Role of Organic Matter and Pore Structure on CO2 Adsorption of Australian Organic-Rich Shales

Carbon dioxide (CO2) injection into shale reservoirs is a potentially efficient technique to reduce greenhouse gas emissions while enhancing methane recovery. Despite research displaying a dominant role of organic matter (OM) in the adsorption behavior of shales, we show in this study that pore-size distribution and accessibility could be more important than merely having more OM. We assessed this hypothesis by conducting a combined set of experiments including (i) CO2 adsorption of Australian shale samples collected from Beetaloo, Perth, and Cooper basins before and after removing their OM using oxygen plasma ashing, at pressures up to 5.0 MPa and two temperatures (30 and 50 °C); (ii) CO2 adsorption of clay minerals (kaolinite, illite, and smectite) at the same pressures and temperatures; and (iii) X-ray diffraction (XRD), mercury injection capillary pressure (MICP), and focused ion beam scanning electron microscopy (FIB-SEM) to examine the effects of the samples' compositional and structural properties with CO2 adsorption. The results of this study highlighted the significant role of microporosity and pore structure/accessibility in the adsorption process of both shale’s organic and inorganic counterparts. Specifically, for the studied samples, it was found that (a) the calculated amount of CO2 adsorbed by OM separated from intact organic-shale samples is multiple times larger than the amount adsorbed by the intact organic-shale samples themselves (from which the OMs were removed); (b) the OM of the Beetaloo, which has higher pore-size distribution and volume than the OMs of the other two samples, adsorbed double the amount; (c) the positive correlation between shales’ adsorption capacity and TOC is mainly controlled by porosity and pore structure of the OM; (d) while shale samples reach their adsorption capacity at relatively low pressures, the adsorption process of clay minerals is faster at lower temperatures; (e) although montmorillonite has a higher total surface area than illite, both adsorb approximately the same amount suggesting that adsorption mostly occurs at external rather than internal clay surfaces; and (f) CO2 has a higher affinity to remain adsorbed in organic shales than in clays.