of square or hexagonal
cross-section - by which the rotary motion is applied to the string from the
rotary table. Mud is circulated from the suction tank through the pumps to
the swivel on top of the kelly; from there down the inside of the drill pipe to
the bit; then up the annulus to the shale-shaker at the surface, and so back to
the suction tank after passing through settling tanks in whicK much of the
solids settles out.
At the bottom of the hole, the mud serves various purposes: it cools the
bit, it assists the drilling process not only by removing the cuttings and keep-
ing the bit clean, but also by actively scouring the bottom of the hole by
virtue of nozzles in the bit that accelerate the mud into jets. The hydraulic
energy of mud accelerated to a velocity of 100 m/s or more makes a signif-
icant contribution to the penetration rate and general bit performance.
The drilling mud has another essential function. Its density (“weight” in
the jargon) can be adjusted so that the pressure it exerts at the bottom of the
hole is greater than that exerted by the formation fluids. It thus excludes
formation fluids from the borehole. Indeed, excessive mud weight can flush
petroleum from the immediate vicinity of the borehole in permeable forma-
tions so that it is not detected in subsequent logging operations.