An integrated study of fluid–rock interaction in a CO2-based enhanced geothermal system: A case study of Songliao Basin, China

Applied Geochemistry 59 (2015) 166–177

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An integrated study of fluid–rock interaction in a CO2-based enhanced geothermal system: A case study of Songliao Basin, China Jin Na, Tianfu Xu, Yilong Yuan, Bo Feng ⇑, Hailong Tian, Xinhua Bao Key Lab of Groundwater Resources and Environment, Ministry of Education/College of Environment and Resources, Jilin University, Changchun 130021, China

a r t i c l e

i n f o

Article history: Available online 4 May 2015 Editorial handling by M. Kersten

a b s t r a c t The reactive behavior of a mixture of supercritical CO2 and brine under physical–chemical conditions relevant to the CO2-based Enhanced Geothermal System (CO2-EGS) is largely unknown. Thus, laboratory experiments and numerical simulations were employed in this study to investigate the fluid–rock interaction occurring in the CO2-EGS. Rock samples and thermal–physical conditions specific to the Yingcheng Formation of Songliao Basin, China, an EGS research site, were used. Experiments were conducted by using of reactors at high temperature and pressure. Six batch reaction experiments injected with supercritical CO2 were designed at temperatures of 150–170 °C and a pressure of 35 MPa. Moreover, a separate experiment at the same experimental conditions without injection of CO2 was also conducted for comparison. Analyses of scanning electron microscopy (SEM) and X-ray diffraction (XRD) of the resulting solids were conducted to characterize changes in mineral phases. Numerical simulations were also performed under the same conditions as those used in the experiments. Significant mineral alterations were detected at the CO2-EGS reservoir, which may change the properties of fluid flow. The presence of supercritical CO2 led to an dissolution of primary minerals such as calcite and K-feldspar and precipitations of secondary carbonate such as calcite and ankerite. The numerical simulations were generally consistent with laboratory experiments, which provide a tool for scaling the time up for long period of reservoir simulations. The information currently available for the mineral alteration at high-temperature natural CO2 reservoirs is generally consistent with those of our lab experiments and numerical simulations. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The concept of Enhanced Geothermal System (EGS) involves creating a reservoir by hydro-fracturing of a hot dry rock, and then mining heat from the artificial reservoir. This method entails pumping cold fluid into the targeted reservoir through the injection well and bringing hot water from the production well to utilize for generating electricity. A Massachusetts Institute of Technology (MIT) report indicated that EGS could become a major supplier of primary energy for U.S. base-load generation capacity by 2050 (MIT, 2006). Many scientific and industrial communities have been working on the development of EGS projects over the past 20 years. Almost all attempts to develop EGS regarded using water as the heat transmitting fluid. Here, a EGS concept is considered, in which CO2 is used instead of water as heat transmission fluid to achieve geologic sequestration of CO2 as an ancillary benefit (Brown, 2000; Pruess, 2006). Numerical simulations of fluid dynamics and heat transfer indicate that CO2 is superior to water for recovering heat from hot ⇑ Corresponding author. Tel./fax: +86 431 88502773. E-mail address: [email protected] (B. Feng). http://dx.doi.org/10.1016/j.apgeochem.2015.04.018 0883-2927/Ó 2015 Elsevier Ltd. All rights reserved.

fractured rock at least in some cases (Pruess, 2006, 2008). Due to less irreversible losses compared with flash cycles for water-EGS, CO2-EGS can produce more power than water-EGS for reservoirs with low recoverable thermal energies (Zhang et al., 2013). CO2 is also advantageous for wellbore hydraulics because it has lower viscosity and larger expansivity than water. Therefore, CO2-EGS would increase buoyancy forces and reduce the power required for the fluid circulation system. Due to the strong buoyancy leading to high self-driven flow rates, Atrens et al. (2011) designed CO2 thermo siphons that could operate without a pump. Even though the prospective indications of the thermal and hydraulic aspects of a CO2-EGS system, however, uncertainties remain in terms of chemicalinteractions between fluids and rocks, particularly during the transition from resident water to supercritical CO2. Although several experimental and reactive transport modeling studies have been conducted (Liu et al., 2003; Ueda et al., 2005; Kaieda et al., 2009; Petro et al., 2012; Xu et al., 2014), experimental investigation and validation of the thermodynamic and kinetic data for mineral–water reactions are limited, particularly for precipitation mechanisms and rate laws. Literature on natural analogues including wall rock alteration in gas-rich geothermal systems is limited (Watson et al., 2004; Giolito et al., 2007; Xu et al., 2014). Apps

