A New High-Resolution Integrated Resistivity and Acoustic LWD Imager in Oil-Base Mud: A Case Study from Offshore Norway

The ability to acquire borehole images in oil-based mud is demonstrated using a prototype LWD tool with dual ultrasonic and electromagnetic measurements in a well drilled offshore Norway. High-resolution ultrasonic images of travel time and amplitude provide a wealth of data about both the borehole condition and the formation. These are similar in resolution to wireline ultrasonic imagers. Where formation acoustic impedance is low contrast, the ultrasonic does not differentiate features so clearly. The electromagnetic images are often complimentary to the ultrasonic images, due to the greater dynamic in the resistivity of the formation. The electromagnetic images from the LWD have a resolution of about 1 inch and highlight the layering in this well. A wireline resistivity imaging equivalent was run subsequently. Interpretation of both imaging tools gives equivalent results from a geological perspective in the mixed clastic sequence of mudstones, sandstones and coals. The higher resolution of the wireline imager provides more information on small scale texture and in the thinly layered mudstones; especially where the layers are thinner than the LWD resistivity imagers resolution and have low acoustic impedance contrast. Conversely, the LWD imager defines the damage in the borehole and fractures more clearly.