A Compact Multisensor LWD Tool Optimized for Unconventional Reservoirs
Authors:   Scientific Drilling International, Langford Cory , Mickael Medhat ,   Innovative Downhole Solutions, Barnett Craig ,   Consultant
Published: January 2024

High-cost deepwater, high-pressure/high-temperature (HP/HT), and extended-reach wells in conventional reservoirs drove impressive developments in logging-while-drilling (LWD) technology in recent decades. However, the length, cost, and mechanical specifications of traditional LWD tools make them generally unattractive for use in onshore unconventional reservoirs, which present very different economic and technical challenges. This has led to the development of a compact integrated LWD tool optimized for geosteering, evaluating, and optimally completing unconventional reservoirs. Azimuthal spectral gamma ray, high-resolution ultrasonic imaging, and azimuthal sonic sensors are incorporated into a single 14.5-ft (4.4 m) sensor collar. Applications of the azimuthal spectral gamma ray sensor include real-time geosteering, organic content evaluation, and clay content determination in uranium-bearing shales and carbonates. The ultrasonic imager provides high-resolution borehole images in both water-based and oil-based muds for fracture and fault detection, stress orientation, formation dip, and borehole stability applications. The ultrasonic imager also provides high-resolution caliper data. Azimuthally oriented compressional and shear slowness measurements from the azimuthal unipole sonic sensor provide important geomechanical, geophysical, and petrophysical information, including Poisson’s ratio, porosity, and VTI shear anisotropy in horizontal wells. Log examples from various North American basins demonstrate the applications of this integrated LWD logging suite. Spectral gamma ray data from the Marcellus Shale differentiates high clay formations from cleaner, organic-rich, uranium-bearing formations, facilitating the evaluation of both organic content and clay content from K, U, and Th data. Ultrasonic imager data from the Marcellus Shale and Permian Basin reveal natural fractures and formation dip, as well as borehole breakout in both normal and thrust fault stress regimes. Azimuthally focused unipole array sonic measurements from a horizontal well in the Wolfcamp Formation resolve intrinsic VTI anisotropy. Together, these measurements allow operators to locate and geosteer in unconventional strata with higher organic content and/or geomechanical properties, which are more conducive to hydraulic fracturing. These measurements also facilitate engineered completions, where frac stages are selectively placed to group together rocks with similar mechanical properties in each individual stage. Incorporating multiple logging sensors with particular value for unconventional reservoirs into a single compact drill collar represents a new direction in LWD technology. In addition to the primary drilling and evaluation applications, recent log data have also revealed several novel uses, including (1) observing the early time progression of borehole breakout by comparing ultrasonic images acquired while drilling and shortly after drilling, (2) detecting gas influx while drilling from decreases in ultrasonic imager amplitude, and (3) monitoring significant variations in mud slowness during various drilling operations using the ultrasonic mud cell. This integrated logging suite has also found applications beyond unconventional reservoirs, including use in fractured granite geothermal drilling and, when combined with LWD resistivity, in conventional offshore wells.