J. Na et al. / Applied Geochemistry 59 (2015) 166–177

and Pruess (2011) proposed chemical interactions of CO2-EGS, which also have natural analogues in mesothermal gold deposits. As shown in Fig. 1, a CO2-EGS system consists of three distinct zones: (1) a central zone, or ‘‘core’’ (Zone 1), in which the reservoir fluid is in a single supercritical CO2 phase, caused by a complete dissolution of the aqueous phase into the supercritical CO2; (2) a transition zone, in which the reservoir fluid is composed of a CO2–water two-phase mixture (Zone 2); and (3) an outer or peripheral zone (Zone 3), in which the reservoir fluid is in a single aqueous phase with dissolved CO2. Rock–fluid interactions in Zones 2 and 3 in this system would be mediated by the aqueous phase. Some information on the relevant processes is available from experiments (e.g., Liu et al., 2003; Ueda et al., 2005; Kaieda et al., 2009; Petro et al., 2012), numerical simulations (Xu et al., 2014), and studies on natural CO2-bearing geothermal systems (e.g., Giolito et al., 2007). Liu et al. (2003) conducted lab experiments at different temperatures from 100 °C to 350 °C in the presence or absence of excess CO2. The results suggested that the addition of excess CO2 facilitates the dissolution of granite and sandstone and the deposition of secondary minerals, particularly above 250 °C. A similar conclusion was also drawn by Ueda et al. (2005). Their results revealed an enhanced release of Ca2+ from minerals, such as the An component in plagioclase, and precipitation of secondary carbonate minerals because of the addition of CO2. Fu et al. (2009) conducted reactor experiments to assess the alkali–feldspar dissolution and formation of boehmite and kaolinite in an initially acidic fluid (pH = 3.1) at 200 °C and 30 MPa. Lu et al. (2013) conducted experiments on alkali feldspars/Amelia albite–CO2–brine interactions at 150–200 °C and 30 MPa. Their analysis of reaction products showed dissolution features on feldspars and precipitation of secondary minerals such as boehmite, kaolinite, muscovite and paragonite on feldspar surfaces. Suto et al. (2007) performed granite alteration with CO2-saturated fluid over a temperature range of 100–350 °C at up to 25 MPa, which showed the dissolution of granite and the deposition of a secondary Na, Ca-aluminosilicate. Munz et al. (2012) combined an approach of flow-through column and batch experiments to investigate the mechanisms and rates of plagioclase carbonation reactions from 100 to 250 °C and 2 to 12 MPa. Ré et al. (2014) conducted hydrothermal experiments to evaluate the geochemical reactions of fractured granite and granite–epidote in contact with thermal water, with and without supercritical CO2, at 250 °C and 25–45 MPa. Their results revealed the precipitation

Fig. 1. Schematic of the three zones created by injection of CO2 into hot fractured rock (after Fouillac et al., 2004; Ueda et al., 2005).

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of illite and smectite, whereas calcite formed during cooling and degassing in the granite–epidote experiment with CO2. Field experiments were conducted at the Ogachi Hot Dry Rock geothermal site, Japan, in which water with dissolved CO2 was injected into a high-temperature borehole (OGC-2, 210 °C) to study CO2 sequestration in solid minerals (Kaieda et al., 2009). During the experiments, mineral changes were observed. Xu et al. (2014) performed reactive transport modeling to study the impact of fluid– rock interactions in CO2–geothermal systems. Their study was not specific to a particular site; geological and geothermal conditions and parameters were taken from various research sites. The aqueous presence in CO2-based geothermal systems causes a combination of mineral dissolution and precipitation effects that could impact reservoir growth and longevity. The long-term behavior of the outer zones is crucial for sustaining energy recovery, estimating CO2 loss rates, and determining trade-offs between power generation and geologic storage of CO2. However, the impacts of geochemical on the reservoir have been rarely studied in the outer zones. In this study, we focus on the investigations of reactivity in rock and aqueous solutions by using the reservoir mineral property of the Yingcheng Formation of the central depression in the Songliao Basin, Northeastern China, which is a Chinese EGS research site. Laboratory batch reaction experiments under in situ reservoir conditions of 160 °C, 35 MPa were used to analyze aqueous chemical evolution and mineral alteration induced by mixtures of CO2 and water. The variations in formation temperature were also considered with various temperatures from 150 to 170 °C. Moreover, modeling analyses were conducted on the basis of the measured data obtained by experiments using the multiphase non-isothermal reactive transport simulator TOUGHREACT (Xu et al., 2006). The objectives of the present work are to understand the chemical behaviors of a CO2-based EGS system for actual reservoir conditions to calibrate the thermodynamic and kinetic data of mineral–water reactions by using the batch experimental data, and to validate the numerical model.

