The Role of Partitioning Inter-Well Traces (PITT) in Determining Remaining Oil Saturation (ROS) and Sweep Efficiency

Inter-well tracer testing is used extensively to map well-to-well communication and sweep in water flooding and enhanced oil recovery (EOR) projects. Recent tracer technology development, including new interpretation methodologies and access to a wide range of new tracers, has further strengthened this application of tracers. One particularly interesting development is the introduction of partitioning inter-well tracers technology, a field-proven and stable in harsh environments. These specialized tracers yield information on the fractional flow of oil and water. The technology has involved the injection of tracers into an Injector well (both portioning and non-partionening) and chase their arrival by collecting and analysing the samples from the Producer well. The scope here is to measure oil saturation in the inter-well region between injector A and producer B. A review recent development and application of PITT in an on-shore sandstone reservoir has been conducted. This requires close collaboration between the operator’s technical team and specialized personnel that perform the test, analyses samples and interpret results. Results are presented and discussed, and best practices for interpretation was given. Finally, an assessment of oil saturation from the tracer test is summarized.

The results show that the partitioning tracer technology provide viable and valuable information on oil saturations in inter-well volumes flooded by water. This technology has been proved as an effective tool in determining the two said properties, namely, the Remaining Oil Saturation (ROS) and communication pathways (Sweep) for future planning of the reservoir including EOR activities.

The recovery of tracer was low, estimated at 1.7% and 0.3% of the total amount injected for the water and partitioning tracer. The low recovery is consistent with the observation that most of the water injected are injected into zones contributing little to the water production. the RTD analysis show that the flow is relatively heterogeneous and that the swept volume is small compared to the inter-well volume between injector and producer. A likely scenario is that most of the water transported in a narrow 1m zone.

Results from interpretation of tracers finds an oil saturation of about 13–17%. The likely explanation for this is that the PITT tracers move in the narrow 1-meter zone and that this zone has been swept by a large number of pore volumes. An estimate based on injection and production logging results and volumetric considerations indicate that the narrow zone is swept by 3–6 pore volumes per year.