Use of Partitioning Tracers To Estimate Oil-Saturation Distribution
Authors: Carpenter Chris 
Published: January 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 179655, “Use of Partitioning Tracers To Estimate Oil-Saturation Distribution in Heterogeneous Reservoirs,” by R.M. Dean, D.L. Walker, V. Dwarakanath, T. Malik, and K. Spilker, Chevron, prepared for the 2016 SPE Improved Oil Recovery Conference, Tulsa, 11–13 April. The paper has not been peer reviewed.
Partitioning interwell tracer tests (PITTs) have been used to estimate remaining oil saturations (ROSs) during waterflooding. Compared with core tests, well logs, and single-well tracer tests, PITTs sample a much larger representative elemental volume (REV) and provide interwell estimates of remaining oil saturation. In this paper, the authors present the information gained from conducting a polymer PITT and the saturation estimated during the PITT. The polymer PITT allows characterization of polymer-flood efficiency and is a useful tool in polymer-flood evaluations in heterogeneous reservoirs.
Introduction
The difference in arrival time between conservative tracers has long been used to estimate the volume of oil remaining in a reservoir after tracer tests. The method-of-moments solution to calculate ROS from mean arrival times of conservative and partitioning tracers is based upon single-phase flow. When large changes of saturations occur in between the initial conservative and partitioning tracers, the volume calculated is hard to interpret. Use of equations with single-phase-flow assumptions makes it a challenge to understand the correlation between the volume calculated and its corresponding time.
The method of moments has been used to calculate oil volumes and saturations. With the method of moments, the mean residence time of the tracer for a given slug duration is calculated from the concentration of tracer in the produced effluent.
Oil volume is normally related to the difference in mean residence volume between a partitioning tracer and a conservative tracer, although two partitioning tracers can be related. The remaining oil volume can be calculated from the mean residence volumes of the conservative and partitioning tracers. When oil cut and saturation change are large, the volume becomes difficult to correct. Often, correcting for production on the basis of the mean residence time of the first tracer gives the right value if oil production is small, but this does not give the correct answer if oil cut is high.
Conducting a PITT with a favorable mobility ratio helps alleviate the problem of high oil cut during tracer production because it allows the partitioning tracer to contact oil at residual saturation.
In order to calculate the volume of oil and saturation more accurately during an active waterflood with mobile oil, a technique is required that estimates oil remaining behind the waterflood. To achieve that objective, modifications to the method of moments, known as the residence-time-distribution- analysis (RTDA) method, were made in order to account for two-phase flow. In this new derivation, oil and water can be produced simultaneously and are accounted for by the fractional flow of each phase.