Nanoscale Chemical Processes Affecting Storage Capacities and

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Nanoscale Chemical Processes Affecting Storage Capacities and Seals during Geologic CO2 Sequestration Published as part of the Accounts of Chemical Research special issue “Chemistry of Geologic Carbon Storage”. Young-Shin Jun,* Lijie Zhang, Yujia Min, and Qingyun Li Department of Energy, Environmental and Chemical Engineering, Washington University, St. Louis, Missouri 63130, United States CONSPECTUS: Geologic CO2 sequestration (GCS) is a promising strategy to mitigate anthropogenic CO2 emission to the atmosphere. Suitable geologic storage sites should have a porous reservoir rock zone where injected CO2 can displace brine and be stored in pores, and an impermeable zone on top of reservoir rocks to hinder upward movement of buoyant CO2. The injection wells (steel casings encased in concrete) pass through these geologic zones and lead CO2 to the desired zones. In subsurface environments, CO2 is reactive as both a supercritical (sc) phase and aqueous (aq) species. Its nanoscale chemical reactions with geomedia and wellbores are closely related to the safety and efficiency of CO2 storage. For example, the injection pressure is determined by the wettability and permeability of geomedia, which can be sensitive to nanoscale mineral−fluid interactions; the sealing safety of the injection sites is affected by the opening and closing of fractures in caprocks and the alteration of wellbore integrity caused by nanoscale chemical reactions; and the time scale for CO2 mineralization is also largely dependent on the chemical reactivities of the reservoir rocks. Therefore, nanoscale chemical processes can influence the hydrogeological and mechanical properties of geomedia, such as their wettability, permeability, mechanical strength, and fracturing. This Account reviews our group’s work on nanoscale chemical reactions and their qualitative impacts on seal integrity and storage capacity at GCS sites from four points of view. First, studies on dissolution of feldspar, an important reservoir rock constituent, and subsequent secondary mineral precipitation are discussed, focusing on the effects of feldspar crystallography, cations, and sulfate anions. Second, interfacial reactions between caprock and brine are introduced using model clay minerals, with focuses on the effects of water chemistries (salinity and organic ligands) and water content on mineral dissolution and surface morphology changes. Third, the hydrogeological responses (using wettability alteration as an example) of clay minerals to chemical reactions are discussed, which connects the nanoscale findings to the transport and capillary trapping of CO2 in the reservoirs. Fourth, the interplay between chemical and mechanical alterations of geomedia, using wellbore cement as a model geomedium, is examined, which provides helpful insights into wellbore and caprock integrities and CO2 mineralization. Combining these four aspects, our group has answered questions related to nanoscale chemical reactions in subsurface GCS sites regarding the types of reactions and the property alterations of reservoirs and caprocks. Ultimately, the findings can shed light on the influences of nanoscale chemical reactions on storage capacities and seals during geologic CO2 sequestration. spaces (residual trapping).2 The CO2 plume movement and the residually trapped amount are dependent on the wettability, porosity, and permeability of the reservoir rocks,4 which can be altered by nanoscale chemical reactions. Solubility trapping occurs when CO2 dissolves into brine and forms aqueous species. CO2 dissolution into brine is influenced by temperature, pressure, brine composition, and injected fluid composition.5 As CO2 dissolves, the brine is acidified to pH ∼ 3, and can be further acidified to pH ∼ 1 due to dissolution of coinjected acidic gases, such as H2S and SO2.6 Under these acidic pHs, minerals can dissolve and release cations; then the pH of brines will slowly increase to around neutral. Precipitation of carbonate minerals leads to mineral trapping, which is considered the most stable trapping mechanism.

I. IMPORTANCE OF NANOSCALE CHEMICAL REACTIONS DURING GCS OPERATION To decrease anthropogenic CO2 emission, geologic CO2 sequestration (GCS) is considered an effective strategy. GCS sites include deep saline aquifers, depleted oil and gas reservoirs, basalts, coal-seams, or enhanced oil/gas recovery sites. The sites require an impermeable (≤10−19 m2) caprock layer on top of a permeable (≥10−12 m2) reservoir rock where CO2 can transport through pores.1 After CO2 is injected, it can be trapped via several mechanisms (Figure 1).2 First, the upward movement of CO2 is hindered by the low permeability caprock (structural/stratigraphic trapping). Nanoscale chemical reactions between CO2 and caprock can cause either opening or sealing of fractures in caprocks, and can affect structural trapping.3 As injected CO2 migrates, the free phase CO2 first displaces brine in pore spaces and is then further displaced by brine, leaving some CO2 trapped in the pore © 2017 American Chemical Society

