Real-Time Distributed Fiber Optic Sensing for Cement Sheath Integrity Monitoring
Authors: Feiyu Su , Yongcun Feng , Xiaorong Li , Saxing Li 
Published: August 2024

ABSTRACT:
The integrity of cement sheath is critical to oil and gas effective extraction, in which the cement displacement efficiency and solidify quality are the most important factors affecting the cementing quality. At present, the conventional monitoring methods can only monitor the integrity of the cement sheath at a certain time and a certain measuring point, which can not meet engineering field test requirement. In this paper, a novel method is introduced based on laboratory experiments to monitor the cement hydration and displacement process in real time using Distributed Acoustic Sensing (DAS) technology. The DAS system is used in a laboratory-scale well model, which has a fiber optic cable installed spirally on the outside surface of a casing string. The strain changes at the cement-casing bond are successfully captured by continuous 24-hour monitoring. In addition, the fluid leakage channels are preset at the second interface between cement sheath and formation of the model, and the fluid leakage simulation experiments under different flow rates are carried out. The results show that the strain rate profile measured by DAS system can accurately determine the Top of Cement (TOC) and the location of fluid leakage.

1. INTRODUCTION
The cement sheath placed in the annular space between the casing and the formation plays the role of sealing the formation, reinforcing the borehole wall and protecting the casing. It is the essential part to prolong the life of the oil and gas well and protect the environment. Once the integrity of the cement sheath fails, it can easily lead to interlayer channeling, casing deformation, and even blowout (Fan et al., 2021; Qi et al., 2011; Zheng et al., 2022; Zheng et al.,2023).
The integrity failure of cement sheath is affected by multiple factors (Feng et al., 2017; Li et al., 2023; Zheng et al., 2022; Zheng et al., 2022; Zheng et al., 2024). First of all, the cement top is insufficient due to less cement slurry injection or fluid leakage. Secondly, the effect of cement slurry displacement is poor during the injection process. Cement slurry cannot effectively wash the residual drilling fluid or mud cake on the wellbore and casing wall. This residue is easy to form the double interface (cement sheath-casing interface and cement sheath-formation interface) weak cementation zones after cement solidification. At the same time, the loss of water and hydration of cement slurry lead to the decrease of hydrostatic column pressure in annulus. Formation fluids will invade the annulus and form fluid leakage channels in the cement sheath once the hydrostatic column pressure is lower than the formation pressure. In addition, micro-cracks may occur in the cement sheath due to factors such as injection and mining operations or aging of the cement sheath in the later production stage. In a word, the integrity of the cement sheath runs through the entire process of completion and production. The sealing effect of cement sheath is crucial for the safe and efficient development of hydrocarbons. Therefore, it is necessary to carry out real-time monitoring of the integrity of the cement sheath in the whole life cycle to warn the risk of underground fluid leakage caused by the failure of the cement sheath in real time.