Through-Tubing Lightweight Cement (TTLWC) Evaluation in Production Wells
Authors:   GOWell, Abdelmoula Hichem , Zhao Jinsong ,   GOWell, Yang Qinshan ,   GOWell, Rourke Marvin ,   GOWell, Sookram Neil ,   GOWell, Zeghlache Mohamed Larbi ,   Saudi Aramco
Published: November 2024

Cement placement and bonding are critical for zonal isolation and well integrity assurance. Cement bond evaluation with sonic logs has been used in the hydrocarbon industry for over 50 years, and for about 30 years, higher-resolution ultrasonic logs have also been available to improve cement placement diagnostics. However, when there is tubing in the well, these traditional technologies fall short because they cannot evaluate cemented casings or liners behind tubing. Reliable through-tubing (TT) cement evaluation is technically very challenging, but finding an effective solution for it has been long desired in the oil and gas industry. The benefits are obvious because it positively reduces intervention time and costs, as operators would not need to remove the tubing string to conduct cement evaluation, as currently required with existing technologies. This TT cement evaluation challenge also increases significantly when there is a need to inspect casing cemented with lightweight cement (LWC) because of its low density and limited acoustic impedance contrast with borehole fluids. This paper presents the study of a new method for TT-LWC measurement and evaluation. Instead of conventional acoustic wave propagation, the proposed method relies on the novel principle of measuring the nonharmonic resonance modes of the coupled elastic structure comprised of the multiple pipe strings in the well. A transducer is used to emit one or more continuous sinusoidal excitation signals to activate the nonharmonic resonance modal shapes in the multipipe structure. The vibration impedance of the structure is tested for the power requirements to maintain stable vibration modals, allowing the power losses corresponding to the energy being coupled across the casing-LWC bond interface and dissipating into the formation to be measured. The research shows that the system resonance frequencies and their corresponding mode shapes are affected by Poisson’s ratio and Young’s modulus of the material behind the casing. The proposed TT-LWC solution overcomes the main challenges associated with the low-density and low-impedance LWC conditions. The new solution uses a pad-free apparatus for cement evaluation applications in wells with or without tubing. An additional challenge in any TT cement evaluation is the effect of tubing eccentricity and its heavy impact on the measurements through the tubing. The paper also covers the tubing eccentricity correction algorithm from sectorized measurements made with the tool. The absence of radial symmetry affects the structure vibration and creates an uneven distribution in the sinusoidal excitation in the tubing-casing annulus. Tubing eccentricity affects the impedance measurement, resonance frequency, and mode shape, resulting in inaccurate bond evaluation. A machine-learning (ML) based algorithm was developed to analyze the eccentricity and the cement bond condition on the impedance measurements from each sector. The solution was evaluated with simulations and lab tests, and the performance and reliability were examined in different pipe combinations, conventional and LWC bond conditions, and tubing eccentricity scenarios. The results confirm the ML algorithm effectively removes tubing eccentricity contribution from the measurements, and this novel nonharmonic resonance technique provides valid TT bond evaluation for conventional and lightweight cement.