Fishing Operations (to be read after Unit 3)

Regardless of the precautions that are taken to avoid the loss of wireline tools in the well, it is inevitable that even the most careful operator will lose wireline and tools in the well occasionally. Well conditions cannot always be known or anticipated.

When tools are lost in the well, the method required to retrieve the tools depends on many different factors. Well conditions, completion design, the tools that were lost, and alternatives to fishing must be considered before beginning the fishing job. Consideration should also be given to the reason the tools were lost. Each fishing job is different because the circumstances are never the same from one well to the next.

If the wireline work is contracted, the next step is for the engineer or operator and wireline specialist to sit down together and discuss the situation. Together they should decide the best way to fish the tools from the well. After a procedure is decided upon, a list of the fishing tools needed should be made. If the needed tools are not on location, arrangements should be made to get them to the location. The success or failure of many fishing jobs is determined by what is done or is not done during the planning stages of the job.

The attitudes of the field engineer and the wireline specialist may play an important role in the eventual success of the job. Many times a simple fishing job becomes complicated simply because the specialist tries to «cut corners» and quickly fish the tools from the well. This often results in something being overlooked because the situation was never adequately analyzed. Once a procedure has been agreed upon and the proper tools are on location, the fishing job may begin.

Although a multitude of situations might require that fishing operations be performed, this discussion is limited to only a few examples of how a fishing job might be performed. Each actual situation should be evaluated, and the procedure used should be matched to the well conditions and downhole circumstances.

One rule is almost universally accepted as the best way to perform a fishing job. Fish the wireline first and then fish the tool string. This requires that the wire be cut from the tools.

Cementing Programme (to be read after Units 4a, b)

The primary purposes of cement are to seal the annulus between casing and formation, and to support the casing strings. The slurry selections and placement techniques vary widely, depending on the types of formation, well temperatures, hole and casing sizes, hole enlargement, formation pressures, and depths and loads of the casing strings.

The well planner must determine all of the conditions and requirements, select slurries that are appropriate, and prescribe placement and evaluation techniques that ensure effective results.

All this should be presented in the cement program in a concise manner, and appropriate instructions should be included in the drilling procedures section.

There is a large body of cementing technology with which the well planner should be familiar. Cementing company handbooks are the most readily available source illustrating the many variations in slurry compositions available for special conditions. Manufacturers catalogues are the most ready source of equipment choices. Cementing technology cannot be covered in depth here, but the principal concerns of the well planner are discussed.

Requirements and Considerations

The casing, hole sizes, depths, and the mud weight and type determined previously are the basic data for cement design.

Surface strings that support subsequent strings should be cemented to the surface with an excess of cement to ensure uncontaminated cement at the surface. Top fill with strong cement is necessary if the primary cement falls

back. This is sometimes advisable even after circulation to the surface in order to place strong, uncontaminated cement at the top of the string. Top filling is done through 1-in. pipe lowered alongside the casing.

For subsequent strings, the desired height of the cement column must be determined and calculated with annular volume to determine cement volume requirements. Caliper logs should be used and some excess (usually 10 to 20%) specified, based on area experience, to ensure cement fill to the desired height. In some cases, only bit gauge is known, and a larger  excess  (usually  50%) should be specified to compensate for washout. This, too, depends on area experience.

The following are other considerations that affect volume requirements:

The cement height should be sufficient to minimize casing buckling during subsequent drilling;

The upper portion of a cement job often shows poor bonding as a result of insufficient contact time (the time that a given depth is flushed by cement). All intervals that must be securely cemented need a minimum (recommended) contact time of ten minutes;

Potential lost-circulation problems often require the use of a light, scouring, lead slurry nearly matching the mud density, followed by  denser, strong (neat) cement through potential producing intervals, or for the lower part of surface and intermediate string annuli. This is also more economical than using all neat cement;

Two-stage cementing may be used with separate volume calculations for each stage;

Liners should be cemented with an excess to be left inside casing. This provides less contaminated cement at the liner top.