What is the process of coiled tubing fishing in well remediation?

I. High-level purpose and where coiled tubing (CT) fishing fits

Coiled tubing fishing is the targeted retrieval or remediation of stuck, broken, or lost in-hole equipment and debris using continuous, reel-fed tubing with the ability to circulate, pump, jar, mill, and convey specialized fishing tools under live-well conditions.

• I.I Purpose: remove obstructions (e.g., parted wireline, broken BHA, scale/metal junk) to restore access, integrity, and production or to enable subsequent remedial work (cleanouts, recompletions, stimulations).
• I.II Value chain position: remediation within well operations, bridging between production operations and workover; often executed to avoid a full workover rig mobilization.
• I.III Why CT: continuous pipe enables circulation and pressure control, operational under pressure with a stripper/CT BOP, and precise depth control. Advantageous in live wells, deviated/horizontal sections, and where rotation is not required or is provided by a downhole motor for milling.

II. Step-by-step process flow (planning through recovery)

II.A Planning and diagnostics

• II.A.1 Define fish: dimensions, top-of-fish (TOF), internal/external profiles, metallurgy, length, mass (estimated), and sensitivity to rotation/jarring.
• II.A.2 Acquire downhole context: deviation, restrictions, completion hardware, pressure/temperature, fluid system, sand/scale risk, sour service.
• II.A.3 Verify barriers: well status, surface and subsurface barriers per policy; select CT PCE stack and kill philosophy (overbalanced, balanced, underbalanced/foam/nitrified as needed).
• II.A.4 Select strategy: engage-and-pull (overshot/spear), wash-over and engage, mill-and-retrieve, or circulate small debris with junk baskets/magnets.

II.B Bottomhole assembly (BHA) design

• II.B.1 Core: hydraulic/mechanical jars, accelerators, bumper sub, flow-through overshot or internal spear with appropriate guides and pack-off grapples.
• II.B.2 Contingency: tapered mills, dress mills, washover shoes, junk basket, high-strength magnet, disconnect (drop ball or hydraulic), downhole motor (if milling required), no-go/centralizers for alignment.
• II.B.3 Telemetry/correlation: CT weight/pressure sensors, optional downhole gamma/CCL for depth correlation, fiber-enabled CT if available.

II.C Surface equipment readiness

• II.C.1 CT unit: reel, injector, gooseneck, control cabin.
• II.C.2 Pressure control: stripper, CT BOP with shear/seal/blind rams, lubricator as required, wellhead adapter, dual barriers, choke manifold, returns handling.
• II.C.3 Fluids/pumping: high-pressure pump(s), fluid mixing, possible nitrogen unit for nitrified/foam operations, filtration, solids control.
• II.C.4 Measurement: weight indicator, depth tracking, pressure/temperature, flow meters, returns monitoring (gas detection for sour/UBD).

II.D Execution

• II.D.1 Pre-job cleanout: circulate to remove fill; condition hole to TOF; verify ECD margin and lift capacity for debris transport.
• II.D.2 Tag and dress: softly tag TOF; if irregular, dress with mill/washover to create a clean profile for grapple engagement.
• II.D.3 Engage fish:
  • II.D.3.a Overshot (external catch): lower with flow; guide over fish; set grapple by slight overpull; confirm with weight/pressure response.
  • II.D.3.b Spear (internal catch): enter fish ID; expand/engage slips; confirm latch integrity with function test overpull.
• II.D.4 Free the fish:
  • II.D.4.a Apply controlled overpull/soak; alternate tension/compression to break differential sticking.
  • II.D.4.b Activate jars: set stretch/slack-off window; execute upward/downward jarring cycles; monitor surface weight signature and pressure pulses.
  • II.D.4.c Circulate alongside jarring to reduce differential sticking and transport debris; consider nitrified fluid to reduce hydrostatic if required.
• II.D.5 Mill/washover if needed:
  • II.D.5.a Run motor + mill to remove scale, collapsed pipe lips, or deformed profiles.
  • II.D.5.b Washover shoe to cut annular pathway and relieve sand/scale bridges prior to re-engagement.
• II.D.6 Retrieve and pull out of hole (POOH): maintain circulation to surface; stage through PCE; lay down debris safely; inspect and measure recovered fish.
• II.D.7 Contingencies:
  • II.D.7.a Use mechanical/hydraulic disconnect if stuck and unable to free BHA; ensure ball-drop path clear.
  • II.D.7.b Switch to alternative catch (e.g., spear to overshot) based on fish condition.
  • II.D.7.c Escalate to washover/milling or plan heavy intervention/workover if economics dictate.

