Dissolution kinetics of selected natural minerals relevant to potential CO2-injection sites – Part 2: Dissolution and alteration of carbonates and feldspars in CO2-bearing brines

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Dissolution kinetics of selected natural minerals relevant to potential CO2 -injection sites – Part 2: Dissolution and alteration of carbonates and feldspars in CO2 -bearing brines Astrid Holzheid Institut für Geowissenschaften, Universität Kiel, Ludewig-Meyn-Straße 10, 24118 Kiel, Germany

a r t i c l e

i n f o

Article history: Received 31 March 2016 Received in revised form 9 July 2016 Accepted 26 September 2016 Keywords: Dissolution Kinetics Carbonates Feldspar group minerals CO2 –sequestration

a b s t r a c t Chemical interaction processes among injected CO2 , saline fluids and potential reservoir materials are experimentally simulated to derive dissolution rates of natural materials (minerals) that can be used as input parameters for modeling of CO2 storage in deep saline formations and risk analyses. In order to study dissolution processes, mineral aliquots were exposed to CO2 -bearing brines at elevated temperature (60, 100, 150 ◦ C) and pressure (85 bar) and at various run durations. Several potential reservoir rocks include carbonates as cement. Calcite and dolomite grains were therefore mainly used as solid starting material. Experiments with the two feldspar varieties alkali feldspar and almost pure anorthite were performed in addition. Grain sizes of the mineral starting materials varied between <63 ␮m and 500 ␮m with most experiments performed at grain size fractions of 160 – 250 ␮m and 250 – 500 ␮m. All experiments run with a complex synthetic brine (total dissolved solids: ∼156 g/l) according to a natural upper cretaceous formation water. Dry ice was used as CO2 -source. All experiments were done in closed batch reactors. These reactors allow mimicking reservoir conditions far from the injection site as well as reservoir conditions after finishing the CO2 injection. The concentration changes during the experiment were monitored by ICP-OES measurements of the initial and the post-run fluids. Dissolution rates were derived based on the concentration changes of the brine. Most of the studied experimental variables and parameters (temperature, run duration, grain size, brine composition – expressed as pH-value and ionic strength) impact alteration of the reacting agents, i.e. they change the chemical composition of the brine, change the surfaces of the mineral aliquots exposed to the CO2 -bearing brine, and induce formation of secondary minerals. Hence, all influencing parameters on dissolution processes have to be considered and time-resolved changes of the dissolution behavior have to be implemented in numerical simulations of processes at CO2 injection sites and CO2 storage reservoirs. © 2016 Elsevier GmbH. All rights reserved.

1. Introduction The present experimental study was performed as part of the CO2-MoPa project (Modeling and Parameterization of CO2 storage in deep saline formations for dimension and risk analyses) within the framework of the special program GEOTECHNOLOGIEN (see also Bauer et al., 2012). The overall aim of the CO2-MoPa project is to investigate dimension and risk analyses of subterrestrial CO2 sequestration on virtual scenarios. The development of numerical, process-oriented and integral model tools provides quantitative conclusions about long-term retention of dense CO2 in deep geological formations. Prognoses on the migration of CO2 in deep as

E-mail address: [email protected]

well as in shallow subsurfaces were established in consideration of hydraulic, geometrical, geochemical and geomechanical processes. To support the modeling attempts accompanying geochemical and geomechanical laboratory experiments were accomplished within the framework of the CO2-MoPa project. The compilation of a validated, self-consistent and comprehensive database is essential (i) to provide a basis for numerical modeling and (ii) to collect and save data, which were gained within the project. Scenario simulations then depict the assessment of potential CO2 storage capacities (dimensional analyses) and possible influences of protected resources during the unlikelihood of CO2 leakage (risk analyses). This study is part of subproject E1 of CO2-MoPa. Geochemical reactions and interactions between minerals occurring at potential injection sites (with the focus on carbonates and feldspar minerals)

http://dx.doi.org/10.1016/j.chemer.2016.09.008 0009-2819/© 2016 Elsevier GmbH. All rights reserved.

Please cite this article in press as: Holzheid, A., Dissolution kinetics of selected natural minerals relevant to potential CO2 -injection sites – Part 2: Dissolution and alteration of carbonates and feldspars in CO2 -bearing brines. Chemie Erde - Geochemistry (2016), http://dx.doi.org/10.1016/j.chemer.2016.09.008

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and CO2 -bearing brines were experimentally studied at prevailing p-T conditions of reservoir rocks far from CO2 -injection sites. Based on the laboratory experiments knowledge about dissolution and/or precipitation processes during the reaction progress was gained and kinetic data like dissolution rates or activation energies were derived. The advantage of the present study is the use of a synthetic brine as fluid phase that is in composition similar to a Lower Cretaceous formation water of the northern German basin. Thus, processes far from the injection site of a hypothetical area in the northern German basin could be simulated. This publication is part 2 of a series of two. Part 1 (Holzheid, 2016 – same issue) places the subject of dissolution kinetics in the context of CO2 sequestration and describes the differences of raised scientific questions and consequential changes of required experimental conditions and parameters. It is here referred to Part 1 for a more general introduction to the subject. Part 1 also provides a comparison of the influence of single experimental variables and material properties (e.g., temperature, pressure, pH-value and ionic strength of the brine, exposed reactive surfaces of the minerals of interest) on dissolution behavior of minerals like carbonates and feldspar group minerals as well as a comparison of the interaction of various variables and material properties on dissolution processes of those minerals. Part 2 mainly focuses on experimentally determined dissolution rates and alteration of carbonates and feldspars during exposure to CO2 -bearing brines at elevated temperatures and pressures. This paper is organized as follows. At first the focus is on experimental details, including experimental and analytical techniques and selection of the starting material. Then derivations of mean chemical composition of the brines, of dissolution rates, and of activation energies are provided. This is followed by a detailed discussion of the dissolution behaviors, alteration features of the minerals’ surfaces, and concluding remarks. 2. Experimental and analytical methodology All experiments were performed using steel autoclaves with teflon inlay. Altogether 130 dissolution experiments with almost pure calcite (CaCO3 ), pure dolomite (CaMg(CO3 )2 ), almost pure anorthite (Ca0.97 Na0.03 Al1.97 Si2.03 O8 ), and an alkali feldspar with a bulk composition of 75 at.% orthoclase and 25 at.% albite (K0.75 Na0.25 AlSi3 O8 ) were conducted. As solvent synthetic model brine was used. The brine composition was similar to a formation water of a theoretical storage horizon within the northern German basin. Dry ice was used as pressurizing medium and CO2 -source. Only in a few experiments N2 gas was used as pressurizing medium to study in general the influence of CO2 dissolved in brines on dissolution kinetics. Fluid phases as well as solid materials were analyzed before and after each experiment to derive dissolution rates based on the changes of chemical compositions. 2.1. Experimental procedure The used steel autoclaves with teflon inlay (from now on called teflon reactors) are batch reactors. They are able to simulate reservoir conditions far from the injection site and also conditions after ending CO2 injection (see Holzheid, 2016; for more details on reservoir conditions and appropriate reactor techniques). Fig. 1 illustrates the closed and static setup of the teflon reactor with approximate dimensions of all relevant parts. The teflon inlay has a total volume of 55.75 ml. A bag made from fine-meshed teflon fabric (Lechleiter, Germany) is freely suspended inside. This bag contains the solid starting material and avoids fresh fracture surfaces on the mineral grains’ surfaces caused by clashing solids during the stirring process (see below for more details). Care is taken to ensure

Fig. 1. Illustration of the used teflon reactors (not to scale). The approximate dimensions of the steel autoclave are length (L) 155 mm, outer diameter (OD) 55 mm, inner diameter (ID) 35 mm. Dimensions of the teflon inlay are L 105 mm, OD 33 mm, ID 25 mm with a total reactor volume of 55.75 ml. The size of the teflon bag inside the teflon inlay is 20 × 25 mm. Figure is modified after Beier (2012).

that the bag is entirely surrounded by the fluid phase. The mesh size of the teflon fabric was small enough to keep the mineral grains inside the bag and large enough to ensure that the mesh openings did not clog and that the minerals were exposed to the fluid all the time of the experiment. The entire reactor is heated up to the desired run temperature by placing the steel autoclave with its interiors in Memmert® heating ovens. The run temperatures were 60 ◦ C, 100 ◦ C, and 150 ◦ C, respectively. The temperatures of 100 ◦ C and 150 ◦ C might be slightly higher than temperature conditions estimated within potential reservoirs but were chosen on purpose to accelerate the chemical reactions. The total pressure of ∼85 bar was generated by dry ice, which also served as CO2 -source. The pressure was calculated based on the thermal expansion of the fluid and dry ice inside the teflon inlay. Due to the lack of pressure control during ongoing experiments nine reference experiments were performed in PARR® reactors under similar conditions, i.e. identical net weights of solid starting material and dry ice as well as same fluid volume and target temperature (100 ◦ C and 150 ◦ C) and target pressure (85 bar). The PARR® reactors are equipped with pressure gauges allowing continuous pressure measurements of the interior. All reference experiments had similar p-T-loops and reached the aimed target pressure, implying that a pressure of 85 bar was reached in the teflon reactor experiments as well. The experiments are stirred three times a day for five minutes with a teflon coated magnetic stirrer using combined stirring and heating plates in order to better mimic natural flow rates in saline, deep aquifers far from the injection site. The stirring speed was 1100 rpm. The calculated Reynolds number of Re = 27465 corresponds to a turbulent fluid flow regime according to e.g., Dreybrodt and Buhmann (1991), Raines and Dewers (1997) or Pokrovsky et al. (2005). Sampling of fluid and solid phases during an ongoing experiment is not possible using the teflon reactors. To gain knowledge regarding the time-resolved progress of dissolution, experimental series were performed with identical run conditions such as net weights of solid starting material and dry ice as well as fluid volume, target temperature and pressure but with various run durations. The run durations were 1, 2, 5, 10, 20, and 30 days.

