Detection and Characterization of Casing-Cement Microannulus Using the Pitch-Catch Method with a Dense Array of Receivers: A Simulation-Experimental Approach

A microannulus is an extremely thin annular gap that may develop between the steel casing and the cement sheath, or between cement and the formation, in a wellbore (Jutten & Hayman, 1993; Guo et al., 2009). Such gaps, often only a few to hundreds of micrometers wide, typically form when the cement debonds owing to temperature, pressure, or mechanical changes after placement (Taleghani & Santos, 2023). For example, Su et al. (2022) predicted that a 0.004 mm (4 μm) microannulus could be generated by downhole cooling stresses during subsequent routine production operations. Detecting microannuli is critical because even a slender fluid channel can undermine zonal isolation. A microannulus furnishes a high-permeability leak pathway for formation fluids or injected gases; for instance, it may allow gas or CO2 to migrate along the annulus rather than being blocked by the solid cement matrix. Therefore, robust cement evaluation must detect these microscopic debonds to ultimately forestall long-term leakage. One established technique for detecting and characterizing microannuli is the pitch-catch acoustic logging method (Zeroug & Froelich, 2003; Van Kuijk et al., 2005). This approach employs separate transmitting and receiving transducers to launch and record guided waves in the casing–cement system. The wave energy couples through the cement sheath and returns information about bonding quality along the logged interval. In practice, a pitch-catch tool generates an ultrasonic flexural (Lamb) wave that travels along the casing wall, and the receivers measure its attenuation and reflection as the signal propagates. Commercial pitch-catch instruments typically incorporate one transmitter and two receivers positioned 3 ft and 5 ft, respectively, from the source. When a microannulus exists between casing and cement, tools with only two receivers display several deficiencies. First, limited spatial resolution—derived from just two measurement points—yields a coarse, averaged response that can overlook subtle waveform modifications induced by a microannulus. Second, the sparse array is more susceptible to misinterpretation under noisy conditions or complex wave interactions. These factors complicate discrimination between well-bonded and poorly bonded zones when acoustic contrast is low or when casing and borehole noise degrade signal clarity. Consequently, conventional two-receiver configurations frequently show restricted sensitivity to changes in microannulus thickness and diminished robustness when detecting these defects with high confidence.