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Waste Disposal Methods 209

method is normally considered more reliable than the annulus pressure
method for detecting leaks in the casing.
Although there are no reliable methods to detect whether or not
water is flowing up the outside of casing, a number of methods are
available that can detect flow in some cases. These methods are
generally limited to large leaks or high fluid flow rates. Noise logs
can detect high-volume fluid movement behind casing, but are sensi-
tive to extraneous sources of sound. Neutron activation logs can detect
water movement in some cases (Arnold and Paap, 1979; McKeon et
al., 1991; Uswak and Howes, 1992). Injecting a boron solution into
the well and logging with a pulsed neutron log to monitor boron
migration has been proposed (Bount et al., 1991). Radioactive tracers
can be injected into the well and the well logged with a gamma-ray
detector to test for tracer movement. Acoustic cement bond logs can
provide evidence of a bad cement bond in some cases.
The possibility of pressure communication between the disposal
zone and overlying formations can be tested by separately completing
an intermediate zone above the disposal zone and below the overlying
freshwater aquifer. If the pressure in the intermediate zone responds
to the injection pressure into the disposal zone, then a leak behind the
casing is indicated (Poimboeuf, 1990).
If a well fails a mechanical integrity test, the well normally must
be repaired before it can be used as an injection well. One method of
repairing a leak in casing is to install a concentric packer to isolate
the leak and allow fluid flow past the bad section of casing (Wilson,
1990). Other methods to repair wells that fail a mechanical integrity
test include squeeze cementing, running a liner, or plugging and
abandoning the well.
Failure of a mechanical integrity test does not necessarily mean that
freshwater aquifers will be contaminated; it only indicates the possi-
bility of water flow up the annulus. No upflow will occur if the
disposal formation is underpressured, e.g., if its flow potential (abso-
lute pressure minus hydrostatic gradient) is lower than that of the
overlying zones. It has been suggested that wells that fail a mechanical
integrity test and are underpressured could still be safely used without
repair if the fluid level in the annulus is continuously monitored to
ensure that no vertical flow of fluids occurrs (Janson and Wilson,
1990). The disposal of water by gravity feed, by which there are no
pumps to pressurize the disposal zone, has also been proposed for such