Environmental Engineering Reference
In-Depth Information
may indicate which zones are most likely to take frac material. If a particular zone
takes a disproportionate share of the injectant, there is time to place a bridge plug or
perform a cement squeeze before proceeding with a frac job. Figure
6.1
illustrates a
gamma ray log used to monitor well treatment.
Tracer Ejector Tool
The tracer ejector tool (also known as the nuclear flolog) is illustrated in Fig.
6.2
.
A casing-collar locator helps to place the tool in depth. A reservoir of radioactive
material is housed inside the tool and small quantities of it may be ejected on com-
mand by the operator at the surface. Beneath the ejector port lie one or two gamma
ray detectors. There are two methods of using this tool, one known as the
velocity
shot
and the other as the
timed-run
.
Velocity Shot
With the velocity shot, the tool is held stationary in the well bore and a “shot” of
radioactive fluid is ejected. The two gamma ray detectors record gamma ray inten-
sity as a function of time. Figure
6.3
shows such a log in an injection well. Note that
the passage of the radioactive slug past each detector causes a gamma ray peak. By
measuring the time difference between the two peaks and knowing the spacing of
the two detectors it is a simple matter to calculate the flow rate.
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