A New Resistivity Measurement System for Deep Formation Imaging and High-Resolution Formation Evaluation

Abstract

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 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.