While they provide a recognized technical advance for wells drilled with oil-based mud (OBM), OBM-adapted microresistivity images of the last 13 years remain far from the geologic interpretability provided by imagers that operate in a water-based mud (WBM) environment. Recently the use of a high-definition WBM imager has been demonstrated in wells drilled with OBM, but its application has been principally limited to high-resistivity formations with excellent hole conditions or to cases where the drilling fluid has been engineered to favor acquisition.

To fill this gap, a new wireline microelectrical imager has been introduced, engineered from the ground up to acquire high-definition, full-coverage images in any well drilled with OBM. The all-new physics architecture includes a strategy to minimize and eventually eliminate the inevitable contribution of the nonconductive fluid and to optimize the mode of operation in accordance with formation parameters. New tool-specific processing steps complement the standard borehole image processing workflow to render highly representative images of the formation.

Examining the measurement response in detail, via both modeling and real-world examples, demonstrates several favorable characteristics, for example, sensitivity to vertical as well as horizontal features, reduction of shoulder-bed effects, and reduced sensitivity to desiccation cracks.

The novel mechanical architecture includes a new sonde design with significant operational advantages. It conveys a sensor array composed of 192 microelectrodes providing 98% circumferential coverage in an 8-in. borehole. The individual microelectrodes are smaller than those of industry-standard imagers for WBM, each with a surface area of only 10.8 mm², which provides excellent spatial resolution.

From a field test comprising more than 40 operations in various OBM fluids, high-definition images were acquired in a variety of environments, from high-resistivity carbonates to shales and low-resistivity clastics, demonstrating the robustness and widespread applicability of the new tool. The examples include challenging environmental conditions and they explore the limits of accurate measurement. Comparison with legacy images demonstrates that the new physics of measurement coupled with the high-resolution, high-coverage sensor array has achieved much more than a microimaging step change. The new images faithfully reproduce formation geology with photorealistic clarity and promise to revolutionize the geologic interpretation of wells drilled with OBM.