Page 42 - Environmental Control in Petroleum Engineering

30 Environmental Control in Petroleum Engineering

are heavy metals, salt, and hydrocarbons. The concentration of these
materials varies significantly. The primary concern arises when the
drilling fluid must be disposed of.

Heavy Metals

Heavy metals can enter drilling fluids in two ways: Many metals are
naturally occurring in most formations and will be incorporated into the
fluid during drilling; other metals are added to the drilling fluid as part
of the additives used to alter the fluid properties. The most commonly
found metals have traditionally been barium from barite weighting agents
and chromium from chrome-lignosulfonate deflocculants.
Heavy metals naturally occur in most rocks and soils, although at
relatively low concentrations. The elemental concentrations of native soils
and gravels on the Alaskan North Slope are summarized in Table 2-2,
Although the concentrations of the major elements will vary from car-
bonate to siliceous rocks, the concentration of the trace elements, including
heavy metals, is probably representative of rocks and soils of many other
areas. Naturally occurring metals of particular concern include arsenic,
barium, cadmium, chromium, lead, and mercury.
Drilling fluids typically contain high concentrations of barium.
Barium is a constituent of barite, which is used as a density control
material. The most commonly used form of barium, however, is barium
sulfate, which is highly insoluble. Because of its low solubility, it will
not leach with groundwater movement, nor will it be taken up by
plants and enter the food chain.
Chromium is another major constituent of many mud additives,
particularly chrome-based deflocculants. Chromium in its toxic hexava-
lent form can be used as a gel inhibitor/thinner, a high-temperature
stabilizer, a dispersant, a biocide, and a corrosion inhibitor. It is
believed, however, that hexavalent chromium is quickly reduced to its
relatively nontoxic trivalent form in a mud system (Campbell and
Akers, 1990). Typical chromium levels in drilling muds are between
100 and 1,000 mg/L (Bleier, Leuterman, and Stark, 1993).
Another significant source of heavy metals in drilling fluid is the
thread compound (pipe dope) used on the pipe threads when making
up a drill string. Pipe dope serves two primary purposes: (1) it prevents
the seizure of the joint from galling at high stresses and (2) it seals
the joint and prevents fluid flow along the threads. Early formulations