Page 39 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 27

corrosion of equipment when present in drilling muds. To prevent these
bacteria from growing, biocides are added to drilling fluids. Common
biocides include paraformaldehyde, chlorinated phenol, isothiazolin,
and glutaraldehyde. The latter two biocides have lower toxicities and
are replacing the former two in popularity (Clark, 1994).

Formation Damage Control

Many formations contain active clays that swell upon contact with
fresh water. These swelling clays can plug pores in the reservoir,
lowering its permeability, or they can cause shale around the wellbore
to slough into the wellbore, "wellbore washout." To prevent these
reactions from occurring, salts are commonly added to the drilling
fluid. These salts prevent water molecules from exchanging with the
cations in the clays. Salts commonly used include sodium and potas-
sium chloride. Potassium acetate or potassium carbonate can also be
used, as well as cationic polymers. Shale stabilization additives based
on glycols have also been successfully used (Reid et al., 1993). A
number of cationic polymer muds having good shale stabilization
properties have also been introduced (Clark, 1994).
A related problem during drilling is that cuttings can ball around
the bit, forming a gummy paste. This paste reduces drilling speed
because it is not easily removed from the bit by the drilling fluid.
Copolymer/polyglycol muds have been successfully used to prevent
bit-balling (Enright and Smith, 1991).
If a well is drilled through a salt dome, a water-based mud that is
saturated in chloride salts may be required to prevent excessive
dissolution of the salt along the wellbore.

Oil-based Drilling Fluids

Various organic fluids are also used as a base for drilling muds. In
some cases, the properties of these "oil-based" muds are superior to
those of water-based muds. Like water, however, these organic fluids
do not have all of the proper physical and chemical properties needed
to fulfill all of the requirements of a drilling mud, so various additives
are also used.
Oil-based muds are often preferred for high-temperature wells, i.e.,
wells with temperatures greater than about 300°F. At temperatures