Integrated Pre-Drill Pore Pressure and Borehole Stability Prediction for Prelude Development
Authors: Tare Uday Arun , Sarfare Manoj Dnyandeo , Azbel Konstantin , Wang Gary , Couzens-Schultz Brent Alan , Adeleye Olayinka , Hansen Kirk S , McNeil Katja V. 
Published: May 2013

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 pressure measurements and drilling events such as kicks from existing wells. Fracture gradients were based on leakoff tests and loss events from offset wells and were not necessarily equal to the minimum compressive horizontal stress, which was calculated from lithology-dependent effective stress ratios. Maximum horizontal stress was inferred from observed breakouts. Pore pressure and stresses were combined with formation properties from well logs and laboratory rock mechanics tests to provide input 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 encountering geopressured formations in high-angle well sections requiring good pre-drill estimates to adequately plan the casing and drilling programs and determine borehole stability. The pre-drill studies can be extended to provide integrated real-time pore pressure and borehole stability while drilling, and the models can be recalibrated following 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 borehole stability models indicate that minimum required mud weight in deviated sections could be up to 20% higher than required to balance formation pore pressure. 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 required mud weights and narrower drilling windows than would be suggested from the PP/FG models by themselves and can therefore contribute to enhanced safety, optimized well designs, and reduction of non-productive drilling time. Lost circulation at mud weights well below the minimum in-situ stress can be explained by reactivation or initiation of shear fractures.

Introduction

Prelude is a gas and gas-condensate field that was discovered in 2007 in the Browse Basin, offshore northwest Australia; it will be developed utilizing the world's first floating liquefied natural gas (FLNG) facility. The Prelude development will require several wells drilled at high angle from approximately the same surface locations to near-horizontal sections within the target reservoir as chosen from among eight potential well paths (Figure 1). The potential for moderately geopressured formations requires good pre-drill estimates of pore pressure (PP) and fracture gradient (FG) to adequately plan the casing and drilling programs and (combined with the high-angle well sections) determine borehole stability. Expected (P50), minimum (nominally P15), and maximum (nominally P85) cases are considered for both PP and FG to account for uncertainty in the predictions.