The definition of unconventional reservoirs continues to evolve over time as advances in technology make it more viable to extract hydrocarbons. The need for reservoir characterization in such reservoirs, however, will continue to increase to optimize wellbore placement and enhance production. For high-angle or horizontal wellbores common in unconventional drilling, obtaining information from wireline technologies may be either too expensive or risky, although obtaining a wellbore stability assessment while drilling provides a key input into the real-time geomechanical model. This paper presents field test results of a new 4¾-in. ultrasonic imaging logging-while-drilling (LWD) tool that provides a real-time assessment of borehole shape and high-resolution caliper and acoustic impedance images in both water-based mud (WBM) and oil-based mud (OBM) applications.

Images from measurements, such as gamma ray, resistivity, or density, are common in LWD applications. However, high-resolution images have historically been limited to WBM applications. This paper describes the sensor physics and tool configuration that enable the acquisition of borehole caliper and acoustic impedance images in all mud types, with examples of logs obtained while drilling in boreholes using OBM. Details of the comparison with wireline data sets are also given.

Vertical and horizontal wellbores covering different lithologies are described, showing that high-resolution images are now available in slimhole OBM applications. Caliper images illustrate small changes in borehole shape, and impedance images can be used to evaluate geological features and determine stratigraphic dip. The evaluation of caliper data with a wireline multifinger caliper illustrates the potential to eliminate a separate wireline run before completing the well. Comparison of while-drilling data with tripping out of hole data provides crucial insight into wellbore deterioration with time.

The technology described addresses key challenges encountered while drilling and evaluating unconventional reservoirs. Real-time wellbore stability assessment enables optimization of drilling parameters and mud weight in all unconventional reservoirs. Identification of faults and fractures provides valuable information to optimize the hydraulic fracturing program in shale gas applications. Inputs into the geomechanical model are valuable in the assessment of tight sand reservoirs with extremely low porosity and permeability. Limestone reservoirs with minor shale content may require OBM to minimize wellbore deterioration with time. Monitoring such deterioration is critical in optimizing the placement of packers and the hydraulic fracturing program design.

Providing the industry's highest-resolution images in all mud types, even under high logging speeds represents a unique method of assessing real-time wellbore stability and enhancing formation evaluation in slim wellbores in unconventional reservoirs.

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