Please cite this article in press as: Holzheid, A., Dissolution kinetics of selected natural minerals relevant to potential CO2 -injection sites – Part 2: Dissolution and alteration of carbonates and feldspars in CO2 -bearing brines. Chemie Erde - Geochemistry (2016), http://dx.doi.org/10.1016/j.chemer.2016.09.008

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Experiments were terminated by removing the autoclaves from the heating oven and blowing compressed air onto the still sealed autoclaves for 15 min. The autoclaves were opened after the outer surface reached ∼40 ◦ C. Every time 20 ml of the homogenized fluids was removed from the teflon inlay and the pH-value was measured with a pH-meter (Type QpH 70, VWR International) at room temperature and atmospheric pressure. Afterwards the fluids were passed through a syringe filter (mesh size: 0.45 ␮m), acidulated by adding 14.35 M nitric acid to chemically stabilize the sampled fluids, and stored at 8 ◦ C until analysis of the fluid’s chemistry (more details are provided below in 2.3). To exclude cross contamination of the teflon reactor, blank experiments with only distilled water were also performed. The post-run teflon fabric bags with the reacted solid phases inside were rinsed thoroughly with deionized water to remove residual brine and to inhibit salt precipitation on the solids. 2.2. Starting material Clear-transparent crystals of almost pure calcite (CaCO3 , from now on called Cc) and almost pure anorthite (An97 Ab3 : Ca0.97 Na0.03 Al1.97 Si2.03 O8 , An) and milky crystals of pure dolomite (CaMg(CO3 )2 , Do) and alkali feldspar (bulk composition: Or75 Ab25 , Or) with exsolution lamellae of albite (NaAlSi3 O8 ) and orthoclase (KAlSi3 O8 ) were used as solid starting materials (all minerals from Krantz, Germany). The solid starting materials were prepared as individual batches of four different grain size fractions (<63 ␮m, 63 – 160 ␮m, 160 – 250 ␮m, 250 – 500 ␮m). All experiments had identical initial mineral-to-brine ratios of 1:200 (by weight) with 0.18 g solid material and 37 ml brine. To recall, the aim of the CO2-MoPa project is to simulate the effects of a CO2 injection into a virtual test site. As virtual, but realistic, site a hypothetical area in the northern German basin was chosen. Two potential storage formations were identified, an upper one in the Räth formation and a lower one of about 30 m thickness in the middle bunter sandstone formation, both composed of fine sandstone layers with porosities of about 0.15 and permeabilities of about 50 mD (Bauer et al., 2012). Müller and Papendieck (1975) provide a detailed description of the northern German basin in terms of its stratigraphy as well as associated formation water compositions. A synthetic model brine that is in composition similar to a Lower Cretaceous brine was therefore chosen as the used fluid phase in our experiments. Although the total dissolved solid value (TDS) of 156 g/L and the ionic strength of 2.6 M is high, the synthetic brine is far from the saturation point and unwanted precipitations can be ruled out. Fig. 2 compares the natural Lower Cretaceous formation water with the synthetic model brine composition used in the experiments. The synthetic brine is simplified in regard to its chemical composition and includes only the 4 main cations Na+ , Ca2+ , Mg2+ , and K+ as well as the main anion Cl− . Salts of A.C.S. reagent grade NaCl and MgCl2 ·6H2 O (Merck), suprapure KCl (Merck), and extra pure CaCl2 · 2H2 O (Riedel-de Ha˜en) were dissolved in deionized water to prepare the brine. A buffer for keeping the pH-value constant was not used. The brine was stored at 8 ◦ C prior to the experiments. Multiple ICP-OES analysis of the brine revealed no chemical alteration during storage time. 2.3. Analytical methods Aliquots of all solid starting materials (fractions of Cc, Do, An, Or) were analyzed by electron microprobe (Jeol Superprobe JXA 8900R, Institute of Geosciences (IfG), CAU Kiel) to verify the minerals’ compositions. Representative post-run charges of calcite and dolomite grains were also analyzed by electron microprobe.

Fig. 2. Compositions of average Lower Cretaceous formation water of the northern German basin after Müller and Papendieck (1975; left) and of the synthetic model brine used in this study (ICP-OES analysis, IfG, CAU Kiel; right). Figure is modified after Beier (2012).

The minerals’ surfaces were inspected pre-run by scanning electron microscopy (CamScan 44/EDX, IfG, CAU Kiel) and again post-run to detect dissolution and/or precipitation phenomena on the minerals’ surfaces formed while exposed to CO2 -containing brines. The attempt to reproducibly determine reactive surfaces of the used mineral fractions by gas adsorption measurements failed (BELSORP, Institute of Inorganic Chemistry, CAU Kiel). Literature values of reactive surfaces of the used mineral phases and grain size fractions were taken instead to allow derivation of dissolution rates (see below). The post-run brines were analyzed by ICP-OES using either Spectro Ciros CCD SOP (IfG, CAU Kiel) or Spectro Ciros Vision (E&P laboratory, RWE Dea AG, Wietze, Germany). The multi-element measurement techniques differ in the two ICP-OES laboratories (CAU Kiel and RWE Dea Wietze) as seawater components are typically measured in Kiel and concomitant waters formed during the oil production are inspected in Wietze. In Kiel the post-run brines were diluted in 2% nitric acid until the solutes’ concentrations were supposed to be similar to seawater concentrations. In addition, each sample was loaded with Yttrium as internal standard. The analysis of an individual fluid sample includes five separate ICP-OES measurements, which are automatically averaged. Instrumental blank measurements are routinely performed and the ICP-OES device is calibrated to in-house as well as to commercial standards (e.g., MWSTD05 or NASS4). In addition, the commercial standard IAPSO K15 was measured as an ‘unknown sample’ after every tenth individual measurement to check for accuracy of measurement of the main components. Any deviation of analyzed composition of IAPSO K15 to its standard composition led to recalculation of all brine compositions measured after the previous IAPSO K15 measurement. The ICP-OES apparatus was rinsed with 2% nitric acid upon completion of each individual brine sample measurement. At the Wietze E&P laboratory the post-run brines were diluted with ultrapure water (ratio 1:100) and 1 ml nitric acid was added. The nitric acid enhances the aerosol formation of the fluid phase and reduces variations of measured values. The analysis of an individual fluid sample includes four separate ICP-OES measurements, which are automatically averaged. The device is calibrated to different

Please cite this article in press as: Holzheid, A., Dissolution kinetics of selected natural minerals relevant to potential CO2 -injection sites – Part 2: Dissolution and alteration of carbonates and feldspars in CO2 -bearing brines. Chemie Erde - Geochemistry (2016), http://dx.doi.org/10.1016/j.chemer.2016.09.008

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Table 1 Compilation of performed experiments and experimental conditions. initial [ml] fluid

Weight [g] solid

ta [days]

pb [bar]

gas phase

Tc [◦ C]

37

0.18

1, 2, 5, 10, 20, 30

85

dry ice

60

Ccd,e

37

0.18

1, 2, 5, 10, 20, 30

85

dry ice

100

Ccd Ane,f Ord,e

37

0.18

1, 2, 5, 10, 20, 30

85

dry ice

150

37

0.18

1, 2, 5, 10

85

N2

150

grain size fraction [␮m] <63

Ccf

63–160

Ccf

160–250

Ccd Ane,f Ord

250–500

Ccd Dod Ane,f Ord Ccf

a

Experimental run duration. Pressure. c Temperature; Cc – calcite, Do – dolomite, An – anorthite, Or – orthoclase. d Runs were repeated twice at identical run conditions and grain size fractions to check for reproducibility of experimental results, yielding to two experimental sets (V–I and V–II). e Solid post-run phases were solely investigated (i.e., post-run brines were not analyzed and are missing in Tables 2A–2C and 3A–3C). f Runs were performed only once. b

diluted standards (commercially available by Bernd Kraft GmbH, Germany) and some of the standards were measured as ‘unknown samples’ to check for accuracy of measurement and allow recalculations of analyzed brine compositions in analogy to the analytical procedure at Kiel. After finishing an individual brine sample measurement the ICP-OES apparatus was rinsed with ultrapure water and instrumental blank measurements are performed. 3. Derivations of dissolved ions’ mean content in the brine, of dissolution rates of minerals, and of activation energies of dissolution reactions 3.1. Mean content of dissolved ions’ in the brine Most experiments were performed twice at identical run conditions to test for reproducibility of the experimental findings, yielding to the experimental sets V–I and V–II. A summary of experiments and their conditions is provided in Table 1. To ensure reliability of the experimental findings, five aliquots of each individual post-run brine sample were analyzed by ICPOES (see 2.3). Concentrations of these five aliquots were averaged to get a mean composition of the each post-run brine. The brine compositions of the experimental sets V–I and V–II at identical run parameters were similar in most cases or differ only by usually less than 10 relative%. Hence, the brine compositions of V–I and V–II were averaged again, providing one brine composition of each experiment. Complete chemical compositions are given in Tables 2A–2C. Averaged concentrations of the ions of interest in the post-run brine of Cc, Do, and Or experiments (Ca for experiments with calcite, Ca and Mg for experiments with dolomite, and Si for experiments with orthoclase), related parameters and calculated dissolution rates are given in Tables 3A–3C. Please note that calcite post-run charges of experiments at 60 ◦ C, all anorthite postrun charges, and orthoclase post-run charges of experiments at 100 ◦ C were only inspected for changes of mineral surfaces. Chemical compositions of the associated post-run brines are therefore lacking. 3.2. Dissolution rates of minerals Dissolution rate RX of an ion X in a mineral is calculated according to R X = cX /(RS·t)

(1)

where cX is the concentration change of ion X based on pre- and post-run fluid composition (in mol), RS is the reactive surface of the mineral (in m2 ) and t describes the experimental run duration (in seconds). Values of cX are derived from ICP-OES analyses and the experimental run duration t is known. As mentioned above, literature values of reactive surfaces of the used mineral phases and grain size fractions were taken. Data provided by Eisenlohr et al. (1999), Finneran and Morse (2009), and Gledhill and Morse (2006b) are used to derive reactive surfaces of carbonates and data of Welch and Ullman (1996) for reactive surfaces of feldspar minerals. More details are given below in the corresponding paragraphs on the calculated dissolution rates of carbonates and feldspars (see 4.1). 3.3. Activation energies of dissolution reactions The activation energy Ea of the dissolution reaction of calcite is calculated in addition to the dissolution rate RCa . The rate constant k has to be known to derive activation energy values. Lasaga and Kirkpatrick (1981) link the dissolution of calcite to a pseudo firstorder reaction. In analogy, the rate constant k can be expressed by −(dcsol /dt) = k·(csat − csol )

(2)

where csol is the actual Ca-concentration (in mg/L) in the post-run fluid at a defined time t (in s) and csat is the saturation concentration of Ca (mg/L) in the post-run fluid. Integration of Eq. (2) yields (csat − csol )/(csat − c0 ) = e−k ·t

(3)

or transposed k = −(1/t)·ln[(csat − csol )/(csat − c0 )]

(4)

with the known values of csol (actual Ca-concentration in the post-run fluid at a defined time t) and c0 (Ca-concentration of the initial (pre-run) brine). Both, pre- and post-run brines, are analyzed by ICP-OES. The saturation concentrations csat can be approximated by using the time-dependent changes of Ca-concentrations. To recall, experiments with identical run parameters but various run durations from 1 day to 30 days were performed. After deducing the value csat from the time series data, the rate constant k can be calculated using Eq. (4).

