Oil Saturation Measurements at Brown and East Voss Tannehill Fields
Authors: O'Brien L.J. , Cooke R.S. , Willis H.R. 
Published: December 2007

The partitioning tracer method was used to measure in-situ oil saturation in the Glorieta formation of the Brownfield and in the Tannehill sand of the East Voss Tannehill Unit. Such measurements are needed to evaluate the economic potential of applying an enhanced oil recovery process. Results obtained with this method also are presented.

Introduction
The amount of oil present in a reservoir is needed to evaluate applications of enhanced oil recovery processes. The residual oil saturation near the wellbore can be determined from cores and logs, but such data can be biase by abnormally high pressure drops and accompanying saturation changes. To measure in-situ oil saturation away from the wellbore, we employed the partitioning tracer method developed and licensed by Exxon Production Research Co. Measurements were made in the Glorieta (dolomite) formation of the Brown field and in the Tannehill sand of the East Voss Tannehill Unit.
Tomich et al. and Bragg et al. have described the partitioning tracer method for measuring residual oil partitioning tracer method for measuring residual oil saturation and the theory behind it. In brief, the over-all procedure consists of three parts: (1) laboratory tests at procedure consists of three parts:laboratory tests at reservoir conditions to determine the chemical partitioning coefficients and rate of hydrolysis of the ester used,a mini-test to check the validity of the reaction-rate measurement and reservoir parameters, andthe main test to determine the oil saturation. The mini-test follows the same procedure as the main test.
The oil saturation found with the mini-test can be the same as that of the main test. However, as the names imply, the volume of the reservoir contacted is different in each test. The mini-test may show that some change should be made in the test procedure and incorporated in the main test.

Test Procedure
The oil saturation measurement in the field requires the following sequence of steps.A slug of water is injected. The first portion of the slug contains both an ester (such as propyl formate) and methanol. The next portion contains only methanol. The selection of the ester depends on reservoir temperature.The slug is kept in the formation for a selected period of time to hydrolyze part of the propyl formate, period of time to hydrolyze part of the propyl formate, forming propanol and formic acid.The well is placed on production and samples of the produced water are analyzed for the tracer chemicals. produced water are analyzed for the tracer chemicals.Injection and production profiles are run several times during the test to determine where the fluids are entering and leaving the wellbore. Such information is necessary to define the reservoir model.The chemical concentrations measured for methanol, propyl formate, and propanol are plotted as a function of production. Then, these are compared with chemical concentration values generated by a computer model of the test application developed from pertinent reservoir and fluid parameters. The residual oil saturation is obtained by the best fit between computed and measured chemical concentration data determined by trial-and-error.
Laboratory experiments using crude oil and brine samples (preferably from the test well) determine the partitioning coefficients of the propyl formate and partitioning coefficients of the propyl formate and propanol at reservoir temperature and pressure. propanol at reservoir temperature and pressure. JPT
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