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Relationships between the compaction state and effective stresses are the basis for most quantitative pore-pressure and stress estimates. Common practice uses only a single element of the stress tensor, the vertical stress, for these calculations; mean stress formulations also exist, although they are less widely applied. Using simple models and field data from two distinct stress regimes, we examined the validity and limitations of the vertical-stress approach as well as a mean-stress approach, showing that in complex stress settings, both can perform very poorly. We evaluated a method for incorporating shear stresses into compaction relations by using state boundary surface (SBS) formulations from soil mechanics and demonstrated how the resulting model may be calibrated and applied to field data. This approach was found to perform much better in the complex stress environment, providing more stable calibration behavior and more reliably extrapolating to stress states beyond those present in the calibration data. Although vertical and mean stress compaction models may work well in simple stress environments, we discovered that incorporation of shear stress is necessary for models in complex stress settings. Although the addition of shear stress significantly improves agreement with field data, it also increases the complexity of the model as well as the requirements for calibration data. We therefore evaluated the settings in which each of these three approaches — vertical stress, mean stress, and SBS — may be most appropriate.

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Summary Pore-pressure (PP) and fracture-gradient (FG) predictions were prepared for Prelude development wells in the Browse basin in offshore northwest Australia. The PP forecasts were based on resistivity- and sonic-based models calibrated with pressure measurements and drilling events, such as kicks from existing wells. FGs were based on leakoff tests and loss events from offset wells and were not necessarily equal to either the minimum compressive principal stress (often considered a lower bound to FG) or the formation-breakdown pressure (often considered an upper bound to FG that includes effects of formation tensile strength and near-wellbore hoop stress). The minimum compressive horizontal stress was calculated from lithology-dependent effective-stress ratios. Maximum horizontal stress was inferred from observed breakouts. PP and stresses were combined with formation properties from well logs and laboratory rock-mechanics tests to provide input for elastoplastic (shales) and poroelastic (sands) borehole-stability (BHS) models. These techniques are applicable to exploration, appraisal, or early-development wells that have potential for encountering geopressured formations in high-angle well sections requiring good predrill estimates to adequately plan the casing and drilling programs and determine BHS. The predrill studies can be extended to provide integrated real-time PP and BHS while drilling, and the models can be recalibrated after each well to provide updated predictions for subsequent wells. There are only minor deviations in the predicted PP and FG among the different well locations considered. Common features include potential loss zones in the shallow overburden, pressure ramp within the Jamieson, pressure regression below the Aptian, and near-hydrostatic pressure within the Upper Swan and below. The BHS models indicate that minimum-required mud weight in deviated sections could be up to 20% higher than that required to balance formation PP. In one well that would cross a suspected fault, the risk of fault reopening or reactivation is low.