Applications of Coil Tubing
1. 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 important 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 that the wellbore is clean. The whole operation could take multiple trips. Coil tubing can be used for running the clean-out assembly and for pumping fluid trains. Tripping the continuous tubing multiple times in the wellbore as needed results in significant time savings.
2. 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 that the wellbore is clear or 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.
3. 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 carrying out milling or clean-out operations, coil tubing can be used for the confirmation run to save tripping time.
4. Perforation: Perforation is the process of creating 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 the flow of fluid 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.
5. Well Stimulation: Well-stimulation is carried out to improve the well productivity by cleaning existing channels and opening new channels for improved flow of hydrocarbon from the reservoir to the wellbore. Common stimulation methods are either injecting acid or fracking to split the rock and pump proppant to keep these fractures open for the flow of hydrocarbons. Coil tubing can be used for well-stimulation operations efficiently and cost-effectively.
6. Nitrogen Lifting: On many occasions, heavier liquid associated with formation fluid could accumulate in the wellbore suppressing 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 created by this 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.
7. Logging and Camera Run: Oil and gas well operations require running several open-hole and cased-hole logs for formation, casing, and cement evaluation. In certain special operations, a downhole camera needs to be lowered into the well for better assessment and planning of remedial action. In most cases, the logging tools or cameras are run on a wireline. However, wireline operations become difficult in more than 55 – 60 degrees wellbore inclination. In such scenarios, running logging tools or cameras on coil tubing is a viable option. In addition to the ability to push logging tools through high deviation sections, coil tubing enables continuous circulation which helps maintain hole stability and reduces wellbore temperature.
8. Drilling: The process of drilling requires rotation of the drill bit, slacking of 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. Common applications for coil tubing drilling are drilling of small diameter wells, sidetracking, or re-entry drilling in existing wells. These situations can be handled in a cost-effective manner using a Coil Tubing Drilling (CTD) unit rather than mobilizing a drilling rig. A coil tubing unit can be used for underbalanced drilling as well, which minimizes formation damage and improves drilling rates.
9. Cementing: Coil tubing is used for secondary cementing operations like 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.
10. 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 the small diameter, spotting a cement plug has minimal chances of contamination. The well abandonment can be carried out in a very cost-effective manner using a coil tubing unit without the need for mobilizing a workover rig.