Page 46 - Environmental Control in Petroleum Engineering

34 Environmental Control in Petroleum Engineering

(American Petroleum Institute, 1990b). The larger flocculates then
settle more rapidly in the mud pits. This process involves the neu-
tralization of the surface charge (zeta potential) on suspended par-
ticles to overcome coulombic electrical repulsion between the par-
ticles and allow aggregates to form. Inducing alternating electrical
currents to overcome the coulombic repulsion has also been proposed
(Farrell, 1991).
If a drilling mud contains gas that is not removed by the solids
separation equipment, a vacuum chamber can be added to the mud
system. This lowers the mud pressure in the chamber and expands the
size of the gas bubbles, allowing them to be separated from the liquid
by gravity more rapidly. In these systems, the mud is typically passed
over inclined planes in thin layers to enhance separation.
If the proper equipment and procedures are not used to remove the
cuttings as they are added to the mud system, the concentration of
cuttings in the mud gradually increases with time, and the mud
properties, such as density and viscosity, are degraded. The maxi-
mum tolerable solids concentration varies with the mud used, but is
generally between 4% and 15% (Wojtanowicz, 1991). To maintain the
mud properties in the desirable range, the mud can be diluted; this
requires the addition of more base fluid, either water or oil, and many
of the chemicals needed to alter its chemical properties. Dilution,
however, increases the volume of drilling waste that must ultimately
be disposed of.
In many cases, shale shakers and settling pits are insufficient to
separate the mud solids from liquids, and further treatment with
advanced technology is required. For example, after separating the
solids from the mud, a significant volume of liquid is normally
retained with the cuttings. Volumetric measurements from offshore
platforms have shown that the total volume of liquids with the cuttings
after discharge can be from 53% to 73% (Wojtanowicz, 1991). In some
cases, further dewatering of the solids may be required before disposal.
Advanced separation methods are discussed in Chapter 6.
One difficulty with using advanced technology for improved separa-
tions at a drill site is the high cost of equipment rental. The expendi-
ture for this equipment can be easier to justify if a good economic
model for their benefits is used. One such model has been proposed
by Lai (1988) and was subsequently verified by field performance (Lai
and Thurber, 1989).