Effects of surface mineralogy and roughness on CO2 wettability of the Mount Simon sandstone; implications for predicting CO2 storage capacity and pore scale transport
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Wettability is a key reservoir characteristic influencing geological carbon sequestration (GCS) processes such as CO2 transport and storage capacity. Wettability is often determined on limited number of reservoir samples by measuring the contact angle at the CO2/brine/mineral interface, but the ability to predict this value has not been explored. In this work, minerals comprising a natural reservoir sample were identified, and the influence of their surface roughness, surface charge, and location in the sample on contact angle was quantified to evaluate controlling mechanisms and predictive models. A core sample was obtained from Mount Simon formation, a representative siliciclastic reservoir that is the site of Department of Energy CO2 injection project. Quartz, microcline, illite, hematite, illite + hematite were identified as dominant minerals in the core, and contact angle (θ) measurements were conducted over a wide range of pressure (290-3625 psi) at 40⁰C. At supercritical conditions, individual minerals and the Mount Simon sample were strongly water wet, with contact angles between 27⁰ and 45⁰ and contact angle generally increased with surface roughness, suggesting that brine is trapped in roughness pits between CO2 and the substrate. There was no relationship between contact angle and surface charge. A thin section of the Mount Simon sandstone was examined with a compound light microscope, and reddish precipitates coating quartz and feldspar grains were apparent. These were evaluated with environmental scanning electron microscopy (ESEM) and energy dispersive X-ray spectroscopy (EDS). ESEM images show precipitate morphology that is consistent with clay coatings. The EDS results identify regions of the precipitate with high iron content. Several predictive models for contact angle were evaluated, including the Wenzel, and Cassie-Baxter models, plus new modifications of these that account for alternative surface roughness geometries and/or the fraction of different minerals comprising the reservoir sample surface. Modeling results suggest the fraction of illite/hematite covering Mount Simon grain surfaces is the most important reservoir characteristics that control wettability. To our knowledge, this is the first study that provides mechanistic insights into the characteristics of individual minerals affecting the wettability of a natural reservoir sample.