Measurements of rock dielectric properties can help to quantify porosity, mineralogy, clay content, texture and especially water saturation of reservoir rocks. Permittivity in the range of MHz to GHz, can be measured rapidly using downhole tools and validated in the laboratory, offers insights into rock flow and mechanical properties not only for conventional reservoirs, but also for tight reservoirs and overburden shales. One barrier to the wider acceptance and adoption of dielectric logging has been the scarcity of laboratory measurements on a range of standard rock types at controlled saturation levels that are verified with independent methods. A second limitation is the lack of well understood and calibrated models for interpreting and inverting advanced, multi-frequency dielectric logs.
CSIRO is embarking on a program to collect standardized dielectric spectroscopy data on a library of different rock types obtained from sources worldwide. We present examples of the data collected so far on typical, (mainly clean, i.e., low clay content) sandstones and carbonates as a function of water saturation, for air-brine and oil-brine systems. We also show how this data can be tied into standard core analysis results and mineralogy, and critically, how it can be cross-matched with NMR T1, T2 and D distributions measured in the same rocks at the same saturations. These are the initial steps in building up a comprehensive database of measurements for pore volume- and saturation-controlled dielectric permittivity and effective conductivity at frequencies from < 10 MHz to 2 GHz in combination with the NMR response.
The combined dataset can be used to build, test and improve upon, dielectric rock physics models based either on mixing laws, effective medium theory or on solutions of electrodynamic equations in 3D digital rocks. Calibrated measurements and interpretation models will maximise the value of advanced petrophysical methods and broaden their application.
Several petrophysical methods can be used to determine the water and hydrocarbon saturation of rocks in the laboratory and downhole, but each is affected differently by matrix properties and mineralogy (especially clays), and each has different sensitivity to water salinity, and hydrocarbon type. In a conventional reservoir where clay content is low or well characterized, and formation water salinity is known, then a standard combination of density-neutron porosity with resistivity is routinely successful (Archie's method, and a plethora of shaly sand models). More "advanced" petrophysical methods, including nuclear magnetic resonance and dielectric logging have found applications where standard methods fail and where particular characteristics such as hydrocarbon type, wettability and mobility are required to discriminate pay from non-pay. They can help to predict key well performance characteristics such as production rate and water cut. Even in "Archie" reservoirs, dielectric tools, sometimes combined with NMR have been used to reduce uncertainty in residual water saturation (Bean et al., 2013; Pillai et al., 2015).