A comprehensive framework for the theoretical assessment of the single-well-chemical-tracer tests Rasoul Khaledialidusti * , Jon Kleppe Norwegian University of Science and Technology, Norway ABSTRACT Single-well-chemical-tracer (SWCT) is the most commonly used ﬁeld method to determine oil or water saturation in one-spot pilot. This method is a complex process due to many effective parameters and non-ideality factors involved. Understanding the extent to which theses parameters might affect the SWCT test proﬁles could help us to manage and design the SWCT test more efﬁcient at different reservoir conditions. This paper proposes a comprehensive framework of a new approach to highlight different aspects of the SWCT tests theoretically before implementing the ﬁeld test. In order to accomplish the task, combining of numerical and analytical solutions have been used. The devised algorithm has been programmed in six different stages. In the ﬁrst four stages, all test design parameters in different investigation regions and retardation factors are calculated. The test design parameters are sizing the test volume, test timing (i.e., injection, shut-in, and production), tracer concentration during the test, and the mean residence volume. In the ﬁfth stage, all criteria are taken into consideration to ﬁnd the most efﬁcient test designs. Then, the achieved parameters are applied in the simulation stage (sixth stage) to investigate the effect of the ester bank and concentration, ester properties, shut-in time. The geochemical speciation code PHREEQC is also used to study the level of pH-variation during shut-in time. The effects of the calcite dissolution, temperature, and initial buffer capacity have been evaluated on the probability of pH-variation. The results show that the probability of pH-variation increases with temperature and lower amount of calcite concentration. To evaluate the workﬂow methodology, two different ﬁeld test cases with different reservoir conditions are employed in order to reﬂect the inﬂuence of different stages of the algorithm. We hope that the workﬂow developed can be used to minimize the uncertainties and improve the quality of the SWCT tests. 1. Introduction Determination of remaining oil saturation (S o ) is vital in managing and selecting the EOR methods for economic exploitation of a reservoir. There are many methods to determine S o including laboratory and ﬁeld methods (Kidwell and Guillory, 1980; Donaldson and Staub, 1981; Blackwell, 1985; Chang et al., 1988; Teklu et al., 2013; Khaledialidusti et al., 2014). The laboratory methods are not representative for the large scale of a reservoir and may not precisely predict S o at reservoir scale even when performed with extreme accuracy. Therefore, ﬁeld methods to determine S o at reservoir scale are more reliable. Recently, a combi- nation of the ﬁeld methods is also proposed in order to determine a more accurate S o (Khaledialidusti et al., 2015b). Tracer methods were intro- duced as the most efﬁcient ﬁeld methods largely due to (1) the mea- surement over large reservoir volume beyond damaged and desaturated regions and (2) non-dependency on porosity (Khaledialidusti et al., 2015b). The earliest tracer technique is the well-to-well method (Cooke, 1971). This method employs two or more non-reactive tracers with different partitioning coefﬁcient  K ¼ Co Cw  , where C o and C w are the tracer concentrations in oil and water phases at equilibrium. Cooke's method includes the injection of a solution of the non-reactive tracers (e.g., 1% for each tracer) with different K-values into the reservoir (i.e., “tracer bank”). Then, this slug pushes through the formation by the volume of water (i.e., “push bank”). Different K-values lead to traveling speeds of the tracers at different velocities and thus different arriving times to the production well. The separation between tracer proﬁles at the production well may be employed to determine S o . The main limi- tations of this method are: (a) long measuring time because of large measured volume between two wells and (b) extreme tracer dispersion especially in layered formations. An SWCT method, which is implemented only in a one-spot pilot, was developed by Deans (1971) to resolve the barriers of the well-to-well method. The SWCT method includes the injection of the chemical reac- tive tracer (ester) bank into the target well. Then, the ester bank displaces away from the wellbore to a radial depth of investigation by the push bank. After the injection step, the well is shut-in for a period of 1–10 days, * Corresponding author. E-mail address: rasoul.khaledialidusti@ntnu.no (R. Khaledialidusti). Contents lists available at ScienceDirect Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol https://doi.org/10.1016/j.petrol.2017.09.027 Received 7 November 2016; Received in revised form 18 April 2017; Accepted 13 September 2017 Available online 18 September 2017 0920-4105/© 2017 Elsevier B.V. All rights reserved. Journal of Petroleum Science and Engineering 159 (2017) 164–181