energies Article
Thermodynamic Modeling of CO2-N2-O2-Brine-Carbonates in Conditions from Surface to High Temperature and Pressure Jun Li 1, *, Raheel Ahmed 2 1 2
*
and Xiaochun Li 1
State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China;
[email protected] Dimue Technology Ltd. Co., Wuhan 430000, China;
[email protected] Correspondence:
[email protected]; Tel.: +86-181-6256-8012
Received: 22 August 2018; Accepted: 27 September 2018; Published: 1 October 2018
Abstract: Nitrogen (N2 ) and oxygen (O2 ) are important impurities obtained from carbon dioxide (CO2 ) capture procedures. Thermodynamic modeling of CO2 -N2 -O2 -brine-minerals is important work for understanding the geochemical change of CO2 geologic storage with impurities. In this work, a thermodynamic model of the CO2 -N2 -O2 -brine-carbonate system is established using the “fugacity-activity” method, i.e., gas fugacity coefficients are calculated using a cubic model and activity coefficients are calculated using the Pitzer model. The model can calculate the properties at an equilibrium state of the CO2 -N2 -O2 -brine-carbonate system in terms of gas solubilities, mineral solubilities, H2 O solubility in gas-rich phase, species concentrations in each phase, pH and alkalinity. The experimental data of this system can be well reproduced by the presented model, as validated by careful comparisons in conditions from surface to high temperature and pressure. The model established in this work is suitable for CO2 geologic storage simulation with N2 or O2 present as impurities. Keywords: thermodynamic modeling; CO2 geologic storage; CO2 -N2 -O2 -brine-carbonates; fugacity; activity
1. Introduction Carbon emission reduction is becoming a more and more important topic for the sustainable development of human beings. Carbon capture and storage (CCS) has been validated as a useful method for reducing carbon emission and the clean use of fossil energy. High cost is one of the main barriers for commercial use of CCS. In the whole chain of CCS, more than half of the cost is consumed by carbon dioxide (CO2 ) capture, because a high purity of CO2 is usually used for real CO2 storage projects [1]. Lowering the CO2 purity could be a way to reduce the CO2 capture cost. According to an International Energy Agency Greenhouse Gas R&D Programme (IEAGHG) report [2], even though there are various impurities obtained from CO2 capture techniques, the main impurities are nitrogen (N2 ) and oxygen (O2 ). When CO2 is injected with impurities, fluid properties such as gas density, viscosity and gas-water-mineral interactions could be different [3,4] compared to the injection of pure CO2 . The presence of impurities decreases CO2 solubility in brine and also affects H2 O contents in non-aqueous phase. A change in fluid property also affects fluid migration and CO2 storage capacity in subsurface porous media. When O2 is injected with CO2 into the subsurface reservoir, the oxidation-reduction environment is changed, which triggers related water-mineral interactions. In this case, water properties (such as composition, pH and alkalinity), porosity and permeability vary consequently [5,6].
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Thermodynamic modeling of CO2 -impurities-brine systems is essential work for understanding subsurface fluid transport. It mainly consists of the modeling of phase partitioning, density and viscosity. Soreide and Whitson [7] constructed mutual solubility of multi-gases (including CO2 , CH4 , H2 S and N2 ) and brine systems. The model shows good accuracy at low temperature and pressure with a salinity less than 2 (molal). Li et al. [3] made modifications and achieved good accuracy at wider ranges of temperature, pressure and salinity for CO2 -CH4 -H2 S-brine systems. Gas solubility modeling of CO2 , N2 and O2 in water and brine has been performed by Duan and Sun [8], Mao and Duan [9] and Geng and Duan [10] based on comprehensive reviews of experimental data. These models can reproduce most of the experimental data over wide ranges of temperature, pressure and salinity. However, they can only consider single gas component cases, and cannot calculate multi-gas and brine system equilibria. Tan et al. [11] developed a model for CO2 , SO2 and brine system equilibria using a “cubic plus association” (CPA) method. Sun and co-workers developed an improved “Statistical Associating Fluid Theory” (SAFT) model for CO2 -brine and water-hydrocarbon systems by introducing a Lennard-Jones (LJ) term (SAFT-LJ models, [12–14]). CPA and SAFT type models usually have high computation accuracy, but are time-consuming and are not practical for reservoir simulation implementations. Carbonates (including calcite, magnesite and dolomite) are important minerals for CO2 geologic storage. When CO2 is injected into porous media with carbonate minerals, dissolution of carbonates into aqueous phase is enhanced. Water property (such as pH, ion concentration, density and viscosity), porosity and permeability are affected consequently. Thermodynamic modeling work for CO2 -brine-mineral systems began in the 1980s. Harvie and Weare [15] and Harvie et al. [16] used the Pitzer model to predict mineral solubility in the system Na-K-Mg-Ca-H-Cl-SO4 -OH-HCO3 -CO3 -CO2 -H2 O. Moller [17], Greenberg and Moller [18], and Christov and Moller [19] further consider the temperature effects for model parameters of the system. The pressure effects are not considered in these models. Duan and Li [20] proposed a thermodynamic model for the quaternary system CO2 -H2 O-NaCl-CaCO3 which predicts the solubility of calcite, CO2 and other properties over wide ranges of temperature and pressure. Li and Duan [21] extended the model to a quinary system with CaSO4 included. Gypsum and anhydrite dissolution, precipitation and interactions with CO2 and H2 O can be considered in the model. This work discusses the phase partitioning modeling of the CO2 -N2 -O2 -brine system based on careful review and validation with experimental data at high temperature, pressure and salinity. The model is then extended to CO2 -N2 -O2 -brine-carbonates phase equilibria. Calcite, magnesite and dolomite are considered to be carbonate minerals of this system. The calculation results given by the model are compared with existing experimental data such as mineral solubilities with CO2 contained, CO2 solubility in brine saturated with minerals, water pH, and ion concentrations at various pressures and temperatures. 2. Thermodynamic Modeling of CO2 -N2 -O2 -Brine Equilibria When gas components (CO2 , N2 and O2 ) are dissolved in brine, chemical potentials approach equality, and the whole system approaches an equilibrium state. The Henry constant [21] is defined as: KH =
fi ai
(1)
