Digital Solutions Improve the Sustainability of Underbalanced Coiled Tubing Interventions
Authors: Fonseca S. Hassig , Molero N. , Sulays A. A. , Khan A. M. , Vodnikov N. , Spesivtsev P. , Ramondenc P. 
Published: February 2025

Abstract
As the global energy transition accelerates, coiled tubing (CT) operations face the challenge of delivering increasingly sustainable well interventions. This can be achieved by reducing operational time and optimizing pumping strategies without compromising operational integrity and safety. This challenge is compounded in depleted fields, where more complex interventions requiring longer operating times and facing high levels of operational uncertainty, such as underbalanced coiled tubing cleanouts (CTCOs), are increasingly needed.
CTCOs in low-pressure wells rely on establishing a slight underbalance to maximize annular velocity and the carrying capacity of solids to the surface. Equivalent circulation density (ECD)—traditionally used in underbalanced drilling—and flowing reservoir pressure (FRP) are estimated in real time using a new CT acquisition digital solution. ECD and FRP are used to optimize the underbalanced environment. FRP helps define drawdown, for which there is a fine optimal operating window. An increase in ECD may incur losses, which can result in stuck CT pipe, extend intervention time, and impact overall efficiency. A decrease in ECD may result in exceeding critical drawdown and produce unwanted solids. Real-time ECD and FRP provide the CTCO operator with the necessary information to fine-tune the pumping schedule for an efficient and sustainable CT intervention.
The new CT acquisition digital solution uses real-time downhole data from sensors on the CT bottomhole assembly, surface data from CT equipment, and a priori information such as the wellbore geometry to estimate critical parameters of CTCOs, including ECD and FRP. These parameters are estimated, aggregated, and plotted in real time. The underbalanced CTCO has been successfully executed in live-well conditions in gas wells with heavy inorganic scale and mixed deposits across the production tubulars, averting losses to reactivate gas production while achieving safe handling of liquid, heavy solids, and gas. Visualization of ECD and FRP enabled the underbalanced operating condition and facilitated optimizing aspects of the CTCO: CT speed, bite size, bottoms-up frequency, and frequency of solids flushing in the flowback setup. The new solution and workflow optimized the cleanout interventions by eliminating nearly 24 working hours, 1,500 bbl of cleanout fluids, and 12,000 gal of liquid nitrogen as compared to the CTCO using the traditional, underbalanced approach. When compared to the traditional overbalanced approach—one which may not even be viable in highly depleted wells—the new solution and workflow yield an effective reduction of 66.7% in CO2e.
Novel digital solutions play a fundamental role in achieving more efficient and sustainable CT interventions in underbalanced conditions. In some cases, when downhole conditions are adverse and at the operating limit, they even make such operations possible in the first place. These underbalanced cleanouts are among the earliest implementations that leverage a new CT digital solution that can be tailored to guide and enhance CTCOs amid tightening working conditions. The work paves the road for the creation of additional execution advisors using the same acquisition architecture and facilitating the delivery of other CT downhole applications.