CO₂ Migration in the Overburden

Publication Overview

This report documents the subsurface processes that may enable CO₂ to potentially migrate from the storage reservoir to within the overburden sequence. The potential rates of migration for each migration pathway and the implications for leakage are discussed. Secondary trapping mechanisms within the overburden are also discussed within the report. The conclusions are focused on tying overburden characteristics to their impact on developing risk assessments. As well as specific pathway structures, five CO₂ storage projects were selected for this review and the characteristics of the overburden sequence that promote trapping and hinder migration at each site are a summarised. The projects chosen were the offshore Sleipner and Snøhvit CO₂ storage projects, the planned storage site in the Goldeneye Field, the onshore Ketzin pilot CO₂ injection project in Germany and the Field Research Station in Canada.

Publication Summary

• 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 highlight 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.