Complex conductivity of tight sandstones

Abstract

Induced polarization well logging can be used to characterize sedimentary formations and their petrophysical properties of interest. That said, nothing is really known regarding the complex conductivity of low-porosity sedimentary rocks. To fill this gap of knowledge, we investigate the complex conductivity of 19 tight sandstones, one bioclastic turbidite, and four sand/smectite mixes. The sandstones and the bioclastic turbidite are characterized by low to very low porosities (in the range of 0.8%–12.3%) and a relatively narrow range of cation exchange capacity (CEC — [Formula: see text]). The sand-clay mixtures are prepared with pure smectite (Na-Montmorillonite, porosity approximately 90%, CEC [Formula: see text]) and a coarse sand (grain size approximately [Formula: see text]). Data quality is assessed by checking that the percentage frequency effect between two frequencies separated by a decade is proportional to the value of the phase lag measured at the geometric frequency. We also checked that the normalized chargeability determined between two frequencies is proportional to the quadrature conductivity at the geometric mean frequency. Our experimental results indicate that the surface conductivity, the normalized chargeability, and the quadrature conductivity are highly correlated to the ratio between the CEC and the bulk tortuosity of the pore space. This tortuosity is obtained as the product of the (intrinsic) formation factor with the (connected) porosity. The quadrature conductivity is proportional to the surface conductivity. All these observations are consistent with the predictions of the dynamic Stern layer model, which can be used to assess the magnitude of the polarization associated with these porous media over the full range of porosity. The next step will be to extend and assess this model to partially saturated sandstones.