An oil field operator was experiencing a high gas oil ratio in two producing wells that was unexpected. The source of gas may have originated from one of two injection wells that were in operation as part of an extensive gas flood campaign. One of the wells was across a fault block that was thought to be sealing and the other was some distance away. Understanding the source of communication would allow the customer to better balance injection gas and maximise its effect throughout the targeted sweep zones.
Tracerco selected unique gaseous chemical tracers for each injection well out of their wide range of proven gas chemical tracer portfolio. Using reservoir and fluid flow data a project plan was developed. As there was uncertainty with respect to cross block communication as well as matrix flow between the injectors and producing wells a sampling strategy was designed with the expectation that rapid breakthrough could occur.
Tracerco mobilised to the field and injected a small amount (less than five kilos) of each of the unique tracers into injection wells I1 and I2 over one day with no disruption to normal operations. After injection, customer staff sampled production wells on a regular basis for a seven month time period.
Sampling and Analysis
Tracer was detected in a number of production wells. The field map in figure one together with directional arrows shows communication pathways between injection and production wells.
The tracer response curve from injector 1 in wells P1, P2 and P4 is shown in figure two. There was no response in any producers from the tracer injected into I2.
Interpretation of Results
Results showed communication pathways between injector well I1 and P4, P2 and P1 in that order. There was no tracer from well I1 detected in well P3. Relative tracer responses showed that 66% of the injection gas was producing at well P4 with 21% at P2 and 13% at P1.
The lack of tracer from well I2 proved that there was no communication across the fault block. The lack of tracer in well P3 highlighted the likely conduction of injected gas along the fault running North South in the field resulting in little sweep of the targeted reservoir and the high gas to oil ratios in wells P1 and P2.
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