where KH is the Henry constant; fi is the gas fugacity of component i; ai is activity of component i in water phase. This equation is valid only when the system is at equilibrium state. Furthermore, f i = Pyi φi , where yi is the mole fraction of component i in gas phase and φi is fugacity coefficient of component i in gas phase. For activity ai = mi γi , where mi is the molality of component i in aqueous phase, and γi is the activity coefficient of component i in aqueous phase. From Duan and Li [20], KH is the function of temperature and pressure: K H ( T, P) = K H ( T, Pre f ) × exp( PF )
(2)
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where Pre f is pressure at reference state and is usually set as 1 atm. PF is Poynting factor, which is defined as, wP V Vm m PF = dP = ( P − Pre f ) (3) Pre f RT RT R is gas constant and Vm is partial molar volume. Vm is average partial molar volume. From Equations (1) to (3), solubility of component i can be calculated as, mi =
Pyi φi V ( P− P ) γi K H ( T, Pre f ) exp m,i RT re f
(4)
In this work, fugacity coefficients are calculated based on the Peng-Robinson (PR) [22] equation. The related parameters for CO2 , N2 , O2 , and H2 O can be found in Tables 1 and 2. Activity coefficients are calculated by the Pitzer [23] model. The model parameters, as well as KH and Vm , are discussed in Section 2. The related experimental work of the concerned fluid system is used for the parameterization of the model. 2.1. Fugacity Coefficients The two-parameter cubic form equation of state has been widely used in the oil and gas industry for phase properties and flash calculations [22,24,25], and has shown good accuracy for non-polar molecules. In this work, fugacity coefficients of gaseous species are calculated using the Peng-Robinson [22] model of the form as follows: P=
RT a( T ) − Vm − b Vm (Vm + b) + b(Vm − b)
(5)
where a( T ) = a( Tc )α( Tr , ω ); a(Tc ) is the Van der Waals’ attraction factor at a critical temperature R2 T 2
c defined as a( Tc ) = 0.45724 Pc c , where Pc is critical pressure; b = 0.07780 RT Pc ; Tr is reduced temperature T Tr = Tc ; Tc is critical temperature; ω is acentric factor; α( Tr , ω ) is a dimensionless function of relative temperature and acentric factor. When gaseous phase has more than one component, the mixing rule is considered for parameters “a” and “b”. a = ∑ ∑ yi y j aij (6)
i
b=
j
∑ bi y i
(7)
i
√ where aij = ai a j (1 − δij ) and δij is binary interaction coefficient of species i and j; yi is mole fraction of species i in gaseous phase. The parameters (critical temperature Tc , critical pressure Pc , and acentric factor ω) used in PR [22] EOS for each individual component are listed in Table 1. Table 1. Parameters of each component for PR EOS. Tc (K) CO2 N2 O2 H2 O a:
a
304.2 126.2 b 154.6 b 647.3 a
Pc (bar) a
73.83 34.0 b 49.8 b 221.2 a
ω 0.2236 a 0.0377 b 0.021 b 0.3434 a
from Soreide and Whitson [7]; b : from PHREEQC database [26].
The binary interaction parameters (δij ) used PR EOS are listed in Table 2.
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Table 2. Binary interaction parameters for PR EOS. H2 O
δij H2 O CO2 N2 O2 a:
CO2
0.1896 a 0.32547 b 0.20863 c
N2 a
O2 0.20863 c 0.1140 d −0.0119 c -
b
0.1896 −0.007 d 0.1140 d
0.32547 −0.007 d −0.0119c
from Soreide and Whitson [7]; b : from Ziabakhsh-Ganji and Kooi [27]; c : from Wang et al. [28]; d: from Li and Yan [29].
2.2. Equilibrium Constant From Equations (2) and (3), equilibrium constant can be expressed as, Vm,i ( P − Pre f ) Ki = K H ( T, Pre f ) exp RT
! (8)
where i denotes a component (i = H2 O, CO2 , N2 , and O2 ). For H2 O, the equation from previous work [3,4] is used, K H2 O
( P − 1)( a6 + a7 T ) = ( a1 + a2 T + a3 T + a4 T + a5 T ) exp RT 2
3
4
(9)
where a1 – a7 are the parameters found in Li et al. [3]. For CO2 , N2 and O2 , Equation (8) is used for the equilibrium constant. K H ( T, Pre f ) is calculated following Appelo [30]: log(K H ( T, Pre f )) = A0 + A1 T +
A2 A + A3 log( T ) + 24 + A5 T 2 T T
(10)
where A0 – A5 are the parameters found in Appelo [30], listed in Table 3. Also, Vm,i is calculated as follows: ! a3,i 104 a4,i 100a2,i Vm,i = 41.84 0.1a1,i + + + − ωi × QBrn (11) 2600 + P ( T − 288) (2600 + P)( T − 288) where a1,i – a4,i and ω i are the parameters found in Appelo [30], listed in Table 3, and QBrn is the Born function, which can be found in Helgson et al. [31]. Table 3. Parameters for equilibrium constants of CO2 , N2 and O2 . A0 CO2 N2 O2
−10.52624 58.453 7.5001
A1
A2
2.3547 × 10−2
3972.8
10−3
−3199 0
−1.818 × −7.8981 × 10−3
A3 0
−17.909 0
A4
−5.8746 × 105 27,460 −2.0027 × 105
A5
a1
−1.9194 × 7.29 10−5 0 7 0 5.7889
a2
a3
a4
ω
0.92
2.07
−1.23
−1.6
0 6.3536
0 3.2528
0 0 −3.0417 −0.3943
2.3. Activity Model Activity or activity coefficient is an important parameter to describe the deviation of solvent and aqueous species from ideal solution at a given temperature and pressure. The Pitzer [23,32,33] model is an accurate model for calculating the activity of water and various aqueous species especially to high salinity of brine. This activity model has wide application to water-salt-mineral system computation and modeling at different temperatures and pressures [16–21,34]. These applications have shown the success of the Pitzer [23] model. In this work, the Pitzer [23] model is used to calculate aqueous species activity coefficients. The details of Pitzer [23] equations have been discussed in previous research
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papers [20,21]. When gas components are dissolved in water, it is assumed that they are in the form of neutral species in aqueous phase. The Pitzer [23] equation of neutral species is as follows [8]: ln γi =
∑ 2mc λi−c + ∑ 2ma λi−a + ∑ ∑ mc ma ζ i−a−c c
a
c
(12)
a
where γi is the activity coefficient of component i; i = CO2 , N2 , or O2 ; λi−c , λi−a and ζ i−a−c are the Pitzer interaction parameters; mc and m a are the aqueous species molality; “c” denotes cation species and “a” denotes anion species. In this work, λi−c , λi−a and ζ i− a−c are parameterized from the gas solubility experimental data. The fitting equation of Pitzer interaction parameters follows Appelo [30]: Param = b0 + b1 × (
1 1 1 T 1 − ) + b2 × ln( ) + b3 × ( T − TR ) + b4 × ( T 2 − TR 2 ) + b5 × ( 2 − 2 ) (13) T TR TR T TR
where b0 – b5 are fitting parameters; T is temperature in (K); TR = 298.15 (K). The parameters are listed in Table 4. Table 4. Parameters of Pitzer interaction equations for gas components and main ion species. b0
. a
λCO2 − Na+ λCO2 − Mg+2 a λCO2 −Ca+2 a λCO2 −SO−2 a 4 λCO2 −K+ a λ N2 − Na+ b λ N2 − Mg+2 b λ N2 −Ca+2 b λ N2 −K+ b λ N2 −SO−2 b 4 λO2 − Na+ c λO2 − Mg+2 c λO2 −Ca+2 c λO2 −K+ c λO2 −SO−2 c 4
ζ CO2 − Na+ −SO−2 4
ζ N2 − Na+ −SO−2 4
0.085 0.183 0.183 0.075 0.051 0.1402 0.2804 0.2804 0.1402 0.0371 0.19997 0.31715 0.35135 0.15022 0.14383 −0.015
b b
b1
b2
b3
b4
b5
−595 −1190 −1190 −595
−4.025 −8.05 −8.05 −4.025
0.01044 0.02088 0.02088 0.01044
−2.131 × 10−6 −4.262 × 10−6 −4.262 × 10−6 −2.131 × 10−6
49,970 99,940 99,940 49,970
−1.16 × 10−2 −0.58 × 10−2 −0.58 × 10−2 −1.16 × 10−2 −1.16 × 10−2 −2.32 × 10−2 −1.16 × 10−2 −2.32 × 10−2
b
ζ N2 − Na+ −Cl − ζ N2 −K+ −Cl − b ζ N2 −K+ −SO−2 b 4
ζ N2 −Ca+2 −Cl − b ζ N2 −Ca+2 −SO−2 b 4
ζ N2 − Mg+2 −Cl − b ζ N2 − Mg+2 −SO−2 b 4
a:
from Appelo [30]; b : this work; c : from Geng and Duan [10].
