Integrated Pre-Drill Pore Pressure and Borehole Stability Prediction for Prelude Development

摘要

Pore pressure (PP) and fracture gradient (FG) predictions were prepared for Prelude development wells in the Browse Basin,offshore northwest Australia. The PP forecasts were based on resistivity- and sonic-based models calibrated with pressuremeasurements and drilling events such as kicks from existing wells. Fracture gradients were based on leakoff tests and lossevents from offset wells and were not necessarily equal to the minimum compressive horizontal stress, which was calculatedfrom lithology-dependent effective stress ratios. Maximum horizontal stress was inferred from observed breakouts. Porepressure and stresses were combined with formation properties from well logs and laboratory rock mechanics tests to provideinput for elasto-plastic (shales) and poro-elastic (sands) borehole stability models.These techniques are applicable to exploration, appraisal, or early development wells that have potential for encounteringgeopressured formations in high-angle well sections requiring good pre-drill estimates to adequately plan the casing anddrilling programs and determine borehole stability. The pre-drill studies can be extended to provide integrated real-time porepressure and borehole stability while drilling, and the models can be recalibrated following each well to provide updatedpredictions for subsequent wells.There are only minor deviations in the predicted PP and FG among the different well locations considered. Commonfeatures include potential loss zones in the shallow overburden, pressure ramp within the Jamieson, pressure regression belowthe Aptian, and near-hydrostatic pressure within the Upper Swan and below. The borehole stability models indicate thatminimum required mud weight in deviated sections could be up to 20% higher than required to balance formation porepressure. In one well that would cross a suspected fault, the risk of fault reopening or reactivation is low.This study indicates that use of integrated borehole stability and PP/FG models can result in higher minimum requiredmud weights and narrower drilling windows than would be suggested from the PP/FG models by themselves and cantherefore contribute to enhanced safety, optimized well designs, and reduction of non-productive drilling time. Lostcirculation at mud weights well below the minimum in-situ stress can be explained by reactivation or initiation of shearfractures.