Please cite this article in press as: Holzheid, A., Dissolution kinetics of selected natural minerals relevant to potential CO2 -injection sites – Part 2: Dissolution and alteration of carbonates and feldspars in CO2 -bearing brines. Chemie Erde - Geochemistry (2016), http://dx.doi.org/10.1016/j.chemer.2016.09.008

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Table 2A Complete compositions of post-run brines (calcite experiments). t [days]

Ca [mg/l]

Cl [mg/l]

T: 100 ◦ C, p: ∼85 bar, grain size fraction: 250 – 500 ␮m 1396(5.2) 87,989(342) 1 2 1410(10) 88,126(879) 5 1370(19) 85,426(1412) 1435(32) 81,636(6025) 10 20 1428(8.1) 87,708(1426) 1422(3.2) 87,328(1190) 30 ◦ T: 150 C, p: ∼85 bar, grain size fraction: 250 – 500 ␮m 1431(7.4) 83,569(517) 1 2 1621(57) 89,011(2686) 1607(13) 86,503(344) 5 1643(21) 88,376(1261) 10 1408(119) 87,004(1106) 20 1624(7.5) 89,383(1612) 30 ◦ T: 150 C, p: ∼85 bar, grain size fraction: 160 – 250 ␮m 1290(172) 86,912(1795) 2 1245(194) 87,865(1000) 5 1497(188) 92,601(6596) 10 20 1440(25) 91,251(563) 1407(25) 89,902(1464) 30 T: 150 ◦ C, p: ∼85 bar, grain size fraction: 63 – 160 ␮m 1367(4.9) 86,391(255) 1 1435(4.2) 97,089(420) 2 1533(1.6) 100,099(236) 5 1484(5.6) 100,075(498) 10 20 1571(4.2) 85,985(166) 2126(9.4) 89,335(273) 30 T: 150 ◦ C, p: ∼85 bar, grain size fraction: <63 ␮m 1 1521(3.5) 101,443(487) 2 1425(3.4) 96,317(295) 5 1427(4.6) 98,315(507) 10 1435(4.4) 96,185(168) 20 1989((4.5) 82,593(290) 30 2059(5.7) 85,761(254) T: 150 ◦ C, p: ∼85 bar, grain size fraction: 250 – 500 ␮m, without CO2 1 1463(2.7) 88,175(278) 1201(3.2) 70,641(147) 2 5 1427(4.6) 86,317(145) 10 1312(1.9) 79,077(241)

K [mg/l]

Mg [mg/l]

Na [mg/l]

128(2.3) 129(1.6) 120(3.7) 128(1.3) 126(7.2) 125(2.0)

53,351(165) 53,315(292) 51,934(710) 50,078(311) 53,237(916) 53,087(769)

94.3(1.8) 137(11) 129(2.5) 135(4.4) 82.4(22) 132(1.7)

51,179(120) 54,211(1529) 52,827(395) 53,913(614) 53,581(470) 54,947(355)

94.1(37) 92.6(43) 108(10) 126(6.4) 105(25)

53,235(646) 53,803(197) 56,305(3516) 55,439(316) 54,820(884)

95.3(1.2) 122(1.2) 132(1.4) 132(1.9) 58.9(1.1) 139(1.8)

52,853(141) 58,811(93) 60,653(88) 60,775(177) 52,557(111) 53,601(131)

262(1.5)

262(1.5) 123(1.3) 125(1.2) 122(1.5) 234(1.8) 244(0.87)

61,267(118) 58,407(114) 59,315(168) 58,415(111) 50,205(106) 51,665(151)

255(1.0) 208(0.62) 251(0.23) 231(1.0)

136(0.48) 80.0(1.0) 132(1.4) 101(0.97)

53,463(54) 43,643(102) 52,655(120) 48,431(93)

Numbers in parentheses are 1␴ standard deviations of the mean. The entry should be read 1396 ± 5.2;
Al [mg/l]

Ba [mg/l]

Ca [mg/l]

Cl [mg/l]

K [mg/l]

Mg [mg/l]

Na [mg/l]

Si [mg/l]

Sr [mg/l]

99,086(1849) 98,627(903) 96,043(996) 98,935(1060) 100,756(1191) 101,808(6309)

288(2.4) 287(13) 278(2.9) 286(3.4) 297(2.7) 294(16)

283(3.3) 282(3.9) 272(2.1) 281(1.9) 291(2.2) 286(15)

59,104(473) 59,013(1308) 58,090(751) 59,276(673) 59,959(522) 60,894(3718)

0.702(0.04) 0.694(0.02) 0.663(0.013) 0.687(0.025) 0.683(0.010) 0.675(0.026)

99,152(1608) 101,293(2161) 100,423(3523) 101,285(2108) 104,479(2790) 102,940(3821)

284(3.9) 296(7.5) 282(4.1) 290(6.4) 301(7.9) 298(8.3)

281(2.5) 288(1.2) 279(3.6) 285(2.4) 297(8.6) 286(3.7)

58,779(569) 60,630(596) 59,742(1166) 60,041(740) 61,301(1806) 60,666(1623)

0.684(0.029) 0.706(0.008) 0.654(0.010) 0.681(0.023) 0.677(0.050) 0.682(0.037)

◦

T: 150 C, p: ∼85 bar, grain size fraction: 250 – 500 ␮m 4.63(0.57) 2.34(0.17) 1423(20) 1 4.66(0.61)

Numbers in parentheses are 1␴ standard deviations of the mean. The entry should be read 4.63 ± 0.57;
Ca [mg/l]

T: 150 ◦ C, p: ∼85 bar, grain size fraction: 250 – 500 ␮m 1410(12) 1 1382(22) 2 1421(48) 5 1366(26) 10 1230(29) 20 1427(5.1) 30

Cl [mg/l]

K [mg/l]

Mg [mg/l]

Na [mg/l]

86,585(1435) 86,665(1756) 87,459(658) 83,328(736) 81,876(2533) 91,013(321)

253(5.5) 254(7.0) 255(3.1) 245(2.7) 228(4.0) 265(1.2)

157(15) 174(9.6) 170(13) 172(4.4) 141(1.1) 178(2.0)

52,777(942) 52,711(985) 53,193(351) 50,877(474) 49,961(1577) 55,065(134)

Numbers in parentheses are 1␴ standard deviations of the mean. The entry should be read 1410 ± 12;
Please cite this article in press as: Holzheid, A., Dissolution kinetics of selected natural minerals relevant to potential CO2 -injection sites – Part 2: Dissolution and alteration of carbonates and feldspars in CO2 -bearing brines. Chemie Erde - Geochemistry (2016), http://dx.doi.org/10.1016/j.chemer.2016.09.008

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Table 3A Ca-content in post-run brines, reaction surfaces, and calculated dissolution rates (calcite experiments). ta [days]

Cab [mg/l]

cc [mol]

RSd [m2 ]

RCae [mol/(m2 s)]

−7.68E-06 −2.05E-06 −1.81E-06 8.14E-06 5.38E-06 2.71E-06

0.00183 0.00183 0.00183 0.00183 0.00183 0.00183

−4.85E-08 −6.47E-09 −2.28E-08 5.15E-09 1.70E-09 5.71E-10

6.22E-06 8.23E-05 7.67E-05 9.12E-05 −2.60E-06 8.33E-05

0.00183 0.00183 0.00183 0.00183 0.00183 0.00183

2.93E-08 2.60E-07 9.70E-08 5.77E-08 −8.23E-10 1.76E-08

−5.00E-05 −6.77E-05 3.26E-05 1.01E-05 −3.20E-06

0.00301 0.00301 0.00301 0.00301 0.00301

−9.60E-08 −5.20E-08 1.25E-08 1.94E-09 −4.10E-10

−1.79E-05 7.32E-06 4.33E-05 2.51E-05 6.50E-05 2.62E-04

0.00296 0.00296 0.00296 0.00296 0.00296 0.00296

−7.00E-08 1.43E-08 3.39E-08 9.83E-09 1.27E-08 3.42E-08

3.89E-05 3.33E-06 4.36E-06 7.32E-06 2.12E-04 2.38E-04

0.00925 0.00925 0.00925 0.00925 0.00925 0.00925

4.88E-08 2.08E-09 1.09E-09 9.16E-10 1.33E-08 9.93E-09

1.92E-05 −8.56E-05 4.78E-06 −4.10E-05

0.00183 0.00183 0.00183 0.00183

1.22E-07 −2.71E-07 6.05E-09 −2.59E-08

◦

T: 100 C, p: ∼85 bar, grain size fraction: 250 – 500 ␮m 1396 1 2 1410 5 1370 1435 10 20 1428 1422 30 ◦ T: 150 C, p: ∼85 bar, grain size fraction: 250 – 500 ␮m 1431 1 2 1621 1607 5 1643 10 1408 20 1624 30 ◦ T: 150 C, p: ∼85 bar, grain size fraction: 160 – 250 ␮m 1290 2 1245 5 1497 10 20 1440 1407 30 T: 150 ◦ C, p: ∼85 bar, grain size fraction: 63 – 160 ␮m 1367 1 1435 2 1533 5 1484 10 20 1591 2126 30 T: 150 ◦ C, p: ∼85 bar, grain size fraction: <63 ␮m 1 1521 2 1425 5 1427 10 1435 20 1989 30 2059 T: 150 ◦ C, p: ∼85 bar, grain size fraction: 250 – 500 ␮m, without CO2 1 1463 1201 2 5 1427 10 1312 a