2.4. Gas-Brine Mutual Solubility Reproduction To validate the above thermodynamic model, comparisons are made between the gas-brine mutual solubility calculated from this model and existing experimental data including CO2 -brine, N2 -brine, O2 -brine and multi-gas-brine systems. The experimental work for CO2 -brine system is extensive and substantial, with a wide range of temperatures, pressures and water salinities. A comparison of the CO2 -brine system mutual solubility obtained from the experimental data and the model proposed in this work is shown in Figure 1. The experimental data for validation was mainly obtained from Todheide and Franck [35], Takenochi and Kennedy [36], Malinin and Savelyeva [37], Malinin and Kurorskaya [38],
mutual solubility calculated from this model and existing experimental data including CO2-brine, N2-brine, O2-brine and multi-gas-brine systems. The experimental work for CO2-brine system is extensive and substantial, with a wide range of temperatures, pressures and water salinities. A comparison of the CO2-brine system mutual solubility in Energies 2018,obtained 11, 2627 from the experimental data and the model proposed in this work is shown 6 of 18 Figure 1. The experimental data for validation was mainly obtained from Todheide and Franck [35], Takenochi and Kennedy [36], Malinin and Savelyeva [37], Malinin and Kurorskaya [38], Tabasinejad Tabasinejad et et al. al. [39]. [39]. As As shown shown in in the the comparison, comparison, the the model model can can well well reproduce reproduceCO CO22 solubility solubility in in aqueous phase with pressure ranging from 1 to more than 1000 (bar), temperature range aqueous phase with pressure ranging from 1 to more than 1000 (bar), temperature range from from 00 to to ◦ more NaCl molality range fromfrom 0 to more (molal). The H2 OThe solubility in CO2 -rich more than than250 250( C), (°C), NaCl molality range 0 to than more6 than 6 (molal). H2O solubility in phase can also be well reproduced by this model. CO2-rich phase can also be well reproduced by this model.
Figure 1. 1. CO [35–39] (dots) (dots) and and results results Figure CO22-brine -brine system system mutual mutual solubility solubility from from experimental experimental data data [35–39] calculated by this model (lines) at various temperatures, pressures and salinities. (a) CO 2 solubility in calculated by this model (lines) at various temperatures, pressures and salinities. (a) CO2 solubility in pure water. (b) CO 2 solubility in NaCl solutions varying with pressure at different temperature and pure water. (b) CO2 at different temperature and NaClmolality. molality.(c)(c) 2 solubility in solutions NaCl solutions varying NaCl molalitytemperatures at different NaCl CO2CO solubility in NaCl varying with NaClwith molality at different temperatures (d) H 2O -rich solubility in CO 2-richpressures phase at different pressures and and pressures. and (d) Hpressures. O solubility in CO phase at different and temperatures. 2 2 temperatures.
A comparison of the mutual solubility of N2 -brine obtained from the experimental data and the results byofthis is shown in Figure 2. The experimental data of N2 solubility water A calculated comparison thework, mutual solubility of N2-brine obtained from the experimental datainand the or NaClcalculated solutions are fromwork, Weibeisetshown al. [40]inand Mishnima et al. [41]. Thedata experimental data ofinHwater results by this Figure 2. The experimental of N2 solubility 2 O in N is from [42] and and Bondareva [43]. It isThe shown that the model of H this or2 -rich NaClphase solutions are Althaus from Weibe et al.Namiot [40] and Mishnima et al. [41]. experimental data of 2O work can well reproduce the experimental work at a wide range of temperatures, pressures and salinities.
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in N2-rich phase is from Althaus [42] and Namiot and Bondareva [43]. It is shown that the model of in Nwork 2-rich phase is from Althaus [42] and Namiot and Bondareva [43]. It is shown that the model of this can well reproduce the experimental work at a wide range of temperatures, pressures and Energies 2018,can 11, 2627 18 this work well reproduce the experimental work at a wide range of temperatures, pressures7 of and salinities. salinities.
Figure 2. N2-brine mutual solubility obtained from experimental data [40–43] (dots) and the results Figure 2. N Nby 2-brine mutual solubility obtained from experimental data [40–43] (dots) and the results Figure 2. mutual obtained from experimental data [40–43] (dots) and the results calculated the model atsolubility various values of temperature, pressure and salinity. (a) N 2 solubility in 2 -brine calculated by the model modelin atNaCl various values varying of temperature, temperature, pressure and salinity. (a) (a) N 2 solubility in calculated at various values of pressure in water. (b) by N2 the solubility solutions with pressure. (c)and N2 salinity. solubility inN NaCl solutions 2 solubility water. (b) N2 solubility solubility in in NaCl NaCl solutions varying with pressure. pressure. (c) N2 solubility solubility in in NaCl NaCl solutions solutions water. (b) N with N varying with at 1 solutions atm. (d) Hvarying 2O solubility in N2-rich(c) phase. 2 NaCl molality 2 varying with NaCl molality at 1 atm. (d) H 2O solubility in N2-rich phase. varying with NaCl molality at 1 atm. (d) H O solubility in N -rich phase. 2
2
The experimental work on the O2-brine system is not as comprehensive as on the CO2-brine The experimental worktemperature onthe theO2O-brine 2-brine system is100 not as comprehensive onCO the200 CO(bar). 2-brine The experimental on system is not as(°C), comprehensive as on the and 2 -brine and N2-brine systems.work The is lower than and pressure isasbelow A ◦ C),100 and N 2 -brine systems. The temperature is lower than (°C), and pressure is below 200 (bar). A N -brine systems. The temperature is lower than 100 ( and pressure is below 200 (bar). A comparison 2 comparison of the model results and experimental data obtained from Stephen et al. [44], Pray and comparison the Yasunishi model results experimental data obtained from et and al. [44], and of the model results and experimental data obtained from Stephen etcan al.Stephen [44], reproduce Pray Stephen [45], Stephen [45],of and [46] and is shown in Figure 3. The model well thePray existing Stephen [45], data. and is Yasunishi is shown in Figure The model can reproduce the existing and Yasunishi [46] shown in[46] Figure 3. The model can 3. well reproduce thewell existing experimental data. experimental experimental data.
