Disposable Fibre Optic Reservoir Monitoring Surveys in CO2 Injection Wells: Operations Planning, Outcomes and Lessons Learnt

Abstract Surveillance data was acquired in four Gorgon CO2 injection wells using Distributed Temperature Sensing (DTS) and Distributed Acoustic Sensing (DAS) technology. The primary objective was to acquire temperature warmback data to quantify the CO2 injection profile and demonstrate containment at the wellbores. The secondary objective was to demonstrate the application of fibre optics in subsurface monitoring for the CO2 injection project by acquiring multi-offset Vertical Seismic Profile (VSP) surveys and continuous DAS data. Temperature warmback surveys were recorded over 3-5 days following a 24-hour injection period to cool the formation. Coincident multi-offset VSP surveys were acquired during the warmback period using a vibroseis truck. A disposable fibre optic technology was selected for these surveys due to its lightweight equipment and small footprint, reducing intervention risk over live CO2 wells. Bare optical fibres are wound within a probe that free falls into the well, unspooling two fibres as it descends. Developed work practices considered safe wellhead access, avoiding phase changes to the CO2, effective pressure control and material selection. The four well program was completed without safety incident or major non-productive time. The probe was deployed to depth in all wells with no observed fibre degradation due to exposure to well fluids. The operator, disposable fibre and seismic survey providers collaborated efficiently to acquire data simultaneously. The acquisition was monitored on site and real-time streamed to decision support centers in Aberdeen and Houston. The warmback surveys were successful in demonstrating that CO2 is contained at the injection intervals; however, challenges were experienced acquiring complete and accurate temperature data across the injection intervals over the extended survey periods. The primary cause was fibre damage and/or breakage due to crossflow between perforated sections. While a qualitative interpretation of the injection profile was possible for most wells, the combination of crossflow and Joules-Thomson cooling combined with fibre damage made quantification difficult. The nature, causes, management, and impact of these are discussed in the paper, along with lessons learned and potential mitigations.