Page 47 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 3S

2,1.4 Reserves Pits

The most common method for the disposal of drilling wastes for
onshore wells is in on-site reserves pits. The contents of reserves pits vary,
depending on the drilling mud and the types of formations drilled.
Reserves pits, however, can cause local environmental impact, particu-
larly older pits that contain materials that are currently banned from
such disposal or that were not constructed according to current regula-
tions. The environmental impact of modern reserves pits are minimal.
The composition of the fluid in a reserves pit may be different from
that of the original drilling fluid. Chemical and physical alterations
of drilling fluids can occur during and after drilling from the heat and
pressure encountered during drilling or from the addition of formation
materials. Other materials may also be added to the pit before closure,
either deliberately or inadvertently. Such materials include caustic
soda, rig wash, diesel fuel, waste oil from machinery, metal and plastic
containers, and other refuse (Powter, 1990). Bad storage and disposal
practices associated with reserves pits have lead to their being a source
of benzene, lead, arsenic, and fluoride, even when these components
were not detected in the active mud system (Wojtanowicz, 1991).
The heavy metals and other dissolved solids contents in both the
water and mud (sludge) phases of 125 reserves pits scattered around
the United States were measured in one study, and the total and water-
soluble (leachable) concentrations were determined (Leuterman et ah,
1988), The mean metals concentrations of all of the pits varied
significantly with species, with mean concentrations on the order of
a few tens of mg/L. These data are summarized in Table 2-3. It was
found that the metals concentrations in the mud phase were generally
higher than in the water phase, indicating that most of the metals were
probably bound to the organic and clay particles and were not readily
available for leaching.
In separate studies, the heavy metals contents of reserves pits in
the U.S. Gulf Coast were also analyzed and found to vary significantly
(Wojtanowicz et ah, 1989 and Deuel and Holliday, 1990). In the latter
study, the pit contents were analyzed by the U.S. Environmental
Protection Agency, the American Petroleum Institute, and in a private
study under Louisiana State guidelines. The analysis protocols and
procedures differed in the three studies and yielded somewhat different
results. The results are summarized in Tables 2-4 and 2-5.