Dual Induction

Key Takeaways

• The physics of induction logging make it particularly effective in fresh to moderately saline mud systems (where the mud resistivity is not too low), in contrast to focused electrode resistivity tools (laterologs) that require conductive (saline) mud to operate effectively: induction tools use electromagnetic induction and therefore function regardless of whether the borehole fluid is conductive or non-conductive, making them the preferred measurement in oil-based mud (OBM) environments and in water-based mud with freshwater or low-salinity formulations; in contrast, laterolog tools require the borehole fluid to conduct electrical current, limiting their use to water-based muds with sufficient salinity; the practical consequence is that the selection between induction-based and laterolog-based resistivity tools is determined by the mud type and formation resistivity range, with induction preferred in fresh mud or oil-based mud and laterolog preferred in highly saline muds where the contrast between the conductive borehole fluid and the resistive formation can be measured more accurately by electrode tools than by the electromagnetic induction approach.
• The invasion profile interpretation from dual induction logs assumes a simplified step-contact invasion model (a sharp boundary between the flushed zone at Rxo and the undisturbed zone at Rt with an invaded zone of transition at Ri) that is a useful approximation but rarely matches the actual gradual transition from drilling-fluid-saturated pore space to native-fluid-saturated pore space: in reality, the capillary pressure, wetting characteristics, and permeability anisotropy of the formation create a complex invasion profile that may include a flushed zone, a transition zone, and a native zone with gradual compositional change across tens of centimeters to meters; the three-curve dual induction interpretation workflow uses chart-book corrections (tornado charts or computer-based inversion algorithms) that relate the measured ILD, ILM, and SFL values to Rt, Rxo, and the invasion diameter di through equations derived from modeling the induction tool response to step-profile invasion; the accuracy of the Rt determination from these corrections is critical because Rt is the primary input to the Archie equation that calculates water saturation (Sw) and hence hydrocarbon saturation in the reservoir.
• Array induction tools (such as the Schlumberger AIT, Baker Hughes HDIL, and Halliburton HRAI) have largely replaced the dual induction log in modern wireline logging because they measure resistivity at five or more radial depths of investigation simultaneously (from approximately 10 inches to 90 inches from the borehole wall), providing a much more detailed characterization of the invasion profile and allowing higher-accuracy inversion of Rt from the multiple measurements; the additional radial resolution of array tools also enables detection of thin resistive beds (less than 3 feet thick) that are below the vertical resolution of dual induction tools due to the large induction coil spacing required for deep investigation; despite the superiority of array tools, the dual induction log remains the standard interpretation model because the majority of historical well log data in legacy field databases was collected with dual induction tools, and consistent interpretation methodology across old and new data requires maintaining the dual induction framework for cross-well comparisons and field-wide reservoir evaluations.
• Environmental corrections applied to dual induction logs account for systematic measurement errors caused by the borehole itself and by adjacent beds that influence the tool's measurement volume: borehole correction removes the effect of the conductive mud column in the borehole (which is within the induction tool's measurement volume and artificially reduces the apparent formation resistivity by contributing a conductive path parallel to the formation); bed thickness correction removes the influence of adjacent low-resistivity beds that blur the resistivity of a thin high-resistivity pay zone (shoulder bed effect) by computing the response that would be obtained for an infinitely thick bed at the logged resistivity value; formation dip correction accounts for the fact that inclined beds present a different effective thickness to the tool than the true formation thickness measured perpendicular to bedding; these corrections are applied in sequence using manufacturer-supplied correction charts or logging company software, and uncorrected dual induction logs in environments with large boreholes, thin beds, or significant invasion can be substantially in error for Rt determination and water saturation calculation.
• The moveable hydrocarbon index (MHI), calculated from dual induction interpretation as the ratio Rxo/Rt normalized to the water saturation derived from Rt, indicates whether the hydrocarbons detected in the formation are recoverable or residual: if the mud filtrate (which displaces original pore fluid during invasion) has itself been displaced by original formation water on the shallow investigation measurement (Rxo approaches Rw, the connate water resistivity), the implication is that the formation contains primarily water and no moveable hydrocarbons; if Rxo indicates high resistivity (hydrocarbon-saturated flushed zone), the mud filtrate has not displaced original fluid effectively and the formation may contain viscous oil or gas-saturated tight rock; the contrast between Rxo and Rt provides a qualitative measure of fluid mobility — a formation where Rt shows high resistivity (hydrocarbons present) and Rxo also shows high resistivity (mud filtrate did not displace the hydrocarbons during invasion) contains light, mobile hydrocarbons that flowed back toward the wellbore as mud filtrate invaded, while a formation where Rt is high but Rxo is low (mud filtrate displaced the original fluid) may contain heavy, immobile oil that the filtrate could displace but that cannot flow on its own under reservoir conditions.