Coiled Tubing Drilling (CTD), particularly Underbalanced Coiled Tubing Drilling (UBCTD), is a strategic technology for redeveloping mature oil and gas fields. It enables economic, low-emissions recovery of bypassed reserves by performing precision re-entries, drilling short-radius laterals, and establishing new reservoir contacts from existing wellheads, thereby leveraging existing infrastructure to lower costs and environmental footprint.

History and Technology Evolution

CTD has evolved from a basic mechanical re-entry tool into a sophisticated directional drilling system. Early systems in the 1990s enabled operations like drilling through tubing and whipstock sidetracks. A major advancement came with wireline-steerable bottomhole assemblies (BHAs), which provided real-time telemetry and control, allowing for precision reservoir navigation and safe underbalanced operations. Modern BHAs now integrate logging-while-drilling (LWD) measurements for geosteering and are ruggedized for harsh environments. Surface equipment has also advanced, with purpose-built CTD units featuring automated control, enhanced safety systems, and integrated pressure control for underbalanced conditions. The natural integration of CTD with underbalanced drilling workflows minimizes formation damage and allows wells to flow during drilling. Today, CTD is delivered as a unified service package encompassing well engineering, real-time operations, and digital workflows.

Engineering Design and Optimization

Successful CTD operations require rigorous, multi-domain engineering. This involves trajectory planning for short-radius wells, BHA design tailored to lithology and steering needs, and coiled tubing string design optimized for fatigue life and torque transfer. Hydraulic modeling is critical, especially for UBCTD, to manage multiphase flow, hole cleaning, and precise bottomhole pressure control using nitrogen or gas. Geomechanical analysis defines safe operational limits. The entire design is a tightly coupled system where changes in one parameter, like tubing geometry, can significantly impact rate of penetration and reservoir exposure, necessitating thorough front-end engineering and dynamic updates during operations.

Operational Applications and Case Studies

CTD and UBCTD have been successfully applied at scale across diverse regions:

• Saudi Arabia: CTD is a scalable development strategy for deep, tight gas reservoirs. Using dedicated rigs and advanced systems, CTD laterals deliver three to four times the production of overbalanced wells without fracturing.
• Norway: A disciplined, phased pilot program first validated CTD in injector wells before expanding to producers, creating a robust template for offshore redevelopment.
• Kuwait: UBCTD was used as a critical response tool to control a blowout, drilling relief well laterals under controlled underbalance to safely redirect flow and reduce pressure.
• United Arab Emirates: A CTD campaign significantly enhanced field production and storage capacity, achieving a 500% increase in producer well productivity and a 100% improvement in reservoir storage by expanding reservoir contact.
• Malaysia: CTD enabled rigless thru-tubing re-entries from small offshore platforms with space constraints, accessing bypassed reserves in depleted reservoirs.

Future Outlook and Strategic Implications

UBCTD is transitioning from a niche technology to a strategic lever for asset redevelopment. It aligns with industry needs for capital efficiency, reduced carbon footprint, and enhanced recovery from aging fields. By minimizing new infrastructure requirements, mitigating formation damage, and shortening cycle times, UBCTD offers a cost-effective pathway to incremental reserves. The