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New High-Resolution Oil-Base Mud Borehole Imaging for Enhanced Interpretation of Deepwater Sediments


Drilling wells in the operationally challenging deepwater environment requires the use of oil-base (OBM) and synthetic oil-base drilling muds (SOBM). The use of such mud types creates very tough challenges for the acquisition of microelectrical borehole images in these wells. Legacy technologies developed for acquisition of high resolution images in these mud systems have not been able to match the high level of image definition and geological interpretability that has been standard for decades in wells drilled with water-base fluids. This paper details the first application of high-resolution, high-coverage borehole images for sedimentological interpretation of deepwater wells drilled with SOBM, using a newly-engineered wireline imaging tool. The tool itself, first described by Laronga et al. (2013 AAPG ICE) employs new physics of measurement to produce high-definition microresistivity images in SOBM from an array of 192 sensors of 3 mm width and 5 mm height. The images, thus produced, have a circumferential coverage of 97% in a borehole of eight-inch diameter. The high resolution of these new images enables more detailed and precise dip determination, thereby lowering result uncertainty. The ‘photorealistic’ visualization offered makes it possible to directly observe sedimentary features in the formation. The paper presents a deepwater Gulf of Mexico case study, using these new images, focused on interpretation of sand and shale depositional mechanisms. Within the sands, the differing modes of sedimentation can be clearly distinguished; thinly-bedded sands are discriminated from irregularly-bedded or chaotic sands; channel scours and rip-up clasts are now clearly observed. Image-based identification of architecture elements allows a more comprehensive depositional analysis. Even in the shales, where previous imagers have been seriously challenged, the new images crisply delineate the laminations and bedding to permit accurate structural dip determination. Additionally, the differentiation of the quiet environment sediments from the high energy deposits – including debris flows – brings into light the depositional changes occurring in the basin. In the deepwater Gulf of Mexico, where drilling a well costs hundreds of millions of dollars, it is crucial to have very accurate geological interpretation. The new images help the asset geologist bring a wellbore interpretation that is much closer to reality than ever before into the geological model.