Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Diatomite Productivity Evaluation Using Dielectric Measurements At Four Depths Of Investigation

Abstract

Dielectric measurements have been used for many years to evaluate water saturation in heavy oil sandstone and diatomite reservoirs. Dielectric permittivity is used to calculate water-filled porosity for comparison with total porosity from traditional density-neutron logs. In heavy oil and low-permeability reservoirs, this methodology calculates oil saturation without requiring knowledge of water salinity. Although this method is commonly used in diatomite reservoirs, it has been seen that zones showing high oil saturation do not necessarily have the highest production rates. New dielectric measurements are now available from a multi-frequency dielectric dispersion tool with multiple receiver spacings, frequencies, and polarizations. One application of this data has been shown to evaluate variations in oil saturation due to oil movement in a wellsite quicklook presentation. The methodology compares the dielectric measurement of water filled porosity at four depths of investigations covering approximately 1 to 4 inches into the reservoir. Comparison of these four water-filled porosities with total porosity yields an oil saturation invasion profile. Oil movement has been observed with this technique in diatomite reservoirs in various fields across California. Zones with high oil movability might be expected to have sidewall core saturations reduced because of flushing by mud filtrate. This has been observed by favorable comparison of core oil saturation to the shallowest reading dielectric oil saturation in zones showing large oil movement. In these cases of high oil movability, the core derived water saturation will reflect flushed zone saturation (Sxo) rather than the true Sw of the zone. This could lead to pessimistic reserve calculations. Zones of high oil movability have been observed in both light and heavy oil diatomite reservoirs. Also in zones believed to be affected by steam fracture stimulations from nearby wells. And observed oil movements have been directly correlated to better production in diatomite wells. The oil movability quicklook provided by the multi-frequency dielectric dispersion tool provides an added dimension to the evaluation of diatomite reservoirs. It improves reserve calculations with the ability to identify core flushing and read deeper into the formation. And offers a straightforward method to evaluate and compare productivity potential in diatomite. It provides a valuable enhancement to the traditional dielectric evaluation workflow.