Experimental run duration. Ca-concentration of the post-experimental fluid sample. Ca-concentration change of the initial model brine and projected to the initial fluid volume. d reactive surface of the calcite samples used in the present study to calculate Ca dissolution rate (according to the BET surface after Gledhill and Morse (2006b), Finneran and Morse (2009), Eisenlohr et al. (1999) and adjusted to the initial solid weight of the present experiments). e Ca dissolution rate after Eq. (1); negative c values cause negative dissolution rates – negative c values are either caused by chemical disequilibria at short time experiments or too minor modifications of the post-run brines’ compositions compared to the initial brine. b c

Table 3B Si-content in post-run brines, reaction surfaces, and calculated dissolution rates (orthoclase experiments). ta [days]

Sib [mg/l]

cc [mol]

RSd [m2 ]

RSie [mol/(m2 s)]

5.20E-07 9.67E-07 1.87E-06

0.0098 0.0098 0.0098

6.14E-11 5.71E-11 7.36E-11

5.57E-07 9.67E-07 1.87E-06

0.0098 0.0098 0.0098

6.58E-11 5.71E-11 7.36E-11

◦

T: 150 C, p: ∼85 bar, grain size fraction: 250 – 500 ␮m

Experimental run duration. Si-concentration of the post-experimental fluid sample. c Si-concentration change of the initial model brine and projected to the initial fluid volume. d Reactive surface of the orthoclase samples used in the present study to calculate Si dissolution rate (according to the BET surface of albite after Welch and Ullman (1996) and adjusted to the initial solid weight of the present experiments). e Si dissolution rate after Eq. (1);
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7

Table 3C Ca- and Mg-contents in post-run brines, reaction surfaces, and calculated dissolution rates (dolomite experiments). ta [days] Cab [mg/l] cCac [mol] T: 150 ◦ C, p: ∼85 bar, grain size fraction: 250 – 500 ␮m 1 1410 −2.37E-06 2 1382 −1.56E-05 5 1421 3.04E-06 1366 −2.31E-05 10 1230 −8.79E-05 20 1427 5.93E-06 30

Mgb [mg/l]

cMgc [mol]

RSd [m2 ]

RCae [mol/(m2 s)]

RMge [mol/(m2 s)]

157.49 174.11 170.31 171.71 140.63 178.19

1.41E-05 2.19E-05 2.01E-05 2.08E-05 6.07E-06 2.39E-05

0.00183 0.00183 0.00183 0.00183 0.00183 0.00183

−1.50E-08 −4.93E-08 3.84E-09 −1.46E-08 −2.78E-08 1.25E-09

8.89E-08 6.94E-08 2.55E-08 1.32E-08 1.92E-09 5.03E-09

a

Experimental run duration. Ca- and Mg-concentrations of the post-experimental fluid sample. c Ca- and Mg-concentration change of the initial model brine and projected to the initial fluid volume. d Reactive surface of the starting materials used in the present study (according to literature data of BET surfaces after Eisenlohr et al. (1999) and adjusted to the initial solid weight of the present experiments). e Ca and Mg dissolution rates calculated after Eq. (1); negative c values cause negative dissolution rates – negative c values are either caused by chemical disequilibria at short time experiments or too minor modifications of the post-run brines’ compositions compared to the initial brine. b

The activation energy Ea can be finally calculated using the Arrhenius’ equation k = A·e−Ea/(R·T)

(5)

(i.e. ions or complexes) from the mineral’s surface. They also trigger chemical alteration of the solid due to ion exchange processes between mineral and brine, resulting in dissolution of the mineral’s surface and precipitation of secondary minerals.

(6)

4.1. Chemical changes of the brines and dissolution rates of predominant cations in carbonates and feldspars

or Ea = −(R·T)·ln(k/A)

with k as rate constant, A as pre-exponential factor (or prefactor, dimensionless), Ea as activation energy (in J/mol), R as the universal gas constant (8.3145 J/mol K), and T as absolute temperature (in K). 4. Results and interpretation In total 76 dissolution experiments with carbonatic samples (calcite Cc and dolomite Do) and 54 dissolution experiments with two feldspar varieties (anorthite An (An97 Ab3 )) and alkali feldspar Or (Or75 Ab25 with exsolution lamellae of albite and orthoclase) were performed. To compare the results of different experimental sets the following experimental conditions were fixed: total pressure (∼85 bar), ratio between solids and fluids (1:200; the initial weights of solids and fluids were also kept constant), and composition of the used synthetic brine. But parameters like temperature, grain size fraction, and run duration varied to estimate the dependencies of these parameters on the mineral dissolution processes: three temperatures (60 ◦ C, 100 ◦ C, 150 ◦ C), up to four grain size fractions (<63 ␮m, 63– 160 ␮m, 160 – 250 ␮m, 250 – 500 ␮m) and run durations of 1, 2, 5, 10, 20, and 30 days were applied. To recall, the run temperatures 100 ◦ C and 150 ◦ C might be slightly higher than temperature conditions estimated within potential reservoirs but were chosen on purpose to accelerate the chemical reactions. Most of the experiments were accomplished twice to demonstrate the reproducibility of the experiments. In addition a few experiments were done with nitrogen – and not with CO2 – as pressure medium at a total pressure of 85 bar, to determine the influence of CO2 dissolved in the brine on the dissolution process of Cc. Representative post-run charges of calcite and dolomite grains revealed no significant changes of these post-run samples compared to the starting materials. Complete chemical compositions of post-run brines are given in Tables 2A–2C. Concentrations of the predominant cations Ca (Cc experiments), Ca and Mg (Do experiments), and Si (Or experiments) in post-run brines and calculated dissolution rates are listed in Tables 3A–3C. Interaction processes between minerals and brines not only result in chemical alteration of the fluid because of minerals’ dissolution processes and subsequent release of various components

The following observations of temperature dependent variations of dissolved ions (see Tables 2A, 2B, 3A, 3B) can be made: (i) neither Cl, Na, and Mg (Cc experiments) nor Cl, Na, Mg, K, and Al (Or experiments) exhibit significant changes of their post-run concentrations compared to pre-run brines’ concentrations irrespective of run duration or run temperature. (ii) fluids of experiments at 100 ◦ C show no significant increase in Ca-content (Cc) and Si concentrations in the fluid were below the detection limit (Or) in most experiments. (iii) Ca-content (Cc) of fluids of experiments at 150 ◦ C increased significantly and Si-content (Or) increased continuously with run duration. Variations of the Ca and Si concentrations in post-run brines are plotted as function of run duration and temperature in Fig. 3A and B. Fig. 3A illustrates the progress of Ca dissolution of calcite and Fig. 3B shows the progress of Si dissolution of alkali feldspar. Both, increasing Ca-content (Cc) and Si-content (Or) of the post-run brines reflect progressing dissolution of calcite and alkali feldspar during exposure to CO2 -bearing brines at elevated temperatures and pressures. This is in agreement with the literature. At first glance, regarding experiments with alkali feldspar it is striking that only Si-content (Or) increased continuously with run duration and K-, Na-, and Al-contents did not increased simultaneously as alkali feldspar contains all four elements (see Table 2B). The reason might be at least twofold: (a) the cations do not reflect congruent dissolution behavior. (b) non-uniform release of cations into the fluid reflects severe alteration of the alkali feldspar’s surface and secondary mineral precipitation during exposure to CO2 -bearing brines. In analogy to soil weathering processes, kaolinite (Al2 Si2 O5 (OH)4 ) might form as first secondary mineral during orthoclase alteration: 4KAlSi3 O8 + 4 H+ aq + 4HCO3 − aq + 2 H2 Oaq → 2Al2 Si2 O5 (OH)4 + 8SiO2aq + 4 K+ aq + 4HCO3 − aq Some of the Si-cations as well as the K-cations and Nacations (Ab-part of the alkali feldspar) might further form other secondary minerals, e.g., illite (K0.65 Al2 Al0.65 Si3.35 O10 (OH)2 ) or Namontmorillonite (Na0.3 Al2 Si4 O10 (OH)2 ·nH2 O), leaving the post-run brine enriched in aqueous SiO2 -complexes, but keeping all other cations and/or their aqueous complexes below detection limits. Dissolution rates RCa (Cc experiments) and RSi (Or experiments) are calculated after Eq. (1). Values of cCa and cSi are derived from

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Fig. 3. Cation concentrations (Ca, Si) in post-run brines and calculated dissolution rates RCa and RSi of calcite and orthoclase. A and B display Ca (A) and Si (B) contents in post-run brines. C and D compare RCa and RSi values of this study with literature values. Experiments of the present study (all data of A and B) are performed at a total pressure (p) of ∼85 bar, a grain size fraction of 250 – 500 ␮m, a stirring rate of 1100 rpm, no fluid flow (0 ml/min), an ionic strength (I) of 2.6 M, a pH value of ∼5, and temperatures (T) of 100 ◦ C and 150 ◦ C. The literature data of C and D are all at p = 1 bar (C) and <90 bar (D), pCO2 < 1 bar, a stirring rate of <427 rpm (C) and a fluid flow of <9 ml/min (D), I < 0.1 M, and pH values from 3 to 5.5. Carroll and Knauss (2005), Chen and Brantley (1997), Oelkers and Schott (1995) and Stillings and Brantley (1995).

