Applications of Coil Tubing
Wellbore Clean-out: Wellbore clean-out is the process of cleaning the well of all the debris after drilling or milling operations. It is an essential step before running the completion string into the well. A cleaned wellbore avoids damage to the completion tools and any adverse impact on the well productivity. Several mechanical tools, like brushes, scrappers, filters, magnets, baskets, etc, are used in the clean-out assembly. Fluid trains with special cleaning chemicals are pumped, and turbidity is checked on the surface to ensure the wellbore is clean. The whole operation could take multiple trips. Coil tubing can run the clean-out assembly and pump fluid trains. Tripping the continuous tubing multiple times in the wellbore as needed results in significant time savings.
Milling: Different types of metal, composite, or cement plugs are placed in the well in different stages of operations. Many of these plugs are later milled out to complete the well. In some cases, there could be obstruction in the well due to a fish or damaged or deformed casing. Mills are run for clearing these obstructions as well. Several runs may be needed to ensure the wellbore is clear of any obstruction before proceeding with other steps. A milling assembly can be run on coil tubing to complete these multiple runs efficiently and cost-effectively.
Confirmation Run: A confirmation run is carried out before running well completion to ensure that the well is clear of any debris or obstruction. Instead of running conventional tubing string for milling or clean-out operations, coil tubing can be used for the confirmation run to save tripping time.
Perforation: Perforation creates channels between the reservoir and the wellbore to flow formation fluid into the wellbore. It involves running explosives charges in perforation guns and puncturing multiple holes in the casing. These holes penetrate deep into the formation, allowing fluid to flow from the formation to the wellbore. Depending on the scenario, the perforation guns are either dropped in the well or spent guns are pulled out of the hole. Perforation guns can be run in and pulled out of the well using coil tubing, saving operation time and cost.
Well Stimulation: Well-stimulation improves well productivity by cleaning existing channels and opening new channels for improved hydrocarbon flow from the reservoir to the wellbore. Standard stimulation methods inject acid or frac to split the rock and pump proppant to keep these fractures open for the flow of hydrocarbons. Coil tubing can be used for efficient and cost-effective well-stimulation operations.
Nitrogen Lifting: On many occasions, heavier liquid associated with formation fluid could accumulate in the wellbore, suppressing the further flow of formation fluid into the wellbore. In these circumstances, Nitrogen is pumped through the coil tubing, lightening the fluid column above. The pressure drawdown lifts the heavier fluid out of the well. Once the hydrostatic pressure of the fluid column in the well is reduced, the formation pressure takes over, allowing the smooth flow of formation fluid into the wellbore.
Logging and Camera Run: Oil and gas wells require several open-hole and cased-hole logs to be run for formation, casing, and cement evaluation. In certain special operations, a downhole camera must be lowered into the well for better assessment and planning of remedial action. The logging tools or cameras are usually run on a wireline. However, wireline operations become difficult in more than 55 – 60 degrees wellbore inclination. Running logging tools or cameras on coil tubing is viable in such scenarios. In addition to pushing logging tools through high deviation sections, coil tubing enables continuous circulation, which helps maintain hole stability and reduces wellbore temperature.
Drilling: The drilling process requires the drill bit's rotation, slacking weight on the drill bit, and the hydraulic energy to lift the cuttings. Using coil tubing for drilling operations limits the weight that can be applied to the bit for crushing the rock. However, using a bottom-hole assembly with a power drive motor makes it possible to rotate the bit at high revolutions and pump fluid. ‘Measurement While Drilling (MWD)’ and ‘Logging While Drilling (LWD)’ tools can also be run as part of the bottom hole assembly for collecting real-time formation and well data. Typical applications for coil tubing drilling are drilling small-diameter wells, sidetracking, or re-entry drilling in existing wells. These situations can be handled cost-effectively using a Coil Tubing Drilling (CTD) unit rather than mobilizing a drilling rig. A coil tubing unit can also be used for underbalanced drilling, which minimizes formation damage and improves drilling rates.
Cementing: Coil tubing is used for secondary cementing operations, such as setting cement plugs for well abandonment, zonal isolation, or well kick-off. It is also effectively used for squeezing cement into perforations or casing leaks to attain well integrity.
Plug & Abandon: When the well has completed its objectives or reached the end of its production life, it is plugged and abandoned safely to avoid any well flow, groundwater contamination, or adverse environmental impact in the future. Coil tubing provides accurate measurements and depth control for installing abandonment plugs. Due to its small diameter, spotting a cement plug has minimal chances of contamination. Well abandonment can be carried out cost-effectively using a coil tubing unit without the need for mobilizing a workover rig.