Improved Resistivity Interpretation Utilizing a New Array Laterolog Tool and Associated Inversion Processing

In many situations the traditional dual laterolog does not supply sufficient information in thinly bedded formations to unambiguously determine true formation resistivity (R t ). In addition, the laterolog-deep measurement may be affected by the presence of highly resistive layers, especially when logging close to the casing (Groningen effect) or when the tool is conveyed on drillpipe. These limitations of the traditional dual laterolog measurement can lead to sub-optimal development decisions and even to missed production opportunities. A recent array laterolog tool, combined with inversion processing, provides answers to these issues. The tool makes several focused measurements with different depths of investigation, in concept similar to an array induction tool. The hardware is optimized to supply maximal measurement-information. The data is interpreted by inversion software using formation models of varying complexities to best approximate the true formation resistivity. All measurements use bucking currents returning to the tool rather than to surface, effectively eliminating the Groningen and drillpipe effects. At the wellsite, R t is provided by a fast one-dimensional (1D) inversion that takes into account only radial resistivity variations and is strictly applicable only in thick beds. The input-curve reconstruction identified inconsistencies between this simplified model and the true formation that would go unnoticed when inverting conventional dual laterolog data. A typical example is the proximity effect of a cap rock in a horizontal well, which produces curve separation that is qualitatively distinct from the separation caused by invasion. In thinly bedded formations, inverting a two-dimensional (2D) axisymmetric formation model improves the interpretation by simultaneously accounting for radial and vertical resistivity variations. The inversion can handle complex invasion profiles and produces a 2D image of the formation resistivity. These methods were applied to surveys of a number of wells drilled in clastic formations, ranging from vertical to highly deviated and horizontal. Comparison with tools operating according to the dual laterolog principle clearly showed the benefit of this new technology. Interpretation improvements were obtained mainly from the increased information content of the array signals. Also, the absence of artifacts associated with distant current returns (such as Groningen-type effects) improved the saturation evaluation.