Evaluation and Comparison of Available Tracer Methods for Determining Residual Oil Saturation and Developing an Innovative Single Well Tracer Technique: Dual Salinity Tracer
Authors: Khaledialidusti Rasoul , Kleppe Jon , Enayatpour Saeid 
Published: May 2016

Abstract
It is generally recognized that the determination of residual oil saturation (Sor) plays a considerable role in managing and selecting the EOR methods. There are a number of methods to determine Sor and each method has advantages and disadvantages. Among these methods, tracers are the most efficient, largely due to the measurement over large reservoir volume and non-dependency on porosity. The objective of this study is the evaluation and comparison of available tracer methods in different reservoir conditions since 1971 until the present time and proposing a new tracer technique to overcome limitations of previous tracer methods.
The first tracer technique to measure Sor, is well-to-well method. The major limitation of this technique is the long measuring time. To get over this barrier, single well tracer techniques (SWTTs) were developed. Due to the reversible nature of tracers, it is impossible to use two or more non-reactive tracers in SWTTs without any irreversibe step. Single well chemical tracer technique (SWCTT) is the first SWTT which utilizes hydrolysis reaction as an irreversible step. While SWCTT overcomes the limitation of well-to-well technique, other limitations are encountered such as difficulty in finding partially reactant tracer and predicting reaction condition in different reservoir conditions (temperature, salinity, and pH), depth of investigation, and drift during soak period. Some other SWTTs are developed to remove some of these limitations, but as far as these techniques utilize hydrolysis reaction, they could not overcome all the limitations of SWCTT.
Since many limitations of SWTTs are as a result of the hydrolysis reaction, the main objective of our innovative technique is to remove hydrolysis reaction and cause an irreversible step by utilizing the dependency of partitioning coefficient to salinity. The sensitivity of partitioning coefficient to changing reservoir condition leads to a delay in arriving back of tracers to the well and Sor determination using the principles of chromatography.

Introduction
It is generally recognized that the determination of residual oil saturation (Sor) plays a considerable role in managing and the selection of EOR methods. There are a number of methods to determine Sor. The detailed of these methods and advantages and limitations of each method are compared (Kidwell et al., 1980, Donaldson et al., 1981, Blackwell, 1985, Chang et al., 1988, and Teklu et al., 2013). The most common method for acquiring Sor is coring which include direct sampling from the rock. This method has some limitations which cause uncertainty. The one is related to sampling technique because the sample taken cannot be considered as a whole reservoir. The other limitation is that coring method can only be used in new drilled wells. Another method for obtaining this information is logging methods which can investigate only a short distance beyond the wellbore. Material balance calculations based on production history are another method. Determination Sor by means of material balance needs more variables than other methods. For example, it is imperative to obtain initial fluid saturation of the reservoir by some other method and information about the source of the recovered flow to utilize material balance. Among these methods, tracers are more efficient largely due to the measurement of large reservoir volume and non-dependency on porosity.