Tracers are increasingly being deployed as an effective reservoir monitoring and surveillance (M&S) tool in the oil and gas industry. In particular, the single-well chemical tracer test (SWCTT) is being widely used to estimate remaining oil saturation (ROS) or residual oil saturation (Sor), especially in improved oil recovery (IOR) and enhanced oil recovery (EOR) operations. The test provides a direct measure of ROS in a wider reservoir volume compared to the measurements made by near wellbore logs or cores.
A SWCTT was designed and implemented in a carbonate field to determine fluid saturations. The reservoir is heterogeneous with layers of high to low permeability limestone, porosity in the range of 20–30%, and interspersed with patchy layers of dolomites and a very competent anhydrite seal. The SWCTT was conducted in a very mature part of the field near the peripheral water injectors, to determine the residual oil saturation before a planned IOR pilot test.
This paper presents the complete design and implementation of the test, operational challenges, and the analyses and interpretation of the results. The depth of investigation was 13–14 feet from the wellbore. Laboratory results of determining the partitioning coefficient of the chemical tracer are described and presented. The design of the tracer test taking into consideration the reservoir properties are shared. Considerable operational challenges in the field and appropriate solutions to overcome them provide a very interesting case study. The returns and analyses of the samples provided a unique set of data that were interpreted using a simple analytical tracer model which was comparable to the reservoir simulations predictions. The Sor was obtained after fitting the tracer return data to a model of the classical convection-dispersion equation. The results indicate that there was no displacement or drift that occurred during the shut-in period. The mass balance of the tracers suggests that more than 98% of the tracers were recovered with negligible tracer adsorption during the test duration. The results are in close agreement with a similar test benchmarked independently by a commercial vendor using a different injection design scheme.