Fish

Key Takeaways

• Fishing tool selection is driven by the fish's geometry and the failure mode that left it in the hole — an overshot (a tool that grabs the outside of the fish's top OD by engaging with a set of grapple slips or a basket grapple) is used when the fish has a clean, accessible top end with known OD that the overshot can engage; a spear (which is inserted into the ID of a hollow fish and engages with an internal grapple or collet) is used when the fish's top end is a box connection or open bore that the spear can enter; a jar accelerator is added above the fishing tool when the fish is expected to be stuck, to provide the impact force needed to free it; a taper tap or box tap is used when the fish's top is in a condition that requires the fishing tool to first dress (machine) the end of the fish before engagement can be made; the selection sequence is logical but must be made carefully because the wrong tool sent to depth on a long wireline or coiled tubing string can make the fishing situation worse if it damages the fish top or pushes it further into the hole.
• Differential pressure sticking — one of the most common causes of a fish — occurs when a portion of the drill string is pressed against a permeable formation by the pressure difference between the mud column (which is heavier than the formation pore pressure in an overbalanced situation) and the formation pore fluid, creating a contact force between the pipe and the borehole wall that friction alone cannot break; the contact force is proportional to the differential pressure and the area of pipe in contact with the filter cake on the borehole wall (the longer the section of pipe lying against the wall in a deviated well, and the thicker the filter cake, the larger the sticking force); freeing a differentially stuck pipe typically requires reducing the differential pressure (by reducing mud weight, which must be balanced against the risk of a kick), applying pipe-freeing agent (spotted oil or glycol solution that dissolves or lubricates the filter cake at the contact zone), and jarring (applying impact force with a downhole jar); if all these fail and the pipe cannot be freed, it becomes a fish that must be backed off above the stuck point and recovered in sections.
• Junk in the wellbore — small metallic debris including broken cone teeth from a tricone bit, fragments of drill collar connections that washed out, dropped hand tools, cone fragments from a worn insert bit, and miscellaneous metal pieces — accumulates on the bottom of the hole and must be recovered before a new bit is run, because any metal debris on bottom will damage the new bit immediately upon contact; junk is recovered using junk baskets (tools with internal baffles and magnets that trap and hold metallic debris as circulation passes through them), junk subs (simple magnets run above the bit to attract ferrous debris), or junk mills (rotating tools that grind debris into fine particles that can be circulated out rather than recovered intact); the presence of junk is typically identified by running a junk detector (a lead impression block or a gauge run to the bottom of the hole to show the shape and size of any objects on bottom) before selecting the appropriate recovery method.
• The fish-or-sidetrack decision is one of the most consequential economic and engineering judgments in well operations — when a fishing operation is ongoing and the fish has resisted multiple recovery attempts, the operator must weigh the cost of continued fishing (rig day rate multiplied by fishing days elapsed and expected days remaining) against the cost of sidetracking (directionally drilling a new wellbore around the fish to reach the planned geological target), minus the salvage value of any tubulars in the fish that might eventually be recovered; the decision threshold depends on the commercial value of the remaining reserves to be reached, the day rate of the rig, the probability of successful fish recovery on the next attempt, and the technical feasibility and cost of sidetracking in the specific wellbore geometry; in deepwater offshore operations where rig day rates exceed $500,000, the economic analysis heavily favors sidetracking over extended fishing operations whenever the probability of recovery on the next attempt drops below about 30-40%; in lower-cost land operations, the lower day rate allows more fishing days before the sidetrack decision becomes economic.
• Prevention of fishing situations is the most cost-effective fishing strategy — the fishing job that never happens costs nothing; the fishing job that lasts three days costs three days of rig time plus the fishing service, plus any wellbore damage caused by the fish or the fishing tools; prevention focuses on eliminating the failure modes that produce fish, including: proper BHA inspection and makeup torque verification before each run (preventing connection fatigue failures), adequate stabilization and BHA design for the formation drilled (preventing spiral or helical buckling that stresses connections beyond design), bit selection matched to the formation (preventing premature cone or insert failure), mud program design to minimize differential sticking risk (thin, low-invasion filter cake, maintaining overbalance within the minimum necessary), and rigorous torque and drag monitoring during drilling to detect developing stuck pipe before it becomes a full sticking event requiring backing off and fishing.