What is the role of coiled tubing in sidetrack drilling?

At-a-Glance: Coiled tubing enables through-tubing sidetrack drilling and re-entries with minimal footprint, rapid mobilization, and the option to drill underbalanced—ideal for restoring or adding reserves without a full drilling rig. Its role spans kickoff creation (whipstock, cement plug, or jetting), lateral drilling, and short-radius multilateral access while managing well control within existing completions.

I. Objective & KPIs

Define how coiled tubing (CT) is applied to plan and execute a sidetrack from an existing wellbore—typically through tubing/casing—to create a new borepath and access bypassed or new reservoir targets with lower cost, lower risk, and minimal downtime.

• 1.1 Throughput/Performance KPIs
  • Footage drilled per 12-hour shift: 50–300 m
  • Average ROP: 1–8 m/h (milling window); 3–15 m/h (soft–medium formations)
  • Planned lateral length vs. achieved: =90%
  • Kickoff depth accuracy: ±1–3 m; azimuth error: =5°
  • Dogleg severity (DLS): =8–12°/30 m (short-radius CTD); tortuosity index minimized
• 1.2 Reliability/Uptime KPIs
  • Operational uptime: =92%
  • NPT: =8% of time; unplanned BHA trips: =1 per 24 hours
  • CT fatigue utilization: =80% of safe cycles at minimum bend radius
  • Motor stalls: =2 per 1,000 m drilled
• 1.3 Cost & Efficiency KPIs
  • Cost per meter drilled: benchmark vs. workover rig and rotary alternatives (target -30 to -60%)
  • Rigless days saved: =3–10 days vs. conventional
  • Fuel/OPEX intensity reduction: =20–40% (smaller footprint)
• 1.4 HSE/Well Control KPIs
  • Zero well control incidents; PCE integrity test to =1.1× MAWOP
  • Losses/returns within managed envelope; differential sticking incidents = 0
  • Emissions intensity: -20–50% vs. rig-based sidetrack (scope 1)

II. Critical Parameters & Target Ranges

Key formulas (LaTeX)

• \( ECD_{ppg} = MW_{ppg} + \dfrac{\Delta P_{ann}}{0.052 \times TVD_{ft}} \)
• \( AV_{ft/min} = \dfrac{24.5 \times Q_{bpm}}{ID_{in}^2 - OD_{in}^2} \)
• \( HHP = \dfrac{P_{psi} \times Q_{gpm}}{1714} \)
• \( RPM_{motor} \approx K_s \times Q \quad;\quad T_{motor} \approx K_t \times \Delta P_{motor} \)
• \( DLS_{^\circ/30m} = \arccos\!\Big(\cos \alpha_1 \cos \alpha_2 + \sin \alpha_1 \sin \alpha_2 \cos(\Delta \phi)\Big)\times \dfrac{180}{\pi} \times \dfrac{30}{\Delta MD} \)

III. Role & Step-by-Step Workflow

III.A What CT Enables in Sidetracks

• 3.1 Through-tubing re-entry: Sidetrack without pulling production tubing; minimal intervention footprint.
• 3.2 Kickoff creation: Set and mill window via whipstock; or kickoff from a cement plug; or hydraulic jetting to initiate departure.
• 3.3 Underbalanced/Managed Pressure Drilling (UBD/MPD): Nitrogen/foam or MPD choke to minimize losses and skin in depleted zones.
• 3.4 Short-radius laterals: Access near-wellbore targets and thin sands with high dogleg capability.
• 3.5 Cost/schedule compression: Rigless or light WO unit; rapid mobilization; lower logistics.

III.B Execution Checklist

• 3.6 Pre-job engineering
  • Well diagnostics: caliper, cement bond, pressure tests; confirm MAWOP and barrier plan.
  • Hydraulics and ECD model; select fluid system (brine/polymer/foam/UBD).
  • Buckling/reach modeling; select CT OD/WT; fatigue life budget.
  • BHA design: CT connector, agitator/oscillator, jars, motor, orienter, MWD/PWD, mills/bits, circulation sub.
  • Method select: whipstock vs. cement plug vs. jetting based on casing condition and target build.
• 3.7 Rig-up & PCE integrity
  • Pressure control equipment: stripper/packer, CT BOP (shear, blind, pipe rams), lubricator as required, flow-T, choke manifold.
  • Function and pressure tests to =1.1× MAWOP; verify accumulator and emergency shutdowns.
• 3.8 Wellbore preparation
  • Cleanout to kickoff depth; scale/sand removal; drift and gauge.
  • Set anchor packer (if whipstock) or spot cement plug; WOC and tag for firmness.
• 3.9 Kickoff execution
  • Whipstock: orient with gyro/MWD; run starter mill; open window to design length; confirm via weight/torque signatures and caliper if available.
  • Cement plug: dress to angle; steer motor to initiate departure; verify deflection with surveys.
  • Hydraulic jetting: use abrasive nozzles to erode window; proceed with pilot bit.
• 3.10 Lateral drilling
  • Control toolface and WOB; maintain AV and cuttings loading =5% by volume at surface.
  • Apply agitator and periodic wiper sweeps; manage ECD/UBD with choke/N₂ as designed.
  • Survey frequency: every 10–30 m; monitor DLS and tortuosity.
• 3.11 Completion interface
  • Condition hole; displace to completion fluid; run liner/patch/screen as applicable.
  • Clean up and flowback under MPD if needed; demobilize.

