Hunka, J.F., and Barber, T.D.,* Schlumberger Well Services;Rosthal, R.A., Logging While Drilling; Minerbo, G.N., Schlumberger-Doll Research; Head, E.A., *Howard Jr., A.Q., and Hazen, G.A., Schlumberger Well Services; and Chandler, R.N., Etudes and Productions Schlumberger
Abstract
A new induction logging system has been developed that represents a fundamental departure from the technology and application of previous induction-based resistivity tools. The AIT* Array Induction Imager tool abandons the concept of fixed-focused sensors and is constructed of several independent arrays with main coil spacings ranging from a few inches to several feet. The AIT tool is operated simultaneously at several frequencies; in-phase and quadrature signals are acquired from every array at the frequencies suitable for that array length. The presented log curves range in median depth of investigation from 10 in. to 90 in. Each log uses all the measured channels, combined with a nonlinear processing algorithm, to virtually eliminate environmental effects such as cave effect, shoulder effect, and skin effect. Reliable logs can be obtained even in difficult cases of bad borehole and extreme invasion. These logs are available at resolution widths of 6 ft and 2 ft. Because of the large number of measurements made by the AIT tool, deep two-dimensional quantitative imaging of formation resistivity is possible. These images expose bedding and invasion features in a clear and quantitative manner. The resistivity in the part of the formation undisturbed by fluid invasion is accurately obtained without making any prior assumptions about the invasion profile. New invasion description parameters conveying more profile. New invasion description parameters conveying more meaningful information about the presence of transition zones and annuli are a result of this imaging capability. Using established interpretation principles, this quantitative information about the invasion can be converted into a two-dimensional image of water saturation.
Introduction
In recent years, the most troublesome environmental features of induction tools have been corrected by tools such as the Phasor* Induction tool. Vertical resolution has been improved from 8 ft to 2 ft; automatic correction for borehole effect is available at the wellsite; and inversion of the three measurements (deep induction, medium induction, and Spherically Focussed (SFL*) log or laterolog) into R, estimates has been made automatic. With the elimination of the grosser environmental distortions, some remaining effects that can introduce errors in difficult logging situations have received more attention. The most troublesome of these are: Cave effect in irregular boreholes, andDetermining Rt in the presence of invasion transition zones. Cave effect is produced when an induction toolen counters a washout or cave in a borehole with high formation resistivity to mud resistivity contrast. Induction arrays normally have response peaks very close to the tool that are very sensitive to conductivity. These responses are cancelled in smooth boreholes, but they do not cancel in rugose holes. Large excursions on the logs can occur when one of these sensitive areas encounters a washout or other irregularity. These "hot spots" are present in all previous induction tools. An invasion transition zone is any radial resistivity profile other than a simple step profile. Transition zones profile other than a simple step profile. Transition zones can be produced by any fingering or mixing of mud filtrate with connate fluids over some radial distance or by the formation of an annulus. Dual induction tools can detect some transition zones, and recent field log evidence suggests that transition zones may be more common than previously realized. previously realized. P. 295
