Dielectric Log-A Logging Method for Determining Oil Saturation

Abstract

Summary This paper presents the principles of dielectric logging by describing measurement of phase shift. laboratory experiments, theoretical calculations for determining the vertical resolution and depth of investigation, and results of application in field practice. Introduction Measurement of oil saturation is an important pan of well logging in the development of- an oil field. It is an important basis for evaluating watered-out reservoirs, estimating the residual oil, and redividing and combining the productive zones. In the development of an oil field by water injection, the salinity of injected water always differs greatly from that of original formation water. For example, the salinity of original formation water in Daqing oil field is about 6,000 to 8,000 ppm, and the salinity of the injected water is as low as 500 to 800 ppm. Thus, the formation water gradually becomes less saline as the amount of injected water is increased. However, the resistivity log and the Neutron Lifetime Log- cannot be used to measure oil saturation in such strata. To solve this problem, it is necessary to introduce other logging methods for measuring oil saturation that are less influenced by the salinity of formation water. Dielectric constant logging is one method to solve this problem and has been described in U. S. and Soviet literature. In the late 1960's, the study of dielectrologs was launched in Daqing oil field. Both theoretical and experimental studies on dielectrologs were carried out. Since 1974, the dielectrolog tool has been used in the field and has been run in more than 100 downhole operations. Field practice has proved that the accuracy for interpreting water saturation of formation and watered-out zones with dielectrolog is +/-8. 8% and +/-5. 7%, respectively. Laboratory Experiment For Determining Dielectric Constant of Rocks The dielectric constants for crude oil and minerals of sedimentary rocks are small (2 to 8), and that for water is very high (80). Therefore, the dielectric constant for water-bearing rocks depends mainly on its water volume. To verify the fundamental principle of this logging method, a laboratory study of main factors affecting the dielectric constant of rocks was performed with core samples from different oil-beefing zones in Daqing oil field. The frequency used here is 60 MHz. Fig. 1 gives the relationship between the dielectric constant and formation water volume of sandstone samples from different regions of Daqing oil field. Air saturation is used instead of oil saturation in these experiments. In Fig. 1 the dielectric constants of rocks correspond very well with the water volume. The dielectric constants of dry samples range from 4 to 7; those of rock matrix, oil, and air are rather small because they depend on shift polarization. JPT P. 1797^