Page 44 - Environmental Control in Petroleum Engineering

32 Environmental Control in Petroleum Engineering

heavy metals. These metals can enter the drilling fluid during drilling
through a formation containing crude oil or if a kick occurs and oil
flows into the well. Metals found in crude oil include aluminum,
boron, calcium, chromium, cobalt, copper, gold, iron, lead, magnesium,
manganese, nickel, phosphorus, platinum, silicon, silver, sodium,
strontium, tin, uranium, and vanadium. Of these elements, vanadium
and nickel occur in the highest concentrations. The concentration of
metals in some crude oils is typically on the order of a few parts per
million to a few tens of parts per million, although concentrations as
high as thousands of parts per million have been reported (National
Research Council, 1985).
A number of other metals are found in drilling fluid additives,
although at lower concentrations. Arsenic can be used as a biocide to
prevent the growth of bacteria. Cadmium is found in some pipe dopes,
The mineral barite, the source for the barium sulfate used for density
control, can have relatively high naturally occurring levels of cadmium
and mercury (Candler et al., 1990). Mercury has also been used in
manometers in the natural gas industry to meter the flow rate of gas,
Zinc is occasionally used as inorganic zinc salts for density control
or as hydrogen sulfide scavengers to minimize corrosion and maintain
human safety.

Another unwanted component of drilling fluid at disposal time are
salts. Salts, like sodium or potassium chloride, are often added to
drilling fluid to protect sensitive formations from reacting with the
drilling fluid. The salt concentration of a drilling fluid can also
significantly increase if a well is drilled through a salt dome or a
formation having water with a high salt concentration.

Hydrocarbons

Except for oil-based muds, hydrocarbons are normally an undesir-
able material in drilling rnud because they contaminate the cuttings.
Hydrocarbons enter a mud while drilling through a hydrocarbon-
bearing formation or when oil is used for a spotting fluid when a pipe
becomes stuck. In general, the deeper the well, the greater the concen-
tration of hydrocarbons that enter the mud.