Changes to Shale Caprock Porosity Induced by Reaction with Supercritical CO2 in Geologic Carbon Repositories

Success of long-term CO₂ storage in geologic carbon repositories will depend in part on caprock integrity.  An effective caprock possesses low permeabilty, such that capillary entry pressures impeding vertical CO₂ migration are high, and contain few open fractures.  In cases where CO₂ leakage through the caprock does occur, contamination of overlying potential drinking water aquifers is a significant concern.  While caprock integrity will be assessed prior to any injection of CO₂ in a sequestration project, changes in caprock integrity with reaction among the CO₂, formation brine and caprock minerals are not currently predictable.               

CO₂ migration into the caprock will acidify the porewaters, enhancing silicate mineral dissolution.  Where mineral dissolution is not accompanied by sufficient carbonate precipitation, caprock porosity will increase, potentially forming high permeability pathways.  Where carbonate precipitation is greater than dissolution, porosity will decrease, promoting self-sealing of the caprock. To investigate porosity changes in caprock material, samples of Maplewood Shale were reacted with brine +/- supercritical CO₂ at temperatures and pressures relevant to CO₂ sequestration.  Changes in microporosity were analyzed using small angle neutron scattering.  Neutrons are effectively scattered by pore-mineral interfaces in rocks.  Scattered neutrons contain information about the number and volume of pores and their arrangement in the sample.  From scattering data, surface or mass fractal dimensions can be determined.    

The shale microporosity increased with reaction from ~2.5% to 4%, with greater contribution from larger pores.  All of the shales exhibit mass fractal behavior with an increase mass fractal dimension from ~2.6 to ~2.8 after reaction with both brine and brine + supercritical CO₂.  Mass fractal dimensions range from 2 to 3 for simple to complicated pore arrangements.  These results can be explained by increased diameter of a greater number of small pores due to mineral dissolution compared to the number of newly created small pores.