Page 218 - Environmental Control in Petroleum Engineering

204 Environmental Control in Petroleum Engineering

When wastewater is discharged offshore, the water is typically
treated to remove only the hydrocarbons. Although the dissolved solids
(salt) concentrations of most produced waters are high enough to be
toxic to even marine life, the rapid mixing and dilution of the dis-
charged water makes the resulting environmental impact negligible.
For near-shore discharges in shallow water, there is less opportunity
for mixing and dilution of the discharged water, and a toxic plume
can exist for some distance away from the discharge point. Such toxic
plumes are of particular concern when discharging a dense, high-saline,
oxygen-deficient brine because it can be trapped in subsurface topo-
graphic low areas. Because this trapped brine can significantly impact
the local marine life, permits to discharge high-salinity brines near the
shore may be difficult to obtain, even if the hydrocarbon content is low.
When wastewater is discharged into onshore freshwater locations, both
the hydrocarbon and dissolved solids concentrations must be low. Because
of the high cost of removing dissolved solids, surface discharge of
wastewater is generally possible only if the initial dissolved solids
concentration of the water is low. Surface discharge into dry stream beds
is a common way to dispose of treated water in arid areas like Wyoming.
Surface discharge into percolation ponds is also used in some areas.
In percolation ponds, the water is allowed to percolate into the under-
saturated (vadose) zone, where it eventually evaporates back into the
atmosphere. Because of the lack of control over where the water goes,
this disposal method is being phased out. Discharge into evaporation
ponds is also an option in many arid areas, particularly if a liner is
used to prevent leaching of dissolved solids.

7.1.2 Disposal of Solids

Waste solids can be discharged directly onto the ground or into the
ocean as long as their quality meets regulatory standards, i.e., the
concentration of contaminants like hydrocarbons and heavy metals is
sufficiently low. Because such discharges are regulated, permits are
required in most areas.

Offshore Discharges

Offshore discharges of treated solids, such as drill cuttings and
produced solids, are permitted in some areas. Offshore discharges,