Interwell Tracer Test To Determine Residual Oil Saturation In A Gas-Saturated Reservoir. Part II: Field Applications
Authors: Tang J.S. , Harker B. 
Published: March 2010

Abstract
This paper entails the implementation and interpretation of the first successful interwell test ever reported in the industry to determine residual oil saturation in a gas-saturated reservoir. Two interwell tests were conducted in Golden Spike to measure the residual oil saturation to gas-flood at two different depths. This method, which involves the comparison of the whole partitioning and non-partitioning tracer curves to derive residual oil saturation values, is an improvement of the original method that compared only the breakthrough times. A chromatographic transformation technique was developed for curve comparison so as to avoid tedious simulation for data interpretation. For layers with different residual oil saturation, the transformation method works only for a pseudo single porosity reservoir with ordered layers, i.e. low residual oil saturation for a high permeability layer. The first test indicated that there were three layers with residual oil saturations of 7%, 15% and 20%. The results from the second test conducted in a lower production interval were masked by the presence of extensive fractures in the production zone. In spite of the interference from the fracture production, the residual oil saturation in some flow channels could still be estimated to be about 12%. Because it is unlikely that the tracers could enter the matrix during the test, the residual oil saturation measured is probably the oil saturation in some secondary channels. Sulphur hexafluoride, F13BI (brome-trifluoro-methane) and F12 (dichloro-difluoro-methane) were selected as the tracers from the previous lab tests. The tracers were pre-mixed and injected as a liquid. A Freon phase behavior program was developed to calculate the exact amount of the Freon's injected.

Introduction
Upon evaluation of various conventional methods, the interwell tracer test(1) has been identified as the most reliable means to determine residual oil saturation in Golden Spike, a low-pressure, low-porosity, gas-saturated carbonate reservoir. The original interwell method disclosed by Cooke(2) in 1971 involved the comparison of the relative breakthrough times of the partitioning and non-partitioning tracers for residual oil saturation calculation. Breakthrough time is not a well-defined quantity, as it is often obscured by dispersion, the detection limit, and most importantly, by the streamline and layer distributions. As a result, the interpretation technique, certainly inadequate, draws criticism(3,4). Consequently, because of the lack of suitable chemicals and interpretation technique, no single test has been tried in the field or, at least, published in the literature. To circumvent the problems anticipated in Cooke's method, our interwell method employed a whole curve comparison to derive a residual oil saturation value at any location on the curve using a simple "landmark" comparison technique. Under ideal conditions, residual oil saturation can be determined by layers. To demonstrate the feasibility of the method, extensive tests were performed in the lab(5). Slim tube displacement results indicated that residual oil saturation could be measured in an accuracy of± 1 % pore volume from the separation of tracers. This paper entails the design, implementation and interpretation of the two field tests conducted in Golden Spike in 1987.