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Partitioning of HCl Between Concentrated Brines and Supercritical CO Under Geologic Reservoir Conditions 2

Miroslaw S. Gruszkiewicz, and David J. Wesolowski ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00033 • Publication Date (Web): 08 Jun 2017 Downloaded from http://pubs.acs.org on June 17, 2017

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ACS Earth and Space Chemistry

Partitioning of HCl Between Concentrated Brines and Supercritical CO2 Under Geologic Reservoir Conditions Miroslaw S. Gruszkiewicz1 and David J. Wesolowski Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 378316110, United States

ABSTRACT: HCl is generated in subsurface brines exposed to high CO2 pressures via reactions such as CO2 + NaCl + H2O ↔ NaHCO3 + HCl. The extent of partitioning of HCl between a concentrated chloride brine and supercritical CO2 (scCO2) has not been measured before and it cannot be estimated with the accuracy sufficient for predictive modeling of subsurface processes. The partitioning of HCl between 4.92 mol/kg aqueous solutions of NaCl and scCO2 was measured at 100 ° C and 150 °C and at pressures between 9 MPa and 16.2 MPa. At P = 15 MPa the concentrations of HCl in the scCO2 phase were 7·10-4 mol/L and 4·10-3 mol/L at 100 °C and 150 °C, an increase by more than 5 and 3 orders of magnitude, respectively, relative to the Notice of Copyright This manuscript has been authored by UT-Battelle, LLC under Contract No. DEAC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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concentrations in water vapor in the absence of CO2. These levels of HCl may accelerate reactions with reservoir rocks, natural seals (shale caprocks) and downhole materials, potentially enhancing or impairing operation and long-term performance of subsurface technologies such as geologic carbon sequestration/storage (GCS), enhanced geothermal systems (EGS) using compressed CO2, and scCO2 extraction of hydrocarbons and other resources in the presence of chloride brines. KEYWORDS: concentrated brines, supercritical CO2, HCl volatility, HCl partitioning, HCl transport, geologic carbon storage, reactive flow.

1. INTRODUCTION Geologic sequestration/storage of carbon in deep sedimentary formations involves injecting compressed carbon dioxide, originating as the gaseous product of industrial processes, at depths of 800 to 3000 m. The best-suited reservoirs are large sedimentary basins, exhausted coal beds, depleted oil and gas reservoirs, and widely distributed deep saline aquifers.1 The expected outcome is long-term storage as a result of a range of trapping mechanisms, including dissolution of the CO2 in the brine and precipitation of carbonate minerals. In order to trap the buoyant supercritical CO2 plume in the subsurface long enough for these more permanent sequestration processes to proceed (hundreds or thousands of years), the technology depends on impermeable rock layers (usually microporous shale caprock) overlying the porous and fractured reservoir to keep the buoyant fluid from escaping long before slower dissolution and precipitation trapping mechanisms can be effective. Since the density of scCO2 is only about 50 to 70 % of that of the aqueous brine at the typical saline reservoir conditions (50 - 100 °C, 150-300 bar), the injected CO2 tends to flow upwards through the aquifer pores and fractures, in continuous contact with the brine, as it displaces brine and also interacts with surface films, until 2 ACS Paragon Plus Environment

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it stops at the caprock. As illustrated in Figure 1, in the stable state after injection, but before large-scale mineralization takes place, the injected CO2 will form a column of fluid saturating the fractures and the pores of the sandstone. This column is held between underlying/adjacent brine-

Figure 1. A column of supercritical CO2 held under a shale caprock.

saturated regions of sandstone and the caprock. The height h of this column depends on the amount of CO2 injected and the density difference between scCO2 and brine, pore geometries, wetting angles and the interfacial tension.2 Thus, the dense scCO2 and concentrated brine will remain in contact over a large interface area, including the borders of by-passed brine-saturated sandstone patches within the scCO2 column. As CO2 behaves as an acid in contact with water, the immediate effect is lowering the pH of the brine from near neutral to below 4.1 In brines containing chloride salts of alkali and alkaline earth metals (primarily Na, K, Mg, Ca), this will effectively lead to the formation of HCl, a potentially volatile acid which is thermodynamically-



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driven to partition between the aqueous and the CO2-rich phase, as illustrated for the NaCl-H2OCO2 system by the following reactions: CO2,sc ↔ CO2,aq