2. Geological setting of the EGS research site The Songliao Basin, located in Northeastern China, is a large Mesozoic–Cenozoic sedimentary basin developed by the initial rifting of the Eurasian Continent during the late Jurassic (Fig. 2). Deep volcanic rocks are widely distributed in this region. The central depression of Songliao Basin is the location of Chinese EGS research project; its average geothermal gradient is approximately 44.5 °C/km. The potential EGS geothermal reservoir is in rhyolite of the Yingcheng Formation in the central depression. The burial depth of the reservoir is approximately 3600 m, and the formation pressure and temperature corresponding to the depth are 35 MPa and 160 °C, which meet the requirements for geothermal development (Huang et al., 2004; Hu et al., 1998). The structure of the rhyolite is porphyritic with crystals of quartz and feldspar, and the matrix consists of cryptocrystalline felsic material with spherulitic texture from devitrification. The alteration features of carbonation are obvious due to the replacement of calcite to phenocrysts and matrix in the deep structure. Clayization is less evident in the rhyolite that is favorable for operation of CO2-EGS. Because clay particles are released and accumulate along the fluid flow path in the reservoir, permeability could be reduced after CO2 injection (Yu et al., 2012). The naturally existing fracture is favorable for generating artificial fractures for the EGS reservoir. According to the geologic data from well Yingshen-2 of the central depression, the mineralogy of deep rhyolite of the Yingcheng formation is mainly composed of quartz, alkali feldspar, plagioclase, and calcite

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J. Na et al. / Applied Geochemistry 59 (2015) 166–177 Table 2 Chemical composition of brine in the reservoir. Components +

K Mg2+ Na+ SO2 4 pH

Concentration (g/L) 1

1.68  10 9.72  103 3.36 4.85  101 8.2

Components

Concentration (g/L)

HCO 3 2+

1.71 3.81  102 4.25 2.23  103

Ca Cl TFe

Table 3 Mineralogical composition in rock outcrop by X-ray diffraction (XRD). Minerals

Mass fraction (%)

Quartz Alkali feldspar Plagioclase

70 20 10

and rhyolite chips obtained from one rock sample were used each time to minimize the effect of heterogeneity on the experiment results. Although calcite is present in rhyolite from the Yingcheng Formation at the 3600 m depth (Table 1), it could not be observed in the rhyolite chips. To accelerate the reactivity, rhyolite and calcite were crushed into grains of 74–100 lm. On the basis of the fluid chemistry of the reservoir, the reaction solution used in experiment was prepared by dissolving CaSO4, MgSO4, Na2SO4, NaHCO3, FeSO4, KCl, and NaCl into distilled water to approximate the in situ composition of Yingcheng Formation water (Table 2). Fig. 2. Location of the central depression of the Songliao Basin, Northeastern China (Xu et al., 2014).

(Table 1). The formation water is NaCl-type, the main aqueous solution compositions are shown in Table 2. The data absent of Si and Al were taken from oilfield water analysis conducted by Daqing petroleum company.

3. Materials and methodologies 3.1. Samples The sample rhyolite chips were obtained from rock samples of massive rhyolite beds from the Yingcheng Formation at the southeastern margin of the Songliao Basin. The petrographic characteristics of the outcrop samples are generally similar to those in the core in situ except for the alteration features of carbonation (Wang et al., 2014). The sample rhyolite chips consist mainly of quartz, alkali feldspar, and albite-dominant plagioclase. Mineralogical and chemical compositions of the samples were determined by X-ray diffraction (XRD; Table 3) and X-ray fluorescence (XRF; Table 4) analyses. Prior to the experiments, rare clay minerals and carbonate minerals were detected in the sample rhyolite chips by scanning electron microscopy (SEM; Fig. 3). The samples used in the experiments were a mixture of outcrop rhyolite samples (90% in mass) and naturally pure calcite (10%) obtained from the crude ore of Iceland spar. In our experiments, fresh calcite

