Coiled tubing drilling: A strategic enabler for mature field redevelopment

PEDRO RANGEL, JOHN RIZAL JENIE and CHUKWUMA OJUKWU, SLB  

INTRODUCTION  

Fig. 1. Coiled Tubing Drilling (CTD) offers precision re-entries, short-radius laterals, and new reservoir contacts from existing wellheads.

Redeveloping mature fields has become central to upstream strategy. Remaining barrels are increasingly trapped by depleted pressure, formation damage, and complex reservoir architecture while capital and emissions constraints demand lower-cost, lower-footprint options. Coiled Tubing Drilling (CTD), especially when executed under underbalanced conditions (UBCTD), offers a distinctive solution—precision re-entries, short-radius laterals, and new reservoir contacts from existing wellheads, Fig. 1.   

By combining continuous-length intervention with modern steerable BHAs, closed-loop pressure control, and real-time telemetry, CTD enables economic, low-emissions recovery of bypassed pay. This article traces the technology’s evolution, engineering practices, operational case studies, and the strategic role that CTD will play in mature-field redeployment.  

HISTORY AND TECHNOLOGY EVOLUTION  

Coiled tubing drilling has undergone significant technological transformations, from a simple mechanical re-entry tool to a fully instrumented directional drilling system capable of geosteering, high-frequency data transmission, and integration within live underbalanced environments. This evolution has been shaped by advances in three core areas: bottomhole assembly (BHA) technology, surface equipment, and full-system integration with managed pressure and underbalanced drilling workflows.  

Early system architecture. The earliest CTD systems introduced in the 1990s were fundamentally mechanical in nature. Directional control was limited, telemetry was either non-existent or dependent on rudimentary surface interpretation, and the application space revolved around basic re-entry activities. Yet even these early systems unlocked critical capabilities that hinted at CTD’s future potential: 

• Drilling through existing tubing strings
• Executing whipstock-based sidetracks
• Setting liners and hangers
• Performing basic coring operations.

These foundational successes demonstrated that continuous pipe, combined with small-diameter BHAs, could perform complex tasks inside constrained wellbores—planting the seed for CTD as a redevelopment tool. 

The wireline steerable era: Real-time control and precision. A step-change occurred with the introduction of wireline-steerable BHAs, transforming CTD from a mechanical intervention system into an actively controllable drilling platform. Electric orienters provided bi-directional toolface control, while the adoption of high-speed wireline telemetry enabled real-time access to a suite of critical parameters, including: 

• Toolface orientation and steering vectors
• Inclination and azimuth
• Gamma-ray and casing collar locator (CCL) signatures
• Annular, internal and differential pressures
• Shock, vibration, and motor stall indicators.

Data rates reaching 100 kbps provided a level of situational awareness that would be challenging for mud pulse telemetry, especially in compressible, gas-rich, or multiphase downhole environments where conventional telemetry fails. This capability repositioned CTD as a viable drilling method for precision reservoir navigation and safe underbalanced operations.  

Modern BHAs: LWD capable and ruggedized. Today’s CTD BHAs have integrated LWD measurements for real-time geosteering and are engineered for extreme performance. Ruggedized electronics, modular architecture, and advanced fatigue-resistant housings allow tools to operate in high-shock, high-temperature environments previously deemed unsuitable for coiled tubing. 

Key upgrades include: 

• LWD tool capacity (such as CTDirect™ Ultraslim LWD Resistivity series) for real-time geosteering
• High-torque orientersdelivering up to 1,900 ft-lbf in short-radius environments 
• Dual-release mechanisms—tension, hydraulic, or electric to safeguard operations during stuck-pipe events
• Extended-run motors and turbinesoptimized for abrasive, high-compressive-strength formations 

These advancements have elevated CTD into a fully integrated LWD-enabled drilling system with the capability to deliver reservoir-quality data in slimhole geometries. 

Rig and surface system evolution. Purpose-built CTD units have undergone multiple generations of improvement. Modern packages feature: 

• Dedicated BHA handlers, eliminating hazardous suspended-load operations
• Taller masts to facilitate BHA make-up without breaking containment
• PLC-controlled surface packages, including injector head and flow control elements (choke, valves, etc), enabling both remote control and autonomous operation modes
• Multi-redundant pressure control equipment (PCE) stacks for sour gas and UBD conditions
• Closed-loop, automated flowback systems for real-time pressure management. 

