coiled tubing drilling

Coiled tubing drilling (CTD) is a drilling technique that uses a continuous steel coiled tubing string (typically 60 mm to 89 mm OD) to convey a positive displacement motor (PDM), measurement-while-drilling (MWD) sensors, and a drill bit into the wellbore without threaded pipe connections, enabling continuous drilling at depths up to approximately 4,500 m in vertical wells without the make-up and break-out interruptions of conventional drill pipe; because the CT string cannot be rotated from surface, directional control relies entirely on the bent housing of the downhole PDM or a downhole rotary steerable system to steer the wellbore, and all weight-on-bit (WOB) is applied by the injector head push force supplemented by the CT string's own weight in deviated wells. In the Western Canada Sedimentary Basin, CTD is used primarily for two applications: through-tubing re-entry drilling in mature producing wells (where a CT BHA is snubbed through the existing production tubing string into the formation to drill a short lateral sidetrack of 300 to 1,500 m into a bypassed pay zone without killing the well, pulling the existing completion, or mobilizing a full drilling rig), and underbalanced drilling in depleted WCSB reservoirs (where nitrogen or foam-nitrified fluid in the CT circulation system maintains wellbore pressure below the depleted formation pressure to prevent lost circulation and formation damage, while a rotating control device at surface contains wellbore inflow during drilling). WCSB Devonian carbonate pools at Pembina, Swan Hills, and Redwater, Cretaceous Cardium and Mannville pools in central and northwest Alberta, and Montney tight siltstone producers in northeast British Columbia represent the principal CTD re-entry targets, with operators including Canadian Natural Resources, Husky Energy, and Surge Energy using CTD to access additional reserves from existing wellbores at 30 to 50 percent of the capital cost of a new vertical well; WCSB CTD operations are governed by AER Directive 059 (Well Drilling and Completion Data Filing Requirements) and Directive 036 (Drilling Blowout Prevention Requirements and Procedures), which require a CTD-specific well program, snubbing BOP certification, and underbalanced pressure management plan approved by the AER prior to operations.

• CTD bottom hole assembly configuration and drilling mechanics for WCSB through-tubing re-entry programs: A standard WCSB CTD BHA for through-tubing re-entry operations (drilled through 73 mm or 89 mm production tubing) includes from top to bottom: CT connector (matched to the CT OD and BHA top sub thread profile), MWD collar (measuring inclination, azimuth, and gamma-ray with mud-pulse telemetry at 1 to 12 bits/second through the CT bore), near-bit stabilizer (keeping the BHA centralized within the small wellbore diameter to prevent buckling), PDM motor (3 to 5 lobe rotor converting 2 to 8 MPa differential pressure across the motor to 100 to 400 RPM bit rotation), and drill bit (PDC preferred for WCSB Cardium sandstone and Devonian carbonate targets at penetration rates of 3 to 15 m/hr with 1 to 5 kN WOB). The bit OD in through-tubing CTD is constrained by the production tubing ID: 73 mm (2-7/8") tubing with 62 mm ID limits the BHA and bit to approximately 45 to 50 mm OD, producing a 52 to 60 mm wellbore that constrains subsequent through-tubing completion options to slim-hole perforating guns and small-bore completion equipment; 114 mm (4-1/2") production tubing allows 90 to 95 mm CTD bits producing 100 to 105 mm sidetracks compatible with through-tubing perforating and acid stimulation at WCSB Devonian reef re-completion targets in Pembina and Swan Hills fields.
• Underbalanced CTD mechanics, rotating control device operation, and fluid systems in WCSB depleted reservoirs: Underbalanced CTD in WCSB depleted Cardium, Mannville, and Devonian pools (reservoir pressure 3 to 10 MPa, original reservoir pressure 12 to 25 MPa, depletion factor 50 to 70 percent) uses nitrogen gas injected into the CT bore at the surface, mixing with drilling fluid in the wellbore annulus to form a nitrified foam or gasified fluid with effective density of 200 to 600 kg/m3 (versus 1,000 to 1,200 kg/m3 for conventional water-based mud), maintaining bottomhole circulating pressure 0.5 to 3 MPa below the depleted formation pressure to prevent lost circulation into the low-pressure reservoir matrix. A rotating control device (RCD) installed above the wellhead BOP stack seals around the moving CT string at surface wellhead pressures of 0 to 5 MPa while the CT is advancing or retracting, containing the reservoir inflow of gas, oil, and water that enters the wellbore during underbalanced drilling; produced fluids flow from the RCD through a choke manifold and four-phase separator to separate gas (compressed and metered), oil (measured and transferred to storage), water (measured and disposed), and drill cuttings (filtered and disposed). WCSB underbalanced CTD in Cardium tight oil pools at Garrington, Wilson Creek, and Edson has demonstrated formation damage reduction of 60 to 80 percent compared to overbalanced CTD (measured by comparing post-drill productivity index to pre-drill open-hole log-predicted PI), attributable to the elimination of mud filtrate invasion into the tight 1 to 10 millidarcy Cardium matrix that would require extensive cleanup production before achieving undamaged flow.
• WOB limitations, helical buckling, and depth constraints for CTD in WCSB wellbores: The maximum achievable WOB in CTD is limited by the onset of helical buckling in the CT string: when axial compressive load on the CT exceeds the critical buckling force (proportional to CT OD, wall thickness, and wellbore curvature but typically 2 to 10 kN for 60 to 89 mm CT in vertical WCSB wellbores), the string begins to spiral against the wellbore wall rather than transmitting additional force to the bit, causing lock-up where further injector push produces no increase in bit force. Maximum achievable WOB without helical buckling in vertical WCSB wellbores is 2 to 10 kN for 60 to 89 mm CT, compared to 50 to 300 kN WOB achievable with conventional drill pipe in the same wellbore, limiting CTD penetration rates in hard WCSB Devonian carbonates (unconfined compressive strength 80 to 200 MPa) to 0.5 to 3 m/hr versus 5 to 20 m/hr for conventional rotary drilling. The maximum CTD depth in WCSB vertical wells is approximately 3,500 to 4,500 m before helical buckling and CT fatigue consumption become prohibitive; in WCSB horizontal and highly deviated wells (inclination above 40 degrees), gravity assists in holding the BHA against the low side of the wellbore, extending the practical CTD reach in the horizontal section to 1,000 to 2,000 m beyond the kickoff point before friction limits further CT advance.
• Directional control, MWD telemetry, and geosteering in WCSB CTD re-entry lateral programs: Directional control in WCSB CTD operations relies on the PDM's bent housing (typically 1.0 to 2.5 degrees of bend) oriented to the desired toolface direction using the CT string's rotational response to wellbore contact friction, which requires the CT operator to inject or retract small amounts of CT (0.1 to 0.5 m increments) to rotate the BHA through friction in the curved wellbore, a slow and indirect process compared to conventional drill pipe where surface rotation directly controls toolface. MWD telemetry in WCSB CTD uses mud-pulse transmission through the CT bore (pressure pulses modulated at the PDM valve traveling at acoustic speed through the drilling fluid to a surface pressure transducer) at data rates of 1 to 12 bits/second, delivering real-time inclination, azimuth, and gamma-ray measurements to the surface control cabin with a 30 to 90 second lag depending on depth and mud pulse velocity; at typical WCSB CTD re-entry depths of 1,500 to 3,500 m with telemetry intervals of 3 to 5 m, the operator receives inclination and gamma-ray updates every 5 to 15 minutes of drilling, sufficient to maintain the lateral within the WCSB Cardium or Mannville target formation (typically 5 to 20 m net pay) using a geological steering approach that adjusts inclination based on gamma-ray log response relative to the expected formation lithology sequence.
• CTD economics and WCSB re-entry program evaluation compared to new well drilling: WCSB CTD through-tubing re-entry programs are evaluated against three alternatives: new vertical well drilling ($1.5 million to $4 million capital in WCSB central Alberta), workover rig re-entry (pulling the existing completion, logging, re-completing in a new zone, re-running tubing, $600,000 to $1.5 million capital), and abandonment (no capital, but loss of remaining wellbore value). CTD through-tubing re-entry capital typically ranges from $400,000 to $1.2 million in WCSB Alberta including CTD unit mobilization ($50,000 to $100,000), BHA rental ($30,000 to $80,000 per re-entry), drilling fluid and nitrogen ($20,000 to $60,000), and AER regulatory filings ($5,000 to $15,000 per re-entry application); the economic threshold for CTD re-entry justification in WCSB operations is typically a minimum incremental production of 30 to 60 barrels per day of oil at $60 to $80 per barrel (WTI), generating $650,000 to $1.75 million per year of incremental revenue against a $600,000 to $1.2 million CTD capital cost for a payback period of 4 to 24 months that most WCSB operators accept for mature field optimization programs with proven geological success from nearby wells.

