An oil field operator wished to better understand waterflood distribution across a stratified carbonate formation at a distance away from the individual injection wells. The information gained would allow a better understanding of vertical sweep efficiency and determine whether Production Logging Tool (PLT) data obtained in water injection wells was representative of actual sweep properties “deep” within the reservoir. Data generated would enable an enhanced understanding of field waterflood flow, allow better design of infill drilling and chemical treatments to maximise hydrocarbon recovery.
Tracerco selected a unique and proven radiotracer that was compatible with the injection water, connate water and formation type to ensure its flow with the water phase through the formation. The Upper Zone, covering six meters (20 feet) of the formation, was selected to test. A suitable injection well was picked that was less than 30 meters (100 feet) away from another well targeting lower zone production. The radiotracer properties ensured emission’s were able to penetrate the cement and casing of the Lower Zone Producer well. This allowed a standard gamma logging tool to be employed to log the Lower Zone Producer twice daily for the first thirty days to determine change in Gamma API units with time across the Upper Zone. Figure 1 shows well bore positioning.
A water injection profile log was carried out in the Upper Zone injector that was to take the radiotracer material to determine vertical injection positioning. Tracerco mobilised to the field and injected the radiotracer over a one hour time period with no impact on normal operations.
A baseline gamma log was taken in the Lower Zone production well immediately prior to injection. Gamma logs were repeated every 12 hours across the Upper Zone to detect the presence of any change in radiation intensity versus time. After a 30 day time period the log frequency was changed to once every day for another 15 days.
Gamma log data was reviewed and specific vertical layers were analysed over time for changes to gamma API units. Tracer was detected in several vertical layers with an increase and decrease with time as the radiotracer moved past the observation position.
Summation of the Gamma API response across each one foot vertical section then allowed a comparison of relative fluid flow past the observation point to be determined. The table below shows this analysis and the resulting injection water flow profile deep in the Upper Zone reservoir.
Comparison of the water injection profile with the relative water flow data deep within the formation showed good correlation. This provided the customer with the confidence that spinner surveys in injection wells could be used to determine vertical sweep in the Upper Zone.
Clearly the presence of channel flow detected in the injectivity gamma logs provided the customer with information on those positions to target with a suitable recovery plan to maximise extraction from the whole of the Upper Zone formation.
The technology can also be applied in a similar manner using gaseous radiotracer technology to injection gas projects or CO2 floods.
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