Benoit Froelich

Ultrasonic leaky-lamb wave imaging through a highly contrasting layer

This article presents an innovative and effective ultrasonic technique to image through a steel layer with immersion transducers. The purpose is to detect scatterers and external interfaces and evaluate the acoustic properties of materials bonded to the steel layer. The technique is based on the lowest-order antisymmetric leaky lamb wave propagating in the steel layer and is implemented through a pitch-catch transmitter-receiver arrangement. The main application presented herein is for a measurement that takes place in an oilfield well to evaluate the quality of the cement sheath that fills the annular region between steel casing and a rock formation. Current ultrasonic imaging techniques for such an application are based on the pulse-echo mode with a single trans-receiver. Because of the large acoustic impedance contrast between the steel and its surrounding media: fluid on the side of the transducers and cement, or fluid/mud on the other, pulse-echo techniques have proven to be limited to probing only the region adjacent to the casing-cement interface. Using field data acquired with an experimental device, we show that the proposed leaky-lamb wave-based imaging technique provides reflection echoes that allow for probing of the entire cement sheath and in particular of the imaging of the external cement-rock interface geometry.

Multimode evaluation of cement behind steel pipe

Oil wells are usually cased with a steel pipe, and cement is injected in the annulus between pipe and rock in order to provide hydraulic isolation of the reservoir. The traditional ultrasonic technique to evaluate cement behind steel pipe has been to excite the pipe thickness mode, and to extract the acoustic impedance of the material from the pulse‐echo response. However, this technique suffers from some limitations, such as a limited depth of investigation or a poor discrimination between mud and light cement. To alleviate such limitations, the thickness mode can be combined with the pipe flexural mode. Although dispersive, the flexural mode has the unique property of a quasi constant group velocity in a specific frequency domain which is high enough to provide azimuthal resolution. Such property provides the means for an accurate measurement of the time of arrival and the amplitude of the different echoes generated by a particular geometry. In particular, the echo traveling within the annulus and refle...

Theoretical model for circular benders in a rigid baffle and radiating in fluid

The bender plate wave equation in vacuum is found to be of a form similar to the usual isotropic plate wave equation. The loading effect of the fluid is then taken into account for a circular bender imbedded in a rigid baffle and acting as a transmitter. This is done using the concept of radiation impedance. The pressure amplitudes generated by two transmitting bender designs are numerically compared. In addition, using the reciprocity relations, the receiving sensitivities of such devices are evaluated.

Cylindrical ultrasonic array for borehole applications

A cylindrical ultrasonic array has been developed for operating environments that can reach extremes of 175 C and 20,000 psi. The array is a key component of the PharUSIT* (Phased Array Ultrasonic Transducer for Inspection of Tubing), a research demonstrator developed for borehole applications. The full array consists of 800 elements (10 rings of 80 elements each) and can provide a whole range of beam‐forming versatilities and capabilities in 3‐D, such as variable focusing, beam steering, electronic scanning, etc, all accomplished without mechanical movements. Special piezo‐composites have been developed for the transduction layer, and new polymeric composites have been formulated for the backing material. The center frequency was chosen to be about 500 kHz to accommodate attenuation of the propagation media. A novel technique utilizing custom flexible circuit provides electrical connections between the array and the front‐end electronics. Special fabrication processes have been developed to construct the...