Resistivity in Carbonates: New Insights

Abstract

Abstract A detailed explanation of the non-Archie behavior of the resistivity index vs Sw curves in carbonates is proposed. The model is based on resistivity index (RI) curves measured at ambient and reservoir conditions, on double or triple porosity micritic and oolitic carbonates. We used the Fast Resistivity Index Measurement (FRIM) method to obtain accurate and continuous RI curves down to very low water saturation (5%). In general, the measured RI curves cannot be described by an Archie's law and the measurements at very low saturation are often a key observation to confirm a non-Archie behavior. A large variety of shapes is observed and the local saturation exponents can vary from 1 up to 3. These shapes can be explained qualitatively and quantitatively using two new models (DPC and TPC) in which the different porosity systems are connected together either in series or in parallel, or a combination of both. The second key feature of the models is the use of NMR and mercury injection curves to estimate the pore volume fraction of each population and the sequence of invasion of each population by oil during the primary drainage process. Hence, only two of three adjustable parameters are needed to model the experimental results. In particular, the saturation exponent of the dominant pore system can be extracted from the analysis. For field applications, we also propose a simple procedure to calibrate resistivity logs with variable saturation exponent. From the experimental results and the model, we conclude that the pore structure plays a key role. In particular, the microporosity can be responsible for the low resistivity contrast in carbonates, although it may represent less than 10% of the total pore volume. The water contained in the microporosity forms a continuous path for the current. When invaded by oil, the conductivity of the microporosity system is weakly modified. Introduction Carbonate reservoir evaluation is a challenging task for petrophysicists and a fine understanding of transport properties in these porous media is still lacking. Relative to siliclastics, carbonates may be simpler in terms of mineralogy but are incomparably more complex in terms of pore structure and surface properties. A large biological origin of the sediments combined with various diagenetic processes yield complex pore structures, which may differ greatly from one reservoir to another. In many carbonate systems, resistivity laboratory calibrations contradict field observations (water-free production, capillary pressure) as well as direct water saturation measurements from preserved cores.