The CarbonNet appraisal well for the Pelican CO2 offshore storage site

Abstract

The CarbonNet Project is operated by the state of Victoria, and jointly funded and managed by State and Federal governments. The Pelican storage site is situated in the shallow marine environment, between 1 to 15 km offshore from the iconic Ninety Mile Beach in Victoria, Australia. A 125 million tonne capacity storage site for greenhouse gas (GHG) has now been confirmed using new state-of-the-art high-resolution 3D seismic data, covering the P90 probabilistic 3D volume of future plume travel in a wide range of scenarios and sensitivities. A new appraisal well (data well) has also recently been completed and is the subject of this paper. The well has confirmed pressure isolation across the main seal interval and suitable sandstone lithologies for storage, with excellent multi-layered reservoirs (up to 10 darcy permeability, over 150m net).

The storage site has undergone careful and diligent appraisal to demonstrate its suitability for storage of CO2 at commercial scale. Initial site screening and characterization relied upon the open-file petroleum database that is available to CarbonNet in this basin – consisting of over 1,500 exploration and development wells and extensive 3D seismic data offshore, supplemented by extensive onshore 2D seismic data. In addition, over 20,000 onshore boreholes targeting brown coal resources allow an excellent onshore-offshore stratigraphic correlation and detailed interpretation of depositional environments and facies geometries.

The Gular-1 appraisal well will inform detailed design of the planned cluster of injection and monitor wells that will offer redundant injection capacity beyond the project requirement of 5 Mtpa , and allow in-zone and above-zone monitoring of injection pressure, temperature and other key parameters. At the present time a nominal set of three mildly-deviated wellbores will be perforated over a variety of stratigraphic intervals to allow load-sharing of the injection and balancing of pressures in several closely-spaced and semi-independent reservoir intervals, separated by intraformational seals.

CarbonNet is following international codes of recommended practice for CO2 site characterization and is working under Australian Greenhouse Gas legislation which contains detailed criteria for development of a secure storage site with a key regulatory step being an application for an Injection Licence, supported by a full site development plan. This will be informed by the results of the current appraisal well.

The topseal at the Pelican site is a sequence of intraformational shales with interbedded coals and minor sands/silts. These are proven by nearby hydrocarbon traps and by the pressure and salinity data from numerous local wells, including the current appraisal well, Gular-1. The state of geomechanical stress is well-characterised from three stress tests over a range of depths in this well. The excellent reservoirs are supported by a world-class aquifer with strong pressure buffering and dissipation capacity which will limit any local pressure increase due to injection. Aquifer performance is quantified by basin-wide pressure responses to a 60-year production history in this prolific petroleum basin.

The Gular-1 well was drilled to a T.D. of 1523 m RT (1473 m ss) and acquired a range of whole core, sidewall core, cuttings, formation fluid samples, and MDT pressure data as well as a modern suite of LWD and wireline log data. The well is calibrated to the Pelican GCN18A 3D seismic data survey by a zero-offset VSP (high-resolution open-hole over the cover section and much of the target interval, and in-casing over the lowermost hole interval). The suite of data collected was planned to cover all the technical requirements of the CarbonNet project, as well as data that may be required or useful for regulatory applications and for demonstration of adherence to international recommended practice.

A water injection test was conducted over one of the planned injection zones for the project to confirm the injectivity predictions of core permeability measurements and petrographic interpretation of well logs. The well was safely plugged and abandoned with a high-quality CO2-resistant metallurgy and cement over all key seal intervals. The site of a future development cluster is not yet finalized but proximity to this well data point would be one significant criterion to be considered. The appraisal well location was therefore chosen to be near-optimal over a range of storage criteria including overall storage efficiency, pipeline and facilities cost, third party infrastructure, and future monitoring.

Keywords: CO2 storage, Site Characterisation, Site Appraisal, 3D seismic, Data Well, Core, Injection test, Monitoring