When invasion is moderate to deep, knowledge of resistivity of the invaded zone (Rxo) is required to derive the resistivity of the uninvaded zone (Rt) from the deep-resistivity measurement. To evaluate a formation with logs, the Rxo/Rt ratio is required for some saturation-estimation methods. In clean formations, a value of the formation resistivity factor F can be computed from Rxo and Rmf if Sxo is known or can be estimated.
Tools designed to measure Rxo have a very shallow depth of investigation, because the flushed zone may extend only a few inches beyond the borehole wall. To avoid the effect of the borehole, a sidewall-pad tool is used. The pad, carrying an array of closely spaced electrodes, is pressed against the formation to minimize the short-circuiting effect of the mud. Currents from the electrodes on the pad must pass through any mudcake to reach the flushed zone.
Microresistivity readings are affected by mudcake; the effect depends on the mudcake resistivity and thickness (hmc). Mudcakes are usually anisotropic, with the resistivity parallel to the borehole wall lower than the resistivity across the mudcake. This increases the mudcake effect on microresistivity readings to make the effective, or electrical, mudcake thickness greater than the physical thickness indicated by the caliper.
Microresistivity measurements have evolved from the first microlog, through the obsolete microlaterolog and proximity-log devices, to the current MicroSFL and Platform Express MCFL microresistivity measurements.