How Does Fishing Work?

I. High-Level Purpose and Where Fishing Fits in the Value Chain

Fishing is the well intervention activity to locate, engage, and recover (or otherwise remediate) stuck, parted, or lost equipment and debris (“fish”) from the wellbore so drilling, completion, or production operations can resume safely and economically.

• I.1 Role in the value chain: Fishing sits within drilling/workover operations. It is triggered by unplanned events (stuck pipe, twist-off, dropped tools, junk) and aims to minimize non-productive time (NPT) and avoid sidetracks or abandonments.
• I.2 Outcomes: Recover the fish, free the stuck string, or condition the well (mill/clean) to continue the program with integrity preserved.
• I.3 Boundaries: Includes drillstring, casing/tubing, wireline, and coiled tubing fishing in both open hole and cased hole. Excludes routine hole cleaning except as needed to facilitate fishing.

II. Step-by-Step Process Flow

1. II.1 Triage, well control, and diagnosis
   1. II.1.1 Secure the well: Verify barriers, BOP status, and maintain safe surface pressures. Stabilize mud properties to hold the well static.
   2. II.1.2 Define the fish: Type (pipe, tool, wireline, junk), dimensions (OD/ID/length), connection type, estimated top of fish (TOF), well geometry, and formation pressures.
   3. II.1.3 Identify sticking mechanism: Cuttings pack-off, differential sticking, keyseat, mechanical failure, collapsed casing, cement/scale, or hydrate/ice in cold operations.
   4. II.1.4 Data gathering: Tally, torque/drag history, last downhole telemetry, recent hydraulics, free-point indicator plan if needed.
2. II.2 Establish TOF and access
   1. II.2.1 Gentle tag: Run a soft tag; if uncertain, use an impression block (lead/soft alloy) to profile the top.
   2. II.2.2 Condition hole: Circulate to clear cuttings; consider viscous sweeps if pack-off suspected.
   3. II.2.3 Calibrate depth: In cased hole, correlate with CCL; in open hole, leverage drift and survey data.
3. II.3 Select the fishing strategy
   1. II.3.1 Recover intact: Overshot (external catch) or spear/taper tap (internal catch) with jars and accelerator.
   2. II.3.2 Free then pull: Jar sequences (up/down), spotting fluids (oil/surfactant, acid/solvent), back-off or chemical/explosive cut, then retrieve.
   3. II.3.3 Bypass and wash over: Washover pipe and mills to free the outside of the fish; then latch and pull.
   4. II.3.4 Destroy/remediate: Mill dress, section mill, junk basket/magnets; condition to continue or prepare for sidetrack.
   5. II.3.5 Wireline/CT fishing: Pulling tools, rope sockets, wire finders, magnets, jars; use lubricator/strippers for pressure control.
4. II.4 Bottom-hole assembly (BHA) design
   1. II.4.1 Engagement tool: Correct overshot or spear size with appropriate grapple (basket/spiral). Include guide or dressing mill if top is irregular.
   2. II.4.2 Impact package: Mechanical or hydraulic jars, plus accelerator/bumper sub to store energy and avoid string damage.
   3. II.4.3 Contingencies: Safety joint, circulating/jetting sub, magnets/junk basket, washover assembly staged at surface.
5. II.5 Execute and confirm engagement
   1. II.5.1 Run in hole and dress TOF: Light WOB with a string/pilot mill to square off and remove burrs or “mushrooming.”
   2. II.5.2 Latch: Slowly lower to engage. Confirm with a pickup test (planned overpull) and a torque test (low torque to verify positive catch).
   3. II.5.3 Freeing attempt: Execute jar cycles per tool specs; apply controlled overpulls within the safe envelope.
   4. II.5.4 Spotting fluid if differentially stuck: Place a pill across the contact zone; soak; re-jar.
   5. II.5.5 Washover/back-off if required: Free-point, back-off at free section; or run washover pipe and free externally, then pull.
6. II.6 Retrieve and clean
   1. II.6.1 Pull out carefully: Monitor hookload, torque, and standpipe pressure; avoid surge/swab losses.
   2. II.6.2 Junk recovery: Magnet/junk basket runs to remove loose pieces; drift and gauge.
   3. II.6.3 Post-job review: Inspect fish; confirm root cause; update practices to prevent recurrence.

