Page 224 - Environmental Control in Petroleum Engineering

210 Environmental Control in Petroleum Engineering

wells (Meyer, 1990). Regulatory approval to use a well that fails a
mechanical integrity test may not be possible in some areas.

7.2.2 Disposal of Solids

Subsurface burial is a common method for disposal of solid wastes.
Drill cuttings and used mud are typically left in reserves pit after a
well is drilled. After the free liquids are removed, the remaining
materials are covered by soil and the site is revegetated. Such on-site
disposal is allowed in most areas, provided there are no hazardous
materials mixed with the waste.
One major concern with the burial of solids is the potential for
heavy metals, hydrocarbons, and salts to migrate away from the site,
Salt buried in reserves pits can migrate both downward and upward
(McFarland et al., 1990). The two metals most commonly found in
drilling muds at concentrations above those found in most soils are
barium and chromium. These metals, along with mercury, are in a
nonsoluble form and have a very limited potential for migration or
plant uptake (American Petroleum Institute, 1983). For pits containing
high salt or hydrocarbon levels, regulations may require the use of an
impermeable pit liner to prevent leaching. The leaching rate for unlined
pits could also be reduced by covering the buried waste with an
impermeable cap to prevent stormwater infiltration (Roberts and
Johnson, 1990).
In arctic regions, the disposal of drilling wastes in pits using below-
grade freezeback has been proposed. In this process, the drilling wastes
are buried in a deep pit dug into the permafrost. After closure, the
materials will freeze, minimizing any migration of soluble components
from the site. Only water-based muds, cuttings, and excess cement can
be successfully disposed of by this method; any freeze-depressing
materials like brines, glycols, or alcohols may be prohibited (Maunder
et al., 1990). The long-term stability of these pits in the event of
climate changes, however, is not known (Fristoe, 1990).
A developing new technology for the disposal of drill cuttings is
to grind them into small particles and inject them into a well as a
slurry (Malachosky et al., 1991; Smith, 1991; Minton and Secoy,
1993). In most cases, annular injection is used for the slurry. If
fracturing is required for the slurry to be accepted by the formation,
it will be necessary to ensure that the disposal zone and any hydraulic