Case Study of Dielectric Logging in the Permian Basin: An Opportunity for Solving Petrophysical Challenges in Tight Formations

Abstract

Abstract High uncertainty of water saturation from petrophysical interpretation has been recognized as a challenge in the Permian Basin for years. Such challenges include, but are not limited to, variable Archie's parameters (m and n) due to the formation heterogeneity, variable formation water salinity, and thinly bedded nature. These challenges make dielectric logging a unique opportunity in tight unconventional reservoirs. Dielectric logging is sensitive to water content and provides water-filled porosity without having to know Archie's empirical parameters or water salinities, which are required with resistivity log interpretation. In addition, dielectric logging offers high vertical resolution logs to address thin-bed issues. Moreover, because of the extremely low permeability of the shale reservoirs, there is effectively no invasion of the borehole fluids into the formation. Thus, in these reservoirs, dielectric logging directly provides water-filled porosity of the undisturbed zone. In this paper, we will share two case studies of dielectric logging in several formations of the Permian Basin which wide-band dielectric measurements on core samples in the lab were also made. A recently developed interpretation model, DCRIM (Dual-frequency Complex Refractive Index Method) is tested and further extended to a wider frequency band. It is observed that the DCRIM method is valid at the frequency as low as 100 MHz in the studied wells. Compared with several traditional interpretation methods such as CRIM, bi-modal, and SMD methods, the new method has significant advantages over the existing approaches since it does not require inputs for either matrix or hydrocarbon permittivity, including kerogen permittivity, to derive water-filled porosity as do existing approaches. The new method enables the elimination of all associated uncertainties with formation mineral models in complex lithologies, unknown mineral permittivity endpoints and, most importantly, the poorly defined permittivity of kerogen. It only requires the relatively well-known input of formation temperature. Thus, the new method provides a more robust, streamlined, and consistent interpretation of the dielectric dispersion logs in tight rocks and potentially reduces the uncertainty on the estimation of hydrocarbons in place. The new method was applied in two selected wells with the availability of water saturation from core samples and other advanced logs such as Nuclear Magnetic Resonance (NMR). After Quality Assurance/Quality Control (QA/QC) the raw dielectric logging data, the water-filled porosity (or water saturation when the total porosity is available) from dielectric logging data is benchmarked against the core data. It is concluded the new method using dielectric logging data from two frequencies presents a good solution to reduce the petrophysics uncertainty and should be considered as a reliable alternative of resistivity-based interpretation when optimizing petrophysical analysis workflows in the Permian Basin.