Economics of Fishing
- C.S. Adkins (Mitchell Energy)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- May 1993
- Document Type
- Journal Paper
- 402 - 404
- 1993. Society of Petroleum Engineers
- 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.11 Drilling Fluids and Materials, 3 Production and Well Operations, 1.12.6 Drilling Data Management and Standards, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.10 Drilling Equipment, 1.6 Drilling Operations, 1.6.9 Coring, Fishing, 1.14 Casing and Cementing, 1.6.1 Drilling Operation Management
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Fishing usually is needed when least expected and brings a sudden halt tooperations, especially if the drillstring becomes stuck. Reaction planningbegins at this time unless the drilling project was planned properly from theoutset. That reaction planning is not good and should not be done is not thequestion. What is important is to establish the facts. What caused thedrillstring to become stuck? What should be done to free the drillstring? Whatwill the cost be? This paper addresses these questions. It also emphasizes theimportance of routine, continuous, but often unrewarded, effort by operationpersonnel. This paper presents one viewpoint for evaluating alternatives tofishing, for retrieving a fish compared with sidetracking, and for usingeconomics and risk factors in the decision-making process. These alternativesare compared in Fig. 1. The discussion includes factors that can preventfishing, such as drillstring inspection, and that can cause fishing, such aspoor mud programs and differential-pressure sticking. An example illustratesdecision-making processes involved in recovering or sidetracking the fish whenthe processes involved in recovering or sidetracking the fish when thedrillstring is stuck by differential pressure. Even though this is only one ofmany causes for fishing, the process used to evaluate the economics may beapplied to many other operations. The goal is to provide a usable wellbore atthe lowest ultimate cost.
Planning Precludes Fishing Planning Precludes Fishing Drillstring. The mosteconomical method of fishing is to develop a drilling plan that precludesfactors and operations that may result in fishing. The most important rule inany drilling operation is to ensure that the drillstring, especially thebottomhole assembly (BHA), is designed for the specific drilling conditions andinspected before a well is begun. Additional inspections may be necessary topreclude a fatigue failure or to ensure that the maximum anticipated loads canbe handled safely. Another critical factor is inspection of all threads onrental equipment and fishing and directional tools planned for use. Thecomplete string, from the top of the kelly to the bottom connection in thedrillstring, should be inspected. Economics or risk evaluation for justifyingcosts can be evaluated by comparing the cost of fishing and the cost ofinspections and the operations required to preclude fishing. The typical costto inspect a drillstring before spudding and two BHA's during drilling of a30-day 12,000-ft well is about $15,000. The cost for fishing at about 10,000 ftto recover part of a BHA in a clean hole is roughly $20,000 (including part ofa BHA in a clean hole is roughly $20,000 (including lost rig time and fishingexpense). Some operators include tubular inspection in prespud costs. The costscited are for land operations in the U.S., but the principle applies tooffshore or remote operations. Inspection is cheaper than fishing.
Circulating Fluid. Another important factor in preventing fishing is use ofa good mud system. A good mud system will circulate cuttings to the surface(clean the hole); provide a thin, impermeable filter cake (prevent differentialsticking); and maintain wellbore stability (control shales). These threecharacteristics have various degrees of importance, depending on the drillingenvironment. It is extremely important to consider a mud system designed toaddress these items in initial drilling economics. A mud system with poorproperties can make the drillstring very susceptible to sticking. This oftencan be avoided with nominal expenditures to improve mud properties.
Operating Practices. Many operating practices developed to preclude fishingare generally accepted and peculiar to a specific preclude fishing aregenerally accepted and peculiar to a specific geologic province. One suchpractice that will minimize the chances of the BHA sticking in a keyseat is toadd a stabilizer or a keyseat wiper (with an OD larger than drillpipe tooljoints and drill collars) at the top of drill collars to guide the BHA aroundthe keyseat. The expense of one stabilizer is minimal compared with the cost torecover a stuck drillstring. Other practices that may prevent fishing includefrequent wiper trips, controlled rates of penetration (ROP's), and the pumpingof viscous sweeps before trips.
Types of Fishing Jobs
Parted Drillstring and/or Tools. Fishing for a parted string can PartedDrillstring and/or Tools. Fishing for a parted string can be extremelyexpensive and may be a reason to consider sidetracking the fish. A normal jobwould consist of running jars on a properly sized overshot/grapple, latchingonto the fish, and pulling (may jar loose first) the fish out of the hole.However, the fish top can be damaged, which will require the fish top to bedressed with a milling tool, or the fish top may be difficult to locate, whichwill mean several trips before the fish is engaged. The fish may become stuckby the time it is actually engaged with the fishing string. Then, a procedurefor removing the free (unstuck) sections of the fish by "backing off"becomes necessary. At this time, an economic comparison between fishing orsidetracking should be finalized. The decision to continue fishing or tosidetrack should be based on a comparison of the estimated cost and associatedrisk of these alternatives.
Stuck Drillstring. Another type of fishing involves a stuck drillstring. Ifdifferential sticking is suspected, the first step usually is to spot a specialfluid around the drillstring at the suspected stuck point to free thedrillstring. (A spotting fluid in the wellbore at the stuck point willpenetrate the filter cake along the pipe and reduce the area subject todifferential pressure. The tension to move the drillstring is decreased and thedrillstring often can be freed.) The cost to free a stuck drillstring with aspotting fluid is minimal compared with backing off a free section and thenwashing over to free additional sections. Therefore, the first step should beto use a spotting fluid. If this procedure does not free the stuck string, afree point with or without a stuck-pipe log (similar to a cement-bond pointwith or without a stuck-pipe log (similar to a cement-bond log) is run todetermine the uppermost stuck point. Then, fishing operations to retrieve thestuck string may begin. Again, the decision to fish or to sidetrack should bebased on a comparison of their estimated costs.
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