Wellhead movement as discussed is related to cementing, reservoir depth, fluid temperature and pressure. I don't believe in can be accurately calculated, at least not during the detail design phase of a project. Most operators rely on similar experience elsewhere when coming up with a design value, and my experience is that the number is always more than reality when actually measured (mostly North sea). I don't believe the age of the well has any particular effect, unless the flow has reduced to practically nothing, and thus negligable thermal and pressure effects.