This rig-scale innovation is particularly critical for H₂S-bearing and depleted reservoirs, where maintaining well control under underbalanced conditions requires robust, highly integrated surface equipment. This shift significantly reduces rig-up time, enhances safety, and standardizes performance across large-scale campaigns. 

Integration with underbalanced drilling. CTD’s natural compatibility with underbalanced drilling enhances its value proposition. Nitrogen or gas-lift systems, closed-loop returns, and MPD/UBD chokes maintain bottomhole pressure below pore pressure while ensuring safe surface containment. This minimizes formation damage, enhances drilling efficiency, improves reservoir connectivity, and enables wells to flow while drilling—an essential feature that contributes to early well productivity understanding.  

Operational integration and digitalization. Modern CTD is delivered as a unified service: well engineering, UBD/MPD design, reservoir modelling, real-time operations, completions integration, and logistics are managed within a single project framework. Delivering CTD as a unified service package enables optimization initiatives, such as remote monitoring and operation, optimized, multiskilled crews, and unified digital workflows for design, well placement, real-time reservoir understanding, and evaluation.  

Fig. 2. Second laterals layout in the sandstone formation; targeting different formation layers.

ENGINEERING DESIGN AND OPTIMIZATION   

The engineering foundation of a successful CTD operation relies on multidomain optimization—mechanical, hydraulic, geomechanical and operational—built through structured design workflows and robust modelling.   

Trajectory planning sets the mechanical envelope: short-radius sidetracks demand anti-collision studies, bending-strain checks, and build-rate feasibility aligned to BHA steering capability. BHA design involves drive system selection, bit type matching lithology and compressive strength, sensor package configuration for real-time decision-making, Fig. 2.   

Coiled tubing string design is a fatigue-life and torque/weight trade-off—choosing OD, wall thickness, and material grade to survive dynamic spooling stresses while limiting torque losses and buckling risk.  

Hydraulics in UBCTD are critical. Multiphase models evaluate liquid/gas ratios, pump schedules, ECD envelopes, and borehole cuttings transport under single- or two-phase regimes. Nitrogen or gas-assisted strategies are sized to maintain a desired bottomhole pressure window and to ensure hole cleaning while preventing formation invasion. Choke strategy, annular compression, and flowback containment are simulated with surge/swab sensitivity runs to protect the formation during makeup and pull-out. 

Geomechanical analysis defines allowable drawdown and fracture margins, informing acceptable underbalance and operational limits.  

The outcome is a tightly coupled engineering package. Small changes in CT string geometry can produce measurable impacts on ROP, cuttings handling, and reservoir exposure. Rigorous front-end engineering, followed by dynamic model updates during the job, is critical to well delivery success.  

OPERATIONAL APPLICATIONS AND CASE STUDIES 

CTD and UBCTD are no longer experimental; they have been applied at scale across varied geologies and use cases, from controlled relief wells to programmatic redevelopment campaigns. The most instructive lessons come from geographic diversity, where governance, geology, and commercial drivers vary. 

Saudi Arabia: Large-scale CTD as a field development strategy. Saudi Arabia has become the global benchmark for CTD deployment, transforming the technology from a specialized intervention method into a scalable development strategy for deep, tight gas reservoirs. With more than 10 dedicated UBD-capable CTD rigs, the Kingdom has drilled over 2 MMft of laterals in high-strength, abrasive formations such as the Khuff and Unayzah. 

Key success drivers include: 

• Advanced bit designsdelivering higher ROP and extended durability 
• Three generations of integrated surface systemscombining rig, MPD/UBD, and flowback functions 
• A unified execution modelenabling real-time monitoring, remote operations, and automated workflows 

The result: CTD lateral wells consistently deliver three to four times the production of wells drilled overbalanced without hydraulic fracturing. This has fundamentally reshaped the development strategy for tight and sour gas assets across the region (SPE-142363). 

Fig. 3. Accurate UBCTD precision in well positioning targets and sidetrack control versus drilled laterals.

Norway: A methodical, low-risk pathway to adoption. Norway’s latest CTD pilot reflects the North Sea’s disciplined engineering culture. The first campaign targeted injector wells—selected specifically to validate the well architecture, drilling methodology, and operational workflow in a controlled setting. 

After demonstrating consistent success, the program expanded into producer wells. This sequential validation built confidence, streamlined operational processes, and reduced risk exposure. Unlike regions that scaled CTD rapidly, Norway’s deliberate approach created a robust technical and operational template ready for larger-scale rollout. 

The results confirm CTD’s suitability for North Sea conditions and reinforce its potential as a scalable redevelopment tool in offshore environments.    

