Milling technique a critical first decision in re-entry drilling

Multilaterals, extended reach, and sidetracks begin with choice between section or window milling

Mike Neff
Weatherford Enterra

• When special sizes and debris eliminate washover options, sidetracking from a whipstock can be the most cost effective means to reach the target.

Re-entering existing wells to side-track and re-drill the well to a new bottom hole location can in many instances be the most economical approach for remedial applications. Older wells often suffer from corrosion, water cutting, chalk or sand infiltration, and other problems.

The advantages associated with re-entering and side-tracking include the ability to replace old completion systems with more efficient ones. Examples include replacing corroded casing with chrome liners and modern "intelligent" completion assemblies. Improved seismic data from existing reservoirs can extend platform life and improve recovery by selecting new bottom hole locations.

Two milling methods exist to start a sidetrack from an existing wellbore. The choice of milling a section or milling a window is generally based on the degree of success that the well planners have experienced in the past with milling techniques. To quantify the process, there are some basic considerations to be taken prior to getting started.

Section milling advantages are:

• Sidetracks can kick off in any direction.

• Lower dog leg severity is created.

• Less eqiuipment is required for remote locations.

• A smoother exit from old well bore is created.

The disadvantages to section milling are:

• Mud properties must be altered.

• Sweeps must be prepared.

• Higher volumes of steel and cement must be handled.

• Bird nesting downtime is a possiblity.

• A cement plug must be set in order to kick-off.

Window milling advantages are:

• A positive kick-off is assured in the required direction.

• Fewer metal cuttings to deal with at surface are created.

• It is not necessary to alter mud properties.

• It is more reliable at greater depths (more than 9,000 ft).

• It is more reliable at higher hole angles (35 degrees).

• There are no cementing costs.

• It is possible to sidetrack through two strings of casing.

• It is more cost effective.

Window milling disadvantages are:

• It creates a higher dog leg severity (3-12 degrees).

• It creates higher initial costs.

• More equipment is required for remote locations.

• The whipstock could turn or become unrecoverable.

Recent trends

Three events have shaped section milling in recent years - new materials, a better understanding of the processes involved and breakthroughs in milling fluids. Improved flow nozzle designs as well as better blades and mill inserts lead to higher reliability of section mills.

The process of milling long sections of casing came to be better understood on a regional and worldwide basis once detailed databases were kept. This allowed job performance to be monitored and tracked by variable.

Lastly, the introduction of improved milling fluids has greatly facilitated improved milling performance - particularly for section milling. The removal of steel cuttings requires different rheological properties than the removal of formation cuttings. The improved lifting characteristics of milling fluids coupled with better nozzle properties have gone a long way toward minimizing "bird-nesting" problems in section milling operations.

Window milling

Window milling and the use of whipstocking technology has enjoyed a rebirth of sorts in recent years due to several factors. Like section milling, new material, and insert technology has reduced overall milling time. So too has the reliance on "database decision making" when pre-planning windows.

The use of MWD technology makes orientating quicker and more reliable. While the average number of trips needed to create a smooth window has decreased it has become commonplace now to retrieve the whipstock. Retrievability of whipstocks offers an elegant solution to maintaining the parent wellbore in excellent condition.

Problem one

This situation occurred upon re-entering an old well by setting a 9 5/8-in. whipstock at 10,500 ft with a hole angle of 67 degrees and a 2.5 degree per 100 ft build. The sidetrack was performed without incident.

The drilling of the 8 1/2-in. section to 17,210 ft and building the hole angle up to 80 degrees was completed 18 hours ahead of the curve. Everyone was in line for a bonus at the end of this well. The 7-in. liner was run to bottom and cemented again without any problems.

All that remained was to drill out the shoe, 60 ft to the top of the reservoir, and 350 ft into the payzone, and run the completion.

However, a problem in the pump room required shutting down for a few minutes. By the time the pumps were restarted, the bottom hole assembly was stuck. After several attempts to jar free, the surface pressure started rising and the drilling string was packed off.

Being on a tight schedule the decision was made to mobilize a wireline company to run a string shot and back-off charge just above the shoe and go fishing. The back-off went as planned leaving a drill collar and a stabilizer looking up to wash over.

Getting over the drill collar was not a problem. But when the stabilizer was encountered, it was discovered that the connections had been manufactured over sized. Adding to the problem, the stabilizer had been dressed to be indestructible during back-reaming.

After two runs with washpipe, it was obvious that without a diamond shoe the situation was hopeless. Due to the special OD of these connections, an off-the-shelf diamond shoe was not available. The debris from the string shot had by now plugged the ID of the drill collar.

The solution was to set a whipstock at 17,000 ft and 80-degree hole angle.

Problem two

A 13 3/8-in. casing string had been run and cemented in a vertical well to 6000 ft. At some time during the pumping of the tail slurry, there was a sudden loss of pressure. It was determined that the casing had parted at a coupling. The depth of the parted coupling was 3500 ft.

Section milling is a good choice for this application.

Problem three

On a platform, a well began producing water. It was decided to sidetrack to a new bottom hole location. This would enhance the recovery reserves of the field and extend the life of the platform.

The objective was to cut and pull 9 5/8-in. casing above the top of cement and mill the remaining casing to allow an open hole sidetrack below the 13 3/8-in. shoe. The 13 3/8-in. shoe was at 6,850 ft and a hole angle of 55 degrees. The top of cement on the 9 5/8-in. casing was calculated to be at 6,600 ft, 250 ft inside the shoe.

The cut and pull operation was halted early due to barite settlement at a depth of 5,650 ft. There was 1,200 ft of 9 5/8-in. casing to mill inside the 13 3/8-in. at a hole angle of 55 degrees. A review of the casing tally revealed there were no centralizers on the 9 5/8-in. casing. Two days later and 200 ft of milled casing, the average removal rate was 100 ft per day. It would take an additional ten days to complete the milling operation. Was there a more cost effective option.

Setting a whipstock and sidetracking would have saved about seven days.

Studies show that the performance of both whipstocks and section milling has continually improved over the past few years. The reliability of both methods has also improved due to better database monitoring of the processes and through the continuous improvement of materials and products.

In general, at shallower depths and in more vertical wellbores section milling is the more effective alternative. Alternately, at greater depths and at higher wellbore angles whipstock sidetracking has shown to be the more cost effective solution.

Evaluating all the variables associated with the well condition aids in the selection of the most effective sidetracking alternative.

Why re-enter existing wells

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