Figure 3.3. O O2-brine -brine mutual mutual solubility solubility obtained obtained from from experimental experimental data data [44–46] [44–46] (dots) (dots) and and the the results results Figure Figure 3. O2by 2-brine mutual solubility obtained from experimental data [44–46] (dots) and the results calculated the model (lines) at various values of temperature, pressure and salinity. (a) O2 calculated by the model (lines) at various values of temperature, pressure and salinity. (a) O2 solubility calculated by the model (lines) at various values of temperature, pressure and salinity. (a) O2 solubility in water. (b) O 2 solubility in NaCl solutions. in water. (b) O2 solubility in NaCl solutions. solubility in water. (b) O2 solubility in NaCl solutions.
For multi-component multi-componentgas gasmixture mixtureand and brine systems, experimental work is rare. Liu al. For brine systems, thethe experimental work is rare. Liu et al.et[47] For multi-component gas mixture and brine systems, the experimental work is rare. Liu et al. [47] proposed CO 2-N 2O equilibrium experiments two temperatures and three pressures with proposed CO2 -N equilibrium experiments at two at temperatures and three pressures with different 2 -H 2 O2-H [47] proposed CO 2-N2-H2O equilibrium experiments at two temperatures and three pressures with different CO 2 -N 2 gas mixture compositions. As shown in Figure 4, the experimental results can CO -N gas mixture compositions. As shown in Figure 4, the experimental results can accurately be 2 2 different CO 2-N2 gas mixture compositions. As shown in Figure 4, the experimental results can accuratelybybethis predicted predicted model. by this model. accurately be predicted by this model.
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Figure 4. CO2-N2-H2O equilibria (i.e., CO2 and N2 mole fractions in brine varying with N2 mole Figure 4. CO2 -N2 -H2 O equilibria (i.e., CO2 and N2 mole fractions in brine varying with N2 mole fractionsiningas) gas) obtained from experimental work al. dots) [47], dots) and the calculated fractions obtained from experimental work (Liu(Liu et al.et[47], and the calculated results results (lines) (lines) by theinmodel in this work atvalues various values of temperature and(a)pressure. (a) and at P=80 bar and by the model this work at various of temperature and pressure. at P=80 bar T=308.15 K; T=308.15 K; bar (b) and at P=120 andat308.15 at T=308.15 P=160 barK;and T=308.15K; P=80 bar (b) at P=120 308.15bar K; (c) P=160 K; bar(c) and (d) at P=80 bar (d) andat T=318.15 K; and (e) atT=318.15 P=120 barK;and T=318.15 (f) T=318.15 P=160 barK.and T=318.15 K. atT=318.15K; P=120 bar(e) and (f) P=160 barK; and
3.3.Gas-Brine-Mineral Gas-Brine-MineralEquilibria Equilibria In section, the themodeling modeling and validation of gas-brine-mineral equilibria are discussed. In this section, and validation of gas-brine-mineral equilibria are discussed. When When a saline aquifer industrialCO CO geologicstorage storage projects, CO CO2 isCO injected into into a saline aquifer in in industrial 2 2geologic CO22 existence existence 2 is injected significantly significantlyinfluences influencesthe thewater-carbonate water-carbonatemineral mineralequilibrium. equilibrium. The The commonly commonlyfound foundcarbonate carbonate minerals minerals in in sedimentary sedimentary environments environments are arecalcite calcite(CaCO (CaCO 3), magnesite magnesite (MgCO (MgCO3 3),), and and dolomite dolomite 3 ), (CaMg(CO The above three minerals are considered in this (CaMg(CO33))22). The this work work even even though though there there are are many many other otherkinds kindsof ofcarbonate carbonateminerals mineralsininnatural naturalenvironments. environments. The Thegas-brine gas-brineequilibrium equilibriummodel modelwas was explained section will focus onon brine-mineral equilibrium modeling andand its explainedinindetail detailininSection Section2. 2.This This section will focus brine-mineral equilibrium modeling its coupling with the gas-brine model. When the above carbonate minerals reach an equilibrium state with water/brine at a given temperature and pressure, the following chemical reactions are observed.:
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coupling with the gas-brine model. When the above carbonate minerals reach an equilibrium state with water/brine at a given temperature and pressure, the following chemical reactions are observed.: CaCO3(Calcite) = CO3 2− + Ca2+
(14)
MgCO3(Magnesite) = CO3 2− + Mg2+
(15)
CaMg(CO3 )2(Dolomite) = 2CO3 2− + Ca2+ + Mg2+
(16)
CaCO3 (0) = CO3 2− + Ca2+
(17)
MgCO3 (0) = CO3 2− + Mg2+
(18)
H2 O = H+ + OH−
(19)
HCO3 − = H+ + CO3 2−
(20)
CO2 (0) = 2H+ + CO3 2−
(21)
MgOH+ = Mg2+ + OH−
(22)
CaOH+ = Ca2+ + OH−
(23)
where superscript (0) denotes neutral aqueous species. Similar to Equation (1), when an equilibrium state is reached, the following equation for each of the above chemical reactions is applicable [30]: Kr =
∏ a υi
(24)
i
where Kr is the equilibrium constant of a chemical reaction “r”; υi is the stoichiometric coefficient of species i of a chemical reaction “r”; ai is the activity of species i of a chemical reaction “r”. For aqueous species i,ai = mi γi , where mi is the molality of species i, and γi is the activity coefficient of species i. Activity of minerals is 1. To solve the speciation of the brine-mineral system at equilibrium state at a given temperature and pressure, K or log K and the activity coefficients of all related reactions and species should be specified. The equilibrium constant is calculated by Equation (8) using the equilibrium constant (Kref ) at reference pressure (1 bar or water saturation pressure) and mole volume (Vm ) of the related reaction. Kref is a function of temperature and can be expressed by Equation (10). Vm is a function of temperature and pressure and can be expressed by Equation (11). For all the above reactions, the related parameters required for Kref and Vm are obtained from PHREEQC database [30]. Activity coefficients (γi ) of the aqueous species are calculated from the Pitzer [23] model. The expressions of the Pitzer [23] model can be found in Li and Duan [21]. The Pitzer parameters (0)
(1)
(2)
ϕ
fall into three categories: (1) β MX , β MX , β MX and C MX for each cation-anion pair of the species; (2) θij for each cation-cation or anion-anion pair of the species; (3) ψijk for each cation-cation-anion or anion-anion-cation triplet of the species, λni for ion-neutral pairs of the species, and ζ nij for cation-anion-neutral triplets of the species. The Pitzer parameters of various water-salt-mineral system have been studied since the 1980s via thermodynamic modeling in conjunction with experimental work. Table 5 lists the sources of the Pitzer parameters for the calculations of this work system.
