Achieving Zonal Isolation in Challenging Formations Using Novel Scrubbing Technology and Foam Cementing

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Abstract Achieving reliable zonal isolation in low-fracture-gradient reservoirs remains a persistent challenge across the Middle East. Elevated hydrostatic and frictional pressures during circulation and cement placement frequently induce losses into weak formations, jeopardizing annular cement coverage and long-term well integrity. Effective drilling fluid removal is critical to ensuring proper cement bonding and hydraulic isolation. This paper presents the design, execution, and evaluation of a novel scrubbing spacer deployed in combination with a low-density foam cement system to address these challenges. Achieving optimum cement bonding while cementing across weak formations prone to lost circulation typically necessitates the use of two-stage cementing, lightweight slurries incorporating low-specific-gravity materials, or foam cement systems—each associated with operational trade-offs. Despite these mitigation strategies, post-job bond logs sometimes reveal suboptimal cement placement and inadequate casing-to-formation bonding. In this study, a specially engineered foam cement slurry with self-healing, expansion and anti-gas migration properties was designed to cover a challenging carbonate hydrocarbon producing interval while minimizing equivalent circulating density (ECD). A novel scrubbing spacer incorporating mechanical scouring particles was pumped ahead of the cement to enhance gelled mud removal and improve cement bonding. Extensive laboratory testing validated foam stability, slurry properties, fluid compatibility, and set cement performance. A modified rotor cleaning test was developed to quantitatively compare mud removal efficiency between conventional spacer systems and the new scrubbing spacer. The scrubbing spacer achieved greater than 93% cleaning efficiency in modified rotor testing, significantly outperforming conventional spacer systems, which averaged approximately 65% efficiency. Field execution included cementing a 7-in. production liner across an 8½-in. open hole section. The operation was completed as designed, with top of cement (TOC) achieved as planned and no losses observed. Cement bond log (CBL), variable density log (VDL), and ultrasonic measurements confirmed effective zonal isolation, with low CBL amplitudes and strong formation arrivals. Compared with prior conventional cementing approaches that exhibited micro-debonding and inadequate mud displacement, the combined foam cement and scrubbing spacer system demonstrated improved annular cleaning, minimized micro-annulus risk, and achieved consistent cement coverage. The results confirm that this integrated approach provides a robust solution for cementing operations in low-fracture-gradient environments, enhancing wellbore integrity and supporting optimized hydrocarbon recovery.

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