Development of Transducer and Electronics Technology for an LWD Ultrasonic Imaging Tool

Abstract

Abstract A new logging-while-drilling (LWD) ultrasonic imaging tool was designed and tested to acquire high-resolution borehole surface images in any type of drilling fluid. The LWD ultrasonic imaging tool uses ultrasonic transducers to acquire pulse-echo signals from the borehole wall and pulser/receiver and acquisition electronics to store the ultrasonic image and caliper data. The transducer, pulser/receiver electronics, and acquisition electronics have been successfully tested to meet functional targets. The design of the transducer incorporated ultrasonic modeling to achieve the desired sensitivity and bandwidth using a composite piezoelectric element, two impedance matching layers, and a high-impedance backing. Two designs variants of the transducer were built and tested. The final design was selected by evaluating the performance of each transducer design before and after environmental testing. The pulser/receiver electronics were designed to deliver wideband excitation with optimal damping to reduce transducer ringdown. The acquisition electronics necessitated a low distortion path, variable gain control, a low noise floor, and consistent channel-to-channel performance. Functional testing on the electronics system verified the analog front-end had low channel-to-channel variation with various input signals and with exposure to 165° C. A Digital Signal Processor (DSP) and a Field Programmable Gate Array (FPGA) managed the high-capacity memory to store the high-resolution ultrasonic image and caliper data with a high spatial sampling rate. The FPGA was verified with Universal Verification Methodology (UVM) and with code coverage and functional coverage metrics. UVM increased design reliability and reduced development time by facilitating the creation of many test vectors. The LWD ultrasonic imaging tool has a sufficient spatial sampling rate to fully sample the borehole in the typical ROP and RPM ranges experienced while drilling. Sample downhole images show the LWD ultrasonic imaging tool compares favorably with wireline ultrasonic and resisitivity images and correlates well to LWD resistivity images in water-based mud.