Ca- and Si-concentrations in the pre-run and post-run brines’ compositions. Only the post-run concentrations at sufficient long run durations, i.e. after achievement of chemical equilibrium (Cc), or approximate values at steady state (Or) were used to calculate cCa and cSi . As pointed out, literature values of reactive surfaces were used. The reactive surface values of calcite are based on experimental data provided by Eisenlohr et al. (1999). Eisenlohr et al. (1999) determined reactive surfaces of 0.0116 m2 /g and 0.0082 m2 /g for limestone samples with a grain size of 250 – 350 ␮m and 350 – 500 ␮m, respectively (BET measurements). As the grain size fraction used in the present study (250 – 500 ␮m) comprises both grain size fractions of Eisenlohr et al. (1999), an average value of the reactive surface values of Eisenlohr et al. (1999) is used to calculate the dissolution rates in the present study. Reactive surface values of Welch and Ullman (1996) are used for alkali feldspar. Welch and Ullman (1996) determined a reactive surface of 0.053 m2 /g for albite with a grain size of 125 – 250 ␮m (BET measurements). Harouiya and Oelkers (2004) report reactive surface values of 0.0955 m2 /g for orthoclase and 0.0846 m2 /g for albite, both minerals had grain size fractions from 50 to 100 ␮m. The albite value of Welch and Ullman (1996) is the only available literature

value of a reactive surface of feldspar minerals with approximate similar grain sizes as of the present study and is therefore used to calculate the reactive surface of alkali feldspar. Tables 3A and 3B summarize the used reactive surface values, additional experimental details and calculated calcite and orthoclase dissolution rates. Dissolution rates RCa and RSi of calcite and feldspar dissolution reactions (this study and literature data) are plotted as function of temperature in Fig. 3C and D. All dissolution rates increase with higher temperature as expected by the temperature influenced increase of Ca and Si contents of the post-run brines. The offsets of the literature values of both sets of dissolution rates (RCa and RSi ) compared to this study are obvious and are most likely related to the differences in experimental procedures. Care was taken in selecting subsets of RCa and RSi values with the most similar experimental parameters/procedures available in the literature regarding to this study’s parameters:

(1) Cc experiments: all plotted RCa values are based on closed batch reactor experiments. As pointed out by Holzheid (2016), the stirring rates influence the flow regimes and therefore the dissolution. For example, Alkattan et al. (1998) or Plummer et al.

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(1978) describe increasing dissolution rates with increasing stirring rates. As obvious in Fig. 3C, the lower stirring rate of the literature studies (<425 rpm) seems to cause faster dissolution rates compared to the dissolution rates of the present study with a stirring rate of 1100 rpm. The literature values are based on rotating disk experiments with a continuous stirring process. But the stirring process of the present study is limited to 3 times a day. Thus, the experiments of the present study run more or less under static conditions without fluid flow and consequently result in slower dissolution rates. Higher experimental temperature seems to increase the calcite dissolution rates of the literature studies by a lesser extent than the present study does. Due to the continuous stirring process the influence of stirring might be – compared to the influence of temperature – the more dominant factor on the dissolution, resulting in a less pronounced increase of dissolution rate with increasing temperature of the studies mentioned in the literature compared to the present study. Grain size fractions also vary between the compared studies. Whereas Alkattan et al. (1998, 2002) and Pokrovsky et al. (2005) accomplished experiments with calcite grain size fractions of <125 ␮m and 100 – 200 ␮m, respectively, the calcite grains in the present study had sizes of 250 – 500 ␮m. These different grain sizes might also contribute to the observed offset of dissolutions rates (see below for more detail). Fluid – to – solid ratios also influence the dissolution rates. If the fluid – to – solid ratio is large, longer experimental run duration is required due to slow dissolution processes. None of the compiled literature data in Fig. 3C do provide information of the prevailed fluid – to – solid ratios. Therefore the influence of fluid – to – solid ratios on dissolution rates cannot be quantified. (2) Or experiments: Regarding the offset of RSi values it has to be pointed out, that most of the literature feldspar RSi values plotted in Fig. 3D are based on flow through reactor experiments due to the lack of closed batch reactor experiments at appropriate experimental conditions in the literature. Any fluid flow increases the dissolution rate, implying higher dissolution rates in flow through reactors than closed batch reactors (present study). The variations in RSi values might hence be caused by the different flow regimes of the used reactors. Holzheid (2016) provide a compilation of different reactor types, associated flow regimes, and resulting mineral dissolution behaviors. Activation energy Ea of the dissolution reaction of calcite is calculated after Eq. (6). The derived value of Ea is 10.65 kJ/mol (2.54 kcal/mol) and in good agreement with Plummer et al. (1978) and Salem et al. (1994). Both mention activation energies of ∼10 kJ/mol. Small activation energies indicate diffusion controlled reactions, implying that mineral dissolution behaviors of experiments with slow or without any fluid flow are influenced by diffusion controlled processes. Activation energies mentioned by Alkattan et al. (1998; Ea = 19 kJ/mol), Finneran and Morse (2009; Ea = 20 kJ/mol) or Gledhill and Morse (2006a; Ea = 21 kJ/mol) are significantly larger. All three studies used flow through reactors with a permanent fluid flow. Mineral dissolution processes in these experiments are not only influenced by diffusion controlled mechanisms, but also influenced by hydrodynamic processes (see Holzheid, 2016; for more details). The mass transport of reactants/reaction products is speed up because of the permanent fluid movement and consequently results in increasing dissolution rates as well as increasing activation energies. To conclude, the flow regime strongly impacts the activation energy. Chemical reactions close to a CO2 -injection site exhibit larger activation energies than chemical reactions far from the CO2 -injection site. Computational simula-

9

tions of sequestration scenarios have to take changing activation energies into account. Only dissolution data of alkali feldspar experiments at 150 ◦ C exist. Hence, derivation of the activation energy Ea of the dissolution reaction of alkali feldspar was not possible. The influence of the grain sizes of the mineral fraction on dissolution of ions in coexisting brine was studied by using up to four grain size fractions (<63 ␮m, 63 – 160 ␮m, 160 – 250 ␮m, 250 – 500 ␮m) while keeping both, total pressure (∼85 bar) and temperature (150 ◦ C) constant. The dependencies of release of Ca (Cc) and Si (Or) on grain size are illustrated in Fig. 4A (Cc) and B (Or). The dependencies can be summarized as follows (see also Tables 2A, 2B and 3A, 3B): (1) neither Cl, Na, and Mg (Cc experiments) nor Cl, Na, Mg, K, and Al (Or experiments) exhibit significant changes of their post-run concentrations compared to pre-run brines’ concentrations irrespective of run duration or grain size fractions. (2) Ca-concentration in the post-run fluid samples (Cc) significantly changes with both, run duration and grain size. (3) Si-content in the post-run brine is above detection limit after 5-day run duration and continuously increases with run duration. Various grain size fractions do not influence the Si-concentrations in the post-run brines. An incongruent dissolution behavior of alkali feldspar and severe alteration of the alkali feldspar’s surface and secondary mineral precipitation during exposure to CO2 -bearing brines could be again the reason of the decoupled release of K, Na, Al, and Si although all these cations are present in alkali feldspar. As illustrated in Fig. 4A the most pronounced Ca release (in total 700 mg/L) as function of run duration is documented in post-run brines of Cc-experiments with grain sizes < 63 ␮m. Post-run brines of experiments with grain sizes from 250 to 500 ␮m have only an absolute increase in Ca-content of 200 mg/L. This might suggest decreasing Ca releases with increasing grain sizes. But (1) as Ca-concentration changes between larger and smaller grain size fractions are not as clearly pronounced for experiments with run durations shorter than 10 days and (2) to back up the statement regarding possible grain size influence on Ca release, a larger variety of grain size fractions and their influences on Ca release needs to be further studied. However, the likely influence of grain size on Ca release with increasing Ca release at smaller grain sizes is also described in the literature and the differing Ca release is linked to the increase of reactive mineral surfaces with decreasing grain sizes (Eisenlohr et al., 1999). Dissolution rates RCa (Cc experiments) and RSi (Or experiments) are calculated after Eq. (1). Values of cCa and cSi are derived from Ca- and Si-concentrations in the pre-run and post-run brines’ compositions. Only the post-run concentrations at sufficient long run durations, i.e. after achievement of chemical equilibrium (Cc), or approximate values at steady state (Or) were used to calculate cCa and cSi . The used literature values of reactive surfaces of the various grain size fractions of Cc experiments are listed in Table 4 (see table caption for additional information). The reactive surface value for albite with grain sizes from 125 to 250 ␮m (0.053 m2 /g, Welch and Ullman (1996)) is used for both grain fractions of our alkali feldspar experiments due to lack of literature data. Fig. 4C and D illustrates the influence of run duration and grain size fraction on dissolution rates RCa (Cc experiments) and RSi (Or experiments). Although the Ca release is most significant in experiments with the smallest grain size, the fastest dissolution rate RCa is calculated for experiments with the largest grain size. The calculated negative dissolution rates of RCa are likely caused by chemical disequilibria at short time experiments. RSi values of experiments with run durations longer than 5 days remain more or less constant. They are not influenced by either longer run duration or different grain size fractions of the alkali feldspar. Comparison of our dissolution rates with literature data regarding dissolution dependences on grain sizes is

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Fig. 4. Cation concentrations (Ca, Si) in post-run brines and calculated dissolution rates RCa and RSi of calcite and orthoclase. All experiments are performed at a total pressure of ∼85 bar, a temperature of 150 ◦ C, and various grain size fractions. A and B display Ca (A) and Si (B) contents in post-run brines. C and D are corresponding dissolution rates RCa of calcite (C) and RSi of orthoclase (D). The negative dissolution rates of RCa might reflect chemical disequilibria at short time experiments.

