Laboratory Evaluation of Inter-Well Partitioning Tracers for the Determination of Remaining Oil Saturation after ASP Flooding

Alkaline Polymer Surfactant (ASP) flooding is an Enhanced Oil Recovery (EOR) technique aiming to mobilise remaining and residual oil. Estimating the average oil saturation prior to and post ASP flooding is a recognised challenge, but is vital to the evaluation of an ASP pilot. The use of oil-water partitioning tracers is a method to determine the average oil saturation in the water-swept regions between wells. The implementation of partitioning tracer technique is known in the industry to evaluate effectiveness of waterfloods, but its applicability in ASP flooding is a topic of investigation due to additional complexities. Often, the partition coefficients of tracers depend on salinity of the aqueous phase. It is equally important to know how the presence of polymer and surfactant may influence the behaviour of partitioning tracers. Laboratory study has been performed to analyse the behaviour of partitioning tracers prior to and post ASP injection for a field that is currently undergoing sea water injection and is being evaluated for ASP flooding later in the field life. A group of six oil-water partitioning tracers have been included in the study. Batch (bottle tests) and dynamic (core flood) experiments were performed to evaluate the partitioning behaviour of the tracers. The residual oil saturation to water (Sorw) before ASP flooding was determined using pulse injections of tritium labelled water (HTO). Several passive tracers were also injected in the core floods and their retention times were compared with that of HTO. Herein we describe evaluation of oil saturation, through the tracer responses, over a range of brine compositions such as soft brine, sea water, ASP and polymer solutions. Batch studies indicate the dependency of the partitioning of the tracers on brine salinity. The results of core floods show earlier breakthrough of anionic passive tracers in comparison with HTO suggesting possible rock/fluid interactions for the anionic passive tracers. The residence time distribution of HTO measured before, during and after the ASP flooding illustrates that the flow paths available for water were changed during ASP flooding. The paper highlights the importance of good laboratory experimental protocols using a more accurate passive tracer such as tritium labelled water. The results from the experiments draw attention to the uncertainties associated with the flow of tracers under changing salinity and chemical composition.