CO2CRC geologists have been researching the geological structures of the CO2CRC Otway Project site since 2003. Following comprehensive site characterisation they found that the geology of the site is well suited to the geological storage of CO2.
Integral to the safe geological storage of CO2 is selection of a suitable site. There are a number of geological storage options for CO2. The geological conditions required are:
- a porous (containing tiny spaces which store the CO2) and permeable (containing pathways that allow the CO2 to flow from the point of injection and through into the spaces) reservoir rock;
- a trap (five basic mechanisms hold the CO2 in place: stratigraphic, structural, residual, solubility, and mineral trapping); and
- an impermeable caprock, which prevents the CO2 from migrating toward the surface.
Site characterisation is the analysis and interpretation of subsurface, surface and atmospheric data in order to assess whether or not an identified site is suitable to store a specific quantity of CO2 for a defined period of time and meet all required health, safety, environmental and regulatory standards. It involves three major processes:
- Reservoir modelling: building a static model of the underground storage reservoir using geological, seismic and engineering data;
- Reservoir simulation: building a dynamic model to show how the CO2 will move through the reservoir; and
- Risk assessment: determining the possible risks of CO2 injection into the reservoir.
Existing data was used to build an initial model and simulation of the Waarre C reservoir. This included sequence stratigraphy, partial cores and some basic wire-line logs from the Naylor 1 and Naylor South 1 and cores and palynology (fossil studies used to determine age of rocks) from nearby wells (eg Boggy Creek-1). There was good quality 3D seismic data available to interpret the presence of faults but this data could not give information on the configuration of the reservoir.
In addition, information about similar sites (analogues) was reviewed. Analogues can provide information about the possible dimensions, orientation and structure of a reservoir body from studies of outcrops or subsurface formations with a similar depositional history. The most likely depositional model is a braided fluvial model. A picture of that type of river system is below.
Models based on the reservoir structure, the permeability, the properties of CO2 and water were constructed to predict the flow of the CO2 in the reservoir and the pressure of the reservoir under injection.
To test the models, they were matched against known information from when the Naylor well was producing. The production rates, reservoir pressure and gas composition was input into the model, and then the permeability, the height of the gas/water contact point and aquifer characteristics are adjusted, and the model run to predict the known wellhead pressure and the well water production.
Once these models were built and verified, the precise location of the CRC-1 well was decided. As the well was drilled, new data was acquired from well logs and core samples. All of this new data was used to further update the reservoir models.
Qualitative Risk Assessment
A detailed qualitative Risk Assessment was successfully carried out at the time the site was initially evaluated. The risks examined included:
- Project planning and pre-implementation risks
- Implementation risks following oil & gas industry standards
- Long term storage and containment risk
Quantitative Risk Assessment (QRA)
Possible risk events considered:
- Leakage through cap rock (permeable zone)
- Leakage through faults
- Well integrity
- Overpressuration at local and regional scales
- Exceeding spill-point (overflow of the reservoir)
- Equipment (compressor, pipeline, wellhead) failure
The risk assessment was peer reviewed by group of experts.
Risks are constantly re-evaluated throughout the main phase of the project:
- QRA 1: Containment (start of the project-2005)
- QRA 2: Pre-injection
- QRA 3: During-injection
- QRA 4: End of injection
- QRA 5: Post-injection
QRA 1&2 have been completed showing that the project has low risk events with minimal consequences.