Table 4 Literature values of reactive surfaces and used in the present study. material

grain sizes [␮m]

reactive surface [m2 /g]

reference

calcite calcite limestone limestone limestone

32 – 63 63 – 425 180 – 250 250 – 350 350 – 500

0.0500 0.0160 0.0163 0.0116 0.0082

Gledhill and Morse (2006b)a Finneran and Morse (2009)b Eisenlohr et al. (1999)c Eisenlohr et al. (1999)d Eisenlohr et al. (1999)d

All data are based on BET measurements. a Reactive surface value of Gledhill and Morse (2006b) is used as the reactive surface value of grain size fractions <63 ␮m prior adjusting to the initial solid weight of the present experiments. b Reactive surface value of Finneran and Morse (2009) is used as the reactive surface value of grain size fractions 63 – 100 ␮m prior adjusting to the initial solid weight of the present experiments. c Reactive surface value of Finneran and Morse (2009 is used as the reactive surface value of grain size fractions 100 – 250 ␮m prior adjusting to the initial solid weight of the present experiments. d The fraction with the largest grain sizes in the present study (250 – 500 ␮m) comprises both other grain size fractions of Eisenlohr et al. (1999, an average value of these reactive surface values of Eisenlohr et al. (1999) is hence used as the reactive surface value of grain size fractions 250 – 500 ␮m (this study) prior adjusting to the initial solid weight of the present experiments.

not appropriate. Even though experiments with varying grain size fractions are described in the literature, other experimental parameters or conditions that strongly influence the dissolution behavior

(e.g., reactor type, temperature, ionic strength or pH-value) are too different to make a comparison useful. As already known from literature data decreasing dissolution rates are caused by increasing grain sizes and hence, decreasing reactive surfaces. However, this assumption is not affirmed by the determined dissolution rates in the present study. The observation is inverse as the largest grain size results in the fastest dissolution rates. In turn, this statement seems not plausible to the observed minor Ca release out of solids with larger grain sizes compared to the increased Ca release out of smaller grains (Fig. 4A). This apparent disagreement is caused by the calculation itself, whereat a concentration change between two time steps is related to a defined reactive surface. In the present study the absolute Ca release out of the solid phase related to the available reactive surface is greater than for experiments performed at larger calcite grains compared to experiments performed at smaller calcite grains. This indicates that the larger grain size fraction exhibits a larger available reactive surface and could be explained as followed: due to the denser packing of smaller grain size fractions inside the teflon net used in the present study the available reactive surface might be smaller compared to larger grain size fractions as a certain amount of surfaces touch each other and consequently these surfaces are not anymore available for fluid-solid interactions as shown by quantitative image analysis of ␮-CT measurements (Kahl and Holzheid, 2010). This then results in the calcite dissolu-

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Fig. 5. Calcium concentrations in the post-run brine as function of run duration. All experiments are performed at a temperature of 150 ◦ C, a total pressure of ∼85 bar, and a grain size fraction of 250 – 500 ␮m. The influence of the mineral phase (calcite and dolomite) on the dissolved cation contents is also shown (calcite-squares; dolomite-diamonds).

tion behavior observed in the present study that is contrary to the decreasing dissolution rates with increasing grain sizes described in the literature. All dolomite experiments are done at a temperature of 150 ◦ C, a total pressure of ∼85 bar, and with a grain size fraction of 250 – 500 ␮m. The influence of the mineral phase (calcite and dolomite) and run duration on the dissolved Ca cations contents in the CO2 -containing brine is graphically shown in Fig. 5 (see also Tables 2C and 3C). The dissolution behavior of Ca in calcite and dolomite differs with increasing run duration. At first the Ca-concentrations in post-run fluids of calcite experiments significantly increase and the release levels off after 2 days. But the Ca-concentrations in post-run fluids of dolomite experiments are independent of run duration and remain similar compared to the initial Ca-content of the model brine. The minor Ca2+ release for dolomite compared to calcite might be related to different dissolution mechanisms (Yadav et al., 2008): the calcite dissolution is based on the destruction of framework bonds and the dissolution of dolomite is based on the detachment of Ca2+ and Mg2+ from the crystal surface. Furthermore the dissolution behaviors are results of the lattice structure. Dolomite exhibits a denser atomic packing of the lattice than calcite and the Ca2+ release might be therefore hampered. The influence of gases dissolved in brines on the geochemical behavior of different solid materials while exposed to these brines has to be taken into account when considering possible CO2 sequestration processes. Dissolution of gases like CO2 in brines causes changes of pH-values in these fluids (see e.g., Huq et al., 2015; and references therein). Dissolution and/or precipitation of solid materials are affected differently by changes of pH-values of the brines. For example, the calcite dissolution process is accelerated at higher pH-value, but the fastest feldspar dissolution process takes place at both, acid and alkaline pH-values. Earlier studies were performed at experimental conditions as well as fluid compositions far from natural conditions (e.g., Chou et al., 1989; Schott et al., 1989; Plummer and Busenberg, 1982; Eisenlohr et al., 1999; Zhang et al., 2007). The pH-values of the brines in former studies were also for instance adjusted by acids or bases (e.g., Yadav et al., 2008; Blake and Walter, 1999; Alkattan et al., 1998; Carroll and Walther, 1990; Chou et al., 1989; Cubillas et al., 2005). To better mimic dissolution processes at potential CO2 sequestration sites the experimentally

Fig. 6. Calcium concentrations and dissolution rates at various brine compositions. Influence of dissolved CO2 in the brine and hence pH-value of the brine on calcite dissolution process expressed as Ca content in the post-run brine (A) and dissolution rates RCa of calcite (B, RCa values of this study are compared with literature values). Experiments of the present study (all data of A, filled symbols in B) are performed at a total pressure (p) of ∼85 bar, a grain size fraction of 250 – 500 ␮m, a stirring rate of 1100 rpm, an ionic strength (I) of 2.6 M, and a temperature (T) 150 ◦ C. The literature data of B are all at p = 1 bar, pCO2 < 1 bar, a stirring rate of <2000 rpm, I ≤ 5.6 M, and T = 25 ◦ C. Data (1) is the RCa value based on CO2 -containing experiments of the present study and data (2) is the RCa value based on CO2 -free experiments of the present study. Berner and Morse (1974), Gledhill and Morse (2004) and Palandri and Kharaka (2004).

used brines should be exposed to CO2 or at least to gases including CO2 . To study the influence of dissolved CO2 in brines interacting with calcite, comparative experiments with CO2 -free brines interacting with calcite and N2 as pressure medium were performed in the present study as well. All other experimental conditions were kept constant. The Ca release of calcite seems to strongly depend on the presence or absence of CO2 in coexisting brines as shown in Fig. 6A. Only calcite exposed to CO2 -containing brine releases significant amounts of Ca to the brine. The Ca-content in CO2 -free brines remains constant in regard to the initial content or even seems to decrease at longer run duration. The differing behavior is most likely caused by the pH-values of the brines and affirms the hypothesis that the dissolution of carbonates is induced by the adsorption of hydrogen ions (H+ ) on the mineral surface (e.g., Alkattan et al., 1998; Gautelier et al., 1999; Golubev et al., 2009; Sjöberg and Rickard, 1984a, 1984b). As dissolution of CO2 in the model brine enhances the amount of available H+ in the brine and hence of the

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formation of H2 CO3 (carbonic acid), dissolution of calcite is intensified in experiments with CO2 -containing brines. The different Ca-release of calcite while exposed to CO2 -containing or CO2 -free brines is also reflected in the dissolution rates calculated after Eq. (1). The reactive surface values are identical to these used in the above-mentioned calculations. Dissolution rates of Ca of literature studies are compared to our results in Fig. 6B. The literature data represent a subset of all available data. The experimental conditions within the subset are similar (temperature: 25 ◦ C; ionic strength: ≤5.6 M; total pressure: 1 bar; CO2 partial pressure: ≤1 bar; stirring rate < 2000 rpm) and RCa values show a non-linear dependence of dissolution rates on pH-value with a slight level off of the dependence at higher values. Data (1) is the RCa value based on CO2 -containing experiments and data (2) is the RCa value based on CO2 -free experiments. Both data are from the present study and follow the trend of decreasing RCa values with increasing pH-value, although they are slightly smaller than the literature values. However, the data basis is not sufficient enough to describe gas-phase dependent alteration processes of either calcite or the brine. It is known that the separation of CO2 from combustion gases of fossil fuel burning power plants does not result in pure CO2 . A certain admixture of other combustion gases (e.g., SOx or NOx ) will always be present. Experimental studies including various other gases are necessary to investigate gas-phase dependent alteration processes in the future. 4.2. Surface alteration of minerals exposed to CO2 -bearing brines In addition to the derivation of dissolution rates of minerals relevant for potential CO2 -injection sites, surface textures of these minerals and formation of secondary minerals were also studied. It is referred to part 1 of the series (Holzheid, 2016) in regard to surface alterations of the carbonate mineral dolomite and the feldspar group minerals anorthite and orthoclase as well as the newly formed secondary minerals on the surface of the feldspar group minerals. The focus here is on the surface alteration of calcite as experiments with calcite of the grain size fraction 250 to 500 ␮m were performed at all three temperatures (60 ◦ C, 100 ◦ C, 150 ◦ C) and run durations of 1, 2, 5, 10, 20, and 30 days. The experiments allow comparison of calcite dissolution as function of time and temperature. Hence, a systematical study of the evolution of surface alteration of calcite minerals is possible. In Fig. 7 representative scanning electron microscopy (SEM) images are compiled to illustrate the time and temperature dependent surface alteration of calcite. In general, calcite crystals have many possible cleavage planes. The lowest energy surface is the ¯ surface (de Leeuw and Parker, 1998, 1997; Hwang et al., {1014} ¯ 2001; Kerisit et al., 2003). Cleavage along the {1014} plane gives rise to the characteristic rhombohedral shape of calcite crystals. ¯ plane contains Ca2+ and (CO3 )2− ions, making it charge The {1014} ¯ neutral. The stable surface and low surface energy of the {1014} ¯ plane is a consequence of the higher density of ions of the {1014} plane compared to other possible neutral planes (Lardge, 2009). As obvious from Fig. 7A the initial rhombohedral shape of the calcite grains with stepped surfaces is conserved during the exposure to CO2 -bearing brines. At 60 ◦ C relative smooth surfaces with widely spaced terraces still remain even after 10 days, but small etch pits formed. Due to faster kinetics at higher temperatures, at 100 ◦ C and only after 5 days the pits already became broad (Fig. 7B). At 150 ◦ C and 5 days even needles with wide basis at the bottom formed as remnants of old etch pit walls which have overlapped with other etch pits (Fig. 7C and D as close-up image). Furthermore, the surface became dominated by kink sites as the terrace area minimizes and