CTD Through-Tubing Sidetrack Recovering Bypassed Cardium Pay at Pembina

A WCSB operator at Pembina field used CTD to re-enter a 1989-vintage vertical Cardium oil well producing 8 barrels per day from the C sand at 1,650 m depth. The well had 73 mm production tubing through which a 44 mm CTD BHA was snubbed under 2.8 MPa wellhead pressure. The PDM drilled a 52 mm sidetrack lateral of 420 m into the previously undrilled D sand Cardium interval confirmed on offset logs to contain 12 m net pay at 11 percent average porosity. Underbalanced circulation with nitrified water (nitrogen-to-water ratio 3:1) maintained bottomhole pressure at 7.2 MPa versus 8.5 MPa measured D sand pressure, eliminating lost circulation. Post-drilling, through-tubing perforating guns fired 4 shots per metre across a 30 m interval. Post-stimulation production from the D sand sidetrack reached 47 barrels per day, adding 39 barrels per day net incremental production at a CTD capital cost of $680,000, yielding a 22-month payback at $75/barrel WTI.

Related Terms

Coiled tubing is the delivery system for CTD; the CT string conveys the BHA to target depth, applies WOB through the injector head, and circulates drilling fluid through the CT bore to power the PDM and lift cuttings in WCSB re-entry programs. Positive displacement motor (PDM) converts drilling fluid hydraulic pressure into bit rotation in CTD; bent housing of 1.0-2.5 degrees provides directional control when the non-rotating CT string is oriented to the desired toolface by friction-based rotation. Underbalanced drilling in WCSB depleted Cardium and Devonian pools uses nitrified fluid circulated through the CT to maintain bottomhole pressure 0.5-3 MPa below depleted reservoir pressure, preventing lost circulation and reducing near-wellbore damage by 60-80% versus overbalanced CTD. Measurement while drilling (MWD) telemetry in CTD transmits inclination, azimuth, and gamma-ray via mud-pulse at 1-12 bits/second through the CT bore, providing real-time geosteering data at 30-90 second lag for lateral placement in WCSB Cardium and Mannville targets. Sidetrack is the wellbore drilled by CTD re-entry from an existing WCSB wellbore into a bypassed pay zone; through-tubing sidetrack OD is constrained by the production tubing ID, typically producing a 52 to 105 mm wellbore diameter depending on tubing size.