Interwell Tracer Test To Determine Residual Oil Saturation In A Gas-Saturated Reservoir. Part I: Theory And Design
Authors: Tang J.S. , Harker B. 
Published: March 2010

Abstract
The residual oil saturation to gas-flood is an important parameter to evaluate the potential of the Golden Spike D3 ‘A’ pool as a candidate for enhanced oil recovery and gas storage. Conventional methods, including logging, sponge coring and single well tracer testing, are not applicable to this low-pressure, low-porosity, gas filled carbonate reservoir. An interwell tracer method which works on the chromatographic separation of tracers with different Henry's law constants was therefore proposed and tested in the lab. Stringent selection criteria based on the detection limit and Henrys law constant were established to screen chemicals for application. According to the criteria, sulphur hexafluoride and halocarbons (Freon), which can be detected in the sub ppm range using a gas chromatograph equipped with an electron capture detector, were selected for this study. Henry's law constants were determined experimentally using a static equilibrium method and a dynamic slim tube displacement method. In the preliminary screening of chemicals for lab testing, the Henry's law constant could be estimated from vapor pressure after correcting for the non-ideal behavior using the regular solution theory. It was demonstrated in the slim tube tests that residual oil saturation could be determined within I pore volume % accuracy from tracer separation using the simple chromagraphic theory. A mixing cell model was also developed to simulate the slim tube test results. This model was also capable of handling dual porosities that are common to carbonates.

Introduction
The Golden Spike D3 ‘A’ pool(1,2), located 29 km southwest of Edmonton in central Alberta, is a carbonate reservoir. With the on-coming of the Beaufort Sea gas development, Golden Spike is being considered as a candidate for Beaufort gas storage and enhanced oil recovery. In as much as both projects have substantial economic incentive and are somewhat adversely affected by each other, the two projects need to be carefully evaluated using the best possible data. Residual oil saturation to gas-flood is the key parameter for the evaluation of either process. Various conventional methods for Sor determination, such as production history, sponge coring, laboratory gas floods, logging and single-well tracer testing, have been extensively reported and compared in the literature(3,4). Unfortunately, these methods are not satisfactory for low-pressure, low-porosity, gas-filled carbonate reservoirs such as Golden Spike. Therefore, an interwell method(5,6) using Freon's with different vapor pressures was proposed and studied in the lab. The interwell method works on the chromatographic separation of Freon in the reservoir. According to the chromatographic theory, the Freon with the highest Henry's law constant (or K value) or the lowest solubility in oil is produced first. Thus, by comparing the production profiles of various Freon's, the average residual oil saturation between wells can be determined. Although the principle of the interwell method was disclosed by Cooke(6) in 1971, not a single test has been reported in the literature due to a lack of suitable chemicals and the potential interpretation problems(3). The Golden Spike test is the first attempt ever in the industry to apply the theory to the field.