the distance between adjacent steps decreases as nicely pictured in Fig. 7D. This is in agreement to observations by e.g., Arvidson et al. (2003) or Lüttge et al. (2013). The ‘etch pit-dominated’ mineral surface morphology gradually converts into a ‘needle-dominated’ mineral surface with significantly increased reactive surface areas. With ongoing dissolution the former pointy needles firstly change to fork-like needles and subsequently to stub point needles (Fig. 7E). Different grains of a single post-run charge, i.e. of one individual experiment, sometimes show a variety of time-resolved stages of surface alteration. This is a direct result of the diversity and distribution of surface sites. Fig. 7F is an example of that: only one solitary needle on an otherwise quite evolved surface is left. The solitary needle is located at the upper right corner of Fig. 7F. The entire variety of needle shapes, i.e. their design and time-resolved development, is nicely documented in Fig. 7G. At further progressive surface alteration the needles finally vanish and a layer parallel to ¯ the {1014} plane completely disappeared. The dissolution continues on the newly formed surface and etch pits form again (Fig. 7H). The nicely pictured shapes of the pits are rhombohedral as their morphology is controlled by the crystallography of the calcite structure. Eventually the dissolution circle starts over and needles would form as the next step of the proceeding surface alteration. 5. Concluding remarks Chemical interaction processes between injected CO2 , saline fluids, and potential reservoir materials immediately start at CO2 injection and continue while CO2 -bearing brines will dilute and disperse around the initial CO2 sequestration site. When minerals of either reservoir or cap rocks come in contact with the CO2 -bearing brines with which they are out of equilibrium, reactions begin as the system seeks to establish a new equilibrium, i.e. a lower energy state. If the brine is undersaturated with respect to the ions of the mineral that is exposed to the brine, the mineral will dissolve until the fluid becomes saturated with the ions of the mineral phase. If the fluid becomes supersaturated with ions of any new mineral phases, these mineral phases may precipitate by nucleation and subsequently grow. The experiments of this study aimed to simulate processes of reservoir rocks that are far from the injection site or after finishing CO2 injection. The main findings are: Carbonates: • Ca-content of the post-run brines increases as function of temperature and run duration/exposure time. This reflects progressing dissolution of calcite. • The most pronounced Ca release as function of run duration is recorded for post-run brines with the smallest grain sizes of calcite. • The dissolved CO2 in brines strongly influences dissolution of calcite, and hence, the Ca release as the pH-values of the brines change due to dissolution of CO2 in brines and subsequent increase of H+ ions in the brine. • Changes of reactive surfaces and habitus of carbonates during interaction with CO2 -bearing brines at elevated temperature and pressure are limited to dissolution features like etch pits and needles. No precipitation of new minerals was observed. • Dissolution rates RCa increase with higher temperature as expected by the temperature influenced increase of Ca contents of the post-run brines. The differences of RCa values of this study and of literature values are based on differences in experimental procedures. Although only closed batch reactor experiments are compared, the stirring rates as well as solid to liquid ratios are different and significantly influence

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Fig. 7. SEM images of post-run calcite grains at different alteration stages. The initial rhombohedral shape of the calcite starting grain is preserved and dissolution features developed with formation of etch pits (A). At progressing alteration pointy needles form on the mineral surface (B) and kink sites dominate the surfaces (C, close-up image D). With on-going dissolution the pointy needles change to fork-like and then to stub point needles (E). Different alteration stages can be observed at individual post-run grains (F, G). The stub point needles finally diminish and the dissolution circle starts over with formation of etch pits (H). SEM images: SEM laboratory, Institute of Geosciences, CAU Kiel.

the dissolution progress and process. The different grain sizes of the compared studies also add to the observed offset of RCa values. The apparent disagreement of faster dissolution rates (greater RCa values) at larger grain sizes can be explained by smaller available reactive surfaces at smaller grain size as larger amounts of surfaces touch each other and consequently these surfaces are not anymore available for fluid-solid interactions.

The different Ca-release of calcite while exposed to CO2 containing brines compared to exposure to CO2 -free brines is reflected in the RCa values that follow the trend of decreasing RCa values with increasing pH-value, although this study’s RCa values are slightly smaller than the literature values. But the data basis is not sufficient enough to describe gas-phase dependent alteration processes of either calcite or the brine in more detail.

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Activation energy Ea of the dissolution reaction of calcite is in good agreement with Ea values of literature studies that used comparable experimental setups and parameters. The dissolution behavior of Ca in calcite and dolomite differs with increasing run duration: the Ca-concentrations in post-run fluids of calcite experiments significantly increase and the release from calcite levels off after 2 days, while Ca-concentrations in postrun fluids of dolomite experiments are independent of run duration and remain similar compared to the initial Ca-content of the model brine. This minor Ca2+ release of dolomite can be explained by the different dissolution mechanisms of calcite and dolomite as the dissolution of dolomite is based on the detachment of Ca2+ and Mg2+ from the crystal surface. In addition, as dolomite exhibits a denser atomic packing of the lattice than calcite, the Ca2+ release of dolomite is hampered. Feldspars: • Si-content of the post-run brines increases as function of temperature and run duration during exposure of alkali feldspar minerals to CO2 -bearing brines, implying progressing dissolution of feldspars. • Contrary to carbonates, alkali feldspar dissolution is not influenced by the mineral’s grain sizes. The continuous increase of Si-content – but not of K-, Na-, and Al-contents – in the post-run brines hints toward incongruent dissolution behavior of alkali feldspar, but could be additionally influenced by the observed severe alteration of the alkali feldspar’s surface and secondary mineral precipitation during exposure to CO2 -bearing brines. Minor alteration of anorthite’s surfaces and secondary mineral precipitation also occur during exposure to CO2 bearing brines. In analogy to RCa values, dissolution rates RSi of Or increase with higher temperature as Si contents of the post-run brines increase with temperature. The offset of this study’s RSi values to the literature values is again related to the differences in experimental procedures: the present study was performed in closed batch reactors, but most of the literatures’ RSi values are based on flow through reactor experiments due to the lack of closed batch reactor experiments at appropriate experimental conditions in the literature. RSi values seem to be neither significantly influenced by run duration (after Si-contents in post-run brines are above detection limit) nor by grain size of the alkali feldspar. To summarize, all studied experimental variables and parameters trigger structural and geochemical alterations of the reacting agents and, consequently, change the dissolution behavior as well as the kinetic data of the starting materials. In consequence, all likely influences on dissolution processes – generally speaking fluid-solid interactions – have to be considered by modeling attempts and time-resolved changes of the dissolution behavior have to be implemented in numerical simulations of processes at CO2 injection sites and CO2 storage. The composition of the injected gas-phase and the associated pH-value of the fluid (brine) are the most important influencing factors in regard to chemical processes at potential CO2 storage reservoirs. The separation of CO2 from combustion gases of fossil fuel burning power plants does not result in pure CO2 . A certain admixture of other combustion gases like SOx or NOx will always be present in trace amounts and severely influence the brine composition. Future studies have therefore to include other combustion gases to better describe gas-phase dependent processes. During CO2 injection as well as during CO2 storage various mechanical and/or chemical reaction processes result in alteration

of the mineral surfaces. These processes crucially influence the reactive surfaces of minerals exposed to the CO2 -bearing brines and hence the dissolution behavior of the considered minerals. Furthermore, the mineral grains at potential CO2 sequestration sites are consolidated and cemented. Mineral surfaces contacting each other are not available for fluid-solid interactions and pore spaces can be separated from open pore space by cementation of the connections. Hence, not all pore spaces are available for fluid-solid interactions and profound knowledge of ‘real’ reactive surfaces is crucial for numerical simulations. In addition, different potential reservoir rocks will have diverse chemical reaction behaviors caused by the variation in grain sizes of mineral paragenesis of the reservoir rocks. Consequently, changes in these chemical reaction behaviors have to be taken into account when deriving dissolution rates. Dissolution rates that are derived by also considering timerelated influences are most suitable for input-parameters for modeling of CO2 injection and CO2 storage in rock formations and risk analyses.

Acknowledgments The dissolution experiments at 100 and 150 ◦ C were performed as part of the PhD thesis of Katja Beier. As Katja Beier moved on, she unfortunately refrained from being co-author. Some sentences of the present publication are literal citations from Beier (2012) but not all are marked individually. The calcite experiments at 60 ◦ C were part of the B.Sc. project of Lisa Diedenhoven at the Institute of Geosciences, University of Kiel. The entire study is part of the CO2 -MoPa project (Modeling and Parameterization of CO2 storage in deep saline formations for dimension and risk analyses). The overall aim of CO2 -MoPa is to investigate dimension and risk analyses for subterrestrial CO2 sequestration on virtual scenarios. CO2 -MoPa is funded by the German Federal Ministry of Education and Research (BMBF), EnBW Energie Baden-Württemberg AG, E.ON Energie AG, E.ON Ruhrgas AG, RWE Dea AG, Vattenfall Europe Technology Research GmbH, Wintershall Holding AG and Stadtwerke Kiel AG within the framework of the special program GEOTECHNOLOGIEN. The author is grateful to Karin Kissling, Karen Bremer, Petra Kluge, Bredan Ledwig, Ute Schuldt, and Birgit Mohr (all Institute of Geosciences, Kiel) for help and assistance in performing the experiments, analyses, and SEM images. The present study would have not been possible without Wolf-Achim Kahl’s (formerly Institute of Geosciences, Kiel, now at University Bremen, Germany) continuous interest, numberless contribution and groundbreaking support. The author also thanks all actively participating scientists of the CO2-MoPa project partners in Kiel and is much obliged to the staff of the E&P laboratory, RWE Dea AG (now DEA Deutsche Erdoel AG), Wietze, Germany, for their help, assistance and efficient cooperation.

