Page 45 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 33

2.1.3 Drilling Fluid Separations

During the drilling process, a large volume of cuttings are generated
and carried out of the well by the drilling fluid. These cuttings must
be separated from the mud liquid so the liquid can be reinjected into
the drill string to remove more cuttings. Cuttings contaminated with
drilling mud are a major source of petroleum industry waste. The
potential environmental impact of such cuttings can be significantly
reduced by separating the solid cuttings from the more toxic mud,
The effectiveness of separating cuttings from the mud depends
primarily on the cuttings size. Separations can be enhanced if the
cuttings size is kept as large as possible. Cuttings size depends on a
number of factors. The most important factor in keeping cuttings size
large is to generate large cuttings at the bit during drilling. The initial
cuttings size is controlled by the bit type, the weight on bit, and the
formation type. A second factor in controlling the cuttings size is to
minimize additional grinding of the cuttings in the well as they are
lifted to the surface. Cuttings removal is controlled by the hydraulic
design of the bit jets, the mud viscosity, the mud velocity, the well
depth, the rotational speed of the drill string, and the mechanical
strength of the cuttings. A third factor controlling cuttings size is
whether the cuttings contain clays which can hydrate (deflocculate)
in the mud before separation. Clay hydration can be controlled by the
mud chemistry. Additives like polyacrylamides, polymers and salts, as
well as oil-based muds, can help control formation reactivity and
minimize degradation of solids.
The first stage of separation is to remove large cuttings from the
mud with a shale shaker. Shale shakers are vibrating screens over
which the mud passes. The liquid and small cuttings pass through the
screens, while the larger cuttings remain on the screen. If the mud
contains gas, the shale shaker will also separate much of it from the
mud. The mud and small cuttings that pass through the screens are
returned to the mud pit, where additional separation of cuttings and
gases occurs from gravitational settling. The effectiveness of vibrating
screens depends on the vibrator placement, vibration frequency, vibra-
tion amplitude, speed of solids as they pass across the screens, and
screen opening size (Hoberock, 1980; Lai and Hoberock, 1988).
Chemicals can be added to the mud that cause the small clay
particles to coagulate or flocculate into larger groups of particles