Advanced Corrections of LWD High-Resolution Resistivity Images in Hostile Downhole Environments

Abstract

Abstract High-resolution resistivity images can be impacted negatively by several factors, especially in high-conductive water-based mud (WBM) systems. Additionally, borehole and drilling environmental parameters like elevated levels of drilling dynamics, multiple rotation regimes of the drill string, the logging while drilling (LWD) imager's eccentricity and excessive salinity are major factors, if uncontrolled while drilling, can result in severe image quality degradation. Even though such control is desired, it is not always possible. To overcome these conditions and their related artefacts, this article demonstrates a recently developed correction method to restore an acceptable definition of fine features from such heavily affected images. The implemented, multi-stage, correction methodology is based on the LWD raw image dataset and comprises a series of sequential corrections based on the examined environmental artefacts and their severities. The image correction is conducted in a strict order to take most effect. For full transparency of the correction, all intermediate steps are available for quality control purposes and enables the user to potentially improve certain correction steps if required. The final correction level is one image with the most detail restored possible, honoring the data contained in the raw dataset. This study focuses on the specific objective of identifying fine structural features in slim lateral boreholes, drilled through carbonate reservoirs under uncontrolled and unfavorable environmental challenges. These conditions are a challenge for high-resolution LWD resistivity imager data. The outcome of the correction methodology results in a remarkably sharper definition of fractures and faults (high-dip and high-contrast features) and reduction of the artefacts of LWD imager/string eccentricity effects. In addition, recovery of most of the lost information caused by severe drilling dynamics and associated image dataset spike artefacts. A comparison of structural features from the final corrected LWD image dataset to wireline (WL) imager was conducted and good repeatability was concluded between both different technologies with similar WBM salinity properties. The methodology enables the LWD WBM high-resolution resistivity imagers to deliver up to standard image quality in extreme downhole drilling and environmental conditions.