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.

Key Messages

• This study was conducted to assess the natural rates of CO2 and fluid migration that occur in the overburden (defined as the entire geological succession above the target reservoir formation with the lowermost stratum forming the primary seal) and the potential rates that may arise in the unlikely event of unintended migration outside a designated storage complex. The aim was to better inform risk assessments for CO2 storage sites by providing relevant information on the effect of large-scale features associated with natural fluid migration analogues in the overburden.

• With appropriate site selection and site characterisation risk-based process, CO2 storage sites are selected to minimise the likelihood and impacts of fluid migration.

• The five case studies in this report hjosirljosirt that storage sites are likely to have numerous secondary storage formations within the overburden with low permeability sequences (e.g. shale) providing secondary seals, in addition to the primary caprock seal, thereby hindering or preventing migration through the overburden.

• The natural migration of fluid in the overburden over geological timescales is evident from the presence of chimneys, gas hydrates and sediment injections (pockmarks, mud volcanoes and mounds are also present offshore). Generally their formation has been well researched although their current in-situ properties (and their impacts on fluid flow) require further analysis due to the lack of data.

• The principal potential geological pathways which may enable the migration of fluids within the overburden are fractures and faults (chimneys and large-scale geomorphological features such as tunnel valleys and mass-movement deposits may also enhance flow in the overburden).

• Ice-loading on bedrock and sediment deposits can cause rafting, fracturing and faulting. Potential fluid migration pathways are created along faulted surfaces and rafting disrupting lateral seals. Evidence of glacio-tectonic deformation is recorded in areas where CO2 storage is operational or planned such as onshore in Canada and offshore in the North Sea.

• The large volume and complexity of the overburden makes modelling potential migration pathways difficult. The characterisation of overburden structures should focus on parameterising elements and quantifying potential fluid flow rates.

• From this report, it is recommended that further in-situ data is acquired during future work, directly sampling overburden features such as faults and chimneys. Direct sampling is required to further refine the potential fluid properties of these structure and their implications for fluid migration. Further research, for example the EU funded STEMM project, will investigate these features and is already underway.