Page 51 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 3§

potential source of contamination through surface runoff and wind
dispersion to adjoining land. Bromacil and tebuthiuron have commonly
been used as sterilization chemicals. These herbicides can become inactive
by applying charcoal to the site at abandonment (Powter, 1990),

2.2 PRODUCTION

The production of oil and gas generates a variety of wastes. The
largest waste stream is produced water, with its associated constituents.
This section reviews both the production process and the wastes that
are generated during production.

2.2.1 Overview of Production Processes

For the oil (or gas) to be produced, a pressure gradient must be
established in the formation on the pore level. This pressure gradient
then forces oil from one pore to the next, and ultimately to the
production well. There are two basic ways for such a pressure gradient
to be established. First is to have a production well with a lower
pressure than that of the surrounding formation. This will cause oil
to flow to the well, where it can be produced. Second is to increase
the pressure in some parts of the formation by injecting fluids. This
will force oil to flow away from the injection wells to lower pressure
production wells. In many reservoirs, a combination of low pressure
at the production well coupled with a high pressure at an injection
well are used.
During production, both water and formation solids are commonly
produced with oil and gas. The produced materials are passed through
separation equipment, where the density differences between the
produced materials are used to separate them.
The first stage of separation normally occurs in a free water knock–
out. This consists of a large tank that allows time for the bulk oil,
gas, and water phases to separate. These tanks are also called wash
tanks, settling tanks, and gun barrels. The output streams from this
equipment consists primarily of gas, water with some oil, and oil with
some water. Solids either settle to the bottom of the tank or are carried
along with the water stream. The performance of these separators has
been reviewed by Powers (1990 and 1993) and the American Petro-
leum Institute (1990a).