Mass Balance Method To Determine Residual Oil Saturation From Single Well Tracer Test Data
Authors: Tang Joseph S. , Harker Brad 
Published: March 2010

Abstract
The single-well tracer test, although fully proven for homogeneous sandstone reservoirs, has several deficiencies when applied to heterogeneous carbonate reservoirs due to their typical complicated pore structure. Although multiple-porosity simulators are available/or result interpretation, because of the large number of parameters involved, they are difficult to use and the residual oil saturation (Sor) obtained by simulation may not be unique. In addition, current existing multiple-porosity simulators typically suffer from being insensitive to the oil saturation in the non-flowing pores. This insensitivity gives rise to an uncertainty in the calculated pore average Sor.
To circumvent the aforementioned problems, a Mass Balance Method was developed for fast, direct solution Sor from the single-well tracer test data. This method makes tedious profile matching unnecessary. It draws on the theory that as oil saturation increases, more ester stays in the oil phase where it is unavailable for hydrolysis. Therefore, with a knowledge of the hydrolysis rate and with proper loss and production time corrections, Sor can be directly calculated from the recovery of the unreacted ester and the produced alcohol. This method has been successfully applied to five tests conducted in three carbonate reservoirs by Esso Resources Canada Ltd. In all cases, the Sor determined by the Mass Balance Method compares favourably with those obtained by simulation, sponge coring and inter-well testing. The field test results confirm the validity and superiority of the method over classical simulation techniques.
An accurate knowledge of hydrolysis rate is essential to the Mass Balance Method, as a 10% uncertainty renders a 3% (pore volume) error in Sor. While ester hydrolysis is believed to be first order in the reservoir, this rate often cannot be measured in the lab due to the deviation caused by auto-catalysis which is triggered by generated acid. Several factors which contribute to the onset of auto-catalysis and a procedure/or prolonging the first-order region so (l5 to improve the accuracy of the hydrolysis rate measurement are also discussed in this paper. All field results indicate that the lab measured hydrolysis rate constant accurately reflects the actual rate in the reservoir, and that pressure has no effect on the hydrolysis rate/or medium temperature and fairly sweet reservoirs.

Introduction
Various conventional methods for Sor determination, such as production history, laboratory waterflood tests, core analyses, logging and single-well tracer tests have been extensively reported and compared in the literature(1–3); each technique offers certain advantages and limitations. Of all the methods available to date, the single-well tracer test is unique in its relatively large and variable depth of investigation, and its insensitivity to near wellbore effects. It has, therefore, been widely accepted as the most reliable means to determine in situ Sor. A typical single-well tracer test(4,5) consists of the following five stages:Injection: injection of a chemical bank composed of an ester, a non-reacting cover tracer and a mass balance tracer.Push: injection of water tagged with mass balance tracer to push the chemical bank to the formation.