Assessing Subsea Well Architecture Impact on A-Annulus Behaviour: Case Study from Ichthys Gas Field, Australia

ABSTRACT: In response to higher reservoir temperatures in the Jurassic Plover formations, significant modifications were made to well architecture and tree hardware to endure temperatures up to 165°C. The new design featured 26" and 36" casings with dual cement return ports. While the lower ports on the "D" annulus were sealed during cementing, the upper ports on the ‘C’ annulus remained open. Production from the Plover wells revealed regular fluctuations in A-annulus pressure, raising concerns about well integrity. A multidisciplinary investigation identified a strong correlation between A-annulus pressure variations, tubing wellhead temperature, and tidal magnitude. The observed pressure fluctuations exceeded the pressure change from tidal effects, suggesting that temperature changes within the annulus, likely due to flow through the open ‘C’ annulus, were influencing pressure. This was confirmed by an ROV (Remotely Operated Vehicle) survey revealing bidirectional flow through the ‘C’ annulus. The study led to the decision to seal the ports with custom metal plugs and apply chemical treatments to address Microbially Induced Corrosion (MIC). Post-installation monitoring confirmed the effectiveness of this intervention. This investigation underscores the value of proactive measures and interdisciplinary collaboration in managing offshore gas field operations. 1. INTRODUCTION The Ichthys gas field, one of the largest discoveries in Australian waters, is in the Timor Sea, approximately 220 km offshore from Broome, Western Australia (Fig. 1). The field connects to the onshore LNG facilities at Bladin Point, near Darwin, through an 889 km subsea pipeline. This vast infrastructure supports the development of two key reservoirs: the Brewster member of the early Cretaceous upper Vulcan formation and the Jurassic Plover formation (Fig. 2). The Plover formation, which consists of fluvial to shallow-marine sandstones, shales, and igneous rocks, presents unique technical challenges, particularly in terms of well integrity due to the higher reservoir temperatures. The Jurassic Plover formation is characterised by elevated temperatures reaching up to 165°C, necessitating significant adjustments to well architecture and hardware design. These design modifications, undertaken in line with INPEX's Well Integrity and Design Manuals, were aimed at ensuring the well's long-term operability and safety under such extreme conditions. The introduction of 26" and 36" casings and dual cement return ports within the low-pressure wellhead system was a critical part of this effort.