Page 41 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 2S

used. A chemical emulsifier (surfactant) is normally added to prevent
the water droplets from coalescing and settling from gravitational
forces. Commonly used emulsifiers are calcium or magnesium fatty-
acid soaps. If further viscosity increases are required, solids can be
added to the mud, including asphalts, amine-treated bentonite, calcium
carbonate, or barite.
The density of oil is significantly lower than that of water, so den-
sity control additives normally must be used. The water in water-in-oii
emulsions only slightly increases the mud density, so solids are norm-
ally added. The same solids that are used to increase the viscosity-
asphalts, amine-treated bentonite, calcium carbonate, or barite—can be
used to increase the density. One limitation with oil-based muds is that
most of the solids that enter the mud, including cuttings, are water-
wet. To prevent the solids from concentrating in the dispersed water
droplets and settling out, chemical wettability agents (surfactants) are
added to change the wettability of the solids to oil-wet. This allows
the solids to be dispersed through the more voluminous oil phase.
One of the advantages of oil-based muds is their compatibility with
water-sensitive formations. Because the continuous phase is oil, only
oil can enter the formation as a filtrate. Water invasion is severely
limited, which minimizes the damage to the formation. Because clay
particles do not flocculate in oil-based muds, bit-balling is also
minimized. If fluid loss becomes too high, fluid loss agents like
bentonite, asphalt, polymers, manganese oxide, and amine-treated
lignite can be used.
Although oil-based muds have a lower corrosion rate than water-based
muds, corrosion can occur, particularly when drilling through a formation
containing CO 2 or H 2S. Like water-based muds, the primary method to
control corrosion is to control the pH of the water phase of the mud. A
common additive for pH control of oil-based muds is calcium oxide.
A number of oil-based muds using organic materials have been
developed as low-toxicity substitutes for diesel (Friedheim and Shinnie,
1991; Peresich et al. 1991). Mineral and synthetic oils are becoming
increasingly popular as a base for drilling mud (Clark, 1994).

Unwanted Components

All drilling muds generally have a number of unwanted components
that can potentially harm the environment. The most common of these