Received: December 31, 2016 Published: July 7, 2017 1521

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affect the capillary entry pressure, which is the pressure difference between the nonwetting phase (scCO2) and the wetting phase (water). As given by the Young−Laplace 2γ cos θ

equation, Pc = PCO2 − Pw = wc R ,4,13 capillary pressure is determined by the pore radius (R indicates porosity and permeability), scCO2−brine interfacial tension (γwc), and the contact angle (θ indicates wettability). The capillary pressure impacts the transport of scCO2 or CO2-saturated brine, the residual trapping of the reservoir rocks, and the structural trapping by the overlying caprocks. Nanoscale chemical reactions of minerals can change porosity and permeability (R), interfacial tension (γwc), and wettability (θ), or change the mechanical properties of geomedia, and thus affect GCS operations. Such an understanding, bridging nanoscale chemical reactions and hydrogeological and mechanical alterations, is helpful for designing safer and more efficient GCS operations.

III. WHAT WE HAVE LEARNED III.a. Chemistry of Reservoir Rocks Figure 1. CO2 trapping mechanisms at GCS sites.

Based on findings from pilot projects such as the Frio formation project, bench scale laboratory experiments of core samples, and geochemical modeling studies, the importance of chemical reactions of reservoir rocks in GCS operation has been recognized.14,15 Even with this crucial recognition, modeling approaches still have to embrace some uncertainties due to the lack of an accurate database of mineral dissolution and precipitation kinetics and thermodynamics. Hence, an accurate database of mineral dissolution kinetics is needed for improving our prediction of the behaviors of GCS sites. To address this need, we conducted experiments under GCS relevant conditions using single mineral experiment systems. We chose to study single mineral systems as a starting point because they allow us to better control the experimental conditions, providing more accurate data that can ultimately help understand multimineral systems in GCS sites. Feldspars, which make up nearly 60% of the Earth’s crust, are abundant in both typical reservoir rocks and caprocks. Considering their abundance and reactivity, and their potential to release cations to form carbonates for long-term CO2 storage, we chose feldspars as model sandstone minerals. The feldspar group contains the Na−Ca series, called plagioclases, and the K−Na series, called alkali feldspars (Figure 2A). The dissolution kinetics of feldspar have been investigated in numerous studies,16 where the dissolution rate of feldspar is usually modeled as a function of reactive surface area, temperature, pH, and the Gibbs free energy of dissolution (ΔG). However, our understanding of the potential roles of their crystallography and aqueous species in chemical reactions remains limited, especially under GCS conditions. Thus, the following section will discuss our findings on their roles in chemical reactions. Effects of Feldspar Crystallography on Its Dissolution. The crystallography of solid phases is a key factor affecting their dissolution kinetics. Yang et al. developed a model to predict the role of T−O−T bond length (average distance between tetrahedral sites (T) and oxygen atoms (O)) and Al/Si ordering in feldspar dissolution.17−19 The framework of feldspar crystal structure is formed by Si and Al tetrahedra (Figure 2B). Each Al or Si is bonded to four oxygens, and each oxygen is shared with another tetrahedron, which forms T−O− T linkages. The Al content (Al/Si ratio) of feldspar determines

Nanoscale chemical reactions affect all the trapping mechanisms, and further influence the storage capacity and seal during GCS.

