Casing Design in Complex Wells: The Use of Expandables and Multilateral Technology to Attack the Size Reduction Issue
Traditional casing design is driven in response to the issue of zonal isolation. Adverse pressures, hole problems, and undesired fluids all need to be cased off to allow successful completion of a well. This approach is successful but has one main drawback – the 25-30% reduction in hole size every time a new casing string is run. Now, expandable casing technology is available to reduce or eliminate the size reduction problem. The use of a multilateral junction, furthermore, usually compounds casing size reduction due to zonal isolation. In new wells, savings anticipated through use of a multilateral well is often consumed by the cost of a corresponding increase in hole size to the depth of the junction. In some cases, a 50% reduction (Well Construction and Completion Aspects of a Level 6 Multilateral Junction Hervé Ohmer, SPE, Schlumberger, Jean-Marc Follini, SPE, Schlumberger, Ricardo Carossino, Schlumberger, Mirush Kaja, SPE, ENI) in hole size occurs owing to the junction. Following passage through the junction, retention of the remaining hole size is all the more important. Clearly, there needs to be a significant improvement in amounts of reduction in casing and junction internal diameters in order to fully benefit from multilateral technology. When applying multilateral technology to existing wells, the problem becomes even more exacerbated. The re-entry multilateral well is constrained to the previous casing size. The retention of hole diameter in these applications becomes paramount. This paper will discuss the application of expandable casing technology to multilateral wells and the operational procedures dealing with the different casing requirements. The use of expandable casing above, below, and through a junction will be reviewed. Examples of well designs will illustrate the technical and economic case for using expandable casing technology in multilateral wells.