Through tubing cement evaluation in a multi-string well has been considered as a cost-efficient way for well integrity evaluation without removing the production tubing. Conventional acoustic cement bond logging methods are not able to operate accurately with the multi-string structure due to an extremely low sensitivity and Signal-to-Noise (SNR) ratio. Therefore, it is important to develop novel technology and apparatus that can accurately and efficiently monitor the cement condition in the multi-pipe cased well. Applications would include use in production, injection, and storage well configurations as well as for plug and abandonment planning.
To this end, a novel through-tubing cement evaluation technology based on a Selective Non-Harmonic Resonance (SNHR) is proposed. Unlike the traditional acoustic wave propagation method (WPM), the new tool emits continuous energy to excites the SNHR of the multi-string structure, considered to be a multi-degree of freedom Duffing system. This includes coupling of the hydraulic pressure in fluid and elastic stress-strain of solid materials. The continuous sinusoidal excitation from the SNHR tool drives the structure in a long burst mode and measures the resonance power loss due to the energy leaking through the cement layer, to represent the casing-cement bond, as well as cement-formation bond condition. The SNHR tool, therefore, has overcome the main challenge, which is the acoustic energy reflections and dissipation through multiple interfaces for existing WPMs. The SNHR tool was validated theoretically and experimentally. Results showed that the SNHR tool can reach high sensitivity (> 10%) and SNR (>10 dB) for variable combinations of pipe sizes up to 14”. This implies that the SNHR is a promising technique for evaluating cement bond integrity in the annulus of an outer-most pipe string when multiple inner pipes and their associated annuli are liquid-filled. In addition, the SNHR tool does not require direct coupling to the first pipe string through pads or extensions, which reduces the engineering complexity of a field-worthy instrument.