With improved equipment and operating practices, downhole failures have decreased in all drilling operations. The cost and time to sidetrack round a fish has been substantially reduced due to improved directional drilling and surveying techniques.

This is especially true for slim hole drilled with downsized conventional equipment. A sidetrack round a fish should take one or two days of rig time and this tends to render all but the most basic fishing uneconomic.

Some of the precautions against fishing, which can be taken in slim holes include:

• Maintaining a thin, hard filter cake at static bottomhole temperature especially immediately prior to trips and wireline logging operations.
• Minimise the amount of low gravity solids in the mud either by optimising solids removal equipment or by aggressively dumping mud, which is often the cheaper alternative, especially in shallow holes. This reduces filter cake thickness and allow the viscosity to be optimised to maximise the power to the BHA and clean the hole. Consider the cleaning of the upper sections of the hole, especially if liners are in use. When drilling from a floater consider cuttings transport in the riser. For higher pressured wells consider the use of a mud-based or high density (formate) brine.
• Only use wireline logging contractors who can supply all tools with downhole tension metres with a surface readout. Logging tools in slim holes tend to have closer clearances than with conventional hole sizes. The mud usually loses some or all of its capacity to suspend solids and maintain a thin filter cake as the well warms up after circulation has stopped. As the logging tool is pulled out of hole, a build up of drilled solids and filter cake is pushed ahead of the tool. This has been known to pack off round the tool in the casing.
• Use a top drive or power swivel and keep the pipe in motion and circulate in/out of the hole at the first sign of overpull, including when inside the casing. The tight tolerances between BHAs and the casing ID, coupled with a solids laden mud losing its suspension capabilities have led to fishes inside casing. The maximum mud temperature during circulation is in the annulus about one third of the well depth from the bottom, and during a trip out of hole this is a danger area for mud packing off between the casing and the BHA.
• Beware of small drilling, evaluation, completion, and fishing tools which have been in store for a long time, or the design and materials specification of which have not been updated within the last five years. Check revision dates on drawings.
• Ensure that procedures are in place for makeup of small diameter tubulars.

Fishing in small holes

This is similar to fishing in conventional hole sizes. Most of these tools are used in workover operations and may not be available in all areas. During the pre-spud planning process a review of tool (and crossover) availability with local fishing companies may be considered. This should include the fishing, and stripping over, of wireline logging tools and directional survey tools.

Fishing of mining drillstrings

This is usually accomplished by means of spears, which would appear to be a sensible approach in hard rock drilling where the hole can be expected to be in gauge. For soft formation drilling, where washouts can some times occur, there may be problems getting the spear into the fish. In these conditions problems tend to snowball, as each successive fishing run tends to wash out the hole some more.

If a spear packoff is run below the spear to direct circulation through the fish there may be some packing off of cuttings round the outside of the fish.

If the twist-off is due to bending, the pipe may have assumed an oval cross section, and similarly its biggest OD may exceed the ID of the previous casing.

When running fishing jars and accelerators with mining pipe consideration should be given to the strength of the pipe threads.

One point of danger is just under a casing seat where the rathole of the previous casing shoe is effectively an overgauge hole potentially leading to buckling and fatigue failure of the drillpipe. It is therefore recommended to consider leaving the shortest possible rat hole if the casing is to be drilled out.

Fishing of the inner core barrels inside the drillpipe

Normally this is done by a wireline similar to the slick line on a conventional rig. An overshot is used to latch onto the core barrel. During inner barrel recovery, drilling fluid is circulated down the drillpipe to counteract any tendency of the inner core barrel to swab the well in, and to minimise the chance of getting the stationary drillpipe stuck. In more critical situations a wireline lubricator should be used.

The degree of pressure containment required during sand line inner barrel handling and fishing operations for different parts of the well needs to be addressed during rig and contractor selection and in the well programme. Lubricator vertical and lateral support should consider the strength of the drillpipe body and threads. Available lubricator length due to surface handling limitations becomes critical if a birds nest and the core barrel have to be fished. If a well has to be killed by circulating through the drillpipe, pump out forces should be considered.

Fishing of downsized conventional drilling systems

This is closer to conventional practice than fishing mining pipe. Most fishing can be done with overshots which are more familiar to oilfield drilling personnel. They are stronger and have a better record of success than internal catch tools and it has been claimed that they account for 90% or more of fishes recovered.

The 2 7/8" drillpipe has 3 1/2" OD tool joints. In 4 3/4" hole these tool joints can be fished by a 4 3/8" Slim Hole overshot and the 3 3/4" OD BHA can be fished with a 4 5/8" Slim Hole overshot. As BHAs with this system can normally be reduced to four or five joints only a "pickup capability" is required. The drillpipe body can be caught with a 4" OD FS or a 3 5/8" XSH overshot. The above catches assume that the tools are dressed with spiral grapples.

In 4 1/8" hole below a 5" liner a different approach is required for the tool joints and BHA.

Baker Oiltools have a 4 1/16" OD "Flexlock" overshot (SHOOT, Slim Hole Overshot One Time) with a maximum catch of 3 3/4", a significant improvement on the 4 5/8" OD of the Bowen series 150 with the same catch. The tool is released downhole by overpull which is transmitted to the tool body as hoop stress by the slips. At release this hoop stress yields the tool body. This is typically about 70,000 lb. axial pull. This means that redressing the tool, after it has caught a fish, is by replacement of the tool body and internal slips. It can be dressed for smaller sizes by changing the size of the slips. The tool does not require rotation to engage or release making it useful for horizontal holes and for coiled tubing operations.

If the tool is used in a shallow vertical hole, precautions should be taken to ensure that any shock wave caused by release by overpull does not damage surface equipment, or cause the string to jump out of the elevators, etc.

To release the overshot from the fish the overshot body should be cut open longitudinally with an oxyacetylene cutter. The tool body round the slips may have some hoop tension locked in and it may spring open during cutting. The attention of the welder should be drawn to this prior to cutting.

The tool can be run with a 3 3/4" hydraulic drilling jars used as fishing jars. If required a safety joint can be run above the overshot in case sufficient overpull to release cannot be generated. The action of hydraulic jars can exceed the allowable forces on the SHOOT overshot as the slim design only allows straight pull.

Drillstring severing charges

The pipe used in slim hole drilling is relatively flexible in torsion, and in many cases it might prove difficult to work left-hand torque downhole for a backoff shot. If a slim well, or series of wells, were to be drilled then it would be prudent to have drillstring severing charges on standby for the drillstring in use.