(1)

CO2,aq + H2O + NaClaq ↔ HClaq + NaHCO3,aq

(2)

HClaq ↔ HClscCO2

(3)

In the aqueous phase, HCl, NaCl and NaHCO3 will be largely dissociated, with the activity coefficients and the extent of ion pairing (e.g. the fraction of neutral HCl0 or NaCl0 species) determined mainly by temperature, total salinity and the chemical composition of the brine. Nevertheless, there will be a finite concentration of the neutral HCl0 ion pair (as well as NaCl0, NaHCO30, etc.) in the brine phase, and our previous studies demonstrate that it is neutral species like these that partition into coexisting water vapor, with HCl0 being one of the most volatile species in geothermal brines.3-6 The equilibrium constants for reactions (1) and (2) can be evaluated reasonably well from existing thermodynamic databases, equations of state and activity coefficient/ion pairing models for the components of this simplified system at geologic reservoir conditions. However, the equilibrium constant for partitioning of HCl between the aqueous phase and the coexisting scCO2 phase according to reaction (3) has not been measured before. Furthermore, the molality of Cl- in mixed chloride brines containing, e.g., Li +, Mg2+ and Ca2+ is not limited by the solubility of NaCl (about 6 mol/kg H2O at room temperature). It can reach much higher values, in particular at elevated temperatures, thus driving the corresponding reactions analogous to reaction 2 even further to the right and leading to further lowering of the pH. If HCl is continuously removed from the brine via partitioning into and migration with the


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scCO2 plume, this sequence of reactions can lead to a sustained production of considerable amounts of highly reactive HCl. Chialvo et al.7 discussed the impact of acid gases such as SO2, H2S, and NOx present in captured industrial-origin CO2 on water-rock geochemical reactions. While it is recognized that modest contents of these volatile substances in injected CO2 have far-reaching consequences that can impact every aspect of subsurface processes in aqueous environments (changes in pH, generation of new dissolved species, modifications of solubility, precipitation, porosity and fluid flow), there is currently not enough property data to adequately predict bulk mixed fluid behavior, before tackling more complex issues of pore confinement and rock-fluid interactions. HCl in the scCO2 is potentially an even more reactive solute, which needs to be taken into account in modeling and planning reservoir operation and predicting the long term integrity of GSC reservoirs. The equilibrium 2 is strongly shifted to the left as indicated by the standard Gibbs free energy change (assuming gaseous CO2 and HCl) ∆GƟ = +81 kJ/mol, a value corresponding to the standard equilibrium constant in the order of 10-14 at 298.15 K. Even though the reaction is thermodynamically unfavorable, it can still produce significant amounts of HCl, if the pressure of CO2 is elevated and the products are continuously removed from the solution, leading in effect to an irreversible process never reaching equilibrium. For example, Forster8,

9

investigated

replacing the conventional ammonia Solvay soda process with a modified process producing HCl instead of CaCl2 according to the reaction: CO2 + 2NaCl(aq) + H2O ↔ Na2CO3(aq) + 2HCl

(4)

Despite the fact that reaction 4, conducted at more alkaline pH than reaction 2, is even more strongly shifted to the left, (the standard Gibbs free energy change of reaction 4 calculated as 5 ACS Paragon Plus Environment


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above is ∆GƟ =+176 kJ/mol, giving a standard equilibrium constant of 10-31), it is possible to design a multistep process separating the products, which is equivalent to reaction 4 and leads to practically complete conversion to HCl. The same processes involving HCl are of concern to the operation (at temperatures that may significantly exceed 150 °C) of enhanced geothermal systems using CO2 as the working fluid instead of water10, 11 (CO2-EGS) and other proposed processes using hot scCO2 such as enhanced oil recovery (EOR), supercritical fluid extraction, desalination of seawater and rare earth element recovery.12 By analogy with the issue of HCl content in the steam produced by geothermal reservoirs such as The Geysers,3,

