A key element in any pore pressure prediction study is the comparison between the predicted pore pressure at the prospect location and the formation pressure measurements in offset wells. In order to understand, and predict, pressure distribution within a basin, pressure data and the sediments within which pressure regimes are confined need to be analyzed within a robust and consistent stratigraphic framework. This comparison is primarily required for: a) calibration for forward modelling and b) to have an idea of the extremes in the various pore pressure scenarios in a particular basin. The formation pressure measurements are acquired in permeable units like sands and are assumed to be in pressure equilibrium with adjacent shales. Thus, the predicted estimates in shales are compared with pore pressure measurements from sands in offset wells. This comparison is imprecise until and unless the offset pressure measurement is normalized to the same frame of reference at the prospect.
The normalization process of offset pressure data can be broadly classified into two types: a) the simplistic depth normalization and b) the more detailed stratigraphic normalization. The simplistic depth normalization involves mere normalization of offset pressure measurements with respect to change in the positioning of the datum reference in the prospect location; this method precludes consideration of the structural difference (current plumbing) of each geological unit or in-depth analyses of depositional setup in offset and prospect locations presuming connected sands with constant overpressure. The second method entails the understanding of the regional stratigraphy, the structural differences in positioning of each stratigraphic unit in the prospect location and offset wells, depositional setting including sand connectivity and overpressure-generating mechanisms. This method can be invoked through appropriate approaches; a) constant overpressure (OP) method, b) constant vertical effective stress (VES) method and c) ratio method (ratio of either constant OP or VES to the normal effective stress i.e. NES). Stratigraphic normalization also includes the correct positioning of the offset pressure data in each stratigraphic unit at the prospect.
Whilst the above normalization methods are extremely useful concepts, they form only part of the geological definition of the pressure system for predicting pore pressure profiles in an undrilled prospect. On the basis of geological understanding, stratigraphic normalization method may be preferred over the simplistic depth normalization method. An optimized normalization process for offset well data integrated with predicted pore pressure estimates at prospect locations illustrates a realistic scenario in terms of bounds of uncertainty.
This paper brings forth a new dimension in predicting pore pressures at new prospects by describing scenarios within the framework of the conceived geological model. Also, the methods discussed provide the scope of capturing a valid uncertainty envelope that is imperative for a robust well design to define possible risks and foresee mitigation measures.