III. Major Equipment/Components and Their Functions

IV. Key Performance Drivers (Efficiency, Cost, Safety, Emissions)

• IV.1 Engagement reliability:
  • Right tool, right size: Overshot/spear selection matched to fish OD/ID and connection profile; dress TOF before attempts.
  • Impact optimization: Jar placement and stroke tuned; accelerators sized for fluid density and temperature.
• IV.2 Hydraulics and hole condition:
  • Cuttings transport: Maintain annular velocity and viscosity to avoid re-pack-offs. Annular velocity: \( V_{\text{ann}} = \dfrac{Q}{A_{\text{ann}}} \), where \( A_{\text{ann}} = \dfrac{\pi}{4}\,(D_h^2 - D_{od}^2) \).
  • Manage ECD: Equivalent circulating density: \( \text{ECD}_{\text{ppg}} = \text{MW}_{\text{ppg}} + \dfrac{P_{\text{ann,fric}}}{0.052 \times \text{TVD}} \). Stay within pore–fracture window.
• IV.3 Mechanical envelope management:
  • Buoyancy factor: For steel in ppg units, \( \text{BF} \approx 1 - \dfrac{\rho_{\text{mud}}}{65.4} \). Buoyed string weight: \( W_b = W_{\text{air}} \times \text{BF} \).
  • Safe overpull (estimated): \( F_{\text{safe}} \approx 0.8 \times T_{\text{min,weakest}} \). Respect connection and rig hookload limits.
  • Free-point via elastic stretch: \( \Delta L = \dfrac{F}{A E} \, L_{\text{free}} \Rightarrow L_{\text{free}} = \dfrac{\Delta L \, A E}{F} \) to estimate free length.
  • Jarring energy (simplified): \( E \approx F_{\text{impact}} \times s \), where \( s \) is jar stroke; hydraulic jars develop \( F_{\text{impact}} \) from pressure-induced acceleration.
• IV.4 Time and cost control:
  • Plan–do–review cadence: Pre-job decision tree, contingencies staged, wellsite “red lines” for overpull, torque, ECD.
  • Rig time economics (estimated): Land: USD 20,000–150,000/day; offshore jackup: USD 80,000–250,000/day; floater: USD 200,000–600,000/day. Each saved day materially improves economics.
• IV.5 Safety and environmental controls:
  • Well control discipline: BOP/LRP readiness; wireline lubricators/grease heads; stripper for CT.
  • H2S/CO2 and high-pressure hazards: Materials selection, gas monitoring, sour service PPE, and barrier verification.
  • Fluid stewardship: Minimize spills; manage oil/solvent spotting fluids with containment and recovery.

V. Typical Challenges/Bottlenecks and Mitigation Strategies

• V.1 Differential sticking
  • Indicator: Sudden loss of mobility with high overbalance; unable to rotate/reciprocate.
  • Mitigation: Reduce overbalance if safe; spot lubricants/surfactants; jar up; back-off above stuck zone if needed.
• V.2 Packed-off hole / cuttings beds
  • Indicator: Standpipe pressure spike, low returns, erratic hookload.
  • Mitigation: Circulate at high annular velocity; viscous/high-lift sweeps; staged washover; rotary shoes.
• V.3 Irregular or “mushroomed” TOF
  • Mitigation: Pilot/string mill to square off before engaging; use guide shoes to centralize overshot/spear.
• V.4 Tortuosity and reach limitations (high inclination/ERD)
  • Mitigation: Friction reducers, roller reamers/centralization; CT or wireline tractors for light fish; manage drag and set-down availability.
• V.5 Collapsed/ovalized casing or tight spots
  • Mitigation: Gauge cutters, reamers, and casing patches; section milling if necessary to re-establish drift.
• V.6 Connection failure/twist-off
  • Mitigation: Free-point; back-off at neutral; catch with overshot/spear matched to connection; adhere to safe torque windows.
• V.7 Debris field after milling
  • Mitigation: Magnet and junk basket runs; reverse circulation tools; clean out to full drift before resuming operations.
• V.8 HPHT/sour environments
  • Mitigation: Temperature-rated elastomers; H2S-resistant metallurgy; strict exposure controls and contingency ventilation.

VI. Why Fishing Matters Economically or Operationally

• VI.1 Cost avoidance: Successful fishing can save multiple rig days versus sidetrack or recompletion. On offshore rigs (estimated USD 200,000–600,000/day), even a 2–3 day recovery materially impacts well AFE.
• VI.2 Schedule protection: Keeps the well on path to TD, completion, or workover objectives; avoids cascading logistics delays.
• VI.3 Asset integrity: Proper fishing prevents further damage to casing/formation, preserving long-term production and intervention flexibility.
• VI.4 HSE risk reduction: A controlled, engineered retrieval minimizes additional trips, handling, and exposure hours.

Quick Rules of Thumb (Estimated)

• Engage clean metal: If TOF is questionable, dress first; a 30–60 minute milling pass often saves hours of failed latching.
• Test the catch: Verify with a modest overpull (e.g., 20–40% of planned maximum) and low torque before committing high jarring forces.
• Jar intelligently: Fewer, well-energized jar hits outperform continuous light bumping; allow jars to fully cock/reset per spec.
• Hydraulics first: If standpipe pressure climbs and returns degrade, suspend jarring and restore circulation; avoid packing off the fish.
• Know your red lines: Keep a live sheet of max hookload, overpull, torque, and ECD limits based on the current string and casing envelope.