4

complete speciation of the brine needs to be taken into

account to predict HCl concentration in the vapor phase. It is known that even salts considered strong electrolytes at lower temperature, such as CaCl2 can produce HCl at high temperature as a result of Ca2+ hydrolysis augmented by the volatility of HCl and low solubility of Ca(OH)2.13 Even though, at temperatures not far above 100 °C, HCl was found in the steam during brine boiling and evaporation experimentally and was confirmed computationally,14, 15 the possibility and the implications of HCl generation in reactions 2 and 3 after injection of carbon dioxide into deep saline reservoirs have not been examined experimentally or theoretically. Likewise, the research of corrosion of steel and cements in the chemical environment of geologic carbon sequestration16, 17 has not been focused on detection of small amounts of HCl which might have been present in laboratory experiments, and which, in the subsurface context, would potentially accumulate in the CO2-rich phase and separate from the aqueous phase by migrating towards the caprock or reactive engineered materials. Accurate quantitative predictions of these processes require geochemical modeling of reactive flow through porous rock, based on experimental constants of homogeneous and heterogeneous


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equilibria in subsurface aquifers that are the primary current targets for large scale supercritical CO2 injection. We do not currently have a thermochemical database sufficient for accurate modeling of reactions analogous to 1, 2 and 3 in mixed hypersaline brines, and heretofore, we had no quantitative data on reaction 3. The most important pieces of information are the equilibrium constant for reaction 3, and, later on, the rate of release of HCl from the brine to the scCO2 phase in a porous medium like sandstone. Here, we report initial experimental data on the partitioning equilibrium 3 for 4.92 molal NaCl at 100 °C and 150 °C and at pressures between 9.0 MPa and 16.2 MPa. 2. EXPERIMENTAL SECTION 2.1 Apparatus and experimental procedure. The volatility apparatus, designed for determination of the partitioning ratios of aqueous solutes between the liquid and vapor phases to 350 °C has been described earlier.3,

18

The principle of operation of the apparatus, shown in

Figure 2a, is based on static equilibration of liquid and vapor phases in a large (~ 700 mL) platinum liner enclosed in a pressure vessel (Figure 2b) with subsequent sampling of both phases. As the vapor phase usually contains only a very small concentration of a “nonvolatile” solute which is almost completely dissociated in the liquid phase, the volume of the vapor sample needs to be relatively large, and, as a consequence, sampling the vapor phase causes a gradual decrease of the pressure. Therefore, vapor samples were withdrawn by a positive displacement pump into a polypropylene syringe containing ~2 to 3 mL of deionized water at a very slow rate (0.2 mL/h to 0.5 mL/h), so that the system remained as close as possible to equilibrium. The time to collect each vapor sample was on average ~10 hours. The total volumes of the CO2-rich phase collected in this syringe at the liner pressure were 6 mL to 7 mL at ambient temperature (20 °C) corresponding to several times larger volumes inside the Pt-lined



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Figure 2. a) Schematic of the ORNL volatility apparatus; b) Ptlined pressure vessel assembly. equilibrium vessel at the equilibration temperature (100 °C or 150 °C). Continuous flow in the capillary sampling Pt tubing (~0.4 mm ID) prevented refluxing of the condensed sample. The apparatus was left to equilibrate for at least 4 days after each change of the temperature and at least 48 hours after withdrawing each sample. In the original application of the apparatus, without any noncondensable gas present, the samples, containing only water from condensed water vapor at ambient temperature, could be immediately depressurized and analyzed. Since in this work the samples contained CO2 at elevated supercritical pressures, an additional step was required consisting of slow release of the gas from the primary sample syringe via an additional polypropylene syringe filled with ~ 4 mL deionized water. The final sample consisted of about 7 mL of water including both the nearlypure water transferred from the primary high-pressure sample syringe and the deionized water


