Dielectric properties of porous rocks with partially saturated fractures from finite-difference modeling

Abstract

Fractures are common features of geologic rocks, and they have a profound influence on the fluid flow and transport characteristics as well as physical properties of the fracture-bearing rocks. Fractures are frequently partially saturated in hydrocarbon reservoirs, and the dielectric properties that are highly sensitive to the fluid types and concentrations are useful tools for the quantitative characterization of reservoir rocks with partially saturated fractures. However, the effects of partially saturated fractures on the dielectric properties of the porous rock are still poorly understood. Based on a 3D finite-difference model, we have bridged this knowledge gap by studying the frequency-dependent dielectric properties of a porous carbonate rock with partially saturated fractures obtained by the brine-displacing oil simulation. We find that the conductivity and high-frequency (higher than approximately 1 MHz) relative permittivity generally increase, whereas the low-frequency relative permittivity and peak dielectric loss decrease with the increasing brine saturation. We also find that the partially saturated fractured rock with lower brine saturation exhibits a greater low-frequency relative permittivity and maximum dielectric loss factor than the cases in which the fractures are completely filled with oil. A systematic investigation based on simplified intersecting fracture models suggests that the oil-brine distribution is the main reason for the abnormal dielectric responses of the low-frequency relative permittivity and the maximum dielectric loss factor at low brine saturations. The results provide new insights for the comprehensive understanding of the frequency-dependent dielectric responses of fractured sedimentary rocks and will pave the way for the quantification of the brine saturation in partially saturated fractures.