A New Multi Laterolog Tool with Adaptive Borehole Correction

Abstract

Abstract The Dual Laterolog (DLL) is one of the most commonly used resistivity logging devices in environments with highly conductive (salt-saturated) drilling mud and high formation resistivity. However, this device produces only two apparent resistivity logs with different radial depths of investigation, and it has a relatively low vertical resolution. Several array laterologs have been developed during the 1990s to improve both the vertical and radial resolution of measured formation resistivity. Each of these devices provides several depths-of-investigation measurements. However, most of these measurements, especially the shallow ones, are subject to borehole and eccentricity effects in very conductive muds and large boreholes. These effects can be severe. Therefore, conventional borehole corrections based only on borehole size, mud conductivity, and a fixed value of tool eccentricity (typically used for DLL measurements) become ineffective. A newly developed array laterolog device, Multi Laterolog, is based on the Dual Laterolog principle but with four independent focused measurements. This device is effectively shorter in total length than the traditional DLL and provides higher vertical resolution. Four measurements with different depths of investigation yield a detailed radial resistivity profile. A hardware-based feedback loop is employed to ensure the required focusing. To overcome the problems of the borehole and eccentricity effects on shallow measurements, we have developed a model-based adaptive borehole correction (ABC) technique. It is based on a radial 1-D inversion approach that properly corrects laterolog measurements for the borehole effect including determining and accounting for the unknown tool eccentricity. At every logging depth, we determine four earth-model parameters: tool eccentricity (Ecc), formation resistivity (Rt), invasion zone resistivity (Rxo), and invasion length (Lxo). After that, we apply the model-based correction by modifying the tool response in a borehole with mud resistivity Rm to a response in a borehole with virtual mud resistivity equal to Rxo (or to Rt if the formation is not invaded). This approach guarantees that in uninvaded intervals all four laterolog curves are properly "stacked", while in invaded intervals they show a reliable and consistent resistivity profile. The forward modeling used in inversion is based on the five-dimensional interpolation of pre-calculated look-up tables of various eccentered tool responses. Therefore, the inversion is very fast and the new borehole correction algorithm can be applied in real time. As a by-product, we output the length and resistivity of the invaded zone in invaded intervals. Thus, this correction scheme can provide Rt, Rxo and Lxo for thick beds together with borehole- and eccentricity-corrected curves at the wellsite. We illustrate the advantages of the Multi Laterolog and the model-based adaptive borehole corrections on synthetic data and on field data examples.