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Fishing Diagram

A fishing diagram is a detailed engineering drawing prepared before any fishing operation (the process of retrieving lost, stuck, or broken equipment from a wellbore) that records the major outside diameter profiles, inside diameter profiles, critical dimensions, connection types, and depths of all tools and equipment that have been left in the wellbore (collectively called the "fish"), drawn to scale or at a consistent schematic scale with accurate dimensional annotations, used by the fishing engineer, the fishing tool selection team, and the rig crew to select the appropriate fishing tools (overshots, spears, jars, bumper subs, mills) that can engage, latch, and recover the fish without creating additional complications such as sticking additional equipment, parting the fishing string, or irreparably damaging the wellbore; the fishing diagram is typically prepared by the wellsite supervisor or company man using information from the driller's tally (a record of every joint of pipe, collar, and BHA component run into the hole with its measured length and connection type), the last drill-floor weight measurement before the pipe became stuck or was dropped, and any caliper or imaging log data available from before the incident, and must be updated if additional attempts to free the fish change the downhole configuration (for example, if the fish is backed off at a tool joint connection, shortening the fish, or if a jar or bumper sub is added to the fishing string and left in the hole); the accuracy of the fishing diagram is directly related to the success probability of the fishing operation, because fishing tool selection (the OD of the overshot grapple must match the fish OD within a narrow tolerance, and the ID of the fishing neck must fit the spear OD) depends entirely on knowing the exact dimensions of the fish at the point where the fishing tool will engage.

Key Takeaways
  • The fishing diagram must document all components of the fish in the correct downhole order from top to bottom (the order in which the fishing tool will encounter them when run from the surface), including the top fishing neck or profile (the OD and ID of the uppermost section of the fish, which is the primary engagement point for overshots and spears), the OD and ID of each drill collar, HWDP, or drillpipe joint in the fish string, the OD of stabilizers and bit subs, the BHA components (mud motor, MWD tool, rotary steerable system, directional sub), and the bit at the bottom; each component's nominal OD (the maximum OD that the fishing tool must fit over, for an overshot) and ID (for a spear) must be annotated, along with the depth of each tool joint and major connection in the fish string (so the fishing engineer knows where the fish can be backed off if it cannot be recovered in one piece); additional critical dimensions include the total length of the fish (to size the fishing string length and predict where the top of the fish will be when the fishing tool engages), the depth to the top of the fish (to calculate the length of fishing string needed), and the estimated weight of the fish in air and in mud (to calculate the overpull available after latching and the jar impact force needed to free the fish).
  • Dimensional accuracy in the fishing diagram is critical because the difference between a successful fishing run and an unsuccessful run often comes down to a few thousandths of an inch in OD or ID: an overshot (the most common fishing tool for pipe recovery, consisting of an outer sleeve with an internal grapple that grips the outside of the fish when the overshot is pushed down and then pulled up) has a minimum OD that must fit over the fish OD and a maximum grapple catch range (typically 0.5 to 1.0 inch above the nominal grapple catch ID); if the fish OD is 5.000 inches but the fishing diagram incorrectly states 4.750 inches, the overshot selected will have a grapple ID range that ends below 5.000 inches and will not latch onto the fish, wasting a full round trip (12 to 24 hours at depth); if the fish OD is 5.000 inches but a stabilizer blade in the fish has an OD of 6.250 inches that was not noted in the diagram, the overshot run to catch the 5.000-inch pipe will be stopped by the stabilizer before reaching the fishing neck, again wasting a round trip; the fishing engineer's first review of any fishing diagram is to identify the maximum OD in the fish (which controls the minimum ID that must be cleared to reach the fishing point) and the fishing neck OD (which controls the grapple selection), then to verify that all connections in the fish are compatible with back-off operations if recovery in one piece is not possible.
  • Junk in the wellbore that is not part of the original fish must also be documented in the fishing diagram, since dropped items (bit cones, broken bit blades, cone inserts, lost couplings, scale debris, or pieces of previous BHA that were milled and not circulated out) can lodge above the fish and prevent fishing tool engagement: a junk basket (a fishing tool designed to collect small debris from the wellbore bottom) is run before the main fishing tool when junk is suspected or confirmed by a junk shot magnet on a previous trip; the fishing diagram for a complex fishing job may include a separate "junk log" section listing each known piece of junk, its size, probable location, and the tool used to attempt its removal; in extreme cases (wellbore full of cut or milled debris from previous unsuccessful fishing attempts), a reverse circulation cement or a junk squeeze job may be required to consolidate the junk and create a stable platform before further fishing attempts, at which point the fishing diagram must be updated to reflect the new downhole configuration after the remediation operation.
  • The fishing diagram is also used during the fishing operation itself as a real-time reference for the driller and company man to interpret the weight indicators, torque readings, and depth measurements as the fishing tool approaches and engages the fish: when the fishing tool (run on the fishing string from the surface) reaches the calculated depth of the fish top (from the fishing diagram), the driller slows the lowering rate and watches for the weight indicator to show a "tag" (an increase in string weight, indicating contact with the top of the fish); the depth at which the tag is observed is compared to the predicted fish top depth on the diagram to confirm that the fish has not fallen further or been picked up by a previous fishing attempt; when the fishing tool latches (weight increases as the grapple engages and the fishing string is picked up), the overpull applied is compared to the estimated fish weight from the diagram to confirm that the latching force is consistent with the fish weight and that the fishing tool has actually caught the fish rather than simply setting down on the fish top; at each stage of the fishing operation, the diagram provides the engineering context needed to interpret the real-time wellbore data correctly and make decisions about whether to continue pulling, apply jar blows, or back off and change strategy.
  • Fishing cost economics in the oil and gas industry are substantial: rig time consumed by fishing operations averages 2 to 5 percent of total drilling time across the industry (varying from near zero in simple, low-pressure formations to 15 to 25 percent in challenging formations with high stuck pipe or twist-off rates), with day rates for deepwater rigs of $500,000 to $700,000 per day making a 5-day fishing job a $2.5 to $3.5 million event before the cost of fishing tools, specialized fishing crews, and deferred production are added; the economic decision to "fish or abandon" (to attempt recovery of the fish versus plugging back above the fish and sidetracking around it) is made by comparing the estimated cost and probability of successful fishing recovery against the cost of a sidetrack and the production value of any reserves behind the fish that would be lost if the fish is abandoned in place; the fishing diagram is the engineering input to this economic analysis, providing the dimensional data needed to assess fishing difficulty (complex multi-component fish with small OD fishing necks and large OD stabilizers is harder and more expensive to fish than a simple drillpipe twist-off with a clean fishing neck), and the condition of the fish (from the fishing diagram's record of any deformation, corrosion, or bending observed during prior fishing attempts) is factored into the probability-of-success estimate.

