Page 38 - Environmental Control in Petroleum Engineering

26 Environmental Control in Petroleum Engineering

If a drill string becomes stuck in a well, a lubricant is usually
circulated through the well to help free it. These spotting fluids have
traditionally been formulated with diesel or mineral oils. Because these
fluids "contaminate" cuttings with a hydrocarbon, the discharge and
disposal options for cuttings is limited in some areas. Water-based
spotting fluids are also available (Clark and Almquist, 1992).

Corrosion Inhibitors

Corrosion is commonly caused by dissolved gases in the drilling
mud, e.g., oxygen, carbon dioxide, or hydrogen sulfide. Optimum
corrosion protection of drilling equipment would include elimination
of these gases from the mud. If elimination is not possible, the
corrosion rate should be reduced. A wide variety of chemicals are
available to inhibit corrosion from drilling mud. These additives are
often used even when the pH is maintained in the desired range.
Corrosion inhibitors do not prevent corrosion, but reduce the cor-
rosion rate to acceptable levels, e.g., below 400 mills per year or
2
0.02 Ibm metal per ft of metal in 10 hours. Inhibitors coat the metal
surface and limit the diffusion rate of corrosive chemicals to the
surface. The most common inhibitors utilize a surfactant that protects
the metal with a coating of oil. High molecular weight morpholines
and filming amines are most commonly used for oilfield applications,
Ethylene diamine tetracetic acid (EDTA) is sometimes used to dissolve
pipe corrosion.
Oil-soluble organic inhibitors applied every 10 hours appear to
successfully reduce oxygen corrosion. These inhibitors are strongly
absorbed on clays and cuttings, however, increasing the amount of
inhibitor required. Water-soluble organic corrosion inhibitors may not
be effective for controlling oxygen corrosion, although they can be
used to reduce pitting from H 2S in the absence of oxygen. A more
complete discussion of corrosion is given by Jones (1988).

Biocides

Sulfur reducing bacteria can grow in many drilling muds, particu-
larly those containing starches and polymer additives. These bacteria
can degrade the mud and can enter the formation, where they can sour
the reservoir (generate hydrogen sulfide gas). Hydrogen sulfide causes