II. KNOWLEDGE GAPS II.a. Laboratory Experiments under Conditions Relevant to GCS

The temperatures and pressures at GCS sites vary greatly with injection depth,7 which can be roughly estimated according to the relationships that T(°C) = 15 + 33d and P(atm) = 1 + 100d, where d is the depth in kilometers.7 Temperature is an important parameter affecting CO2 dissolution and mineral dissolution/precipitation kinetics and mechanisms. The CO2 pressure will also affect the dissolved aqueous CO 2 concentration, changing the brine pH and bicarbonate concentration. Although studies have investigated mineral dissolution under ambient conditions, investigations on the effects of GCS relevant temperatures and pressures on the nanoscale interactions between minerals and scCO2−H2O(l) (scCO2-saturated water) or scCO2−H2O(g) (water-saturated scCO2) are relatively limited. Further, brines in subsurface sites usually have high salinities, varying between several mg/L to several hundred g/L.8,9 High concentrations of cations are present in the brine.8−10 Sodium (Na+) is the most abundant, constituting 70%−90% of total cations by mass.9 In addition to cations, inorganic anions, such as sulfate (0.01−0.05 M), and organic anions, such as acetate (0.016−0.25 M) and oxalate (0−0.005 M), are abundant in formation waters.8,9,11,12 As reported for the Frio formation, the concentrations of some organic compounds (mainly formate and acetate) can even increase after CO2 injection, because scCO2 is a good organic solvent.11 Thus, investigations on the nanoscale brine−mineral interactions under various water chemistries related to subsurface conditions are needed. II.b. The Roles of Nanoscale Chemical Reactions in Changes in Hydrogeological and Mechanical Properties at GCS Sites

Information is limited on the roles of nanoscale chemical reactions in changes in hydrogeological and mechanical properties at GCS sites. The hydrogeological properties greatly 1522

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Figure 2. (A) Members of the feldspar group. (B) “Crankshaft chain” structure in feldspar’s framework and illustrative pH dependence of the release of different types of Si atoms. Four types of Si atoms differ in reactivity if the intercrankshaft-chain T−O−T linkages are assumed identically reactive for all Si atoms. (C) Results from feldspar dissolution experiments (I), structure refinements based on synchrotron X-ray diffraction (1 − QT, where QT represents the distribution of Al between T1 and T2 sites. QT = 0 if completely ordered, and QT = 1 if Al is randomly distributed), and Fourier transform infrared (FTIR) spectroscopy analyses (δω is the displacement of the peak at 650 cm−1, which is proportional to the square of the difference in Al/Si ordering). The dissolution incongruence (shown as I, where I = 0 means congruent, and I = 1 if rSi/rAl = 0) obtained from water chemistry experiments correlates closely with the pattern of the variation in the degree of Al/Si ordering for the feldspar specimens. (D) pH dependence of plagioclase dissolution at various anorthite contents. The Al/Si ordering varies with anorthite contents as shown in (C). Reproduced from refs 18 and 19. Copyrights 2014, Elsevier Ltd.

modeled as a function of Al/Si ordering state (Figure 2C and D).18 These new models provide a valuable perspective on feldspar dissolution, which links the crystallography and the dissolution kinetics in surface and subsurface environments. Effects of Cations on Plagioclase Dissolution. In addition to the effects of crystallography, until recently, the impact of high concentrations of cations in brine on feldspar dissolution has been unclear. A previous study found that plagioclase dissolution rates at 25 °C were strongly inhibited by 0.1 M Na+, and a model was provided to predict the effects of Na+ on plagioclase dissolution, based on competing adsorption between Na+ and protons, and the diffusion across the surface hydration layer on plagioclase.20 However, the applicability of these findings to GCS system is uncertain, because the concentration of Na+ in formation brine can be as high as 3.4 M. To reduce this uncertainty Min and Jun investigated the dissolution of anorthite in 0−4 M NaCl at 35−90 °C and 100 atm CO2.21 The inhibition effects of Na+ were particularly significant at >0.1 M concentrations, and were 50% stronger at 35 °C than at 60 °C. At high salinity conditions, the surface hydration layer on plagioclase was not a diffusion barrier. Therefore, Min and Jun modified the recent model to be applicable to concentrations as high as 4 M. These findings showed that high concentrations of cations in formation brine

the average T−O bond length. The dissolution rates were found to be a linear function of the average T−O bond length, a relationship which is applicable from the pure end-member of anorthite (Al/Si = 1) to the pure end-member of albite (Al/Si = 1/3), and even extendable to quartz (Al/Si = 0). Furthermore, the dissolution rate of feldspar is related to Al/ Si ordering. Each Al or Si has four closest neighbor T sites. Due to the Al avoidance principle, all four neighbors of Al must be Si. However, the neighbors of Si can be Al or Si. With different numbers of Si neighbors, Si sites are divided into SiI, SiII, SiIII, and SiIV (Figure 2B). The fractions of these four configurations are determined by the Al/Si ordering, which in turn affects the dissolution kinetics. First, Al−O−Si linkages are weaker than Si−O−Si linkages. Therefore, SiI, SiII, SiIII, and SiIV have different dissolution rates, and the overall dissolution rate of Si is the combination of all four rates. Thus, the dissolution rate of feldspar has been modeled as a function of Al/Si ordering.17−19 Second, the hydrolysis of Si−O−Si linkages is weakly affected by protons at acidic pH. Conversely, the hydrolysis of Al−O−Si linkages can be promoted by proton adsorption. Hence, with different numbers of Al−O−Si linkages, the SiI, SiII, SiIII, and SiIV have different pH dependencies (Figure 2B).19 The relatively fast dissolution of Al compared to Si, which is quantified as the incongruency of feldspar dissolution, was 1523

