In 2018, coiled tubing (CT) interventions in the Norwegian sector witnessed a rise in the adoption of high-grade quench and tempered (Q&T) CT strings. These interventions primarily focused on multistage fracturing stimulation, involving tasks such as sliding sleeve manipulation, fishing, and, as the wells aged, underbalanced CT cleanouts using nitrified fluids due to reservoir depletion. Recent CT interventions in wells revealed significant wellbore abrasion, manifesting as longitudinal grooves, where dogleg severity builds.
This study aims to challenge the perception that high-grade CT, with its increased material hardness, is the primary cause of wellbore abrasion damage. An analysis of over 360 CT runs in 22 wells and more than 30 recent caliper logs sought to identify patterns behind this abrasion. Wells experiencing erosion were categorized by intervention type, wellbore environment (wet or dry) during CT work, CT normal force profiles, abrasion severity from caliper logs, and CT grade used during the work scope. Weighting these data allowed for identifying the primary factor contributing to wellbore abrasion during CT interventions.
The study found that high-grade CT is not the primary contributor to abrasion. Instead, the leading causes of wear are a dry or partially dry wellbore environment during interventions and high CT normal forces. Most affected wells with significant abrasion experienced reservoir pressure depletion, resulting in proppant instability and chalk debris entering the wellbore, hindering production. These wells used CT for chalk and proppant cleanout via underbalanced cleanout with nitrified fluids (base oil with nitrogen and/or gas). The wellbore became dry due to sub-hydrostatic conditions, and partially dry during nitrified cleanout. This dry environment increased the abrasion risk as the CT interacted unlubricated with the production tubing. Caliper logs revealed that abrasion primarily occurred at depths between 250-m and 500-m measured depth, where the well deviation and dogleg severity increased. Normal forces magnitude, tied to stiffness and trajectory, spiked at these shallow depths when running in, and amplified while pulling out of hole. This concentrated force, combined with the dry environment due to deeper liquid levels in sub-hydrostatic wells, compounded the abrasion issue. Furthermore, high overpull operations, like fishing or sleeve shifting at deeper depths, elevate normal forces at shallower depths, raising the abrasion risk.
These findings sparked a significant shift in intervention planning and execution, with the development of local mitigation measures to reduce abrasion risk; these measurements and the analysis of normal forces are now integrated into the CT operations design process, influencing mature wells intervention planning, cleanout strategies, production management, and completion lifetime expectancy; and are considerations potentially influencing future field completion strategies, including well trajectory adjustments, and stimulation techniques selection. This study opens the potential of developing methods to quantify the abrasion rate for each CT run through data analysis and testing.
