A Comparison Of Several Techniques For Measuring Residual Oil Saturation
Authors: Bragg J.R. , Hoyer W.A. , Lin C.J. , Humphrey R.A. , Marek J.A. , Kolb J.E. 
Published: December 2007

A test well was drilled in 1976 to measure the residual oil saturation in two watered-out zones of the Loudon field, Fayette County, Illinois. Five methods were used to determine residual oil saturation — pressure coring, electric logging, pulsed neutron logging (log-inject-log), carbon-oxygen logging, and the single-well tracer test.

In one slightly-stratified zone, results from logs, cores, and the single-well tracer test were in good agreement, with average residual oil saturations ranging from 17 to 23 percent of pore volume. In the second zone, which was highly stratified, the porosity-thickness average from cores and logs ranged from porosity-thickness average from cores and logs ranged from 27.4 to 33.5 percent of pore volume; however, the permeability-thickness average from the tracer test was 19 permeability-thickness average from the tracer test was 19 percent. Results from this second test indicate that percent. Results from this second test indicate that the injected tracers did not sample the total formation interval sampled by cores and logs, but instead sampled zones of lower oil saturation. A comparison of all results suggests that more than one method should be used to most accurately determine the amount and distribution of residual oil.

In addition to these tests, a log-inject-log test was conducted in a perforated interval of a known water sand. Results indicate that injection during log-inject-log tests in cased, perforated wells may not result in complete water exchange in the nearwellbore region. Hence, erroneously high oil saturations may be interpreted.

Introduction

As part of a program for evaluating the potential of enhanced oil recovery, a test well was drilled in 1976 to determine the oil saturation remaining after waterflood in a portion of the Loudon field, Fayette County, Illinois. This well, the Mabel Mills 16, served the dual purposes of evaluating the suitability of the well site for a surfactant flood pilot test and also of providing an opportunity to compare several different methods for measuring residual oil saturation under nearly identical borehole and formation conditions. The various methods tested included pressure coring, electric logging, pulsed neutron pressure coring, electric logging, pulsed neutron logging (log-inject-log), carbon-oxygen (C/0) logging, and the single-well tracer technique. While pressure coring is considered to be an accurate method for obtaining a volumetric measure of residual oil saturation, it is expensive and requires that a new well be drilled. A major objective of this program was to determine which of the other methods would also provide reliable estimates of residual oil saturation provide reliable estimates of residual oil saturation in completed wells.

Many investigators have reported results from tests conducted with electric logs, log-inject-log procedures, pressure cores, and the single-well procedures, pressure cores, and the single-well tracer technique. However, there is some uncertainty in comparing the accuracy of these various methods based upon tests in separate wells because each method is affected differently by properties of the formation rock, fluids, and well completion scheme. The test program reported here was therefore designed to permit a more direct comparison of these methods for the same reservoir environment.

In addition to the question of accuracy, the depths of investigation and vertical resolutions of the various methods are usually different, so saturation estimates may be different even for the same formation. It is therefore probable that no single method can give a complete, "correct" measure of the residual oil that may be a target for enhanced oil recovery. Even if two methods yield different values for the residual oil saturation, both values may be correct, and both may provide important measures of the quantity and distribution of residual oil. For instance, the single-well tracer technique measures an average residual oil saturation that is weighted according to the product of thickness times effective permeability to brine at residual oil saturation for permeability to brine at residual oil saturation for the various strata sampled by injected tracers. it is important to determine how this saturation compares with porosity-weighted saturations for the same interval measured by methods such as pressure coring and logging that give a vertical profile of residual oil.