Determination of Residual Oil Saturation with Pulsed Neutron Logs - A Field Experiment
Authors: Smith T.J. , Stieber S.J. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
This paper was prepared for the 49th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Houston, Texas, Oct. 6–9, 1974. Permission to copy is restricted to an abstract of not more than 300 words. illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussions may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
The Log-Inject-Log method of determining residual saturation in cased holes, previously documented by Richardson, et al., was recently field tested with the Schlumberger Dual Spacing Thermal Neutron Decay Time Log (TDT-K). A companion test with a Pulsed Neutron Capture (PNC) device specially modified by Schlumberger resulted in saturation uncertainties of three percent pore volume or less. Measurement with percent pore volume or less. Measurement with the standard TDT-K resulted in residual oil saturation uncertainties of 10 to 20 percent pore volume. The test procedures for both pore volume. The test procedures for both devices are presented with recommendations designed to minimize potential errors in future applications.

Introduction
The number of new domestic oil discoveries is declining, thus, it is increasingly important to produce the maximum quantity of oil from known reservoirs. This, coupled with current higher crude prices, makes tertiary recovery much more attractive. The most elemental datum for evaluation of the potential of these projects is the swept zone residual oil saturation projects is the swept zone residual oil saturation (Sor). One of the most promising of the available Sor determination techniques is the Log-Inject-Log (LIL) method utilizing Pulsed Neutron Capture Logs (PNC). In view of cost, accuracy and overall applicability, this method compares favorably with the other available open and cased hole techniques for S determination.
Robinson's study of the application of the PNC device to Sor determination revealed as did PNC device to Sor determination revealed as did a previous review by Richardson, et al., that existing standard devices do not achieve the required accuracy. However, he showed that with certain modifications the PNC device could obtain the desired accuracy without the use of departure curves to correct for borehole and diffusion effects.
After Schlumberger conducted laboratory tests which confirmed Robinson's findings, Shell conducted a field test using both the modified and conventional tools and compared results.

THEORY
The theory associated with this test is explained in detail by Robinson. Basically, the method involves the stepwise injection of fluids with contrasting capture cross-sections into the formation and logging after each injection.