Interpretation of the Non-Archie Resistivity Behaviour in Vuggy Carbonates: Use of Pore Structure Parameters

Understanding the petrophysical properties of vuggy carbonate rocks is rendered particularly challenging by the extreme complexity of their pore space. The diverse post-depositional diagenetic processes give rise to complex pore microstructures and porosity at multiple scales in vuggy carbonates. Pores in vuggy carbonates can range from less than a micron (matrix pores) to several millimeters (vugs) and different pore populations (micropores, mesopores and vugs) may or may not form percolating networks. Such structural complexity typically results in a relationship between water saturation and resistivity index that deviates significantly from the classical Archie behavior that is common for clastic rocks. Non-Archie behavior has been qualitatively explained by theoretical models, based on simple mixing laws [Petricola and Watfa, 1995; Fleury, 2002] or effective medium approximation (EMA) [Sen, 1997], but these models have received limited validation against experimental data. To better understand this non-Archie behavior we carry out electrical resistivity index measurements during drainage on a suite of vuggy dolomite rocks and synthetic vuggy porous media of controlled microstructure. The later are created by mixing monodispersed glass beads and much larger calcium carbonate particles in known proportions, consolidating the mixture in an oven and then dissolving the carbonate particles by flowing hydrochloric acid [Padhy et al., 2005]. An in-housebuilt four-electrode resistivity cell is employed to measure the electrical resistivity at ambient conditions using a continuous injection method. Non-Archie behavior is observed in samples containing significant amounts of vuggy porosity. Novel measurements of the matrix and vuggy porosity and pore size distribution by a combination of DDIF-NMR and 3D single-point magnetic resonance imaging (SPI-MRI), in combination with drainage capillary pressure measurements by MIP, provides information on the amount and accessibility of different pore populations. A theoretical resistivity index model based on interconnectedness of two main pore populations (matrix and vug) of vuggy carbonates is quantitatively tested against the experimental resistivity index measurements during drainage. It is concluded that a more robust model is required to predict the non-Archie behavior of these vuggy carbonates. However, quantification of the vug porosity using the independent methods has certainly brought insight towards understanding of the non-Archie behaviour and would serve as an important calibrating/characterizing parameter for various log responses from the complex vuggy carbonates.