SURFACE CASING DESIGN CONSIDERATIONS WITH EMPHASIS ON THE BASS BASIN

Abstract

The main purpose of surface casing and the associated blowout preventer (BOP) system is to contain any well pressure that may be encountered while drilling to total depth or the next casing depth. Not only does this mean that the casing and BOP should be sufficient to withstand the pressure exerted upon them, but also the depth of the casing shoe should be such that the formation will not break down under the influence of this pressure. The latter condition could lead to an internal blowout and subsequent cratering to surface.

Design consideration for surface casing must encompass fracture propagation theory, pore pressure and kick volume estimates, and associated casing pressure calculations. A worked example from the Bass Basin gives a safe and effective design when these are applied.

These design considerations also have an impact on the procedures for pressure testing casing, BOP, and formation which are valid when compared with the existing statutory requirements of Australia, Norway and the USA.

Although the operator is responsible for correct casing design, the various statutory bodies have a role to technically challenge and modify drilling and casing programs, and the procedures outlined give the foundations on which to build an acceptable casing scheme.