Economics of Fishing
Authors: Adkins C.S. 
Published: December 2007

Introduction

Fishing usually is needed when least expected and brings a sudden halt to operations, especially if the drillstring becomes stuck. Reaction planning begins at this time unless the drilling project was planned properly from the outset. That reaction planning is not good and should not be done is not the question. What is important is to establish the facts. What caused the drillstring to become stuck? What should be done to free the drillstring? What will the cost be? This paper addresses these questions. It also emphasizes the importance of routine, continuous, but often unrewarded, effort by operation personnel. This paper presents one viewpoint for evaluating alternatives to fishing, for retrieving a fish compared with sidetracking, and for using economics and risk factors in the decision-making process. These alternatives are compared in Fig. 1. The discussion includes factors that can prevent fishing, such as drillstring inspection, and that can cause fishing, such as poor mud programs and differential-pressure sticking. An example illustrates decision-making processes involved in recovering or sidetracking the fish when the processes involved in recovering or sidetracking the fish when the drillstring is stuck by differential pressure. Even though this is only one of many causes for fishing, the process used to evaluate the economics may be applied to many other operations. The goal is to provide a usable wellbore at the lowest ultimate cost.

Planning Precludes Fishing Planning Precludes Fishing Drillstring.

The most economical method of fishing is to develop a drilling plan that precludes factors and operations that may result in fishing. The most important rule in any drilling operation is to ensure that the drillstring, especially the bottomhole assembly (BHA), is designed for the specific drilling conditions and inspected before a well is begun. Additional inspections may be necessary to preclude a fatigue failure or to ensure that the maximum anticipated loads can be handled safely. Another critical factor is inspection of all threads on rental equipment and fishing and directional tools planned for use. The complete string, from the top of the kelly to the bottom connection in the drillstring, should be inspected. Economics or risk evaluation for justifying costs can be evaluated by comparing the cost of fishing and the cost of inspections and the operations required to preclude fishing. The typical cost to 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 of a BHA in a clean hole is roughly $20,000 (including lost rig time and fishing expense). Some operators include tubular inspection in prespud costs. The costscited are for land operations in the U.S., but the principle applies to offshore or remote operations. Inspection is cheaper than fishing.

Circulating Fluid.

Another important factor in preventing fishing is use of a good mud system. A good mud system will circulate cuttings to the surface(clean the hole); provide a thin, impermeable filter cake (prevent differential sticking); and maintain wellbore stability (control shales). These three characteristics have various degrees of importance, depending on the drilling environment. It is extremely important to consider a mud system designed to address these items in initial drilling economics. A mud system with poor properties can make the drillstring very susceptible to sticking. This often can be avoided with nominal expenditures to improve mud properties.

Operating Practices.

Many operating practices developed to preclude fishing are generally accepted and peculiar to a specific preclude fishing are generally accepted and peculiar to a specific geologic province. One such practice that will minimize the chances of the BHA sticking in a keyseat is to add a stabilizer or a keyseat wiper (with an OD larger than drillpipe tool joints and drill collars) at the top of drill collars to guide the BHA around the keyseat. The expense of one stabilizer is minimal compared with the cost to recover a stuck drillstring. Other practices that may prevent fishing include frequent wiper trips, controlled rates of penetration (ROP's), and the pumping of viscous sweeps before trips.

Types of Fishing Jobs

Parted Drillstring and/or Tools.

Fishing for a parted string can Parted Drillstring and/or Tools. Fishing for a parted string can be extremely expensive and may be a reason to consider sidetracking the fish. A normal job would consist of running jars on a properly sized overshot/grapple, latching onto 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 be dressed with a milling tool, or the fish top may be difficult to locate, which will mean several trips before the fish is engaged. The fish may become stuck by the time it is actually engaged with the fishing string. Then, a procedure for 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 associated risk of these alternatives.

Stuck Drillstring.

Another type of fishing involves a stuck drillstring. If differential sticking is suspected, the first step usually is to spot a special fluid around the drillstring at the suspected stuck point to free the drillstring. (A spotting fluid in the wellbore at the stuck point will penetrate the filter cake along the pipe and reduce the area subject to differential pressure. The tension to move the drillstring is decreased and the drillstring often can be freed.) The cost to free a stuck drillstring with a spotting fluid is minimal compared with backing off a free section and then washing over to free additional sections. Therefore, the first step should be to use a spotting fluid. If this procedure does not free the stuck string, a free point with or without a stuck-pipe log (similar to a cement-bond point with or without a stuck-pipe log (similar to a cement-bond log) is run to determine the uppermost stuck point. Then, fishing operations to retrieve the stuck string may begin. Again, the decision to fish or to sidetrack should be based on a comparison of their estimated costs.

JPT

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