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Table 5. Literature sources of Pitzer parameters. Pitzer Parameters (0) β Ca−CO3 ,
ϕ (1) (2) β Ca−CO3 , β Ca−CO3 , CCa−CO3 (0) (1) β Ca− HCO3 , β Ca− HCO3 ϕ (0) (1) β Ca Energies 2018, 11, x −Cl , β Ca−Cl , CCa−Cl ϕ (0) (1) β Mg−Cl , β Mg−Cl , C Mg−Cl (0) (1) ( 0) (1) , C Mg -Cl , ,Mg Cl Mg -Cl β Mg − HCO3 β -Mg − HCO3 (0) (0) (1) , (1) (2) β Ca−OHMg, -βHCO , β-Ca HCO 3 Ca3−OH Mg −OH ϕ (2) (0(0) ) (1)(1) , H−CaCl-OH − βHCa−-Cl ,C OH, ,β H CaCl -OH ϕ (0) (1) β Na−CO3 , C H -− H(0)-,Clβ,Na−H(1) -Cl , CNa Cl CO3 CO3 ϕ (0) (1) (1) (0) β Na− HCO3 , β , C , , Na C Na HCO3 HCO3 -CO 3 Na −Na -CO 3 Na − -CO 3 ϕ (0(0) ) (1)(1) βNa , C CNa Na−-OH −OH OH3, ,β Na Na− - HCO HCO 3 , Na - HCO 3 ϕ (0)(0) (1) β Na , β (1) , C C−Cl Na−-Cl −-Cl OH , Na Na OH , Na Na -OH θ H − Na , θOH , θCl −CO3 , (0)−Cl , θOH (1) −CO3 , , C Na -Cl Na -Cl Na -Cl θCl − HCO3 ,θ HCO3−CO3
H - Na , OH -Cl , θOHCa-CO 3 ,, θCa Cl − -CO 3 , Cl - HCO 3 , HCO 3-CO 3 −H Na θ CaCa Ca - Na - H−, Mg ψOH −Cl − Na ,ψOH − HCO3 Ca - Mg− Na , ψCO3−Cl − Na , ψCO3− HCO3− Na , ψCO3−Cl −Ca , ψOH −Cl −Ca , −COOH OH -Cl - Na , OH - HCO 3- Na−, Na 3-Cl - Na , CO 3- HCO 3- Na , ψCO3
ψCl − Mg− ,ψ , ψ Ca−, H−CO ψ-Cl , CaCO Cl-− −Ca Ca3-, Na 3-Cl CaNa OH -Cl -Cl OH− H − Mg
Cl - Mg -Ca , Cl - Na -Ca , Cl - H -Ca , Cl - H - Mg 3.1. Mineral Solubility
Literature Sources Duan and Li [20] Appelo [30] Christov and Moller [19]
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Christov and Moller [19] Christov andChristov Moller [19] Greenberg and Moller [18], and Moller [19] Greenberg and Mollerand [18], Christov Christov Moller [19]and Moller [19]
Christov and Christov andMoller Moller[19] [19] Polya et al.Moller [48] [19] Christov and Polya etetal.al.[48] Polya [48] Pabalan and Pitzer [49] Polya et al. [48] Pitzer Pabalan and[50] Pitzer [49] Li and Duan [34] Pitzer [50] Li and [34] Christov andDuan Moller [19] Pabalan and Pitzer [49] Christov and Moller [19] Pabalan and Pitzer [49] Li and Duan [34], Duan and Li [20] Li and Duan [34], Duan and Li [20] Appelo [30] Appelo [30]
3.1. Mineral Solubility
With the thermodynamic model discussed above, the solubility of carbonate minerals the thermodynamic model above, in theaqueous solubilityphase of carbonate minerals (calcite, (calcite,With magnesite and dolomite) candiscussed be calculated (with/without gases in magnesite at and dolomite) can be calculated in aqueous phase (with/without gases in equilibrium) at equilibrium) various temperatures and pressures. To validate the thermodynamic model, the existing various temperatures and pressures. To validate the thermodynamic model, the existing experimental data of carbonate mineral-gas-water equilibria should be reproduced. Predictions of experimental dataare of also carbonate equilibria shouldpressure be reproduced. Predictions mineral solubilities made mineral-gas-water at various values of temperature, and NaCl molality. of mineral solubilities are also made at various values of temperature, pressure and NaCl molality. Calcite solubility experiments have usually been conducted with CO2 in equilibrium. A comparison Calciteof solubility experiments have usually been conducted with calculated CO2 in equilibrium. A of the values calcite solubility obtained from experimental data and those by the proposed comparison of the values of calcite solubility obtained from experimental data and those calculated model at a CO2 partial pressure of 12 (bar) at different temperatures and NaCl molalities is shown by the proposed model at a CO2 partial pressure of 12 (bar) at different temperatures and NaCl in Figure 5. This model can well reproduce the experimental data of calcite solubility, as shown in molalities is shown in Figure 5. This model can well reproduce the experimental data of calcite the comparison. solubility, as shown in the comparison.
Figure 5. Calcite solubility in aqueous solution with CO2 in equilibrium obtained from experimental Figure 5. Calcite solubility in aqueous solution with CO2 in equilibrium obtained from experimental data [51,52] and the calculated results using this model. (a) Calcite solubility varying with NaCl data [51,52] and the calculated results using this model. (a) Calcite solubility varying with NaCl molality molality at different temperatures. (b) Calcite solubility varying with temperature at different NaCl at different temperatures. (b) Calcite solubility varying with temperature at different NaCl molalities. molalities.
