Practical Applications of a New MuItichannel and Fully Digital Spectrum Induction System
Authors: Beard Dave , Zhou Qiang , Bigelow Ed 
Published: December 2007

The performance of the high-definition induction logging tool is demonstrated with practical examples and synthetic models. For improved formation evaluation, this newly developed multichannel and fully digital spectrum induction system acquires a complete data set at multiple receiver coils over a broad band of frequencies. The broad range of investigation depths defines invasion profiles in reservoir rocks with superior quality over other tools. The improved evaluation of the flushed and transition zones allows for a better prediction of hydrocarbon mobility. The more reliably determined true formation resistivity leads to more accurate log analyses and reserve estimates.

Based on the abundance of information in the data set, flexible processing methods are developed. The tool responses are processed to achieve optimized resolution with minimal sensitivity to undesirable effects such as those caused by irregular borehole size and conductive invasion. Matched-resolution curves are also available for enhanced and uniform vertical resolution at all depths of investigation.

Because of the complete set of data, the new tool allows for synthesis of the measurements from other existing induction tools. This is advantageous for comparison to offset logs or performing field studies. In addition, the system uses data redundancy to supply self-consistency and data-quality checks as well as noise reduction. The redundancy also makes the tool tolerant to situations such as the loss of one receiver channel.

With more wells being drilled at large angles, the accurate estimation of formation resistivity in deviated boreholes, or wells penetrating formations with appreciable apparent dip, becomes more important. The multitude of receivers and wide band of frequencies makes it possible to reliably focus the tool measurements in such an environment. The processed log will have a response similar to that of logging a well penetrating the same formation perpendicularly.

The data completeness ensures future developments in other processing techniques. Integrated with other logs and reservoir information, the new induction measurements will significantly enhance the reservoir evaluation capability.

Introduction

Induction log technology has evolved significantly since its introduction by Doll. In recent years, induction devices consisting of one or two complex coil combinations have been replaced by tools with multiple, but simpler, coil configurations. These new induction devices are commonly referred to as array-type induction tools. The older style tools attempt to focus the tool response using carefully selected coil arrangements. The focusing is therefore fixed by the tool design. In array-type tools, the signals from the various coils are combined in software to achieve focusing. This software focusing is more flexible than hardware focusing in that it can be adjusted to suit a particular situation. Software focusing can also create curves that are free of artifacts, such as the cave effect, which cannot be eliminated in hardware-focused devices.

The high-definition induction log takes full advantage of the flexibility and improved performance of software focusing. In addition to the features shared by other array-type tools, this tool offers several additional features. By providing a deeper investigating measurement, the user can establish Rt in the presence of deep invasion. The instrument acquires a complete data set; i.e., the formation conductivity information is fully sampled spatially and in the frequency domain. This ensures the measurements' usefulness for advanced processing techniques that may be developed in the future. This feature also allows for synthesizing the response of other induction instruments; a useful capability when well-to-well comparisons are important, such as in equity negotiations. Finally, redundancy is also built into the system.

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