Radioactive Tracer Logs - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 83

Monitoring Natural Radioactive Deposits
In some reservoirs, the rise of the water table can be monitored very simply by
running a gamma ray log at time intervals throughout the life of a well. The rising
water table will make itself visible on the repeated gamma ray logs as an increase
in radioactivity. It is thought that the formation water carries with it dissolved
radioactive salts that deposit on the casing as precipitates, which build up over a
period of time.
Studies suggest that a considerable volume of water production or that flow past the
well bore is required before any detectable change can be seen on a GR log. Figure 6.5
shows a time lapse gamma ray log run in a well before and after production. The oil/
water contact (OWC) is shown by the radioactive scale (Uranium salts) that have been
deposited where water has been flowing. The contact appears to have risen to XX667 ft
between the first gamma ray log and the second—a period of 11 years. This tech-
nique is by no means universal but should be taken advantage of when it works.
Fig. 6.5 Monitoring OWC changes with a GR log