Evaluation of accessible mineral surface areas for improved prediction of mineral reaction rates in porous media

Abstract

The rates of mineral dissolution reactions in porous media are difficult to predict, in part because of a lack of understanding of mineral reactive surface area in natural porous media. Common estimates of mineral reactive surface area used in reactive transport models for porous media are typically ad hoc and often based on average grain size, increased to account for surface roughness or decreased by several orders of magnitude to account for reduced surface reactivity of field as opposed to laboratory samples. In this study, accessible mineral surface areas are determined for a sample from the reservoir formation at the Nagaoka pilot CO2 injection site (Japan) using a multi-scale image analysis based on synchrotron Xray microCT, SEMQEMSCAN, XRD, SANS, and FIB-SEM. This analysis not only accounts for accessibility of mineral surfaces to macro-pores, but also accessibility through connected micro-pores in smectite, the most abundant clay mineral in this sample. While the imaging analysis reveals that most of the micro-and macro-pores are well connected, some pore regions are unconnected and thus inaccessible to fluid flow and diffusion. To evaluate whether mineral accessible surface area accurately reflects reactive surface area a flow-through core experiment is performed and modeled at the continuum scale. The core experiment is performed under conditions replicating the pilot site and the evolution of effluent solutes in the aqueous phase is tracked.Various reactive surface area models are evaluated for their ability to capture the observed effluent chemistry, beginning with parameter values determined as a best fit to a disaggregated sediment experiment (Beckingham et al., 2016) described previously.Simulations that assume that all mineral surfaces are accessible (as in the disaggregated sediment experiment) over-predict the observed mineral reaction rates, suggesting that a reduction of RSA by a factor of 10-20 is required to match the core flood experimental data. While the fit of the effluent chemistry (and inferred mineral dissolution rates) greatly improve when the pore-accessible mineral surface areas are used, it was also necessary to include highly reactive glass phases to match the experimental observations, in agreement with conclusions from the disaggregated sediment experiment. It is hypothesized here that the 10-20 reduction in reactive surface areas based on the limited pore accessibility of reactive phases in core flood experiment may be reasonable for poorly sorted and cemented sediments like those at the Nagaoka site, although this reflects pore rather than larger scale heterogeneity.

Pore Network Model Predictions of Darcy‐Scale Multiphase Flow Heterogeneity Validated by Experiments

“…Micro-CT has been a valuable tool for experimental characterization of pore space geometry, (Lin et al, 2016) mineralogy (Lai et al, 2015;Menke et al, 2015;Beckingham et al, 2017;Al-Khulaifi et al, 2019), wettability (Iglauer et al, 2012;Bartels et al, 2019;Lin et al, 2019), residual trapping (Herring et al, 2013;Chaudhary et al, 2013;Al-Menhali & Krevor, 2016;Øren et al, 2019), and curvature-based capillary pressure (Armstrong et al, 2012;Herring et al, 2017;Garing et al, 2017;Lin et al, 2018;T. Li et al, 2018).…”

Reactive Transport in Evolving Porous Media

“…13). As discussed in more detail below (see case study by Beckingham et al 2016Beckingham et al , 2017, mineralogical heterogeneity and the evolution of reactive surface area can also substantially affect overall rock reactivity and solute release.…”

“…The first case study by Poonoosamy et al (2015Poonoosamy et al ( , 2016 focuses on permeability evolution in response to mineral-dissolution precipitation reactions. The second case study by Navarre-Sitchler et al (2009) evaluates the effect of weathering on diffusivity on the pore scale and the continuum scale, while the third case study by Beckingham et al (2016Beckingham et al ( , 2017 focuses on the progress and evolution of mineral dissolution reactions in a heterogeneous rock matrix. These three case studies have provided successful comparison between experimental data and continuum approaches, incorporating details from pore-scale observations.…”

“…A case study for evolving reactivity is provided by Beckingham et al (2016Beckingham et al ( , 2017 on highly reactive volcaniclastic sediments from the Nagaoka pilot CO 2 injection site in western Japan. The investigators carried out batch and coreflood experiments on a sample of sediment from the site to determine the time evolution of the sediment and fl uid as a result of injection of CO 2 -saturated fl uid.…”

“…However, one modeling study based on reactive flow experiments with calcite conducted in capillary tubes showed that it was necessary to consider the formation of high surface area precipitates in order to capture the evolving reactivity of the system (Noiriel et al 2012). In a study of CO 2 injection into reactive volcanogenic sands, it was shown the use of a grain size distribution could capture at least part of the evolution of the reactivity of the system (Beckingham et al 2017).…”