Calcite solubility in pure water and in NaCl solutions at various values of pressure, temperature and NaCl salinity are shown in Figure 6. Calcite solubility in pure water increases with pressure and decreases with temperature. At a given temperature and pressure, within the lower NaCl molality range (0–1.5), calcite solubility increases with NaCl molality, but decreases in the higher NaCl molality range (1.5–6). Experimental data for magnesite solubility is rare. Bénézeth et al. [53] conducted synthetic
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Calcite solubility in pure water and in NaCl solutions at various values of pressure, temperature and NaCl salinity are shown in Figure 6. Calcite solubility in pure water increases with pressure and decreases with temperature. At a given temperature and pressure, within the lower NaCl molality range (0–1.5), calcite solubility increases with NaCl molality, but decreases in the higher NaCl molality range (1.5–6). Energies 2018, 11, x 11 of 18 Experimental data for magnesite solubility is rare. Bénézeth et al. [53] conducted synthetic magnesite solubility product measurements at temperatures ranging from 50 to 200 (◦ C) with 0.1 (molal) NaCl (molal) NaCl and with CO2 partial pressure under 30 (bars). A comparison of18Mg2+ Energies 2018,solutions, 11, x 11 of solutions, and with CO2 partial pressure under 30 (bars). A comparison of Mg2+ molality of the molality of the solution in equilibrium with magnesite between the experimental data and calculated 2+ solution in equilibrium with and magnesite between experimental data and calculated results is shown (molal) NaCl in solutions, with CO 2 partialthe pressure under and 30 (bars). A comparison of Mg results is shown Figure 7. Generally, the calculated values measurements are comparable in Figure 7. Generally, the calculated values measurements areexperimental comparable data withand calculated values molality of the solution in equilibrium withand magnesite between the calculated with calculated values a little bit higher than the measurements. Magnesite solubility in pure water is shown 7. Generally, the calculated values and measurements comparable a littleresults bit higher than in theFigure measurements. Magnesite solubility in pure water or NaCl are solutions at various or NaCl solutions at various temperatures, pressures and NaCl molalities of the solutions are shown with calculated valuesand a little bit molalities higher thanof thethe measurements. in pure water temperatures, pressures NaCl solutions areMagnesite shown insolubility Figure 8. The magnesite in Figure 8. solutions The magnesite solubility in pure waterand increases with pressure and decreases with or NaCl at various temperatures, pressures NaCl molalities of the solutions are shown solubility in pure water increases with pressure and decreases with temperature. In NaCl solutions, temperature. In NaCl solutions, magnesite solubility increases with NaCl molality in the lower in Figure 8. The magnesite solubility in pure water increases with pressure and decreases with magnesite solubility increases with NaCl molality in the lower salinity range (0–1.5) but decreases in salinity range (0–1.5) but decreases the higher salinityincreases range (1.5–6). temperature. In NaCl solutions, in magnesite solubility with NaCl molality in the lower the higher salinity range (1.5–6). salinity range (0–1.5) but decreases in the higher salinity range (1.5–6).
Figure 6. Calcite solubility in pure water or NaCl solutions calculated by this model at different
Calcitesolubility solubility in in pure pure water water or or NaCl NaCl solutions solutions calculated calculated by by this this model model at at different different Figure 6. Calcite temperature, pressure and NaCl molality. (a) Calcite solubility in pure water varying with pressure with pressure at temperature, pressure and and NaCl NaCl molality. molality.(a) (a)Calcite Calcitesolubility solubilityininpure purewater watervarying varying with pressure at different temperatures. (b) Calcite solubility in NaCl solutions varying with NaCl molality at different temperatures. (b) Calcite solubility in NaCl solutions varying with NaCl molality at 343.15 (K) at different temperatures. (b) Calcite solubility in NaCl solutions varying with NaCl molality at 343.15 (K) and various pressures. and various pressures. 343.15 (K) and various pressures.
Figure 7. Magnesite solubility calculated by this model (lines) and obtained from the experimental work of Benezeth et al. [53] (dots).
Figure experimental Figure 7. 7. Magnesite Magnesite solubility solubility calculated calculated by by this this model model (lines) (lines) and and obtained obtained from from the the experimental work [53] (dots). (dots). work of of Benezeth Benezeth et et al. al. [53]
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Figure 8. Magnesite solubility in pure water or NaCl solutions calculated by this work at various Figure 8. Magnesite solubility in pure water or NaCl solutions calculated by this work at various Figure of 8. Magnesite solubility in pure or NaCl solutions calculated this worksolubility at variousinvalues values pressure, temperature and water NaCl molality of the solution. (a) by Magnesite pure values of pressure, temperature and NaCl molality of the solution. (a) Magnesite solubility in pure of pressure, temperature and NaCl molality of the solution. (a) Magnesite solubility in pure water water varying with pressure at different temperature. (b) Magnesite solubility in NaCl solutions water varying with pressure at different temperature. (b) Magnesite solubility in NaCl solutions varying with pressure at different temperature. (b) Magnesite solubility in NaCl solutions varying with varying with NaCl molality pressures. varying with NaCl molalityatat343.15 343.15(K) (K)and and various various pressures. NaCl molality at 343.15 (K) and various pressures.
ToTo address the et al. al.[54] [54]measured measurednatural naturaldolomite dolomite address theso-called so-called“dolomite “dolomite problem”, problem”, Bénézeth Bénézeth et To address the so-called “dolomite problem”, Bénézeth et al. [54] measured natural dolomite (CaMg(CO 3)23))2solubility 50 to to 253 253 (°C) (°C)with with0.1 0.1(molal) (molal)NaCl NaCl solutions (CaMg(CO ) solubilitywith withaatemperature temperature range range of of 50 solutions (CaMg(CO with a temperature rangelogarithmic of 50 to 253concentrations (◦ C) with 0.1 (molal) NaCl solutions 3 )2 ) solubility 2+ and 2+,2+Ca 2+ and using a hydrogen electrode Mg , Ca using a hydrogen electrodeconcentration concentrationcell. cell. The The logarithmic concentrations ofofMg H+Hin+ in 2+ , Ca2+ and H+ using a hydrogen electrode concentration cell. The logarithmic concentrations of Mg thethe solution at at equilibrium in the the literature literature[54]. [54].AAcomparison comparison between solution equilibriumwith withdolomite dolomiteare are provided provided in between in the solution at equilibrium with dolomite are provided in the literature [54]. A comparison between experimentalresults resultsfrom fromBénézeth Bénézeth et et al. al. [54] [54] and the is is thethe experimental the results results calculated calculatedusing usingthis thismodel model the experimental results from Bénézeth et al. [54] and the results calculated using this model is shown shown Figure Thetrends trendsofofthe themeasurements measurements varying varying with byby shown in in Figure 9.9.The withtemperature temperatureare arealso alsoobtained obtained in Figure 9. The trends of the measurements varying with temperature are also obtained by this model. this model. this model.
Figure 9. Aqueous species concentration in equilibrium with dolomite obtained from experimental results and the calculated results by this model at various Figure 9. [54] Aqueous species concentration in with dolomite obtained from from experimental experimental Figure 9. Aqueous species concentration in equilibrium equilibrium with temperatures. dolomite obtained results [54] and the calculated results by this model at various temperatures. results [54] and the calculated results by this model at various temperatures.
The presented model is used for the calculation of dolomite solubility at various values of temperature, pressure andisNaCl shown in of Figure 10. From the figure, it is shown that of The presented model usedmolality, for the as solubility at various values presented model calculation dolomite dolomite solubility in water increases with pressure and decreases with temperature. In NaCl temperature, temperature, pressure pressure and and NaCl NaCl molality, molality, as shown in Figure 10. From From the the figure, it is shown that solutions, dolomiteinsolubility increases with with NaCl molality in the lowerwith salinity range (0–1.5) dolomite solubility water increases pressure and decreases temperature. In but NaCl solubility in water increases with pressure and decreases with temperature. In NaCl solutions, decreases in the higher salinity range (1.5–6). solutions, dolomite solubility increases with NaCl molality in the lower salinity range (0–1.5) but
dolomite solubility increases with NaCl molality in the lower salinity range (0–1.5) but decreases in decreases the higher the higherin salinity rangesalinity (1.5–6).range (1.5–6).