Fast Facts

Fishing operations have been a part of well drilling since the earliest cable-tool wells of the 19th century, where broken tools left in the wellbore could only be retrieved by running improvised hooks, grabs, and magnets on the drilling cable; the first specialized fishing tools (socket taps for threaded connections, overshots for smooth pipe sections) were developed in the 1870s and 1880s by oil well supply companies in Pennsylvania and Ohio, with the fishing tool catalog rapidly expanding through the early 20th century as rotary drilling replaced cable-tool methods and the variety of equipment that could be left in the hole increased dramatically; the formalization of the fishing diagram as a standard preparation step before any fishing run was established as best practice by the major oil companies and drilling contractors in the 1950s and 1960s, codified in drilling engineering handbooks and company drilling manuals; today, the fishing diagram is a required element of the fishing work plan (the formal document prepared before any fishing job that specifies the proposed fishing approach, tool selection, overpull limits, and decision criteria) required by most national oil companies and major operators as a condition of approval for fishing operations; specialty fishing service companies (Fishing Tool Rental, Smith International Fishing and Rental, Baker Hughes Fishing Services, and their successors) provide both the engineering expertise to prepare fishing diagrams and the specialized tooling inventory (overshots from 1.5 to 12 inch grapple range, die collars, spears, jars, bumper subs, milling tools) required to execute the recovery plan the diagram supports.

What Is a Fishing Diagram?

A fishing diagram is a scaled engineering drawing of all tools and equipment left in a wellbore (the "fish"), annotated with outside diameters, inside diameters, connection types, lengths, and depths of each component in the fish string from top to bottom. Prepared before any fishing operation, the diagram is the primary engineering reference for selecting the correct fishing tools (overshots, spears, jars), calculating the required fishing string length and overpull capacity, and interpreting real-time weight and torque data during the fishing run. Dimensional errors in the fishing diagram directly cause failed fishing runs and wasted rig time.

Fishing diagram is also called a fish diagram, wellbore fish sketch, or BHA diagram. Related terms include fishing (the process of retrieving lost, stuck, or broken equipment from a wellbore; includes overshot runs, spear runs, jar operations, milling, and back-off operations; consumes 2-5% of industry drilling time on average; the fishing diagram is the primary engineering preparation document for any fishing operation), overshot (a fishing tool that fits over the outside of the fish and latches onto it using an internal grapple mechanism; the most common tool for recovering drill pipe and drill collars; grapple OD range must be matched to the fish OD from the fishing diagram), fish (any object left in the wellbore that must be retrieved or dealt with before drilling can continue; includes stuck or broken drillpipe and collars, lost BHA components, dropped hand tools, bit cones, and failed completion equipment; the subject of the fishing diagram), jar (a downhole tool that delivers impact loads to the stuck fish when activated by applying overpull (hydraulic jar) or slack-off (mechanical jar) at the surface; the jar impact force supplements the pull available from the rig to help free stuck pipe; sized based on the estimated stuck point from the fishing diagram), and sidetrack (drilling a new wellbore around a fish that cannot be recovered, using a whipstock or cement plug to deflect the bit in a new direction; the economic alternative to continued fishing when the probability of recovery falls below the break-even point versus the cost of fishing).

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