Kuwait: UBCTD as a critical response tool. In Kuwait, underbalanced CTD was deployed in one of the most challenging contexts: a blowout in a shallow, water-bearing reservoir producing 47,000 bpd, 10 MMscfd, and 2% H₂S. The operator drilled eight laterals across four relief wells under carefully controlled underbalance to prevent surface blowout propagation. Robust engineering and precise operations resulted in redirecting flow by ~70% and reducing surface pressure by ~90% in the incident well, enabling safe re-entry and abandonment, Fig. 3 (SPE-221847). 

Fig. 4. Well #2 lateral strategy and execution.

United Arab Emirates: Reservoir management for storage and production gains. In the UAE, the operator’s goal was to enhance the production of the field and increase its storage capabilities.  The campaign exceeded expectations by achieving 1.5 x  the planned footage and delivering a 500% increase in producer well productivity, while the injector wells showed a 100% improvement in reservoir storage and a 60% increase in injection rate capacity. These results were achieved by expanding the reservoir contact, conducting 7 OH sidetracks, and significantly improving well-to-reservoir connectivity, Fig. 4 (SPE-222915; SPE-222415). 

Malaysia:  Rigless thru tubing re-entry sidetrack with CTD in a small platform using the catenary system. In the South China Sea, CTD has been used to access the bypassed reserves through existing production tubing and completed with a slim completion. Brownfields with low recovery factors often contain substantial bypassed reserves in shallow, high-angle, and depleted reservoirs that are uneconomical to access with conventional drilling. 

The candidates presented additional challenges, due to their smaller, aging platforms with limited crane access, restricted deck space, and load limits, increasing operational complexity and risk. 

A phased strategy customized to address the challenges was developed, covering candidate selection, surface equipment setup with the vessel, and the execution sequence.  

This strategy was developed using lessons learned from previous CTD campaigns in the South China Sea. 

FUTURE OUTLOOK AND STRATEGIC IMPLICATIONS  

As upstream portfolios age and capital discipline tightens, Underbalanced Coiled Tubing Drilling (UBCTD) is moving from niche technology to a strategic lever. With nearly all major producing regions now dominated by aging reservoirs, operators are increasingly focused on extracting more value from existing assets safely, economically, and with a reduced carbon footprint. This shift has made recovery enhancement technologies mission-critical rather than optional. 

UBCTD directly aligns with this priority. Its capability to perform precision re-entries, drill short-radius laterals, and access stratigraphically trapped or bypassed pay zones near existing wellbores enables incremental recovery at materially lower CAPEX. By leveraging existing completions and surface infrastructure, UBCTD minimizes well construction costs, mitigates formation damage through underbalanced mechanics, and shortens cycle time from intervention to production. As capital efficiency becomes an industry-wide mandate, UBCTD delivers a scalable, cost-efficient pathway to additional barrels without the full field-development overhead associated with new well drilling programs. 

Strategically, the industry is also witnessing a philosophical shift. UBCTD is no longer viewed merely as an intervention tool but as a fully-fledged drilling methodology. This repositioning is especially evident in the Middle East, where operators are leveraging UBCTD to reduce formation damage, improve reservoir connectivity, and deliver higher production with lower completion complexity. UBCTD allows operators to unlock reserves that would otherwise remain uneconomic or inaccessible with conventional overbalanced techniques. Looking ahead, UBCTD adoption will accelerate as operators pursue greater efficiency, sustainability, and recovery optimization.   

PEDRO RANGEL serves as CTD champion and Sales lead at SLB, where he drives commercial strategy and technical excellence in coiled tubing drilling solutions. With deep expertise in well intervention and drilling technologies, Mr. Rangel partners with clients to deliver innovative solutions that optimize performance and reduce operational risk.  

JOHN RIZAL JENIE is a seasoned oilfield professional with more than 28 years of experience at SLB, where he currently serves as CTD Domain champion. His career spans leadership roles across Asia and the Middle East, with deep expertise in well intervention and coiled tubing technologies. Mr. Jenie has authored multiple technical papers for industry conferences and is recognized for advancing operational efficiency and safety in complex drilling environments. 

CHUKWUMA OJUKWU brings extensive drilling operations expertise, supported by an MSc (Eng) in Environmental Engineering & Project Management, and an MBA. His SLB career spans Drilling & Measurements and CTD BHA operations in Saudi Arabia, and he now serves as the Global Product Champion for CTD BHA based in the Middle East.