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.

Our limited understanding of mineral reactive surface area contributes to significant uncertainties in quantitative simulations of reactive chemical transport in subsurface processes. Continuum formulations for reactive transport typically use a number of different approximations for reactive surface area, including geometric, specific, and effective surface area. In this study, reactive surface area estimates are developed and evaluated for their ability to predict dissolution rates in a well-stirred flow-through reactor experiment using disaggregated samples from the Nagaoka pilot CO 2 injection site (Japan). The disaggregated samples are reacted with CO 2 acidified synthetic brine under conditions approximating the field conditions and the evolution of solute concentrations in the reactor effluent is tracked over time. The experiments, carried out in fluid-dominated conditions at a pH of 3.2 for 650 hours, resulted in substantial dissolution of the sample and release of a disproportionately large fraction of the divalent cations. Traditional reactive surface area estimation methods, including an adjusted geometric surface area and a BET-based surface area, are compared to a newly developed image-based method. Continuum reactive transport modeling is used to determine which of the reactive surface area models provides the best match with the effluent chemistry from the well-stirred reactor. The modeling incorporates laboratory derived mineral dissolution rates reported in the literature and the initial modal mineralogy of the Nagaoka sediment was determined from scanning electron microscopy (SEM) characterization. The closest match with the observed steady-state effluent concentrations was obtained using specific surface area estimates from the image-based approach supplemented by literature-derived BET measurements. To capture the evolving effluent chemistry, particularly over the first 300 hours of the experiment, it was also necessary to account for the grain size distribution in the sediment and the presence of a highly reactive volcanic glass phase that shows preferential cation leaching.

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A comprehensive study on the dynamics of dissolved elements (Mg, Al, Si, P, Ca, V, Cr, Mn, Fe, Ni, Zn, As, Sr, Y, W, and U) in Lake Biwa was carried out using a clean technique. Lake water samples (n ϭ 523) were collected from six stations in the North Basin and three stations in the South Basin. River water samples (n ϭ 178) were collected from 14 major rivers flowing into the North Basin. Rainwater samples (n ϭ 89) were collected at Otsu. The river water was enriched with Mn, Al, Fe, P, and Zn and the rainwater was enriched with Zn, Al, Fe, and Mn compared to North Basin water during winter mixing. The residence times of dissolved species were estimated on the basis of input through the rivers and rain. The residence times for Ca, Mg, and Sr were about 8 years, the same as that for water. Mn, Al, Fe, and Zn showed the shortest residence times (0.05-0.19 year). A budget calculation suggested that more than 60% of the input of dissolved Si, P, V, Cr, Mn, Fe, Ni, and Zn was scavenged and retained in the lake sediments and/or discharged as suspended particles.

Sequestration of Carbon Dioxide (CO2) into saline aquifers has been proposed as one of the most practical options of all geological sequestration possibilities. When saline aquifers are to be used to sequester CO2 for long periods, it will be necessary to monitor the migration and diffusion of CO2 in those reservoirs. Monitoring of geological sequestration has been identified as one of the highest priority needs in several recent international conferences on greenhouse gas control technologies. Monitoring is necessary to confirm the containment of CO2, to assess leakage paths, and to gain understanding of interactions between CO2, the rock-forming minerals, and formation fluids. Recently CO2 monitoring has moved to next stage for the purpose of leakage detection and quantification of CO2 stored in reservoirs. What kinds of monitoring methods we could use and do the methods have sufficient resolution and detection levels need to be addressed urgently. Seismic surveys provide the most attractive approach for obtaining the spatial coverage required for mapping the location and movement of CO2 in the subsurface. However, from the first Japanese pilot project, time-lapse sonic logging results showed P-wave velocity becomes less sensitive when the CO2 saturation up to 20%, while resistivity kept increasing with increase in CO2 saturation. This paper describes the results of P-wave velocity and resistivity measurements when injecting CO2 into water-saturated porous sandstones at laboratory and the results of comparison between P-wave velocity and resistivity changes obtained from both laboratory- and field-scales. Introduction Monitoring is the major challenge in CO2 geological sequestration. A number of techniques can be used to monitor the distribution and CO2 migration in the subsurface (IPCC, 2005). However, the effectiveness of these techniques depends upon many factors, including the contrast between the physical properties of CO2 and resident formation fluids, the lithology and structure of the reservoir, formation fluid pressure and temperature variations, source and receiver locations, well spacing, and injection patterns (Hoversten and Myer, 2000). Seismic surveys provide the most attractive approach for obtaining the spatial coverage required for mapping the location and movement of CO2 in the subsurface. Seismic techniques basically measure the velocity and energy absorption of waves, generated artificially or naturally, through rocks. By taking a series of surveys over time, it is possible to trace the distribution of the CO2 in the reservoir. Time-lapse 3D seismic surveys at Weyburn (Canada) and Sleipner (Norway) demonstrated the usefulness in CO2 monitoring at the large CO2 injection sites (Arts et al., 2004; Li, 2003). The annual injection rate at Weyburn and Sleipner is around 1 million ton CO2 per year.