Time-Lapse Variations of Multi-Component Electrical Resistivity Measurements Acquired in High-Angle Wells

Moinfar, Ali
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Measurements of electrical conductivity anisotropy are becoming increasingly popular in LWD and open-hole environments. At the same time, field experience indicates that invasion and time of logging can have a significant effect on multi-component induction measurements acquired in high-angle wells, specifically in the presence of highly permeable formations and large contrasts of fluid density and viscosity. Some efforts have been made to study these effects, but the actual spatial distribution of fluid saturation due to invasion has not been considered in previous works. We use a commercial multi-phase fluid-flow simulator to reproduce invasion behavior in high-angle wells. Simulations provide us with an environment to reliably examine time-lapse effects of different geometrical and petrophysical parameters on the ensuing spatial distribution of fluid saturation. The study considers the effects of both water- and oil-base muds invading hydrocarbon-bearing formations. Simulations show that gravity segregation causes off-centered and asymmetric spatial distributions of electrical resistivity in high-angle wells. Furthermore, no-flow boundary conditions at impermeable shale boundaries cause high local concentration of mud-filtrate. These effects can vary significantly with time. Depending on the time of measurement acquisition after the onset of invasion, three-dimensional (3D) effects originating from invasion can distort the matrix of multi-component measurements. Consequently, inversion techniques should not be used blindly to synthesize all the measurements in the estimation of anisotropic electrical conductivity. It is imperative that 3D effects be diagnosed and separated in the analysis prior to estimating electrical resistivity anisotropy parallel and perpendicular to bedding plane. We suggest that opposite signs of cross-coupled components as well as non-monotonic variations of co-planar components over time could be used to discern invasion effects from other 3D effects.