7 - Coiled Tubing Drilling...Should We Still Consider It?

While the idea of a continuous drill string started in the early 20's, it is not before the mid 90's when Coiled Tubing Drilling (CTD) gained some popularity, especially in Alaska and the Middle East.

While Coiled Tubing is usually used for well lifting and cleanup, large Coiled Tubing units can be used for well drilling applications (~ 1 7/8" - 4 1/2").

As any technology, improving success rate requires finding the right application. So before touching on the advantages and challenges of CTD, let's first list the applications where usually CTD is considered a strong candidate:

• Under Balanced Drilling (UBD) and Managed Pressure Drilling (MPD)

• Re-entries

• Slim-hole drilling

• Shallow Horizontal drilling

• Restricted surface location (e.g. remote location, agricultural are,...etc)

• High Build Up Rate (UBR)

• Open Hole Completion

So What Makes Coiled Tubing Drilling A Strong Candidate For Such Application?

• Injector Head and choke offer the containment needed to control bottom-hole pressure

• The smaller Coiled Tubing diameter makes it ideal for re-entries and slim-hole drilling

• Faster tripping since there are no connections

• Less equipment handling which makes it safer in terms of personnel safety (e.g. no makeup or breakup of drill pipe connections)

• Smaller Footprint which is usually 50 m x 50 m

• Ability to continuously circulate while tripping in/out (wash down and pump out).

• Usually lower ECD due to the larger annular space

• Ability to achieve high doglegs (up to 100 deg/30 m)

• Hardwired telemetry which provides Real-Time Down-hole Data

What Are The Challenges Associated With Coiled Tubing Drilling?

• Shorter life span: coiled tubing is exposed to more pipe fatigue compared to conventional drill pipes

• Limited maximum depth: Coiled Tubing buckling limits maximum depth due to helical lockup (when friction equal push)

• Poor hole cleaning: Since the coiled tubing is not rotating, hole cleaning becomes more challenging especially in deviated sections (30 deg - 60 deg). Also, the lower annular velocity might not be enough to lift cuttings and clean the hole

• Limited WOB: which is mostly provided by the weight of the CT in the vertical section

• Limited Over-Pull Capacity: as the primary stresses and pumping pressure limits the maximum allowable tension

• Restrictions in future interventions: if a high dogleg is achieved, it might be extremely risky to intervene in the future with a rotary drilling rig (with more rigid drill pipes)

• Equipment & Experience Personnel Availability.

• Cost: Coiled Tubing Units and Equipment price might be prohibitive, specially for short drilling campaigns

How to overcome these challenges?

In order to mitigate the impact of such challenges, we recommend to:

• Perform frequent short trips to clean the well-bore (every 3-5 meters)

• Frequently pump High Viscosity pills (Hi-Vis)

• Use lubricant in the drilling fluid to reduce drag

• Apply CTD in a large drilling campaign to realise the benefits from the learning curve

• Plan for the drilling campaign at least 18 months in advance to ensure equipment and personnel availability

Final Words

Each technology, Technique, or practice needs to be applied in the right environment and designed application. Coiled Tubing Drilling (CTD) is one of these activities that can have an outstanding success, technically as well as economically, if it is carefully designed, planned, and executed.

Also, We will be happy to share more details on this post, so please don't hesitate to contact one of our team members.

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