A Generic Electrical Conduction Model For Low-Contrast Resistivity Sandstones Available to Purchase

Based upon the Archie and other often used electrical conduction models, low-contrast resistivity (LCR) formations have high calculated water saturations. However, many LCR formations produce oil and/or gas with a very low watercut. Our conductive rock matrix model (CRMM) correctly describes the basic features of the productive LCR formation. Subsequent studies of LCR sandstones show that conductance in capillary bound water can be a major contributor to the bulk electrical conductivity of LCR rocks and should be treated quantitatively. Scanning electron microscope micrographs, capillary pressure curves, and proton magnetic resonance (PMR) data for samples of LCR sandstones from different geographical regions show directly or by inference a common characteristic - a bimodal distribution of large and small pores. The micrographs show small pores line the surface of the primary pore regime and are formed primarily by clay minerals, and the large and small pore regimes are interconnected. Mineral analyses show that all the common clay minerals can contribute to a network of smaller pores. The capillary pressure curves also suggest a sequential desaturation in the sequence of large to small pores. The bulk conductivity of the LCR sandstones studied does not correlate to a significant degree with either the amount or the type of clay minerals in the rocks. Log-log plots of resistivity index versus partial water saturation for most LCR sandstones are nonlinear, with the nonlinearities being of the degree and type predicted by a generalization of the CRMM that we call GCRMM. GCRMM treats capillary-bound water conductance quantitatively and separately from matrix conductance, which includes surface conductance; whereas, the CRMM treats capillary-bound water as a part of matrix conductance. This generalization better describes how the observed physical properties that seem to characterize LCR rocks affect their bulk electrical conductivity.