Page 40 - Environmental Control in Petroleum Engineering

28 Environmental Control in Petroleum Engineering

above that level, many of the additives used with a water-based fluid
can break down.
Oil-based muds are also used in wells containing water-sensitive
minerals, e.g., salt, anhydrite, potash, gypsum, or hydratable clays and
shales. Using an oil-based mud in a reactive formation can reduce
wellbore washout by more than 20% (Thurber, 1990). Reducing the
amount of washout reduces both the volume of drill cuttings to be
disposed of and the volume of drilling fluid required to drill the hole.
Reducing interactions between the drilling fluid and formation minerals
by using an oil-based mud also limits the degradation of cuttings into
smaller particles, which improves the efficiency of separating the
solids from the drilling fluid.
Oil-based muds are also used in wells containing reactive gases like
H 2S. When oil-based muds are used, corrosion is minimized
CO 2
because the continuous oil phase does not act as an electrolyte. These
gases are prime contributors to corrosion of drilling equipment in
water-based mud systems.
Another application of oil-based muds is in wells requiring unusu-
ally high levels of lubrication between the drill pipe and the formation.
These wells include deviated or horizontal wells, where the drill pipe
rotates against the formation over long intervals. Oil-based muds are
also useful for freeing pipe that has become stuck in the well.
Oil-based muds are generally more expensive than water-based
muds and have a greater potential for adverse environmental impact.
The benefits of oil-based muds, however, can result in a significant
savings in the cost of drilling a well. Because of their superior
properties, drilling can often be completed faster, which may result
in lower overall environmental consequences than those of water-based
muds. Because oil-based muds are more expensive, they are also more
likely to be reconditioned and reused than water-based muds.
Historically, the most common base oil used has been # 2 diesel. It
has an acceptable viscosity, low flammability, and a low solvency for
any rubber in the drilling system. Diesel, however, is relatively toxic,
making the environmental impact of diesel-based muds generally
higher than those of water-based muds.
The most common additive used in oil-based muds for viscosity
control is water in the form of a water-in-oil emulsion. Small, dis-
persed drops of water in the continuous oil phase can significantly
increase the mud viscosity. Water contents of typically 10% have been