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Figure 3. (A) Evolution of nanoscale precipitates on phlogopite surfaces after reaction with CO2 in 1 M NaCl at 95 °C and 102 atm CO2 (tapping mode AFM). Green areas in height profiles are precipitates. (B) Observations of biotite basal planes with contact mode AFM before and after reaction of biotite flakes in 1 M NaCl at 95 °C and 102 atm CO2. The cartoon inset in B-22 h shows the swelling caused by Na+−K+ ion exchange. The dots represent interlayer K in biotite, and the triangles represent hydrated Na+ that entered the biotite interlayer through ion-exchange. The height cross sections below the images correspond to the white dotted lines. (C) Schematic diagram of the process of biotite surface layer cracking and detachment. Reproduced from refs 30 and 31. Copyright 2010 and 2011 American Chemical Society.

the dissolution of anorthite and subsequent Al-containing mineral precipitation at 60 °C and 100 atm CO2, with 1 M NaCl and an in situ pH = 3.1.27 They tuned the pH of the solution with sulfate to be the same as the pH value without sulfate, which is 3.1, so that they could focus on the effect of sulfate without influence of pH. The sulfate anions formed mononuclear monodentate surface complexes on anorthite surfaces. The vibration frequencies of these complexes predicted by DFT calculations were consistent with the frequencies observed using ATR-FTIR. The dissolution rate of anorthite was enhanced by a factor of 1.36 by 50 mM sulfate. This information can be helpful for understanding the chemical reactions at sulfate-abundant GCS sites and for developing scientific guidelines for the amount of SO2 coinjection that is compatible with the mineralogy in selected GCS sites.

can influence mineral dissolution as strongly as pH and temperature, a significant contribution to the geochemical modeling database. Effects of Sulfate on Plagioclase Dissolution. The role of inorganic anions in reservoir rock dissolution at GCS sites has been little understood. In particular, the reported concentration of sulfate in subsurface environments can be as high as 50 mM.8 Sulfate concentrations can be further increased by coinjection of SO2 to alleviate the cost of gas separation. However, the high reactivity of SO2 could raise concerns about the formation’s integrity. Under ambient pressure and temperature, previous studies found that sulfate can significantly enhance dissolution of gibbsite and predicted similar enhancing effects on other Al-containing minerals.22−24 Nevertheless, several other studies reported inhibited dissolution of iron (hydr)oxides and predicted inhibition effects on aluminosilicates.25,26 To clarify the effects of sulfate on feldspar dissolution, Min et al. investigated the effects of sulfate on

III.b. Chemistry of Caprocks

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have stronger enhancing effects on mineral dissolution than monocarboxylic acid ligands.33,36 Zhang and Jun reported that biotite edge surfaces were more reactive than basal surfaces, and oxalate further increased the relative reactivity ratio of biotite edge surfaces to basal surfaces by forming bidentate mononuclear surface complexes on edge planes, while acetate did not impact the relative reactivity.12 This study indicated the importance of considering the structural anisotropy of phyllosilicates, including other clay minerals, when interpreting the dissolution rates, because their reactive surface area is not the total surface area. Effects of Water Content. In addition to chemical reactions between scCO2−H2O(l) and minerals, interactions between scCO2−H2O(g) and minerals are important. Shao et al. examined the effects of water content by comparing scCO2− H2O(g)−phlogopite and scCO2−H2O(l)−phlogopite interactions at 95 °C and 102 atm CO2.32 The small amount of water vapor in scCO2 is reported to be quite reactive toward mineral dissolution under simulated GCS conditions.32 While dissolution pits were the predominant surface feature on phlogopite reacted in both the systems, the pits were deeper in the scCO2−H2O(g) system. This observation suggests that scCO2−H2O(g) is more reactive in dissolving phlogopite and creating pits. This higher reactivity can be explained by an adsorbed water film on mineral surfaces, which allows CO2 to diffuse into more easily than into the bulk water in the scCO2− H2O(l).37 This finding emphasizes the pivotal role of local water content in predicting mineral dissolution.

