Page 122 - Petroleum Geology

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While rotary drilling was a great advance in the engineering aspects of the
petroleum industry, it did not benefit the geologist in his study of the rocks
penetrated by the drill. Cuttings there are, but they have been so abused by
the time they reach the surface that they may be almost useless. When a
more substantial sample of the subsurface rock is required, cores are obtain-
ed either by conventional coring, or by taking samples from the wall of the
borehole. Conventional coring is done with a special bit and core barrel. The
bit drills an annulus, leaving a core of rock to pass into the core barrel. When
the length of the core barrel (less a little) has been drilled, the core is broken
off by fast rotation of the bit. The broken-off core should then be retained
in the barrel by spring retainers. The core assembly is pulled slowly and care-
fully. Each stand of pipe (3 X 30 ft, joints of drill pipe about 27 m in all) is
unscrewed not by spinning the string in the hole with the rotary table as nor-
mal, but by unscrewing the stand above the slips. The expense of a core in
terms of rig time alone requires justification for the core sample; and its ex-
pense demands careful treatment of it. The core is extracted from the barrel
not by hanging it over the core boxes and hammering, but by taking the
barrel off the drill floor and extracting it with a hydraulic pump in a horizontal
position. Government regulations may require a fixed or minimum core pro-
gramme.
The penetration rate while drilling is of general interest. The action of the
bit on the rock at the bottom of the hole is a matter of mechanical engineer-
ing, rock mechanics and geological engineering. In the final analysis, what
matters is not so much the length of hole drilled per bit, or the rate of drill-
ing per bit, but the overall performance of the operation with its associated
down-time for round trips, reaming an under-gauge hole, and other delays.
From the geologists’ point of view, optimization of the penetration rate,
which is a parameter in the overall economy, is concerned largely with the
optimization of the destruction of the rock and the removal of the cuttings.
The main parameters of the penetration rate are: (1) bit tooth geometry;
(2) weight on bit; (3) rotation rate; (4) hydraulic energy and properties of
the mud in circulation; (5) the fluid potential gradient across the bottom of
the borehole; and (6) the “drillability” or competence of the rock at the
bottom of the hole under the stresses existing in it and imposed upon it. If
the first four are kept constant, the penetration rate reflects changes in the
last two parameters. Geological use can be made of a penetration-rate log.
The elimination of formation fluids from the borehole is one of the essen-
tial features of rotary drilling; but it also denies the geologist the insight he
used to receive from the variations in the static water table. He is no longer
aware of the hydraulic properties of permeable formations, nor of their
capacity to yield water. Formation fluids only enter the borehole if their
energy is sufficient to displace the mud column. Although the petroleum
geologist is no longer in touch, as it were, with the static water table, the
geology of interstitial fluids in the subsurface is one of his major concerns.