Formation Evaluation Through Casing - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 169

Reservoir Monitoring Time-Lapse Technique
Pulsed neutron logs are useful for monitoring the depletion of a reservoir. The time-
lapse method is used. A base log is run in the well shortly after initial completion
but before substantial depletion of the producing horizons. A few days, weeks, or
even months of production are required to clean up near well bore effects of the
drilling operation, such as mud-filtrate invasion, etc. Once a base log is obtained,
the well may be re-logged at time intervals over the life of the field. Typically, a log
will be run every six months or once a year, depending on production rate. Successive
logs may be overlaid so that changes in saturation can be easily spotted by changes
in sigma. A good example of this is given by Fig. 11.30 , which shows a base log and
three additional logs at roughly 6-month intervals.
Fig. 11.30 Time-lapse logging