caprocks. Mica is an important mineral for understanding the chemistry of clay minerals, a major component of caprocks at GCS sites.28 They have perfect cleavage along the {001} basal planes, and the properties of basal and edge planes differ greatly. To study the reactivity of caprocks, phlogopite (Mgcontaining mica) and biotite (Mg, Fe-containing mica) were used as model minerals.29−34 Shao et al. reported the dissolution/precipitation on phlogopite surfaces in 1 M NaCl solution.30 Dissolution of phlogopite was the predominant reaction during the entire experimental course (159 h), with dissolution pits observed on phlogopite basal surfaces by atomic force microscopy (AFM). Surprisingly, although the bulk solution was not supersaturated, nanoscale precipitates formed on the reacted surfaces, probably due to a concentration gradient between the near surface and bulk solution; a lower free energy required for heterogeneous nucleation on phlogopite surfaces; or a higher local pH near the surface to facilitate precipitation. They also observed the relocation of nanoparticles from the edges of dissolution pits to other areas (Figure 3A). On the other hand, biotite is a more reactive mica mineral than phlogopite. Under the same condition, Hu et al. reported fast precipitation of fibrous illite on biotite basal surfaces (Figure 3B), and instead of dissolution pits on the phlogopite surfaces, cracks formed due to ionexchange reactions and CO2 intercalation on the reacted biotite surfaces, detaching illite, and further cracks detached from surface (Figure 3C).29−31,34 The significant dissolution and surface morphology changes of phlogopite and biotite called for more attention on investigating their nanoscale chemical reactions under various water chemistries and water contents relevant to GCS sites. Effects of Water Chemistries. The brine at GCS sites has high salinities and high concentrations of cations and anions that affect brine−mica interactions. Shao et al. compared the reactivities of phlogopite after reaction in water and 1 M NaCl solution.32 High amounts of Na+ in 1 M NaCl solution facilitated ion-exchange reactions between Na+ and phlogopite. In view of the inhibition effect of Na+ on plagioclase dissolution discussed above,21 we suppose that competing adsorption between Na+ and protons at a higher concentration of Na+ might also inhibit further dissolution of phlogopite. Formation of secondary minerals was less favored in NaCl solution because of the low activity of aqueous species in NaCl solution due to the high ionic strength. The locations of particle formation also differed: Particles formed near the edges of dissolution pits in 1 M NaCl solution because of lower supersaturation, whereas in water, nanoparticles formed on both terraces and pit edges. Therefore, if those nanoparticles grow at the pore throats or if nanoparticles formed by chemical reactions are transported along with the fluid,35 they may plug the pore throats, decrease the permeability, and consequently hinder the transport of CO2. These findings further provide useful insights into the effects of cations in brine, suggesting their roles in affecting surface morphology of minerals and secondary precipitation under GCS relevant conditions. To study the influence of organic ligands in the distinctive reactivities of mica basal and edge surfaces, Zhang and Jun quantitatively analyzed the dissolution differences between these two surfaces with/without acetate and oxalate under conditions relevant to GCS sites.12 Acetate is the most abundant organic ligand in subsurface environments, with a reported concentration as high as 0.1 M.9 Oxalate forms bidentate ring structure surface complexes, and was observed to

III.c. Linking Chemical Reactions and Hydrogeological Responses

To link nanoscale chemical reactions of biotite with subsequent hydrogeological changes (e.g., wettability alteration), Zhang et al. systematically studied biotite dissolution in 0, 0.1, 0.5, and 1.0 M NaCl solutions under 95 °C and 102 atm of CO2 conditions for 70 h.29 The contact angles (CA, an indicator of wettability) of the reacted biotite samples were measured with a high-temperature and high-pressure apparatus in 0.1 M NaCl solution at 48 °C and 102 atm of CO2, which minimized any possible chemical reactions during CA measurements. Enhanced biotite dissolution at higher salinities resulted in increased roughness, more negatively charged surfaces, and higher densities of hydroxyl groups on the biotite surfaces. Thus, the biotite basal surfaces became more hydrophilic (Figure 4). A lower CO2 adhesion incidence and contact angle hysteresis were observed for more reacted samples (Figure 4C). These findings provide insights into the inconsistencies of previously reported contact angles of minerals at different salinities.38−42 Chemical reactions during CA measurements, which were not considered in those studies, can contribute to the uncertainties. Precipitation can also change mineral surface wettability, either by affecting surface roughness or by coating with different secondary mineral phases, such as amorphous silica, kaolinite, Illite, or iron (hydr)oxides.12,30,31,43 In real GCS sites, we also expect that reactions of mica or clay minerals at pores or close to fractures will induce chemistry and morphology changes of the exposed surfaces, and further affect their wettability, which determines the large scale fluid mobility and residual trapping of CO2. III.d. Linking Nanoscale Chemical Reactions and Mechanical Property Changes of Subsurface Materials

