Historically, running large diameter surface and intermediate casing in the shallow sections of a well with a high build rate and inclination has proven to be very challenging, with a high risk of getting stuck off bottom. At the same time, with limited drilling margins, it has become important to be able to successfully set large diameter casing under these conditions to enable the well to reach target objectives and/or avoid subsurface hazards.
A comprehensive analysis reveals that a major contributing factor to the difficulty of running large diameter casing is due to the large end loads resulting from the high inherent stiffness of the casing as it is bent through dog-legs or ledges. Finite element analysis indicated that these forces were high enough to cause the shoe to dig in to softer formations, and that the end forces increased substantially as casing size and weight increased, or as dog-leg severity and inclination increased.
A fairly simple solution was identified: a composite flexible shoe run at the bottom of the casing reduced the side loads at the bottom of the casing when running into the hole. Effective communication and cooperation between designer and operator allowed for a product that was thoroughly tested and examined from an operational standpoint to identify risks and address possible failure modes. Smaller tools were also proven out in higher build rate horizontal wells prior to running in a higher cost deepwater environment.
Several case histories are presented that demonstrate the ability to run 18, 16, and 14-inch casing smoothly to target depth through build sections with doglegs of more than 4°/100ft, in soft formations, and to inclinations of more than 60° using a composite flexible casing shoe.
The ability to reliably set large diameter casing through build sections and into high inclination wellbores expands the envelope of design possibility when planning deepwater wells.