Interwell partitioning tracer test has been recognized as a reliable method for determining residual oil saturation Sorw between wells. While sophisticated streamline or finite difference simulators have been increasingly used to determine Sorw distribution from the tracer data, simple analytical methods such as Tang's chromatographic transformation and UT's moment analysis method are still useful in providing a direct, unambiguous estimate of average Sorw along the tracer flow path. Upon comparison of these two methods, it can be shown theoretically that the transformation method is more accurate than the moment analysis method when the test is terminated at an early stage before the entire tracer profile is produced. Both methods converge to the same limit as tracer production approaches completion. Therefore, the moment method finds its use mainly in lab tests and small-scale tests such as underground NAPL (Non-Aqueous Phase Liquid) measurement for pollution monitoring. This paper also modified the semiquantitative Brigham 5-spot model for partitioning tracer propagation and Sorw determination.
Interwell tracer test has been recognized as a reliable method for determining residual oil saturation between wells. The method involves the injection of a slug of partitioning and non-partitioning tracers into an injector and production of the tracers from nearby producers.Partitioning of tracers between phases slows down the partitioning tracers in a phenomenon known as chromatographic retardation from which the residual oil saturation can be determined. Oil distribution between wells is derived by matching the tracer production profiles using a 3D finite difference simulator such as UTCHEM1,2 or by a streamline model3,4. To circumvent the technical problems in simulation, a chromatographic transformation method was proposed by Tang5–8 and a moment analysis method by Pope et al. 9–13 to calculate Sor directly by comparing the relative separation of tracers. Chromatographic transformation was employed by Tang5–8 to calculate Sor for the Golden Spike, Judy Creek and Leduc interwell partitioning tracer tests. The moment analysis method was applied mainly to laboratory tests or small-scale NAPL (Non-Aqueous Phase Liquid) tests where the complete profiles were obtained within few days. The semi-empirical Brigham's homogeneous 5 spot model14–16, which is a popular tool for tracer design17,18 and tracer data interpretation, lies between these two extremes. The original Brigham model only handles non-partition tracer and it is modified in this paper to include partitioning tracer for Sor determination.
Tang5–8 has proposed 3 different forms of chromatographic transformation, namely, landmark comparison, equal recovery and equal normalized recovery for direct calculation of Sor from tracer data. Landmark comparison as formulated in Equation (1) explicates that at equivalent landmark, i.e., same normalized concentration for both tracers, the ratio ofpartitioning (PT) to non-partitioning tracer (NT) production times is equal to the delay factor 1+ β defined in Equation (3). Different values of Sor can be generated at various landmarks.
Equation (1) (Available in full paper)
where t=NT production time <t>, τ= corresponding PT production time <t>, Cn,max and Cp,max = NT and PT peak concentrations <WL−3>respectively.