After achieving a better understanding of nanoscale chemical processes of well-characterized minerals, Li et al. expanded our 1525

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beside the intact cement core, a portlandite-depleted zone was formed due to dissolution of Ca2+ from portlandite (Ca(OH)2) and partially from calcium silicate hydrate. The Ca2+ formed a carbonated layer via secondary precipitation with carbonate species on the outer side of the depleted zone. The outer surface of the carbonated zone was dissolved by acidic brine to form a porous surface layer. The structure and the arrangement of the chemical-reaction-induced layers at nanoscale were critical for the overall macroscale strength of the media. Interestingly, although nanoindentation results suggested that the carbonated layer’s hardness was twice that of the intact core, the overall flexural strength of the cement samples was found to be determined by the wide and weak portlanditedepleted zone that had reduced hardness and abundant (micro)cracks. In subsurface environments, fracturing of the geomedia can be caused by external stress, such as a pressure gradient, or by internal stress caused by chemical dissolution and precipitation. In the cement systems, CaCO3 precipitation was localized in the carbonated layer and had a high precipitation rate. The expanding volumes of these precipitates stressed their surroundings and caused cracks. When cracks formed, not only was the overall material’s strength reduced, but the flow paths, fluid pressure and velocity distribution, and chemical reaction rate profiles were also altered. This complex relationship was further investigated in a following study by Li et al. which highlighted the striking significance of nanoscale interfacial chemical reactions in influencing overall mechanical alterations.45 This study showed that in the presence of SO42− ions, the dissolution of the outer layer of the carbonated layer was inhibited due to either SO42− adsorption or/and a nanoscale layer of CaSO4 coating on the CaCO3 grains. This nanoscale effect was able to mitigate CO2induced deterioration of the bulk cement from 80% to only 50% of the unaltered materials. The intertwined relationship of chemical reactions and mechanical properties and processes is an important clue for the safety of subsurface CO2 storage under high pressure. Our studies on cement highlight the fact

Figure 4. (A) AFM images of biotite basal planes after reaction for 70 h in various salinity solutions at 95 °C and 102 atm CO2. The height scale is 60 nm. Rq is the root-mean-square roughness of the corresponding sample. (B) Static CAs measured at 48 °C and 102 atm CO2 in 0.1 M NaCl brine for prereacted biotite flakes, as in the top AFM images. (C) Dynamic contact angle measurement process. Reproduced from ref 12. Copyright 2016 American Chemical Society.

investigation to address how nanoscale chemical interactions with scCO2 under GCS conditions can affect the mechanical property changes of wellbore cement, a heterogeneous geomedium at larger scales.44,45 Wellbore cement deterioration is important for wellbore integrity, and can provide insightful information about chemistry−mechanics linkages in reservoir rocks and caprocks. The CO2-reacted cement matrices were found to develop several zones, shown in Figure 5. Immediately

Figure 5. Chemical and mechanical alteration of wellbore cement. Reproduced from ref 45. Copyright 2015 American Chemical Society. 1526

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Figure 6. Factors affecting mineral nanoscale chemical processes discussed in this Account. Numbers are cited references. Thicker arrow indicates more significant effects.