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Figure 10. Dolomite solubility in pure water or NaCl solution calculated by this work at various Figure Dolomitesolubility solubility in in pure pure water water or NaCl solution calculated bybythis at at various Figure 10.10.Dolomite NaCl solutionsolubility calculated thiswork work various temperatures, pressures and NaCl molalities.or(a) Dolomite varying with pressure at temperatures, pressures and NaCl molalities. (a) Dolomite solubility varying with pressure at temperatures, pressures and NaCl molalities. (a) Dolomite solubility varying with pressure at different different pressures. (b) Dolomite solubility varying with NaCl molality at 343.15 (K) and various different pressures. (b) Dolomite solubility varying with NaCl molality at 343.15 (K) and various pressures. pressures. (b) Dolomite solubility varying with NaCl molality at 343.15 (K) and various pressures. pressures.
3.2. Mutual Effects of Dissolutions of Gases and Minerals 3.2.3.2. Mutual Effects Mutual EffectsofofDissolutions DissolutionsofofGases Gases and and Minerals Minerals When CO2 is injected into a carbonate aquifer, CO2 and carbonate mineral solubility in water When 2 is injected into a carbonate aquifer, CO2 and carbonate mineral solubility in water WhenCO CO 2 is injected into a carbonate aquifer, CO2 and carbonate mineral solubility in water are affected by each other. It has been observed in subsurface hot water recovery projects that areare affected by other. in subsurface subsurfacehot hotwater waterrecovery recoveryprojects projects that affected byeach other. ItIt has has been been observed observed in that carbonate mineraleach deposition usually triggers a decrease in permeability, and the subsurface fluid carbonate mineral decrease in in permeability, permeability,and andthe thesubsurface subsurface fluid carbonate mineraldeposition depositionusually usually triggers triggers a a decrease fluid can be blocked [2]. CO2 injection into water can re-dissolve the carbonate minerals. The geochemical cancan bebe blocked [2]. CO 2 injection into water can re-dissolve the carbonate minerals. The geochemical blocked [2]. CO2 injection into water re-dissolve the carbonate minerals. The geochemical model constructed in the previous sections can be used to evaluate the solubility effects of CO2 and model constructed used to to evaluate evaluatethe thesolubility solubilityeffects effects 2 and model constructedininthe theprevious previoussections sections can be used of of COCO 2 and carbonated minerals. The CO2 solubility in pure water and carbonate mineral (calcite and magnesite) carbonated minerals. water and andcarbonate carbonatemineral mineral(calcite (calcite and magnesite) carbonated minerals.The TheCO CO2 2solubility solubility in in pure water and magnesite) saturated solutions at various values of temperature and pressure is shown in Figure 11. The results saturated solutionsatatvarious variousvalues values of of temperature temperature and 11.11. The results saturated solutions and pressure pressureisisshown shownininFigure Figure The results show that thethe dissolved mineral effects on the CO solubility in water are insignificant. This isThis because 2 CO show that dissolved mineral effects on the 2 solubility in water are insignificant. is is show that the dissolved mineral effects on CO2 solubility in water are insignificant. This thebecause solubility of carbonate minerals is not significant to change the solution properties. the solubilityofofcarbonate carbonateminerals minerals is not significant properties. because the solubility significantto tochange changethe thesolution solution properties.
Figure 11. CO2 solubility in pure water or carbonate mineral saturated solutions at different temperatures and pressures. Solid lines are CO 2 solubility in puresaturated water. Dashed lines at represent Figure 11. CO different Figure 11. CO22 solubility solubility in in pure pure water water or or carbonate carbonate mineral mineral saturated solutions solutions at different calcite saturated solutions. Dash-dot linesare represent magnesiteinsaturated solutions. temperatures and pressures. Solid lines CO 2 solubility pure water. Dashed lines represent temperatures and pressures. Solid lines are CO2 solubility in pure water. Dashed lines represent calcite saturated saturated solutions. solutions. Dash-dot represent magnesite magnesite saturated saturated solutions. solutions. calcite Dash-dot lines lines represent
To evaluate the effects of CO2 on solubilities of carbonate mineral influenced by CO2, carbonate mineral (calcite,the magnesite and in pure water andmineral CO2 saturated solutions of CO 2CO on2solubilities of carbonate mineral influenced by CO 2, carbonate To evaluate evaluate theeffects effects ofdolomite) on solubilities solubilities of carbonate influenced by are CO2 , calculated using the presented model at various values of temperature and pressure (Figure 12). It isare mineral (calcite, and dolomite) solubilities in pureinwater and CO solutions carbonate mineralmagnesite (calcite, magnesite and dolomite) solubilities pure water and2 saturated CO2 saturated solutions evident from that the solubility of all carbonate water 12). withIt is calculated usingthe the comparison presented model at various values of three temperature andminerals pressurein(Figure
are calculated using the presented model at various values of temperature and pressure (Figure 12). It is fromthe thecomparison comparison solubility all carbonate three carbonate water with evident from thatthat the the solubility of all of three mineralsminerals in water in with dissolved
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CO2 is significantly increased. The increase is higher at the lower temperature (25 ◦ C) than at the higher dissolved CO2 is significantly increased. The increase is higher at the lower temperature (25 °C) than temperature (150 ◦ C). at the higher temperature (150 °C).
Figure 12. Carbonate mineral solubility in pure water or in CO2 saturated solutions. (a) Calcite Figure 12. Carbonate mineral solubility in pure water or in CO2 saturated solutions. (a) Calcite solubility. solubility. (b) Magnesite solubility. (c) Dolomite solubility. (b) Magnesite solubility. (c) Dolomite solubility.
3.3. 3.3. Impurity Impurity Effects Effects on on Gas-Water-Mineral Gas-Water-Mineral Equilibria Equilibria In In CO CO22 geologic geologicprojects, projects,the theevaluation evaluationof ofthe theeffects effectsof of impurities impurities (such (such as as N N22,, O O22)) in in terms terms of of water properties(density, (density, viscosity, pH), gas-water-mineral equilibria, and reservoir property water properties viscosity, pH), gas-water-mineral equilibria, and reservoir property change change (porosity and permeability) is an important since decreasing 2 purityreduces efficiently (porosity and permeability) is an important topic, sincetopic, decreasing CO2 purity CO efficiently the reduces the2 cost of CO 2 capture.to Compared to CO2-water-mineral N2 orthe O2CO affects the CO 2 cost of CO capture. Compared CO2 -water-mineral systems, N2systems, or O2 affects dissolution 2 dissolution and solute concentration in water. At the same temperature and pressure, the presence and solute concentration in water. At the same temperature and pressure, the presence of N2 or O2 of 2 or O2 inchanges gas phase changes of theCO fugacity of CO2, and the CO2 solubility in water (Figure 13). in N gas phase the fugacity 2 , and the CO2 solubility in water (Figure 13). The carbonate The carbonate dissolution isaffected significantly by CO2 concentration in water as shown from the dissolution is significantly by COaffected 2 concentration in water as shown from the calculations in calculations in the previous section. the previous section.
