What is Coiled Tubing?

        Coiled Tubing (CT), refers to a continuous, flexible steel pipe that can be spooled onto a reel, typically made of high-strength alloy steel. It is a continuously integrated tubing manufactured by producers to specified lengths that can be spooled onto a reel. The outer diameter of the tubing typically ranges from 0.75 inches to 5 inches, and the yield strength of the steel used varies from 55,000 Psi to 120,000 Psi. Currently, the longest coiled tubing that can be spooled onto a single reel for commercial applications reaches over 9,000 meters. It is a key piece of equipment widely used in the oil and gas industry, applied in operations such as drilling, well completion, workover, well cleaning, and reservoir stimulation. Unlike traditional jointed tubing, which requires section-by-section connection, coiled tubing enables continuous operation due to its integrated, seamless structure, significantly improving operational efficiency.

Main Components of Coiled Tubing Equipment

• Reel: Stores and conveys the coiled tubing.

• Injector Head: Provides power for running in and pulling out the coiled tubing.

• Operation Cabin: Where the equipment operator monitors and controls the coiled tubing.

• Power Pack: The hydraulic power source required to operate the coiled tubing equipment.

• Well Control Device: The wellhead safety device used during pressurized operations with coiled tubing.

Well Control Device   

        The well control device is another critical component in coiled tubing operations. A typical coiled tubing well control device includes a Blowout Preventer (BOP) stack and a stuffing box connected to the upper part of the BOP. (High-pressure coiled tubing operations usually require two stuffing boxes and a backup BOP.) All these devices must be selected based on their pressure ratings and suitable temperature ranges during on-site operations.

        The stuffing box is equipped with sealing elements to isolate the pressure system inside the wellbore. It is typically installed between the BOP and the injector head. The sealing of the stuffing box is divided into dynamic sealing and static sealing. The stuffing box is designed with a side-door structure, facilitating the replacement of its sealing elements while the coiled tubing is in the well.

        The BOP is connected to the lower end of the stuffing box and can also be used to control wellbore pressure. According to the requirements of coiled tubing operations, BOPs are usually designed with special features, including several pairs of rams, each of which has its specific function. The four-ram system is the most commonly used BOP in operations.

What Are the Characteristics of Coiled Tubing?

• Continuous and Efficient Operation: Its reelable design allows for uninterrupted deployment and retrieval, eliminating the need for frequent connections or disconnections, thus reducing downtime and enhancing operational efficiency.

• High Strength and Flexibility: Made from special high-strength steel, it combines excellent mechanical properties with good flexibility, enabling it to navigate complex wellbore trajectories (such as horizontal wells or deviated wells) while withstanding high pressure, high temperature, and corrosive downhole environments.

• Versatility: It supports a wide range of downhole tools and technologies, making it suitable for multiple operations including well intervention, fracturing, logging, and sand washing, adapting to diverse oilfield development needs.

• Cost-Effectiveness: Compared to traditional jointed tubing, it reduces labor and time costs associated with disassembly and assembly, especially in long-term or large-scale well operations.

• Enhanced Safety: The integrated structure minimizes leakage risks caused by connection issues, and its controlled deployment/retrieval process improves operational safety in harsh downhole conditions.