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from the secondary syringe used for scrubbing the remaining HCl which could be present in the released CO2. 2.2 Materials. Water was distilled and deionized to the specific resistivity of 18.2 MΩ·cm as indicated by the built-in conductivity cell of the NANOpure deionization system (Barnstead). Samples of the deionized water kept in polypropylene syringes showed Na and Cl contents below the detection limits by ion chromatography (> 0.01 µg/mL). The sodium chloride salt, used to make (by mass) the concentrated solution of final molality m0 = 4.92 mol/kg placed inside the Pt liner, was Fisher Scientific Certified ACS Crystalline (lot 155659), 99.9% assay, dried overnight under vacuum at 140 °C. The carbon dioxide was Air Liquide Alphagaz 1 grade, 99.99% purity. 2.3 Temperature and pressure measurements. Temperature was measured by a calibrated Pt-resistance thermometer attached outside the pressure vessel and by a K type 0.8 mm OD thermocouple (Ari Industries Inc., certified and tested by the manufacturer) placed in a fitting well surrounded by the brine inside the Pt liner. These probes agreed to 0.1 K at thermal equilibrium. Pressure was measured to 0.01 MPa by 3 strain gauges (Precise Sensors, Inc.) with digital readouts, calibrated to mutual agreement and indicating the pressures inside/outside the Pt liner and inside the water-filled buffer vessel during initial pressurization of the sample syringe. 2.4 Sample analysis. All samples were analyzed for Cl and Na content by ion chromatography (Dionex ICS-5000+ Reagent-Free High-Pressure IC with a conductivity detector). Our previous studies showed that HCl partitions much more strongly into coexisting water vapor than does NaCl. However, in addition to a likely non-zero equilibrium partitioning of NaCl into scCO2 equilibrated with the 5 m NaCl solution, there is also the possibility of aerosol entrainment of liquid droplets during sampling, which might represent a major source of contamination.



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Therefore, we analyzed both the Na and Cl contents of the recovered samples, and assumed that the concentration of volatilized HCl in the scCO2 phase would be equal to [Cl] – [Na] in the CO2 phase, regardless of the source of NaCl in that phase. The routine instrumental limit for reliable quantitative analysis was 0.05 µg/mL. Calibration with standard NaCl solutions over the chloride concentration range between 0.05 µg/mL and 300 µg/mL showed that sufficient accuracy for this work (better than 20 %) could be achieved to at least 0.02 µg/mL.

Accurate nonlinear

instrumental calibration curves were used to account for the slight departure from linearity at the dilute end of the range which covered most of the sample concentrations. 3. RESULTS After drawing several initial samples, which were rejected, the measured concentration of sodium in all remaining samples of the CO2-rich phase samples equilibrated with 4.92 mol/kg solution was indistinguishable from blank samples of deionized water. Since the volatility of sodium at neutral and lower pH at 150 °C is negligible, this indicates that there was no mechanical carry-over of the concentrated brine to the samples of the lighter CO2-rich phase during the experiment, and therefore all chloride was equivalent to HCl.

If such transfer

occurred, as a result of, e.g., boiling or rapid gas desorption causing splashing of the liquid phase inside the equilibrium vessel, caused by too fast rate of sample withdrawal or pressure change, it could strongly contaminate the samples and thus render the results useless. Preventing vapor sample contamination was possible due to relatively large size of the equilibrium vessel with the distance between the surface of the brine and the inlet of the sampling capillary in excess of 10 cm and slow operation close to equilibrium at all times.


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Figure 3 shows the results in terms of concentration (mmol/L) of chloride on the logarithmic

a)

b)

Figure 3. Measured concentrations of Cl- in the CO2-rich phase equilibrated with a 4.92 mol/kg solution of NaCl (red symbols), and corresponding linear correlations 5 (red curves) at 100 °C (a) and 150 °C (b). The blue curves show the corresponding measured pressure (right vertical axes). 11 ACS Paragon Plus Environment


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scale as a function of the density of CO2. The densities were calculated from the equation of pure CO2 built into the REFPROP database,19 without corrections for dissolved water in the scCO2 phase. At the conditions studied, the vapor pressure of water was a small fraction of the total pressure and therefore PCO2 determined as the difference between the total pressure and the vapor pressure of pure water at the same temperature is a sufficiently good approximation of the partial pressure of CO2. The relationship between the density of scCO2 and the measured pressure is also plotted in Figure 3 as continuous blue curves. Also included in Figure 3, as data points at zero CO2 pressure, are the concentrations of chloride in water vapor (without CO2) over the same 4.92 mol/kg solution of NaCl at neutral pH, as predicted by sodium chloride partitioning constant correlation determined earlier:5 log(Kp m0) =

v (l ± )

=

v . v (l ± )

= −33.31 + 33829 / T + 7.152 log(ϱl) + 20.728 log(ϱv),

(4)

where T is the temperature (K), subscripts l and v denote liquid and vapor phases, γ± is the mean activity coefficient of ionized sodium chloride, m are the molalities (mol/kg), c is the concentration (mol/L), m0 is the unit molality, and ϱ are the densities of water (kg/m3). All experimental results are listed in Table 1 as concentrations in the scCO2 phase with average pressures and corresponding densities for each sample. The concentrations cv calculated from eq 4 are listed at P = 0.


