Increased development of naturally and hydraulically fractured unconventional reservoirs from horizontal wells, drilled with oil-based muds, has created a need for high-resolution logging-while-drilling (LWD) borehole imaging tools capable of resolving fractures in this borehole environment. A new LWD ultrasonic borehole imager has been developed and tested to meet this need.

Borrowing from wireline ultrasonic imaging technology, a 250 kHz piezo-electric transducer was adapted to an LWD drill collar. The single transducer serves as both transmitter and receiver: transmitting an ultrasonic pulse, and measuring both the amplitude and two-way travel time of the acoustic reflection from the borehole wall. The LWD tool takes advantage of drill string rotation making a 360-degree scan of the borehole with a single fixed transducer. Finite element modeling and laboratory testing in artificial formations and a large limestone block were used to determine the spatial resolution of the image, as well as the sensitivity to downhole acquisition variables such as standoff, tool eccentricity, and mud attenuation. Prototype tools were then field tested in several horizontal wells to verify the functionality and image resolution under actual drilling conditions.

The borehole images from horizontal wells in unconventional and conventional reservoirs in the Middle East and the UK verified that tool responded as designed. These images, recorded in both oil-based and water based muds, revealed open and cemented natural fractures, drilling induced fractures and borehole breakout, fine-scale bedding, and other textural geological features such as vugs and stylolites. A variety of drilling-related borehole artifacts were also observed, including keyseats, stabilizer impressions in the borehole wall, tool marks from a rotary steerable tool, and gouges made by the bit rotating off bottom. The amplitude image proved more sensitive to fractures, bedding, and other geological features, while the travel time image, combined with input mud compressional velocity, provided a 360-degree borehole caliper image, showing the borehole size and shape.

Although high-resolution LWD electrical imagers have been available for years, these can only operate in conductive, water-based, muds. As most horizontal wells in both conventional and unconventional reservoirs are now drilled with oil-based muds, the development of a high-resolution ultrasonic imager capable of identifying natural and hydraulic fractures, fine-scale bedding, secondary porosity, and other small scale features in wells drilled with oil-based muds fills an important gap in LWD technology.

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