No Seal, No Deal: the Importance of Understanding Caprock Integrity During Long-Term CO2 Storage

In order to curb anthropogenic CO₂ emissions, Carbon Capture and Storage (CCS) technology has been put forward as a way to achieve a large reduction in emissions by capturing CO₂ at source, transporting it and injecting it into the subsurface for geological storage. Suitable formations considered for CO₂ storage are depleted oil and gas fields, saline aquifers and coal seams. The former are particularly of interest due to the presence of an existing surface and subsurface infrastructure, detailed understanding of the reservoir-caprock system and low pore pressures prior to CO₂ injection. However, prerequisite to ensuring safe, long-term storage of CO₂is maintaining caprock integrity, by preventing fracturing of the caprock, as well as by ensuring stability of any pre-existing faults cutting the seal.

Many potential depleted hydrocarbon fields under consideration for CCS, and most current CCS sites, are capped by shaly caprocks. The complex nature of this rock type, both mineralogically, hydraulically and mechanically, combined with the highly reactive properties of CO₂-rich fluids, makes for an interesting, yet difficult, system to investigate. On the short-term, it has been shown that CO₂ can lead to swelling of (swelling) clays present in the shale matrix, which may lead to the generation of outward-directed swelling forces. In addition, on the long-term (100-1000’s of years), CO₂/brine/rock interactions may lead to mineralogical changes in the material. Such processes can lead to changes in the mechanical and frictional behavior of shale, which may or may not enhance loss of containment in response to the new state of stress resulting from CO₂injection.

We studied the effect of these long-term chemical alterations on the mechanical properties of intact shale and clay-rich faults. Our study focused both on typical shale analogues, such as the Opalinus Clay (Mont Terri, Switzerland), as well as clay-rich caprock obtained from a natural CO₂ field (Green River, Utah, USA), which was exposed to CO₂ for > 400 ky. We performed mechanical experiments, using a novel technique, to study the behavior of intact, low permeability rocks, as well as shear experiments to understand the frictional stability of simulated fault gouges. This presentation will give an overview of the results obtained to date.