Numerical simulations of ultrasonic flexural waves in cased wellbores and evaluations of the cement bond quality

Cement bond quality is significant to productivity and security of a cased wellbore in the oil/gas field. The ultrasonic flexural-wave evaluation is a potential way to detect the debonding which appears at the cement sheath interfaces. In this paper, we present the ultrasonic wavefields in a multilayered cased hole and reveal the flexural wave responses to defects at the cement annulus. To excite and record Lamb waves of the flexural modes, an inclined pitch-catch sonic system is arranged inside the steel casing. The wavefields in the layered media are modeled by the 2D/3D finite difference methods. It is found that two groups of flexural waves are prominent in the wavefields. They are the primary flexural (PF) waves arriving early and the secondary flexural (SF) modes received by the transducers later. If a fluid-filled channel appears at the casing-cement interface, the PF-wave attenuation will decrease with the increasing width of the debonding. In contrast, the energy of SF waves will be greatly enhanced if the channel is located between the cement and the formation. For a 250-kHz sonic transmitter, the resolution of detection for a sector defect is about 10 degrees. Hence it is possible to effectively evaluate the bond quality of the cement annulus by extracting the properties of ultrasonic flexural waves.