Single Well Tracer Test
In a single well tracer test, a solution of chemical tracer ester (ethyl formate) is pumped downhole, displaced with brine, shut-in to allow hydrolysis of a portion of the ester to ethanol, then produced back. Since ethyl formate will partition into the oil phase and the other tracers, including the produced ethanol, do not, the ethyl formate will arrive back at the producer later. The amount of separation depends on the amount of oil present in the pore space and the ester partition coefficient K which is equal to the ratio of concentration of ester in oil to that in water at equilibrium conditions. With measurement of the separation in producing profiles of ethanol and unreacted ester, or the retardation factor in terms of retardation time between the arrivals of two tracers in the field, the residual oil saturation can be estimated.
At TORP, tracer test practice for field application is established in the laboratory to verify tracer test methods with analytical methods, core flow test and computer modeling. Field test design and practice has also been implemented in two different field projects with an independent vendor. Factors to consider in design include;
- Radius of investigation
- Injection volume
- Produced back volume
- Soaking time
- Suitable ester
- Separation factor
- Partition coefficient
- Temperature