Introduction
At CO2 storage sites, large-scale overburden heterogeneities and introduced man-made pathways (e.g. wells) could potentially breach the sealing strata and promote the migration of injected CO2 in the dense, gaseous or dissolved phase. The ability to fully characterise the overburden would therefore allow identification of the preferred CO2 vertical migration pathways which will then encourage more complete risk assessments. Overall this will allow for more focussed monitoring efforts and will lead to the deselection of sites prone to vertical migration.
The overburden is very site-specific but at most CO2 storage projects it consists of a thick sequence of sedimentary rocks and young, poorly consolidated sediments. Heterogeneities in overburden sequences may allow fluids to flow vertically across stratal boundaries and provide fluid flow pathways to shallower depths or laterally beyond the immediate vicinity of the fluid source. In most cases the overburden will inevitably include internal heterogeneities and features (e.g. gas chimneys, and glacial landform) and many of these structures could serve to either hinder or promote fluid migration.
Understanding and quantifying potential migration of fluids within the overburden is difficult because of limited in-situ data from common large-scale features over large areas. Incorporation of overburden heterogeneities into risk assessments, given large uncertainties associated with some structures, presents an additional challenge for site selection and characterisation. IEAGHG published a report in 2016 reviewing the permeability of faults and this report develops on this work to look specifically at the overburden and other associated structures.
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Key Messages
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