II.E Post-job verification

• II.E.1 Confirm clear to depth with drift/gauge; verify completion integrity.
• II.E.2 Debrief: compare planned vs actual jarring loads, ECD, TOF changes; update well file and lessons learned.

II.F Critical calculations and formulas used

• II.F.1 Annular velocity (AV)

  To assess transport of debris and cuttings:

  \( AV = \dfrac{Q}{A_{ann}} \) where \( A_{ann} = \dfrac{\pi}{4}\left(D_{ID}^2 - d_{OD}^2\right) \)

  Target AV (estimated): 2.5–4.0 ft/s for viscous fluids; 4.0–6.0 ft/s for light/nitrified fluids in deviated sections.

• II.F.2 Equivalent circulating density (ECD)

  Manage to avoid losses/induced sticking:

  \( ECD\;[\text{ppg}] = MW\;[\text{ppg}] + \dfrac{\Delta P_{ann}\;[\text{psi}]}{0.052 \times TVD\;[\text{ft}]} \)

  Annular friction (Darcy–Weisbach, estimated): \( \Delta P_{ann} = f \dfrac{L}{D_{eq}} \dfrac{\rho v^2}{2} \)

• II.F.3 Force balance for freeing fish

  Required overpull to overcome sticking:

  \( F_{req} = F_{stick} + W_{fish,wet} + F_{drag} \)

  Surface setpoint (estimated): \( F_{surface} = F_{req} + F_{CT\,friction} - F_{buoyancy} \)

• II.F.4 Jarring energy

  Approximate upward jarring impact energy:

  \( E \approx F_{impact} \times s \) where \( s \) is the jar stroke (typically 6–12 in)

  With an accelerator, stored elastic energy can be approximated: \( E \approx \tfrac{1}{2} k x^2 \) where \( k \) is spring stiffness and \( x \) is compression.

• II.F.5 Hydrostatic pressure and nitrification

  Hydrostatic (balanced kill check): \( P_h = 0.052 \times MW \times TVD \)

  For nitrified fluids, effective density (estimated): \( \rho_{mix} = \phi_l \rho_l + \phi_g \rho_g \) where \( \phi \) are phase volume fractions at downhole conditions.

• II.F.6 CT buckling limits (tension/compression window)

  Helical buckling onset (vertical, estimated): \( F_{crit} = \dfrac{2 \sqrt{E I W}}{D_c} \)

  Maintain operating loads below buckling/coil yield; confirm against CT string design curve.

III. Major equipment/components and their functions

III.A Surface spread

• III.A.1 CT reel and injector head: convey continuous tubing and apply controlled WOB/overpull; injector provides traction and depth control.
• III.A.2 Pressure control equipment: stripper, CT BOP (shear/seal/blind rams), lubricator/flow-T; enables live-well intervention and emergency shear/seal.
• III.A.3 Pumps and fluid systems: high-pressure pumps, mixing tanks, filtration; nitrogen unit for nitrified/foam; choke/flowback manifold and separators.
• III.A.4 Monitoring and control: CT weight indicator, depth encoder, surface pressure transducers, flow/return sensors, gas detection.

III.B Downhole BHA (typical fishing stack)

• III.B.1 Guide shoe/centralizers: help enter fish and navigate deviations.
• III.B.2 Hydraulic/mechanical jars: deliver high-impact blows to free stuck fish (upward/downward).
• III.B.3 Accelerator (spring/elastomer): stores energy to amplify jar impact and protect CT from shock.
• III.B.4 Bumper sub: provides axial travel for setting jars and engaging grapples.
• III.B.5 Catching tools:
  • III.B.5.a Overshot (external catch) with pack-off/grapple cones.
  • III.B.5.b Internal spear (slip-type, releasable) sized to fish ID.
  • III.B.5.c Taper tap/screw-in sub for irregular IDs.
• III.B.6 Milling/wash tools: dress/taper mills, washover shoes, junk baskets, high-strength magnets.
• III.B.7 Motor/power section: provides rotation for milling where needed; nozzle configuration for dual-purpose circulation and cuttings transport.
• III.B.8 Emergency disconnect: ball-drop or hydraulic release to recover CT if BHA becomes irretrievably stuck.
• III.B.9 Instrumentation: downhole pressure/temperature/force sub or fiber-enabled CT for real-time diagnostics (if available).

