A New Asymmetrical Array Induction Logging Tool

Abstract

Abstract While array induction tools are fast approaching the 20 year mark for commercial service, a few nagging questions persist in the minds of induction tool designers. These questions relate to some of the compromises in hardware design and data processing that were necessary in the previous generation of tools. These compromises can give rise to various uncertainties in the data that ultimately degrade log responses. A new array induction tool has been designed that addresses these issues. All array induction tools promise radial resistivity curves with focal depths ranging from 10 inches up to 120 inches. However, few users are familiar with the potential uncertainty that stems from the design of the fundamental sub-arrays. In the new tool, dedicated sub-arrays are closely associated with each computed radial curve. By doing so, the new tool produces radial curves with a minimum of extrapolation. The payoff for the user is more accurate radial resistivity curves and a higher fidelity description of the formation and the near wellbore environment. This paper also describes the progress made in calibration methods and correction for thermal effects. While these subjects typically receive little attention in the literature, they are critical to the accuracy and repeatability of array induction measurements. The rigorous calibration method described in this paper assures accuracy across a large population of tools and also insures insensitivity to the calibration environment. The complexity and magnitude of thermal effects on array induction tools is examined alongside a robust, thermodynamic method for correcting them. Finally, several other methodologies are highlighted. A skin effect correction method uses only the in-phase signal measurements and corrects for the skin effect in the aspect of the apparent conductivity value and in the aspect of geometric factors as well. A real time borehole correction method computes the borehole effect with adaptively inverted borehole size, tool eccentricity, and mud resistivity. A new inversion methodology improves the efficiency and stability of the realtime radial inversion.