Natural analogues for geological carbon storage
Abstract
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.