Ensuring Protection of Critical Turtle Nesting Habitat: 3D Geomechanical Modelling Estimates Production-Induced Subsidence at Torosa Field, Browse Basin, Australia

ABSTRACT: Coupled geomechanical approaches are imperative for optimizing field development planning. This paper concerns a subsidence study for Torosa field, Western Australia with a focus on the turtle nesting habitat at Sandy Islet—a sand accumulation located on the reef above the gas reservoirs. This involved building a 3D geomechanical model incorporating structural complexity and rock heterogeneity, followed by adopting a one-way coupling scheme to assess outcomes of three proposed development plans. For the scenario named "REF Case", the maximum predicted subsidence at the end of depletion is less than 51.8 mm at the centre of the field and less than 3 mm at Sandy Islet. The sensitivity analyses performed to account for uncertainty involved scaling down the Young's modulus and Poisson's ratio by 10% throughout the overburden. The results indicate that subsidence is insensitive to the variations of the overburden deformation properties but affected by the intensity and distribution of depletion. 1. INTRODUCTION The petroleum industry had little appreciation in the early days of the field scale extension of geomechanical influences in the reservoir. The most evident manifestation of the consequences of production-induced stress changes is "subsidence"—a sinking of a surface, such as the ground level in an onshore area or the mudline in a submarine setting—relative to a stable reference point. Subsidence was first noticed in 1918 at the Goose Creek field, Texas, USA. Since then, petroleum geomechanics has come to play an increasingly important role in predicting and quantifying the effects of operations, with the obvious focus on mitigating the economic ramifications of subsidence. This paper concerns the offshore Torosa gas field, Western Australia, with the key Jurassic reservoirs lying approximately 4000 m TVDSS beneath Scott Reef. The field had undergone an extensive appraisal drilling and seismic acquisition campaign between 2005 and 2012, and the Browse Joint Venture is seeking to develop Torosa field. The integrated geomechanical study presented herein is important to predict the impact of subsidence on a turtle nesting habitat at Sandy Islet—a sand accumulation located on the reef above the gas reservoirs.