Downhole scale removal is known to be an uneasy task but removing scales in wells with sub-hydrostatic pressures puts it in a different league of challenges. Conventional descaling techniques fall short in sour wells due to cases of uncontrolled hydrogen sulfide (H2S) gas releases at surface, which is a major Health Safety and Environment (HSE) issue, topped with the inability to maintain fluid circulation which causes formation leak off and inevitably induced reservoir damage. This paper discusses under-balanced coiled tubing intervention of live wells for descaling operations.

The descaling technology has evolved on the backs of the recent advances made in real-time downhole monitoring capabilities using fiber optics. This system includes a bottom-hole assembly (BHA) with a full array of real-time sensors (pressure, temperature, casing collar locator, gamma ray, load measurements), and powerful wellbore cleaning tools that can penetrate and remove hard scale deposits. These BHAs’ are designed to withstand high level of shocks and vibrations with bottom-hole temperatures exceeding 300°F. Assisting the fiber-optic telemetry is a fluid hydraulics simulation software that accounts for solids (i.e. scales removed) loading in the fluid stream in equivalent circulating density (ECD) calculations and maintaining a predetermined underbalance condition throughout the descaling process. Since there is high chance for H2S leakage on surface in an underbalanced, flowing well condition, it becomes essential to have a gas-tight, error-free well testing equipment package deployed to meet this challenge.

Presented in this paper is a detailed account of the operational planning, risk assessment, and execution of the underbalanced CT descaling operation. The under-balanced approach proved instrumental in restoring wellbore accessibility and productivity, surpassing the limitations of conventional acid de-scaling methods. Comparisons between conventional descaling and the under-balanced operation highlight the advantages of the latter, including improved efficiency, cost savings exceeding 20%, a 30% production gain, and full restoration of well accessibility. The under-balanced method showcases its adaptability in addressing challenges posed by hard scale accumulations, demonstrating its superiority over conventional techniques. This paper contributes valuable insights into the evolution of descaling interventions techniques, showcasing the success of under-balanced operations as a transformative approach in the realm of live descaling methods. The comparative analysis provides a comprehensive understanding of the operational efficiencies and benefits associated with adopting innovative under-balanced techniques for well interventions.