Demonstrating permanent storage of CO2 in the form of residual and dissolution trapping
Residual saturation and dissolution test (Otway Stage 2B)
The physical structure of a storage formation involves a porous and permeable reservoir rock often overlaid by an impermeable layer called cap rock. In addition to structural trapping underneath the cap rock, there are two further mechanisms that play integral roles in permanently storing CO2 in underground reservoirs. These are residual and dissolution trapping and were the focus of the CO2CRC Stage 2B research project.
- Residual trapping occurs when CO2 saturation in the pore space is less than residual gas saturation (sgr). Sgr is the minimum required saturation of the injected CO2 to be mobile. The residually trapped CO2 will be permanently stored in the reservoir. The video below shows how a bubble of injected CO2 can be trapped in pore space.
- Dissolution trapping occurs when super critical CO2 dissolves into formation water.
A single-well injection test methodology was developed by CO2CRC’s research partners Lawrence Berkeley National Laboratory and CSIRO to obtain field scale estimates of residual and dissolved CO2 as part of its Stage 2B project. This methodology can be used for site appraisal before large scale injection for both CO2 storage and enhanced oil recovery projects.
Residual and dissolution trapping are important because they reduce the possibility of buoyant upward CO2 migration, ensuring the plume remains well within the storage formation.
Measuring residual and dissolved CO2 at field scale
A single-well injection test methodology was developed to obtain field scale estimates of residual and dissolved CO2. This methodology can be used for site appraisal before large scale injection for both CO2 storage and enhanced oil recovery projects
Resulting Publications
Development of the methodology
The method was developed through field trials at CO2CRC’s Otway International Test Centre. The whole project involved the injection of approximately 150 tonnes of pure CO2 into a single-well as well as approximately 450 tonnes of CO2 saturated formation water, intended to saturate the formation water with CO2. A variety of tests for measuring the residual CO2 saturation and dissolution rate in the formation water were applied including:
- Pressure measurement
- Thermal testing
- Noble gas tracing
- Dissolution testing
- Pulsed neutron logging
- Core testing
- Oxygen isotope analysis
Achievements
- Developed new methods to measure residual and dissolved CO2 storage capacity within a formation.
- Verified residual trapping and dissolution as storage mechanisms.
- Demonstrated that each method provides a different depth of analysis, with both advantages and disadvantages. These need to be considered when selecting characterisation methods for site appraisals.
- Revised concepts on residual and dissolution trapping.
Ongoing value
The methodology can be used and built upon for industrial scale CCUS and EOR projects. It allows for a more representative prediction and assessment of residual and dissolved CO2 at the reservoir scale than laboratory works.
