TRANSPORT AND SEALING PROPERTIES OF CLAY-RICH LITHOTYPES EXPOSED TO CO2

In the ongoing CO2Seals project study the influence of CO₂ on both the matrix and fracture flow properties of clay-rich sealing lithologies. Samples used in this study comprise well-characterized clay/shale samples from underground laboratories, one outcrop sample and samples from a prospective storage site.

Within this project various experimental and analytical methods are applied to assess rock properties relevant to the sealing capacity of tight lithologies. These comprise

• single-phase permeability experiments with CO₂-saturated brine on intact & fractured/sheared material
• gas breakthrough tests with He and/or supercritical CO₂, where the water-phase is displaced from the previously brine saturated sample
• CO₂ high-pressure sorption experiments (up to 25 MPa)
• batch and flow reactor experiments with CO₂-saturated brine on single-clay minerals
• CO₂ contact angle & interfacial tension measurements

All rock samples are analysed before and after the exposure to CO₂ with respect to their porosity, specific surface area and mineralogical composition. Water samples collected in distinct time intervals during the single-phase flow experiments are analysed for their elemental composition.

The experimental data show that CO₂ is able to migrate into cap rocks, but at very low transport velocities. Absolute permeability coefficients extend down to the 10^-23m²-range. In many instances, transport is restricted to diffusion only. CO₂ sorption capacities on pure clay minerals and natural shale samples may be as high as 0.4 and 0.7 mmol/g (dry state), respectively. Analyses after exposure to CO₂ revealed no substantial mineral alterations, which is certainly due to the fact that flow and reaction rates are very low and that only a small proportion of the sample gets in contact with the permeating fluid/CO₂.

Acknowledgements: The CO2Seals project is incorporated into the GEOTECHNOLOGIEN R&D program funded by the German Federal Ministry of Education and Research (BMBF, Funding no.: 03G0681A). CO2Seals is co-funded and accompanied scientifically by the industry partner Shell International Exploration and Production, Netherlands. We would like to thank several companies and institutions for kindly providing the sample material (Shell, the CO2SINK project consortium, NAGRA, ANDRA, SCK-CEN/ONFRAF-NIRAS, Fa. Holcim).