Defining Acceptance Criteria for Inflatable Packers in Irregular Completion Profiles
Authors: Lyashkov Victor Vladimirovich , Dementyev Anatoly E 
Published: March 2015

Abstract
Mechanical packers are usually very strong and robust, but they have a very limited ability to expand in through-tubing well interventions because of physical limitations. The commercially available swellable packers require a long set-on time for the completion of the osmosis process. Hydraulically set inflatable packers for coiled tubing interventions are free of those limitations; development of these was initiated in the late 1980s, when the first 2 1/8-in. outside diameter commercial models became available for the cased hole completions. The best cased hole packer designs are reliable and are able to expand with a 3:1 ratio and hold up to ~2000 psi of the differential pressure in 7-in. casings. However, the inflatable packers used in openhole (OH) completion in different applications can be significantly less reliable. The primary reason for worse performance is the complex geometry of the open hole. The geometry parameters for OH wellbores can be gathered from the multi-arm caliper log. Identifying the worst case scenario for geometry in a given OH completion and matching it against the physical packer's limits can predict its performance and assist in making a go/no-go decision; this is essential during the job planning stage. The simple acceptance criteria described in this paper will assist in minimizing the unnecessary risks and may help to prevent inflatable packer failures in both OH applications and collapsed casing conditions.

Introduction
Openhole (OH) completions bring uncertainty in the analysis of their profiles as read by calipers. Understanding the geometry of the OH completion is crucial for jobs that involve inflatable packers on coiled tubing (CT). Identification of an inadequate profile is essential to minimize the risk of packer failure due to improper setting. OH geometry is always somewhat irregular, and profile interpretations usually employ mathematical models. In this study, the state of an inflated packer inside the OH employs a Voronoi diagram—a well-known mathematical algorithm in computational geometry. A Voronoi diagram is a way of dividing space into a number of regions. It is especially helpful in solutions of the irregular geometrical shapes. Using this concept, a go/no-go acceptance criterion was developed.
An example of the flexible metal structure of a cased hole inflatable packer is shown in Fig. 1; in this packer, metal slats form an expandable anchoring body. The packer's flexible structure (Fig. 2) can expand radially to anchor and seal the required space in a wellbore. CT packers with slats usually have the highest expansion ability and thus maximum expansion ratio versus other designs. OH wellbores are known to be less certain and more risky in comparison to typical cased holes, and packer response differs as well. OH inflatable packers with slats are often single-set tools and so can be set reliably only once. If an attempt to set an inflatable packer is unsuccessful, the packer often cannot be re-used. Typically, there is no second chance to set an inflatable packer; the job should be executed in the first attempt. A real OH example is shown in Fig. 3. It is a challenging setting for inflatables because the expandable slats may not open uniformly, and there may be damage to the inflating elements. An alternative design with a metal-braided carcass is not necessarily ideal, as it may not develop sufficient anchoring force and has a tendency to create "pockets" when sealing; the uneven distribution of contact stresses between the inflatable and the rough wellbore surface may additionally cause localized deformation of the metal structure. Similar conditions are present in damaged casings, where even packer metal carcasses with very high elasticity may experience localized stresses that exceed ultimate tensile stress and lead to initiation of a fracture.