Effect of microannuli on ultrasonic pulse-echo resonance and flexural Lamb-wave cement-evaluation measurements

In the oilfield industry, subterranean wells are constructed by placing steel tubes, so-called casings, inside the borehole and filling the space between casing and rock with cement. After the cement has cured, its proper placement is often verified through ultrasonic measurements (such as modes exploiting the casing thickness resonance or flexural/extensional Lamb waves) from the accessible interior of the casing. Because of environmental effects such as cement shrinkage or changes in static pressure, the mechanical bond between casing and cement can be altered with significant effects on the acoustic measurement response. The casing-to-cement interface can open a gap, a so-called microannulus, measuring in thickness from submicrons to hundreds of microns and filled with either gas or liquid. A direct and unambiguous characterization of these microannuli through an ultrasonic measurement is rendered difficult due to the subwavelength dimensions of the microannuli. We have studied the experimental signature of ultrasonic pulse-echo thickness resonances and flexural Lamb waves for liquid-filled microannuli for various annulus materials and steel casing thicknesses. This characterization enables us to statistically link measurement results to microannulus thicknesses. For the highest-precision measurements, we used in-situ laser interferometry to characterize the exact microannulus thickness through transparent annulus samples.