High-quality borehole imaging is of great value during the evaluation of borehole conditions, such as wellbore stability, fracture identification, and dip mapping. Obtaining borehole images from logging-while-drilling (LWD) tools provides real-time information and allows access to highly deviated wells, which could be difficult to access using wireline tools.
Borehole images can be acquired in water-based mud (WBM) systems with both resistivity and ultrasonic measurements. However, oil-based mud (OBM) systems limit the use of resistivity and pose great challenges to the ultrasonic method because of the high acoustic attenuation of the muds. This high attenuation can be overcome in wireline tools using larger ultrasonic transducers with representative dimensions of 2 in. diameter and 2 in. length. However, corresponding transducers intended for LWD tools, particularly for 4¾-in. collar sizes, should be smaller and sufficiently rugged to withstand the drilling environment.
This paper describes a transducer developed by the authors to meet these output, size, and ruggedness constraints. The transducers have high acoustic output and low inherent noise to produce sufficient output to provide reliable imaging, even in heavy muds. Four of these transducers and associated electronics are integrated into an LWD tool. A real-time ringing removal algorithm was implemented to record low-noise echo waveforms in pulse-echo mode. These waveforms are then processed automatically to generate both time-of-flight and reflection amplitude measurements. The former can be used to gauge the borehole shape, while the latter characterizes the acoustic impedance and rugosity of the borehole wall, which, in turn, reveal fractures and formation bedding.
During the development phase we conducted laboratory measurements to illustrate the high signal-to-noise ratio (SNR) of the transducer, which is vital when operating in the transmitting mode. We obtained satisfactory results that demonstrate high sensitivity and well-collimated directionality in receiving mode. During laboratory tests, the transducer was able to detect a 0.1-in. fracture in a cement block immersed in water. In addition, results from a prototype tool incorporating the transducers run in 9.5-lbm/gal OBM in a test well, which are comparable to wireline quality ultrasonic results, are discussed.