Abstract
The recent introduction of a downhole logging device to estimate rock multi-frequency complex permittivity has motivated the development of corresponding laboratory measurements. Various devices for permittivity measurement are described in the literature. Those instruments generally handle powder samples, millimeter-range thin discs or centimeter-range cylindrical plugs. In carbonate rocks, heterogeneity can be observed from micronic to kilometer scales. Laboratory measurements at similar resolution of the downhole tool can provide useful and justifiable input for the interpretation of the logged data.
A dielectric device which can measure 1.5 in. diameter and 2.0 in. long core plugs with a large coaxial probe is presented in this paper. The repeatability of the measurements are shown to be within 2% of the material dielectric constant value of dry rock plugs and 3% of wet rock plugs normalized with porosity and salinity. The range of confidence for the inferred cementation factor from dielectric measurements is also quantified and the results show that this cementation factor is not sensitive to the brine salinity. Error bars are displayed over dielectric dispersion curves and used as quality control.
Various materials have been tested with this large probe. The measurements on fused quartz, Teflon and ceramic materials match the available published results. Data acquired on pure calcite, dolomite and anhydrite crystals provide a good estimation of the limits of the dielectric properties range for calcium carbonate rocks of complex lithology. In the literature, the properties of these minerals vary over a broad range, which limits their practical use for reservoir characterization.
The design of the probe allows averaging the horizontal heterogeneity of the plug, but by flipping the plug upside down the identification of vertical heterogeneity can be put forward. The heterogeneity can be caused by numerous factors such as lithology, pore system, and fluid saturation variations. The case study presented in this paper shows an example of porosity heterogeneity identified by the dielectric spectroscopy measurements, and confirmed by NMR 1D porosity profiling and CT-scan imaging.
