Subsurface impact of CO2: Response of carbonate rocks and wellbore cement to supercritical CO2 injection and long-term storage. Geologica Ultraiectina (310)

Capture of CO2 at fossil fuel power station coupled with geological storage in empty oil and gas reservoirs is widely viewed as the most promising option for reducing CO2 emissions to the atmosphere, i.e. for climate change mitigation. Injection of CO2 into such reservoirs will change their chemical and mechanical state, for example through acidification of the pore fluid or changes in the state of stress. Similar effects may influence the integrity of overlying caprocks and of old, plugged wells. To fully assess storage system integrity, these effects must be understood. The research reported in this thesis addresses the mechanical and chemical response of carbonate rich reservoir rocks, and of wellbore cements to CO2 injection and storage. State of the-art experimental techniques were employed. These included compaction experiments on simulated porous carbonates in order to investigate time-dependent compaction phenomena, compression tests to determine the mechanical failure behaviour of chalks and wellbore cements, and reaction experiments to explore the long-term chemical effects of CO2 on cement. The results show that CO2 can accelerate long-term compaction. However, under the saline conditions of typical limestone reservoirs, the effect will be negligible if the reservoir was stable before CO2 injection. Highly porous chalks are well known to be unstable to water injection. However CO2 has little further effect. Results obtained for wellbore cement (Class A Portland cement) similar to the cements used in depleted Dutch gas fields, such as the De Lier field, showed that the stress changes accompanying injection will not lead to mechanical damage. In addition, experiments on chemical interaction have shown that reaction of CO2 with the cement precipitates calcium carbonate into voids and cracks, thus improving the sealing properties. Provided such wells are plugged properly upon abandonment, mechanical and chemical integrity will be preserved