Interwell Tracer Tests To Determine Residual Oil Saturation To Waterflood At Judy Creek Bhl'a'pool
Authors: Tang J.S. 
Published: March 2010

This paper describes the implementation and interpretation of two successful interwell tests conducted consecutively within five months at the same location in the R5 Zone of the Judy Creek Beaverhill Lake "A" Pool to determine the residual oil saturation to waterflood. An accurate knowledge of Sam was required for the evaluation of the miscible flood performance at Judy Creek. Upon exomining all the conventional methods for Sorw determination, the interwe11 test, though still largely an unproven technique at the time, was deemed to be tire most cost-effective and technically feasible. For the two interwell tests, tritiated water (THO), tritiated n-butonol (TNB) and C-I4 tagged i-amyl alcohol (CIA) were used as tire nonpartitioning and the two partitioning tracers. A "salt-out method was developed for separating and analyzing the tracers in produced water. A chromatographic transformation technique with forced normalization was formulated for direct calculation of Sorw by comparing the separation of the tracer profiles. Since an Sorw value was estimated independently from each of the two partitioning tracers, the reliability of the method was greatly improved.

From the results of the first and second tests. Sorw values were determined to be 27 ±1% and 12 ±1% respectively. These two numbers, though vastly different, were self-consistent. Therefore, it is not likely that the discrepancy came from interpretation errors, Further examination of the core analysis data, also supported by the breakthrough times, suggested that 26% of the flow was conducted through a 0.12 m thick high permeability streak, resulting in a high water frontal velocity of 64 m/d and hence a high capillary number (1.2 × 10–6). Based on the published data(29) on the capillary desaturation curve of Caddo limestone this Nc was high enough to mobilize residual oil during the five months of prolonged water flood at high rate prior to the second test. Leaching of light/water soluble components and cooling of the reservoir by the injected water only accounted for 4% PV and 2% PV declines in Sorw in the second test. As well, the close agreement between the Sorw calculated from the TNB and CIA profiles indicated that there was no detectable mobile oil during the tests; the persistent 10% oil cut registered throughout the tests came from the bottom zones (R2-R4). This argument was indirectly proved in the second test by the immediate start-up produced tracer concentrations.

The constant Sorw calculated over the whole production curves for each test proves against non-equilibrium tracer transfer during the tests. Based on the data published in the literature, tracer transfer between oil and water should be fast enough to be at near equilibrium conditions. As for the tracer transfer between flowing and dead-end pores, equilibrium can be achieved both at extremely fast and slow transfer rates. Since the contact time and hence the dimensionless mass transfer rate i. e. Damkohler number (equation available in full paper) is directly proporational to the system length L, it is likely that at reservoir-scale the transfer is a fast process.