Page 222 - Environmental Control in Petroleum Engineering

208 Environmental Control in Petroleum Engineering

on planning, installation, operation, and maintenance of disposal wells
have been provided by the American Petroleum Institute (1978). The
cost of drilling and completing a disposal well can be a significant
expense in wastewater disposal.
Disposal wells must be completed in a formation that is permeable
and porous, and has a low pressure and a large storage volume. The
disposal formation must also be geologically isolated from any fresh-
water aquifers. To prevent the water from plugging the formation, the
water must normally be treated to remove free and emulsified oil,
suspended solids, and some dissolved solids, such as iron and scale,
prior to disposal.
One disposal method that is growing in popularity is annular injec-
tion in existing wells. In this process, the wastewater is injected down
the annulus of an existing injection or production well and into a
formation above the existing completion. A packer is used to isolate
the disposal zone from the existing injection or production zone. This
disposal method can eliminate the cost of drilling a separate disposal
well. The disposal zone must still meet all requirements for protecting
freshwater aquifers.
A major concern with underground disposal of water is the potential
for the well to provide a vertical communication path from the disposal
formation to any overlying freshwater aquifers. Possible communica-
tion paths include flow up the inside of the casing through leaks in
the casing and flow up the outside of the casing through a bad cement
bond. The presence of leaks in the casing can be detected through
mechanical integrity tests. Unfortunately, there are no reliable ways
to detect the flow of water up the outside of the casing.
Mechanical integrity tests are required in the United States to
determine whether leaks are present in casing. These tests are con-
ducted with tubing set in casing. Two types of tests are commonly
used (Kamath, 1989). In one type of test, the level of any liquid in
the annulus above the packer is monitored for changes. In most cases,
the fluid level will rise as fluid leaks from the higher-pressured
disposal zone to the lower-pressured zones overlying it. In the second
type of test, the annulus is pressurized and its pressure is then moni-
tored. If there is a leak, the pressure in the annulus will decline. The
annulus pressure method, however, requires that the well be isothermal
and that there are no interactions between the liquid and gas in the
annulus. Because these requirements are rarely present, the fluid level