IV. Key performance drivers (efficiency, cost, safety, emissions)

• IV.1 Engagement success rate: correct sizing of overshot/spear and profile preparation; accurate depth correlation; clean TOF.
• IV.2 Jarring effectiveness: adequate overpull window without exceeding CT limits; correct jar placement and accelerator selection; controlled cycle timing and count.
• IV.3 Hydraulics optimization: AV in transport window; ECD below fracture pressure; appropriate fluid rheology (viscosified vs. nitrified/foam) to balance transport and pressure.
• IV.4 CT string integrity: operating within tension, compression, pressure, and bend radius envelopes; minimize fatigue cycles at injector/gooseneck.
• IV.5 Time efficiency: minimal trips by staging BHA contingencies; pre-job cleanout; rapid tool change procedures; clear decision tree for escalate/exit criteria.
• IV.6 Safety and well control: barrier integrity; gas management; H2S/CO2 protocols; emergency shear/seal readiness; ignition control on flowback.
• IV.7 Emissions and fuel: pump/N2 duty optimization; fluids recycling; right-sizing equipment and minimizing idle time; selection of foams to reduce fluid volumes where justified.
• IV.8 Cost control: accurate fish diagnostics to avoid unnecessary milling; contingency tools on first run; clear thresholds for switching to workover options.

V. Typical challenges/bottlenecks and mitigations

• V.1 Sand/scale bridges
  • Mitigation: pre-cleanout; switch to washover; use higher AV and visco-elastic sweeps; deploy motor + mill for hard scale; consider chemical dissolvers where compatible.
• V.2 Differential sticking on permeable zones
  • Mitigation: reduce hydrostatic (nitrified fluids), spot lubricants/pill; maintain circulation; jar cycles with flow; avoid excessive static hold time.
• V.3 Irregular or damaged fish tops
  • Mitigation: dress mill to square shoulders; transition to taper tap; use centralizers and guide shoes; improve depth correlation with gamma/CCL.
• V.4 Limited CT overpull capacity in deep/deviated wells
  • Mitigation: position jars close to fish; add accelerators; reduce friction with roller centralizers/oscillators (if compatible); staged jarring; evaluate tubular buoyancy by fluid density adjustment.
• V.5 ECD-induced losses leading to stuck pipe
  • Mitigation: throttle rates; lower viscosity; optimize nozzle/jetting; foam/nitrified system; use choke control and monitor returns density/flow.
• V.6 CT fatigue and BHA shocks
  • Mitigation: verify fatigue history; use accelerators; limit jar count per run; adjust injector speed/traction; avoid high-frequency oscillations.
• V.7 Sour/HPHT environments
  • Mitigation: NACE-compatible materials; temperature-rated elastomers; continuous H2S monitoring; contingency kill; hydrate control in subsea with methanol/MEG if required.
• V.8 Debris recirculation at surface
  • Mitigation: effective solids control/filtration; magnet on flowline; periodic bottoms-up and pit management to avoid re-ingesting metal fines.

VI. Why CT fishing matters economically and operationally

• VI.1 Reduced downtime: rapid mobilization and live-well capability cut non-productive time versus killing and rig-based workovers.
• VI.2 Cost avoidance: averts sidetracks or workover rig campaigns by restoring access with minimal footprint; smaller crews and lighter logistics.
• VI.3 Production recovery: clears restrictions enabling immediate return to service, improving asset uptime and deferment recovery.
• VI.4 Risk reduction: controlled pressure operations lower kill-induced damage risks; precision engagement minimizes further downhole damage.
• VI.5 Sustainability: optimized fluids and shorter duration reduce emissions and waste volumes relative to heavier interventions.

Key highlights

• Plan the catch, not the trip: exact fish definition and BHA contingencies drive success.
• Hydraulics first: AV and ECD windows set the operational envelope and safety margin.
• Jar smart: energy management and coil limits are paramount—impact without exceeding CT envelopes.
• Decide fast: clear thresholds for switching tactics prevent time and fatigue overruns.