High-Resolution Geological Interpretation in any Mud Type Using LWD Acoustic Borehole Image Logs

Full geological interpretation is now possible from logging-while-drilling (LWD) in oil-based mud following the advent of high-resolution acoustic borehole imaging technology. The new service provides acoustic amplitude images of the borehole wall, in addition to borehole shape in a resolution that greatly surpasses previous capabilities in oil-based mud. The measurements include acoustic amplitude (this response is a mixture of petrophysical properties and borehole shape) and acoustic travel-time (showing the stand-off of the borehole wall from the sensor). From the travel-time measurement, we calculate the borehole profile which is shown in a tube plot and is used to project borehole shape properties. In water-based mud, the acoustic imaging tool can be run in combination with the existing high-resolution electrical imager. This combination is particularly useful for characterizing fractures with the dual resistivity and acoustic properties for a more confident interpretation. We show examples from wells with a range of geological complexity, including fractures, faults, geomechanical features, sedimentological variation and changes in borehole shape. Contrast between lithologies and structural features and the background is similar to that seen in electrical measurements in the same environments.

Fractures with a high acoustic amplitude response show a marked contrast with the background lithology. In many cases, they reveal the processes of formation including cataclasis, intersections, age-determining relationships and impact on reservoir connectivity. The inclusion of borehole shape with the acoustic amplitude image gives a new value to fault interpretation, where post-drilling stress release can be interpreted from the shape of the borehole surrounding a fault. In this example, micro-slippage along a fault can be seen directly in the protrusion into the borehole of one side of the fault. Amplitude images are detailed enough to allow full image facies characterization similar to that possible from wireline image equivalents, but with the benefit of measurement of the formation before the build-up of mudcake. We show examples in clastic and carbonate lithologies that include primary sedimentary structures, secondary remobilization processes and diagenetic overprints. Sedimentary detail in coal beds is unprecedented, including bedding and variation in mineralogical components. Applications based on borehole shape are shown, including geomechanical features, borehole stability, time-lapse logging and hazard mitigation.