IV. Risks & Mitigations

• 4.1 CT buckling/lock-up
  • Mitigate with higher Q for drag reduction, agitator/oscillator, friction reducer, tapered CT, optimized trajectory (limit DLS), and incremental reaming/wiper trips.
• 4.2 Hole cleaning in high inclination
  • Maintain AV targets; periodic high-vis sweeps; step-rate tests; real-time cuttings load model validation.
• 4.3 Losses and instability (depleted/fragile formations)
  • Use MPD/UBD, foam or mist systems; LCM pills; minimize ECD by nozzle optimization and motor ?P balance.
• 4.4 Motor stalls and BHA failures
  • Stall detection via ?P spikes; automatic pump ramp-down; torque-limited ROP; temperature management.
• 4.5 Well control/H₂S
  • Dual barriers; sour service PPE and materials; continuous gas monitoring; MPD choke drills; ensure shear capability of BOP for CT OD/WT.
• 4.6 Casing/whipstock damage
  • Mill selection and stabilizer spacing; verify orientation; control WOB and vibration; confirm window geometry before proceeding.
• 4.7 CT fatigue and ballooning
  • Track cumulative cycles; pressure-step management; adhere to collapse/ballooning envelopes; replace section if margin <20%.

V. Optimization Levers

• 5.1 BHA optimization
  • High-speed motors with low bit aggressiveness for milling; higher torque motors for hard streaks.
  • CT orienter with closed-loop toolface control; short-gauge PDC or hybrid mills for stable kickoff.
  • Add agitator and jars for friction and stuck-pipe contingency.
• 5.2 Hydraulics tuning
  • Nozzle configuration to maximize HHP at bit while maintaining ECD margins.
  • Use friction reducers/low-solids systems; for UBD, tune N₂ injection to hold BHP slightly below pore pressure (e.g., 50–150 psi underbalance).
• 5.3 Data-driven control
  • Real-time models for ECD, cuttings transport, and torque/drag with alarms.
  • ROP optimization via DOC limiters and stall-avoidance logic using motor ?P and vibration signatures.
• 5.4 Debottlenecking surface systems
  • Booster pumps for higher Q; efficient solids control (desilters/desanders) to protect motor and maintain rheology.
  • MPD choke manifold integration and high-resolution Coriolis flowback metering.
• 5.5 Trajectory management
  • Plan low-tortuosity build-and-hold; avoid excessive DLS that increases drag and limits reach.
  • Survey quality: gyro in cased hole; EM or pulse in open hole depending on lithology and fluid.

Additional formulas (LaTeX)

• \( Q_{gpm} = 42 \times Q_{bpm} \)
• \( ECD_{psi/ft} = 0.052 \times ECD_{ppg} \)
• \( \Delta P_{motor} = \Delta P_{surf} - \Delta P_{ann} - \Delta P_{nozzles} - \Delta P_{CT} \)

VI. Verification & Monitoring Plan

• 6.1 Before operations
  • PCE pressure tests and function tests logged; leak-off test or integrity confirmation at kickoff depth.
  • CT string tally, ovality, wall thickness, and fatigue baseline recorded.
• 6.2 During operations (real-time)
  • Surface: pump rate, standpipe pressure, returns flow, gas breakout, density/viscosity, cuttings volume.
  • Downhole: MWD inclination/azimuth, DLS, annular pressure (PWD), motor ?P, vibration.
  • Computed: ECD, AV, torque/drag margin, CT fatigue usage, stall index.
  • Frequency: 1–5 s streaming; KPIs plotted hourly; survey every 10–30 m MD.
• 6.3 After operations
  • Post-job CT inspection and remaining life assessment; BHA inspection (bit dull grading, motor condition).
  • Window geometry verification (if applicable) via caliper/imaging; trajectory QA/QC vs. plan.
  • Performance report: ROP, cost/m, NPT root causes, lessons learned for next sidetrack.

Summary: Role of CT in Sidetrack Drilling

Coiled tubing’s role is to deliver efficient, controlled sidetracks through existing completions by providing a continuous conduit for circulation and mechanical energy (motors/mills) under robust well control, with the flexibility to operate underbalanced or managed pressure. It excels at short-radius kickoffs, cased-hole window milling, cement-plug departures, and economical laterals where conventional rigs are impractical or cost-prohibitive—unlocking reserves while minimizing downtime, risk, and emissions.