Managing debris in cutting multilateral well windows

Casing exit problems slow flow, reduce flexibility

Clifford Hogg
Baker Oil Tools

The potential reservoir benefits of multilateral systems have long been recognized, and very simple, open-hole multilaterals have been around since the 1950's. Unfortunately, these basic open-hole designs have limited functionality and are not applicable for many of the world's wells. It is only since early 1996 that drilling and completion technologies have evolved to the point that more complex multilateral systems have now become possible.

Currently, there are proven commercial multilateral well completions that offer a full range of mechanical and hydraulic support options with both production isolation and re-entry potential for all wellbores.

Much attention has been focused on the perceived risks of these new, complex, high-tech multilateral systems and the effects these risks will have on projects' economic viability. One particular area of concern has been the issue of downhole debris created during the multilateral drilling and completion process.

Unmanaged downhole debris can impede the well's performance if it blocks production from any of the laterals or if the debris prevents completion equipment from functioning properly. Debris-related problems occurred in some of the earliest attempts at complex multilaterals, resulting in less-than-ideal-completions. Fortunately, after much study and testing, several solutions have been successfully utilized in recent jobs that have greatly minimized the impact of debris on the final completion.

Pre-milled windows

When creating a multilateral well in a cased-hole environment, the first step is to create a casing exit or "window" at the kick-off point of the main bore, using a whipstock, pilot mill and watermelon mills (Figure 1 [46,037 bytes]). There are currently two main techniques being used to control debris at this stage and to prevent it from interfering with later completion operations.

One method is to use a pre-milled, composite-wrapped window in order to minimize the actual debris being created. Unfortunately, there are several drawbacks to this approach:

The composite window must be run during the initial well completion, therefore limiting this technique to new wells.
Proper orientation and depth placement of the casing becomes much more vital and complex.
The long-term strength and stability of the composite wrap when subject to downhole conditions must be taken into consideration before selecting this method.

Standard-casing exits

The second approach to casing exits assumes the window is being created in a joint of standard material casing. With this approach, the emphasis is on properly removing and controlling the debris that is created.

Assuming the debris can be adequately removed from the wellbore, the flexibility offered by a casing exit system that can be deployed at any depth or azimuth to form a window in the casing or liner becomes very attractive. Following this approach, the window can be created at any depth in the wellbore and with any orientation desired.

In addition, the multilateral is no longer limited only to new wells, but can now be completed in older re-entry wells as well. To summarize, in order to optimize the flexibility of the multilateral, proper debris management becomes imperative. Debris management consists primarily of two phases:

Removing as much debris as possible
Minimizing the impact of any remaining debris through the use of downhole tools.

Debris removal

The first area to consider in debris removal is the debris material itself. In the past, the mills used to create casing exits have relied on high weights on bit to actually grind the metal away. This type of milling can result in non-uniform cuttings of varying size that can often be difficult to circulate out of the hole.

The milling process has been greatly improved with the introduction of a new generation of mills that use proprietary cutter technology to actually cut rather than grind away the material. The resulting debris consists of very small, very uniform chip-tytpe cuttings which tend to eliminate "bird nesting" and are much easier to circulate out of the hole.

In order to most effectively remove cuttings, the milling fluid must also be carefully considered. Alternating high- and low-viscosity fluid slugs has proven successful in removing cuttings from the well. The low-viscosity slugs assure proper face cleaning at the bit, improve hydraulic efficiency and assist in cooling and lubricating the mill itself.

The high-viscosity slugs, on the other hand, provide the high yield point and laminar flow regime that can most effectively carry the debris from the well. As the debris is removed, it is imperative that the cuttings be collected at surface through the use of screen or magnets. The information obtained from monitoring cuttings can provide indications as to how well the debris is being removed and how much may still be in the wellbore.

After the casing has been exited and any asssociated washover or milling work completed, a cleanout trip should be run in order to remove any remaining debris. Reverse circulating tools and junk baskets have been used in the past, with positive results.

However, a new generation of tools is beginning to appear that are even more effective at cuttings removal. These tools employ varying combinations of screens in conjunction with differential pressure regimes to effectively catch any debris that may still be present in the wellbore.

Controlling the residual

Utilizing these techniques should result in the removal of the large majority of downhole debris. What residual debris remains can be easily controlled with a variety of downhole accessories.

One such accessory is a downhole excluder cup. This tool is simply an upward-facing rubber element that is run below the whipstock to prevent debris from moving below it and interfering with packers or other completion equipment already in place below the kick-off point. The excluder cup is recovered at the same time the whipstock itself is removed, and it further serves as a debris sweeper for the wellbore as it is pulled to surface.

Another effective technique for preventing debris from interfering with completion operations is to utilize high-viscosity pills on top of all critical downhole components such as packers, plugs, or valves, to keep debris suspended and away from them. The pills and any trapped cuttings can be reversed out of the hole just prior to final completion, leaving a clean and accessible bore above the completion equipment.

A third method of controlling debris is through the use of a downhole debris catcher, which consists of a slotted joint sub landed and located in the bore of the mainbore and lateral bore packers. Typically, the packers are run with these debris catchers already in place. They serve two main purposes:

To prevent debris from migrating further downhole and potentially interfering with other completion equipment
To serve as a very effective debris management "scorecard." The presence of large quantities of debris in the debris catcher indicates that other debris removal and management procedures have not worked effectively. Fortunately, this has not been the case in recent wells. The debris catchers have actually contained minimal debris, proving that debris management policies are indeed working as intended.

Debris tolerance

The techniques described above effectively remove debris from multilateral systems and also trap what debris may still remain after exiting casing and washing over. A third area of debris management involves the multilateral equipment itself.

The latest generation of multilateral equipment has been designed such that it is extremely tolerant of debris. For example, a new orientation packer/liner hanger has a full-drift bore in order to minimize the chance of plugging due to debris. In addition, the orientation profiles that allow proper orientation of the completion equipment have been designed with debris management in mind.

The fit between the orientation packer and anchor is not so tight as to be hindered by a small amount of debris (Figure 2 [26,392 bytes]). There are several other completion tools that have been similarly designed with debris tolerance in mind.

From a long-term perspective, the ultimate goal is to provide a multilateral system that generates no debris, while continuing to offer the full flexibility and options demanded by the industry. There are several tools on the horizon that offer this potential. Among these is the new Formation Junction(tm), scheduled for introduction later this year, that offers a true debris-less system.

The junction assembly is actually created and carried downhole as a single component. A combination of pressure and mechanical force is then used to reform the junction to its fullbore size (Figure 3 [40,743 bytes]). The resulting junction is fully sealed, full bore ID throughout, and can be run without the need for any debris creation. This system fits the requirements of a Level 6 system as defined by the TAML (Technology Advancement of Multilaterals) Group and will be the first true Level 6 system offered in the industry.

In conclusion, debris management is certainly an issue that needs to be considered and planned for when designing multilateral completions.

Author

Cliff Hogg is a Senior Applications Engineer with Baker Oil Tools, based in the company's Houston Emerging Technologies/Multilaterals group. Hogg joined Baker Oil Tools in 1993 and worked as a Field Engineer in West Texas and Oklahoma prior to his current position. He holds a BS in petroleum engineering from Texas A&M University.