The Gorgon Project off north-west coast of Australia plans for the injection of more than 2 trillion cubic feet (TCF) of reservoir CO2. For this purpose, the Dupuy Formation under Barrow Island has been selected as the target reservoir, as it is interpreted to have sufficient capacity, containment and injectivity. In order to gather information to narrow a number of key reservoir uncertainties, the Gorgon Project Joint Venture participants decided to drill a well through the proposed injection target. This paper describes the log and core acquisition programmes of the ‘Gorgon CO2 Data Well’ and shows why the information obtained in this well is essential for the success of the project.
The CO2 injection project has been extensively documented and subjected to public comment as part of the Gorgon Project Environmental Impact Assessment Process. Following this process the Western Australian Environmental Protection Authority found that the environmental risks associated with the carbon dioxide injection project were acceptable and recommended that CO2 injection must proceed as an integral component of the Gorgon Project.
The pre-CO2 Data Well Dupuy Formation log data base was of mixed quantity, quality and vintage, resulting in wide ranges of key subsurface uncertainties. The logging programme for the well was designed to answer questions about lithology, mineralogy and reservoir quality in order to reduce the impact of the ranges of these uncertainties. Logs ran included nuclear magnetic resonance (NMR), image, dipole sonic, spectroscopy, tri-axial induction logs as well as a comprehensive formation tester program including a number vertical interference tests and medium-scale formation fracture tests, known as ‘mini-fracs’. Pre-CO2 Data Well routine and special core analysis was also very limited in the proposed injection target, so an extensive core program was undertaken. The analysis on the 500m of CO2 Data Well core consisted of a comprehensive set of routine core measurements, permeability at different scales, geochemical analyses, petrography, NMR, mercury injection capillary pressure (MICP) and electrical measurements.
As well as providing updated and additional input data to static and dynamic models, the data acquired in the CO2 Data Well has resulted in a shift in reservoir modelling focus as well as giving the team an improved understanding of the reservoir. Some examples of how this was manifested are as follows:
- Observed variations in core permeability were analysed and explained through a series of detailed core and log analyses - this has had a direct impact on the modelled distribution of injected CO2 through various parts of the aquifer.
- Core and log evidence have significantly narrowed the range of uncertainty in residual oil saturation (Sor) and reduced its absolute value to zero for most of the reservoir, resulting in a reduction in the number of planned wells.
- Geomechanical tests on core in combination with insitu tests in the CO2 Data Well have greatly increased the understanding of the geomechanical properties of the Dupuy Formation, resulting in increased confidence in containment of the injected CO2.