Abstract There are diverse approaches to casing setting depth determination via kick tolerance, which are inconsistent with each other and involve apparently arbitrary design values. This paper presents a risk based approach to setting depth selection during design which explicitly considers the variables contributing to influx volume and their uncertainties. The use of a risk based approach provides a unifying theme across a range of hole sizes and operational variables allowing comparisons of options where none was possible using previous design approaches.
IntroductIon During well planning an important aspect of casing point selection is the ability of the design to accommodate well control requirements. An approach to this is by the specification of an acceptable kick tolerance value. Unfortunately there are many definitions of ‘kick tolerance’, and also differing views on what constitutes acceptable design practice within each of those definitions. In general the definitions fall somewhere on figure (1). A tolerable influx volume with no underbalance (point A), a maximum overpressure with respect to mud weight in the hole (kick intensity, point B), or some combination of the two (point C) have all been used. Influx volumes have sometimes been related to presumed kick detection capabilities and on occasion recast in terms of maximum overpressure or kick intensity. For a particular well control technique, hole and drill string geometry, pore and fracture pressures, a boundary can be drawn on figure (1), one side of which represents shoe, or open hole weak zone failure, the other a successful well kill. For most geometries the choice of any of points A, B or C uniquely determines the other two, e.g., choosing A defines Point D on figure (1). But there is no general correspondence across all hole sizes and bha geometries that allows immediate comparison between designing for a point A of say 70 bbls, a point B of say 1 ppg, or a point C of 0.5ppg and 20bbls. Such arbitrary design values are difficult to reconcile: defining an influx volume does not reflect hole size, while defining an absolute kick intensity can have quite different implications depending on depth.
Much previous work has focused on the well control aspect of simulation of inflow into an underbalanced well bore and its subsequent circulation. This paper describes an approach to setting depth determination which makes use of simulation of an influx and its circulation to help define appropriate locations for points A, B, or C by providing an estimate of the notional probability of shoe failure. This riotional probability should be regarded as a comparitive engineering measure rather than as an absolute measure. This distinction is clearly drawn in the paper by Banon et al however it is of course desirable that notional probabilities should be of comparable magnitude to observed probabilities where the latter are available.