CO2 Sorption Capacity in Clay-Rich Shales with Moisture Content

Abstract

Injected CO2 in geological sites may contact the impermeable caprock/shales overlying the injection reservoirs. In such cases, the shales may provide additional storage capacity due to CO2 adsorption in the microstructures. Shales have heterogenous mineral compositions and nano-scale microstructures (pore width < 200 nm), which result in complex wetting properties and low permeability. In such nanoporous materials, a large percentage of gas in place is stored by adsorption to the high surface areas. The complex mineralogy of shales, such as clay, quartz, carbonate and organic matter, will control sorption properties in addition to environmental pressure and temperature effects. The goal of this study is to determine the mineralogy and moisture effect on CO2 accessible surface area using subcritical adsorption and low pressure adsorption. Our results indicate that the total organic content (TOC) is a controlling factor of micropore volume in shale. We find a positive correlation where the excess micropore volume increases with increasing TOC of the shale. The hydrophobic nature of organic matter (OM) shows preferential adsorption when tested with different fluids such as nitrogen, hexane and water. With the micropore volume estimated from CO2 subcritical adsorption, CO2 storage potential in Utica shale is estimated to be double of previous theoretical estimation by Godec et al (2014). Moisture effect on shale and zeolites is studied to image the effect of irreducible water saturation, where the CO2 adsorption is significantly reduced with the presence of moisture.