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Table1. Concentrations of HCl in the scCO2 phase average pressure

density of CO2

(bar)

(kg/L)

Log[c/(mmol/L)]

T = 100 °C 0.0000

-8.63a

126.16

0.2598

-3.31

131.89

0.2767

-3.54

137.38

0.2933

-4.00

142.69

0.3096

-3.44

148.21

0.3268

-3.81

153.57

0.3435

-3.10

158.84

0.3601

-3.42

163.82

0.3756

-3.03

0.000

T = 150 °C 0.0000

-6.01a

87.46

0.1249

-3.50

96.63

0.1399

-3.11

140.16

0.2159

-2.71

148.96

0.2320

-2.84

151.52

0.2367

-2.32

158.05

0.2488

-2.85

0.000

a

Values calculated from eq 4.



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As the measured concentrations of chloride in the supercritical CO2 phase, including the values at PCO2 = 0, calculated as c0v from eq 4, appear to be approximately proportional to the density of CO2, the results support a linear correlation: log(cv) = log[c0v(T)] + S(T) ϱCO2

(5)

where log[c0v / (mol/L)] and the slope S / (kg/L), determined from the data in Figure 3, are equal to (-8.63, 15.7) and (-6.01, 14.6) at 100 °C and 150 °C, respectively. It was assumed that eq 4 has much less uncertainty then the current data, and accordingly the fits of eq 5 were constrained to pass through the c0v data points at zero CO2 pressure (density). The aggregate experimental uncertainties of the chloride concentration in the scCO2 phase were assigned the values of either ±1 or ±0.5 log unit based on the scatter of both chloride and sodium analyses for each experimental series. The sources of random error were not identified, but they likely originated from contamination during both sampling and chromatography analysis. The uncertainties assigned to each data point are marked in Figure 3. The difference between the slopes S in eq 5 at 100 °C and 150 °C is insignificant, and it can be assumed that S remains constant within the temperature range investigated.

It should be

remembered, however, that the linear correlation in eq 5 is not valid at low densities (pressures) of CO2, where no measurements were made. In fact, it is expected that at low pressures the decrease of pH, which is not linear as a function of CO2 density or pressure, would be the main factor driving the increase of HCl concentration in the gaseous CO2 phase.

As the density of

CO2 increases to the supercritical fluid range where the measurements were made, the pH will approach a plateau, determined by the dissociation constant of the carbonic acid, but additional factors are expected to become important, such as the effect of the large concentration of CO2(aq) on HCl association in the brine, and the effect of nonideal interactions between the HCl 14 ACS Paragon Plus Environment

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solute and the scCO2 + H20 fluid solvent. There is currently no basis for accurate prediction of HCl partitioning as a function of pressure between very low pressures and the pressures investigated in this work. Eqs 4 and 5 reflect only the overall effect of the scCO2 at these elevated pressures on the volatility of HCl relative to the aqueous system without CO2, where the chloride present in the vapor phase originates only from the (very low) volatility of NaCl. The linear equation allows for estimation of the concentration of HCl in the CO2-rich phase over concentrated NaCl brine as a function of brine molality and partial pressure of CO2 in the vicinity of the conditions investigated (m = 4.92 mol/kg, PCO2 from 9 MPa to 16 MPa, and T from 100 °C to 150 °C). 4. DISCUSSION The experimental results obtained in this work provide information on the expected nonideal interactions of HCl in the scCO2 + H2O phase, which can affect the partitioning of HCl to this phase over the density range between 0.1 kg/L and 0.4 kg/L. These interactions may be negligible at low (gas-like) densities of CO2. It would be possible to calculate the partitioning of HCl between concentrated brine and CO2 using existing models based on Pitzer coefficient databases for the aqueous phase, but accurate results could be only expected at low pressure, when the CO2-rich phase can be treated as a gas mixture without significant deviation from ideal mixing. In this case the model would yield the fugacity of HCl based only on the model of the aqueous phase, while the fugacity of HCl in the CO2 phase would be of course equal to its very low (

Partitioning of HCl Between Concentrated Brines ... - ACS Publications

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