Assessing subsurface geochemical processes when storing CO2 with impurities (Otway Stage 2B Extension)
Separating flue gas into its component gases is a costly undertaking, particularly if the objective is to obtain high levels of CO₂ purity. The cost of carbon capture and storage (CCS) can be reduced if small levels of residual flue gases are accepted for injection with CO2 in geological storage. CO2CRC investigated the impact of storing CO2 with very low concentrations of the common impurities found in flue gas created from industrial processes – oxygen (O2), sulphur dioxide (SO2) and nitrogen dioxide (NO2). The project assessed the potential for these impurities to contribute to slight acidification of formation water due to oxidisation as well as other geochemical impacts on the reservoir.
This research project was undertaken at CO2CRC’s Otway National Research Facility with the support of and in collaboration with Callide Oxyfuel Services Pty Ltd, and the Australian and Japanese governments using CO2 from the Callide A oxy-combustion CO2 capture demonstation plant in Queensland.
Comparisons were made between two samples of formation water:
- The first sample was taken after water saturated with pure CO2 was injected.
- The second sample was taken after water saturated with CO2 and low concentrations of O2, SO2 and NO2 was injected.
The experiment was conducted at CO2CRC’s Otway National Research Facility at a depth of approximately 1400m in the CRC-2 well. Water samples were taken using a U-Tube system and analysed both on site and in a laboratory.
The following conclusions were made:
- The slight acidification of formation fluid, due to the oxidation of the dissolved impurities, was buffered by the dissolved carbonate or carbonate alkalinity of the formation. The most remarkable geochemical impact was caused by oxygen, which oxidised rapidly with minerals such as pyrite and siderite and dissolved ions such as iron. Samples showed enrichment of dissolved sulphate and depletion of dissolved iron.
- Bromide and lithium were identified as suitable liquid tracers that can be added to water produced from the formation and held at surface for later re-injection. These can be used to determine any mixing of injection water and undisturbed formation water.
- The overall impact on the geological formation, of storing CO2 containing low levels of common impurities was minimal. Understanding the geochemical processes at any given CO2 storage site is an important part of site characterisation. However, in general, reservoirs have adequate buffering capacity to minimise acidification from co-injection of CO2 with common impurities.
Vu, H, Black, J and Haese, R, (2017). Changes in formation water composition during water storage at surface and post re-injection. Energy Procedia, vol. 114, pp. 5732-5741.
link to publication reference
Todaka, N. and Xu, T., 2017. Reactive transport simulation to assess geochemical impact of impurities on CO2 injection into siliciclastic reservoir at Otway site, Australia. International Journal of Greenhouse Gas Control, vol. 66, pp. 177-189.
link to publication reference