Well Design for Shallow Gas

Abstract

Shallow gas and its associated operational procedures have been the subject of much opinionated debate. Previous work in the area has focused either on regulatory issues or on the steps that can be taken while drilling to minimise the consequences of a shallow gas kick. Although some attention has been given to modelling the behaviour of wells under shallow gas blowout conditions, this has not resulted either in a practical design tool nor in a structured procedure for checking well designs. This paper puts forward a provisional framework which highlights the design checks that should be made to ensure that rig systems, well designs and operational procedures are safe and fit for purpose. The framework should permit engineers to approach the problem of shallow gas well design in an analytic and structured fashion, using a specified set of analysis techniques. These techniques are reviewed and transient well unloading and quasi-steady blowout simulations are identified as critical elements. The basis and operation of a practical software tool which addresses these elements is described. Some examples of its use are given and areas where modelling improvements could be made are suggested.