changing the historical conditions of water content, temperature, and pressure on chemical reactions should be conducted. Second, the anisotropic structure of clay minerals means they may also have anisotropic wettability. The physicochemical properties of basal and edge surfaces of clay minerals differ, and thus their preferences for CO2 or water will differ. Also, because of the different reactivities, the extent of edge surface alterations will be higher than that of basal surfaces, leading to more significant wettability changes of edge surfaces. In addition, careful consideration of the mobility of clay minerals through fractures is required because they could be released into brine by dissolution of carbonates,3 mica,12,31,33 or sandstone.48 Furthermore, quantitatively predicting the extent of chemical reactions, such as layered silicate swelling, the amount of secondary precipitation, and the time frame to impact CO2 storage, remains an interesting scientific question. Third, studies are still needed on the dissolution and secondary precipitation of minerals along the fractures and faults in reservoir rocks and caprocks. Fractures and faults expose new mineral surfaces to contact with scCO2 or scCO2saturated brine. Dissolution of minerals in these structures will further increase the pathways for reactive fluid transport. On the other hand, secondary mineral precipitation might be able to self-heal the fractures, depending on the size and location of the precipitates and the fractures. Studies on the roles of dissolution and secondary mineral precipitation in fractures and faults and their propagation can be important to ensure the integrity of storage sites. Fourth, there is limited study on the significance of molecular scale and nanoscale reactions on large scale mechanical changes. For example, nanoscale features also affect internal stress distribution. The dissolution of materials enlarges pores and reduces flow rate, increasing the local pressure, and thus

that chemical and mechanical properties are closely related, and molecular/nanoscale mechanisms and macroscale observations are connected.

IV. FUTURE DIRECTIONS AND IMPLICATIONS Successful GCS should ensure CO2 storage capacity and seal integrity to prevent CO2 leakage from GCS sites, which could cause profound impacts on geological, chemical, and biological processes in groundwater and in soil/sediment environments. Our studies have elucidated the effects of environmental factors and mineralogies on the nanoscale chemical reactions of minerals and cement under GCS relevant conditions (summarized in Figure 6), and linked the chemical reactions to hydrogeological and mechanical property changes. Previously, Shiraki and Dunn reported permeability decreases of rock cores due to chemical reactions during CO2 flooding in the Tensleep Formation (Wyoming),15 supporting our experimental results that the reactions of minerals and cement can potentially change the reservoir’s properties. However, many outstanding scientific questions about the chemistry of GCS are still waiting to be explored. Some examples are given below. First, more thorough studies on the chemical reactions of minerals in water-saturated scCO2 are needed. Recent studies reported scCO2, with water dissolved in it, reacted with Ca-, Mg-, or Fe-bearing silicates.46,47 This process can be significant, because much of the contact area between caprock and scCO2 is in the absence of an aqueous phase. In addition, the supercritical phase fluid has low viscosity and can enter small pore throats, which aqueous phase fluid cannot enter. For these reasons, the reactions between water-saturated scCO2 and minerals can be as important as in the aqueous phase. Moreover, a systematic investigation about the effects of 1527

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Accounts of Chemical Research

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causes local pressure gradients that generate stress. Precipitation behaves oppositely in generating pressure gradients. Precipitation can also cause strains by volume expansion.49 If precipitation can be hindered by surface adsorption, cracking by expansive products can be reduced. While we introduced work in the framework of GCS systems, findings from GCS studies can also be applicable to other energy-related subsurface engineering systems, such as shale gas production, enhanced gas/oil recovery, geologic nuclear waste disposal, and geothermal energy programs, which all have environments similar to GCS sites.

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AUTHOR INFORMATION

Corresponding Author

*Mailing address: One Brookings Drive, Campus Box 1180, St. Louis, MO 63130. E-mail: [email protected]. Phone: 314935-4539. Fax: 314-935-7211. Web: http://encl.engineering. wustl.edu/. ORCID

Young-Shin Jun: 0000-0003-4648-2984 Notes

The authors declare no competing financial interest. Biographies Young-Shin Jun is the Professor of the Department of Energy, Environmental & Chemical Engineering (EECE) at Washington University in Saint Louis (WUStL). She holds a Ph.D. in Environmental Chemistry from Harvard University (2005). Lijie Zhang is pursuing her Ph.D. degree under the supervision of Prof. Young-Shin Jun in EECE at WUStL. Yujia Min is pursuing his Ph.D. degree under the supervision of Prof. Young-Shin Jun in EECE at WUStL. Qingyun Li received her Ph.D. in EECE from WUStL in 2016, where she worked under the supervision of Prof. Young-Shin Jun. She is currently a postdoctoral researcher at Stanford University.

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ACKNOWLEDGMENTS We are grateful for support received from the Center for Nanoscale Control of Geologic CO2, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-AC02-05CH11231 and National Science Foundation’s CAREER Award (EAR-1424927).

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REFERENCES

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