Natural analogues for geological carbon storage
In CO2 storage sites, seal has a vital role in inhibiting migration of the supercritical CO2 to other geological strata. The major difficulties in studying seals include two aspects: lack of available samples (especially for saline aquifers), and the difficulty to study over geological time and spatial scales. The analysis of natural analogue has been chosen to overcome these difficulties. Hydrocarbon fields are used to investigate the pore throat radii, which is the major factor for capillary sealing of caprocks, using newly established statistics model. Natural CO2 springs at Green River, Utah are used to study how the long-term CO2 charge triggers chemical reactions and migration in shales. One of the major sealing mechanism of caprocks is capillary sealing. Pore throat radius, as the main factor to decide the capillary sealing, has been investigated in this study. As an alternative to the traditional method of mercury injection porosimetry, a statistical model for effective pore throat radii determination has been established. The cumulative percentage and the probability distribution of the effective pore throat radii of the shale caprocks in the UK North Sea oil fields are obtained, which would be used as a reference for the saline aquifers in CO2 storage siting in the future work. Monte Carlo simulation is utilised to get the distribution of the effective pore throat radii. The cumulative distribution from this study has been compared with the distribution by Yang and Aplin (1998). The distribution by the statistical model enables to narrow down the range of effective pore throat radii to 37nm~1700nm, and help to make a better prediction on the pore throat radii. The correlation between the controlling factors of faulting, burial depth, caprock thickness and the pore throat radii have been examined. Good correlation between the depth less than 3000m and the effective pore throat radii indicates clay diagenesis should be the major factor for shallowly buried caprocks. Faulting and caprock thickness present no significant correlation with the effective pore throat radii. Crystal Geyser is used as an ideal natural analogue to study the Mancos Shale alteration. The interacted fluid that deposited travertine is important for the study. Hence, carbonate veins and reduction zones that associated with the activity of the main fault are used as records of the geochemistry of the paleo-fluid, the features of which are compared to the present spring water. The result shows the paleo-fluid was much more saline than the present fluid, with greater flow-rate. The decreased flow-rate might be owing to the self-healing of the fault during the time. Mancos shales outcropped in the hanging wall of the Little Grand Wash fault were sampled to investigate on the alterations triggered by the CO2-charged fluid from the fault and fractures. The result shows the alteration radius of the Mancos has limited within the distance of 20m away from the fault. CO2-rich fluid could interact with deformed shale (both dissolution and precipitation might happen), but no evidence shows the intact Mancos has been altered. The calcite cements in Mancos derived from CO2 sequestration could reach up to 27% (%weight of the whole rock) at 15m away from the fault. The conclusion facilitates the carbon storage siting criteria by Chadwick et al., (2009), who proposed the cautionary thickness of the caprock to be 20m.