References Alkattan, M., Oelkers, E.H., Dandurand, J.-L., Schott, J., 1998. An experimental study of calcite and limestone dissolution rates as function of pH from −1 to 3 and temperature from 25 to 80◦ C. Chem. Geol. 151, 199–214. Alkattan, M., Oelkers, E.H., Dandurand, J.-L., Schott, J., 2002. An experimental study of calcite dissolution rates at acidic conditions and 25◦ C in the presence of NaPO3 and MgCl2 . Chem. Geol. 190, 291–302. Arvidson, R.S., Ertan, I.E., Amonette, J.E., Lüttge, A., 2003. Variation in calcite dissolution rates: a fundamental problem? Geochim. Cosmochim. Acta 67 (9), 1623–1634. Bauer, S., Class, H., Ebert, M., Feeser, V., Götze, H., Holzheid, A., Kolditz, O., Rosenbaum, S., Rabbel, W., Schäfer, D., Dahmke, A., 2012. Modeling, parameterization and evaluation of monitoring methods for CO2 storage in deep saline formations: the CO2 -MoPa project. Environ. Earth Sci. 67, 351–367.

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Beier, K., 2012. CO2-sequestration on Laboratory Scale: Geochemical Interactions Between Injected CO2, Saline Fluid Phases, and Potential Reservoir Materials, PhD Thesis. University Kiel, Germany, 180 p. Berner, R.A., Morse, J.W., 1974. Dissolution kinetics of calcium carbonate in sea water IV. Theory of calcite dissolution. Am. J. Sci. 274, 108–134. Blake, R.E., Walter, L.M., 1999. Kinetics of feldspar and quartz dissolution at 70–80◦ C and near-neutral pH: effects of organic acids and NaCl. Geochim. Cosmochim. Acta 63 (13/14), 2043–2059. Carroll, S.A., Knauss, K.G., 2005. Dependence of labradorite dissolution kinetics on CO2 (aq) Al(aq), and temperature. Chem. Geol. 217, 213–225. Carroll, S.A., Walther, J.V., 1990. Kaolinite dissolution at 25◦ C, 60◦ C, and 80◦ C. Am. J. Sci. 290, 797–810. Chen, Y., Brantley, S.L., 1997. Temperature- and pH-dependence of albite dissolution rate at acid pH. Chem. Geol. 135, 275–290. Chou, L., Garrels, R.M., Wollast, R., 1989. Comparative study of the kinetics and mechanisms of dissolution of carbonate minerals. Chem. Geol. 78, 269–282. Cubillas, P., Köhler, S., Prieto, M., Chaïrat, C., Oelkers, E.H., 2005. Experimental determination of dissolution rates of calcite aragonite, and bivalves. Chem. Geol. 216, 59–77. de Leeuw, N.H., Parker, S.C., 1997. Atomistic simulation of the effect of molecular adsorption of water on the surface structure and energies of calcite surfaces. J. Chem. Soc. Faraday Trans. 93 (3), 467–475. de Leeuw, N.H., Parker, S.C., 1998. Surface structure and morphology of calcium carbonate polymorphs calcite aragonite, and vaterite: an atomistic approach. J. Phys. Chem. B 102, 2914–2922. Dreybrodt, W., Buhmann, D., 1991. A mass transfer model for dissolution and precipitation of calcite from solutions in turbulent motion. Chem. Geol. 90 (1–2), 107–122. Eisenlohr, L., Meteva, K., Gabrovˇsek, F., Dreybrodt, W., 1999. The inhibition action of intrinsic impurities in natural calcium carbonate minerals to their dissolution kinetics in aqueous H2 O-CO2 solutions. Geochim. Cosmochim. Acta 63 (7/8), 989–1002. Finneran, D.W., Morse, J.W., 2009. Calcite dissolution kinetics in saline waters. Chem. Geol. 268, 137–146. Gautelier, M., Oelkers, E.H., Schott, J., 1999. An experimental study of dolomite dissolution rates as a function of pH from −0.5 to 5 and temper ature from 25 to 80◦ C. Chem. Geol. 157, 13–26. Gledhill, D.K., Morse, J.W., 2004. Dissolution kinetics of calcite in NaCl-CaCl2 -MgCl2 brines at 25◦ C and 1 bar pCO2 . Aquat. Geochem. 10, 171–190. Gledhill, D.K., Morse, J.W., 2006a. Calcite dissolution kinetics in Na-Ca-Mg-Cl brines. Geochim. Cosmochim. Acta 70, 5802–5813. Gledhill, D.K., Morse, J.W., 2006b. Calcite dissolution kinetics in Na-Ca-Mg-Cl brines. Chem. Geol. 233, 249–256. Golubev, S.V., Bénézeth, P., Schott, J., Dandurand, J.L., Castillo, A., 2009. Siderite dissolution kinetics in acidic aqueous solutions from 25◦ C to 100◦ C and 0 to 50 atm pCO2 . Chem. Geol. 265, 13–19. Harouiya, N., Oelkers, E.H., 2004. An experimental study of the effect of aqueous fluoride on quartz and alkali-feldspar dissolution rates. Chem. Geol. 205, 155–167. Holzheid, A., 2016. Dissolution kinetics of selected natural minerals relevant to potential CO2 -injection sites – Part 1: a review, submitted to Chemie der Erde – Geochemistry on 31/03/2016, http://dx.doi.org/10.1016/j.chemer.2016.09.007. Huq, F., Haderlein, S.B., Cirpka, O.A., Nowak, M., Blum, P., Grathwohl, P., 2015. Flow-through experiments on water-rock interactions in a sandstone caused by CO2-injection at pressures and temperatures mimicking reservoir conditions. Appl. Geochem. 58, 136–146. Hwang, S., Blanco, M., Doddard, W.A., 2001. Atomistic simulations of corrosion inhibitors adsorbed on calcite surface I. Force field parameters for calcite. J. Phys. Chem. B 105, 10746–10752.

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Kahl, W.-A., Holzheid, A., 2010. Estimated and true geometric surfaces and their possible impact on experimentally and thermodynamically derived mineral dissolution and precipitation rates in CO2 -brine-mineral reactions. In: Annual Meeting German Mineralogical Society, Münster, Germany, 20.−22.09.2010, Abstract #S04-T02. Kerisit, S., Parker, S.C., Harding, J.H., 2003. Atomistic simulation of the dissociative adsorption of water on calcite surfaces. J. Phys. Chem. B 107, 7676–7682. Lüttge, A., Arvidson, R.S., Fischer, C., 2013. A stochastic treatment of crystal dissolution kinetics. Elements 9, 183–188. Lardge, J.S., 2009. Investigation of the Interaction of Water with the Calcite {104} Surface Using Ab-initio Simulation, PhD Thesis. University College, London, United Kingdom, 152 p. Lasaga, A.C., Kirkpatrick, R.J., 1981. Reviews in Mineralogy (Volume 8): Kinetics of Geochemical Processes. Mineralogical Society of America, 398 p. Müller, E.P., Papendieck, G., 1975. Zur Verteilung, Genese und Dynamik von Tiefenwässern unter besonderer Berücksichtigung des Zechsteins. Zeitschrift für geologische Wissenschaften 3 (2), 167–196. Oelkers, E.H., Schott, J., 1995. Experimental study of anorthite dissolution and the relative mechanism of feldspar hydrolysis. Geochim. Cosmochim. Acta 59 (24), 5039–5053. Palandri, J.L., Kharaka, Y.K., 2004. A compilation of rate parameters of water-mineral interaction kinetics for application to geochemical modeling. Technical report, U.S. Geological survey, Open File Report 2004–1068. Plummer, L.N., Busenberg, E., 1982. The solubilities of calcite, aragonite and vaterite in CO2 -H2 O solutions between 0 and 90◦ C, and an evaluation of the aqueous model for the system CaCO3 -CO2 -H2 O. Geochim. Cosmochim. Acta 46, 1011–1040. Plummer, L.N., Wigley, T.M.L., Parkhurst, D.L., 1978. The kinetics of calcite dissolution in CO2 -water systems at 5◦ to 60◦ C and 0.0 to 1.0 atm CO2 . Am. J. Sci. 278, 179–216. Pokrovsky, O.S., Golubev, S.V., Schott, J., 2005. Dissolution kinetics of calcite, dolomite and magnesite at 25◦ C and 0 to 50 atm pCO2 . Chem. Geol. 217, 239–255. Raines, M., Dewers, T., 1997. Mixed kinetics control of fluid rock interaction in reservoir production scenarios. J. Petrol. Sci. Eng. 17 (1–2), 139–155. Salem, M.R., Mangood, A.H., Hamdona, S.K., 1994. Dissolution of calcite crystals in the presence of some metal ions. J. Mater. Sci. 29 (24), 6463–6467. Schott, J., Brantley, S.L., Crerar, D., Guy, C., Borcsik, M., Willaime, C., 1989. Dissolution kinetics of strained calcite. Geochim. Cosmochim. Acta 53, 373–382. Sjöberg, E.L., Rickard, D.T., 1984a. Calcite dissolution kinetics: surface speciation and the origin of the variable pH dependence. Chem. Geol. 42, 119–136. Sjöberg, E.L., Rickard, D.T., 1984b. Temperature dependence of calcite dissolution kinetics between 1 and 62◦ C at pH 2.7 to 8.4 in aqueous solutions. Geochim. Cosmochim. Acta 48, 485–493. Stillings, L.L., Brantley, S.L., 1995. Feldspar dissolution at 25◦ C and pH 3: reaction stoichiometry and the effect of cations. Geochim. Cosmochim. Acta 59 (8), 1483–1496. Welch, S.A., Ullman, W.J., 1996. Feldspar dissolution in acidic and organic solution: compositional and pH dependence of dissolution rate. Geochim. Cosmochim. Acta 60 (16), 2939–2948. Yadav, S.K., Chakrapani, G.J., Gupta, M.K., 2008. An experimental study of dissolution kinetics of calcite dolomite, leucogranite and gneiss in buffered solutions at temperature 25 and 5◦ C. Environ. Geol. 53, 1683–1694. Zhang, R., Hu, S., Zhang, X., Yu, W., 2007. Dissolution kinetics of dolomite in water at elevated temperatures. Aquat. Geochem. 13, 309–338.

Please cite this article in press as: Holzheid, A., Dissolution kinetics of selected natural minerals relevant to potential CO2 -injection sites – Part 2: Dissolution and alteration of carbonates and feldspars in CO2 -bearing brines. Chemie Erde - Geochemistry (2016), http://dx.doi.org/10.1016/j.chemer.2016.09.008