Integrated Predrill Pore-Pressure and Borehole-Stability Prediction for Prelude Development

Abstract

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.

Combined Effect of Pore Water Overpressure, Far-Field Stresses, and Strength Parameters in Wellbore Stability

“…This constant σ c is lower than the value indicated by Asaka and Holt (2021). Hansen et al (2014) estimated, with log correlations, the profile of the uniaxial compressive strength and friction angle of Prelude 1 A well (Figure 14B). The uniaxial compressive strength in the shaly formations, depth interval z = 2,200-3,960 m, is σ c = 15-55 MPa, and the friction angle is practically constant ϕ′ ≈ 30 °.…”

“…The comparison between the estimated uniaxial strength by Hansen et al (2014) and Mandal et al (2021) in the depth range z ≈ 3,080-3,520 m indicates a large difference. In contrast, the uniaxial strength reported by Asaka and Holt (2021) is the upper limit.…”

“…The author proposed a revised normal pore pressure gradient SG = 1.02 at these depths and concluded that the prediction method might lead to inaccurate pore pressure estimates. Hansen et al (2014) noticed instances of circulation losses in well Concerto 2 (in the same area) at depths of z ≈2,300-4,000 m (shaly formations), with an estimated pore pressure SG = 1.32. Furthermore, they noticed the absence of any observed breakouts in shales.…”

“…The basin is subject to high overpressure and a strike-slip regime with significant stress anisotropy (Khaksar, 2011;Asaka et al, 2016;Asaka and Holt, 2021;Mandal et al, 2021). The pressure ramp starts within the Jamieson shaly formation (z ≈ 2,200 m), reaching a maximum at the top of the Aptian shaly formation (z ≈ 3,200 m) and then regresses to the hydrostatic condition in the sand reservoir (z = 3,900 m) (Hansen et al, 2014;Mandal et al, 2021). = 1.32) close to Mandal et al's (2021) value.…”

Estimation of Rock Mechanical Parameters Using Well Log Data in the Well Poseidon 1, Lower Cretaceous, Browse Basin, Northwest Shelf, Australia

“…The reservoir geomechanics studies mentioned beforehand use as inputs rock mechanic properties that in the case of the Browse Basin has not been publicly disclosed and in the Poseidon 1 well has not been fully carried out [3], [6]. At basin scale the studies done in the area uses valid, but apparently are limited approaches to determine some of the parameters and studies at well context only have been focused on the Prelude 1 and 1A wells [7].…”