Abstract
Accessible surface areas (ASAs) of individual rock-forming minerals exert a fundamental control on the maximum mineral reactivity with formation fluids. Notably, ASA efficiency during fluid-rock reactions can vary by orders of magnitude, depending on the inflow fluid chemistry and the velocity field. Due to the lack of adequate quantification methods, determining the mineral-specific ASAs and their reaction efficiency still remain extremely difficult. Here, we first present a novel joint method that appropriately calculates ASAs of individual minerals in a multi-mineral sandstone. This joint method combines SEM-image processing results and Brunauer-Emmett-Teller (BET) surface area measurements by a Monte-Carlo algorithm to derive scaling factors and ASAs for individual minerals at the resolution of BET measurements. Using these atomic-scale ASAs, we then investigate the impact of flow rate on the ASA efficiency in mineral dissolution reactions during the injection of CO2-enriched brine. This is done by conducting a series of pore-scale reactive transport simulations, using a two-dimensional (2D) scanning electron microscopy (SEM) image of this sandstone. The ASA efficiency is determined employing a domain-averaged dissolution rate and the effective surface area of the most reactive phase in the sandstone (dolomite). As expected, the dolomite reactivity is found to increase with the flow rate, due to the on average high fluid reactivity. The surface efficiency increases slightly with the fluid flow rate, and reaches a relatively stable value of about 1%. The domain averaged method is then compared with the in-out averaged method (i.e the “Black-box” approach), which is often used to analyzed the experimental observations. The in-out averaged method yields a considerable overestimation of the fluid reactivity, a small underestimation of the dolomite reactivity, and a considerable underestimation of the ASA efficiency. The discrepancy between the two methods is becoming smaller when the injection rate increases. Our comparison suggests that the result interpretation of the in-out averaged method should be contemplated, in particular, when the flow rate is small. Nonetheless, our proposed ASA determination method should facilitate accurate calculations of fluid-mineral reactivity in large-scale reactive transport simulations, and we advise that an upscaling of the ASA efficiency needs to be carefully considered, due to the low surface efficiency.
Highlights
IntroductionNatural or engineered geological systems, such as enhanced geothermal systems (EGS) (Althaus and Edmunds, 1987; Pandey et al, 2015) and carbon capture, utilization, and storage (CCUS) systems (Xu et al, 2003; Gaus, 2010; Luhmann et al, 2014; Tutolo et al, 2015; Randolph and Saar, 2011; Saar et al, 2012; Adams et al, 2015; Garapati et al, 2015; Fleming et al, 2020; Ezekiel et al, 2020), often involve fluid-rock reactions
A key parameter controlling the rates of these surface reactions is the effective surface area (ESA) of the minerals, which is defined as the area of the mineral surfaces that is actively participating in the reactions
This study introduces a new approach to accurately quantify mineral-specific accessible surface area (ASA) in a multi-mineral natural rock, and examines the effect of fluid flow rate during reactive transport on the reaction efficiency of such Accessible surface areas (ASAs)
Summary
Natural or engineered geological systems, such as enhanced geothermal systems (EGS) (Althaus and Edmunds, 1987; Pandey et al, 2015) and carbon capture, utilization, and storage (CCUS) systems (Xu et al, 2003; Gaus, 2010; Luhmann et al, 2014; Tutolo et al, 2015; Randolph and Saar, 2011; Saar et al, 2012; Adams et al, 2015; Garapati et al, 2015; Fleming et al, 2020; Ezekiel et al, 2020), often involve fluid-rock reactions. The complex pore structures in natural porous media often induce a wide range of local fluid flow ve locities and degrees of fluid mixing, experienced by all migrating solutes (Nissan and Berkowitz, 2019) The reaction between these solutes and the heterogeneously distributed mineral surfaces can result in a signif icantly different overall behavior of reactive transport, compared to that in homogeneous media or to those found in pore-scale observations. Pore-scale reactive modelling, in combination with high-resolution image scanning (e.g., X-Ray Computed Tomography (XRCT), scanning electron microscopy (SEM), etc.), has been proposed as a promising tool to address the aforementioned issues During this approach, image scans provide structural and mineralogical informa tion, such as pore geometry, mineral distribution, and mineral accessible surface area (ASA) (Poonoosamy et al, 2019), which can be used in pore-scale reactive transport simulations. These results can improve geochemical calculations in larger models
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