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Figure 13. Phase equilibria of CO2-brine-calcite and CO2-N2-brine-calcite at 100 (°C) and 300 (bar). (a) Figure 13. Phase equilibria of CO2 -brine-calcite and CO2 -N2 -brine-calcite at 100 (◦ C) and 300 (bar). (a) 2+ molality and pH varying with NaCl molality; solid CO2 molality varying with NaCl molality. (b) Ca2+ CO2 molality varying with NaCl molality. (b) Ca molality and pH varying with NaCl molality; solid 2+ lines are Ca molality, and dashed lines are pH. lines are Ca2+ molality, and dashed lines are pH.
4. Conclusions 4. Conclusions 2 geologic storage with impurities (such as N 2 and O2) is an important choice for reducing COCO 2 geologic storage with impurities (such as N2 and O2 ) is an important choice for reducing the the cost of the full chain of CCS. However, the phase behavior of CO2-impurities-mineral systems is cost of the full chain of CCS. However, the phase behavior of CO2 -impurities-mineral systems is key to key to understanding the migration, storage capacity and safety when CO2 and impurities are understanding the migration, storage capacity and safety when CO2 and impurities are injected into injected into subsurface porous media. Rodrigues et al. [55] analyzed the CO2 utilization and storage subsurface porous media. Rodrigues et al. [55] analyzed the CO2 utilization and storage mechanisms mechanisms of different technologies (such as CO2 enhanced coal seam recovery and shale gas of different technologies (such as CO2 enhanced coal seam recovery and shale gas recovery). recovery). From the analysis in their work, fluid-mineral interactions and CO2 storage capacities are From the analysis in their work, fluid-mineral interactions and CO2 storage capacities are dramatically dramatically different in CO2 utilization and storage projects with different technologies. Fluid different in CO2modeling utilizationisand projects with different technologies. Fluid equilibrium modeling equilibrium a storage basis for the analysis. In this work, a thermodynamic model of is aCO basis for the analysis. In this work, a thermodynamic model of CO -N -O -brine system equilibria 2 2 range 2 2-N2-O2-brine system equilibria is established with a temperature from 0–250 (°C), a is established with afrom temperature rangeand from 0–250molality (◦ C), a pressure range from 1–1000 and aThe NaCl pressure range 1–1000 (bar) a NaCl range from 0 to more than (bar) 6 (molal). molality from 0with to more thanexperimental 6 (molal). The model validated with data model range is validated existing data fromissubsystems, suchexisting as CO 2experimental -brine, N2-brine, from subsystems, such as CO -brine, N -brine, O -brine, and CO -N -brine, in terms of gas solubility O2-brine, and CO2-N2-brine,2 in terms 2of gas solubility in brine and 2 2 2H 2O solubility in gas phase. The in brine and H2 Oshows solubility in gas phase. Thedata comparison shows that theby experimental data can be well comparison that the experimental can be well reproduced the presented model. reproduced the presented WhenbyCO 2 is dissolvedmodel. in an aquifer, water properties are changed in terms of pH, alkalinity, cation and anion concentrations. Mineral water dissolution and precipitation thealkalinity, water When CO in an aquifer, properties are changedare in affected terms ofby pH, 2 is dissolved property change. In this work, the thermodynamic model of CO 2 -N 2 -O 2 -brine system is extended to cation and anion concentrations. Mineral dissolution and precipitation are affected by the water CO2-N2change. -O2-brine-carbonate (mainly calcite, magnesite and dolomite) systems. With the new property In this work, the thermodynamic model of CO 2 -N2 -O2 -brine system is extended geochemical(mainly model, the following calculations can be performed: to gas-water-mineral CO2 -N2 -O2 -brine-carbonate calcite, magnesite and dolomite) systems. With the new The mineral solubility can be calculated with CO 2 and/or other impurity gas components (N 2 gas-water-mineral geochemical model, the following calculations can be performed: and O2)mineral dissolved. The comparisons with existing show that carbonate (calcite, The solubility can be calculated withexperimental CO2 and/ordata other impurity gas components magnesite and dolomite) solubility can be reproduced well by the presented model. It is also shown (N2 and O2 ) dissolved. The comparisons with existing experimental data show that carbonate by the calculations at various values of temperature and pressure that carbonate solubilities raise (calcite, magnesite and dolomite) solubility can be reproduced well by the presented model. It is considerably with CO2 dissolved in brine. also shown by the calculations at various values of temperature and pressure that carbonate solubilities Gas solubility in brine affected by carbonate dissolution in water is evaluated by the model. raise considerably with CO2 dissolved in brine. The effects of carbonate dissolution are not significant, because there is a lower level of mineral Gas solubility in brine affected by carbonate dissolution in water is evaluated by the model. solubility in water. The effects carbonate dissolution are be notcalculated significant,bybecause thereatisvarious a lowertemperatures, level of mineral solubility Ion of concentrations and pH can the model pressures in water. and salinities. The effects of gas impurities on pH and Ca2+ concentration are evaluated in this work. Ion concentrations and can be calculated by the model at various temperatures, pressures When CO2 is mixed with N2pH or O 2, less mineral dissolution, lower Ca2+ concentration and higher pH and salinities. The from effects gas impurities on pH and Ca2+ because concentration are2 evaluated in this work. can be observed theofcalculation. The change is mainly of the CO fugacity reduction in When CO2 iswith mixed N2 orofON22, or less lower Ca2+ concentration and higher pH gas phase thewith presence O2mineral , and thedissolution, effect of N2 or O2 dissolution is not significant. can be observed fromthe the validation calculation.ofThethe change is mainly the COsupply reduction in To address model, futurebecause work of should experimental 2 fugacity gasmeasurements phase with the of N2 orofOmore the effect of N2 at orvarious O2 dissolution is notand significant. of presence phase behaviors complex systems temperatures pressures, 2 , and such as phasethe equilibrium 2-brinefuture and CO 2-N2should -O2-brine, and experimental carbonate dissolved water To address validationofofCO the2-O model, work supply measurements (i.e., different ion concentrations, pH and/or alkalinity) with different levels of CO 2, N2 as of properties phase behaviors of more complex systems at various temperatures and pressures, such
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phase equilibrium of CO2 -O2 -brine and CO2 -N2 -O2 -brine, and carbonate dissolved water properties (i.e., different ion concentrations, pH and/or alkalinity) with different levels of CO2 , N2 and/or O2 in the system. In real geologic systems, geochemical reactions are much more complicated. More work is still needed to assess the impurity effects on gas-water-mineral geochemical behaviors, especially when there are redox reactions in the systems. In future work, minerals with different chemical valences such as sulfides and sulfates will be considered to evaluate influences of oxygen. Author Contributions: J.L. conducted the modeling work, wrote and revised the manuscript. R.A. worked on part of the coding and reviewed the manuscript. X.L. reviewed and revised the manuscript. Funding: This research was funded by National Natural Science Foundation of China (Grant No. 41502246) and National Key R&D Program of China (Grant No. 2016YFE0102500). Acknowledgments: Thanks to two anonymous reviewers for constructive suggestions. Conflicts of Interest: The authors declare no conflict of interest.
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