Table 1 Mineralogical composition in cores by X-ray diffraction. Minerals

Mass fraction (%)

Calcite Quartz Alkali feldspar Plagioclase

10 70 12 8

3.2. Experiments 3.2.1. Experimental set-up and procedures The experiments were conducted in a high-temperature and high-pressure reactor with a volume of 0.5 L, which allowed for external control and monitoring of pressure and temperature (Fig. 4). The maximum operating temperature and pressure were 200 °C and 40 MPa, respectively. The crushed rocks were rinsed with distilled water and were dried in an oven at 80 °C for 8 h prior to the experiment. A mixture of 50 g of rock and a 250 mL aqueous solution with a water/rock mass ratio of 5:1 was poured into the reactor. During the experiments, the pressure reached 35 MPa by adjusting the initial CO2 pressure after heating the batch reactor to 150–170 °C. In addition, a separate brine–rock experiment was conducted for 12 days without the injection of CO2. The system was closed throughout the experiment; no sampling was conducted until the end of experiment. A total of seven batch reactions were performed under various conditions. A summary of the experimental conditions is given in Table 5. The enhancement of dissolution was guaranteed in all experiments by turbulent stirring at a speed of 250 rpm. After the reaction, the reactor was cooled down to ambient temperature over 1 h or 2 h. Gradual depressurization was then allowed by opening the exhaust valve attached to the reactor. The autoclave was then opened, liquid samples and the rock materials were collected. 3.2.2. Analytical methods Fluid samples taken from the reactors were analyzed for pH and HCO 3 content by using a PHS-3c pH meter and by titration immediately at room temperature and at atmospheric pressure. The liquid samples for metal analysis were filtered through a 0.45 lm filter and were then acidified to a pH of 2 with pure HNO3 to prevent the precipitation of metals. The contents of dissolved Si, Ca2+, Mg2+, Na+, and K+ were determined by using inductively coupled plasma atomic emission spectroscopy (ICP–ES); that of dissolved

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J. Na et al. / Applied Geochemistry 59 (2015) 166–177 Table 4 Chemical composition by X-ray fluorescence.

x (B) (%) SiO2

Al2O3

Fe2O3

FeO

MnO

P2O5

TiO2

CaO

MgO

K2O

Na2O

LOI

Sum

78.55

11.01

0.65

0.005

0.001

0.012

0.11

0.19

0.023

5.57

2.87

0.56

99.56

A

B

C

D

Fig. 3. Scanning electron microscopy photographs of rock sample surface prior to the experiments: (A) quartz in rhyolite, (B) K-feldspar in rhyolite, (C) albite in rhyolite, (D) natural calcite.

Table 5 Experimental condition settings.

Fig. 4. The experimental system. 1: Mixer speed control cables, 2: temperature sensor cable, 3: pressure sensor cable, 4: permanent magnet rotating mixer, 5: exhaust valve, 6: sample basket, 7: liquid sampling valve, 8: inlet line, 9: intermediate container, 10: constant speed and pressure pump, 11: water, 12: CO2 cylinders, 13: multi-function console.

Experiment

Temperature (°C)

Total CO2 pressure (MPa)

Duration (days)

Experiment 1 Experiment 2 Experiment 3 Experiment 4 Experiment 5 Experiment 6 Experiment 7

160

35

3

160

35

6

160

35

9

160

35

12

150

35

12

170

35

12

160

CO2 free

12

4. Numerical modeling of the experiments Fe was determined by ICP–mass spectrometry (MS). The reacted solid samples after 12 days at 160 °C and 170 °C were retrieved and characterized through XRD and SEM analyses. The specific surface areas of the minerals were measured by using a physical and chemical adsorption analyzer.

With the same pressure and temperature used in the experiments, we performed numerical modeling analyses for specific selected minerals, and we compared the simulation results with the measured data.

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4.1. Approaches In the present modeling study, all simulations were performed with the TOUGHREACT code, a comprehensive, publicly available reactive transport simulator (Xu et al., 2006). Spatial discretization was implemented by employing an integral finite difference approach (Narasimhan and Witherspoon, 1976). Flow, transport, and geochemical reaction equations were solved separately by means of a sequential iteration approach similar to that used by Yeh and Tripathi (1991). A new fluid property module based on that reported by Spycher and Pruess (2010) was also used. This module provides an accurate description of the thermophysical properties of water and CO2 mixtures under conditions typically encountered in the CO2–water systems of interest, such as 12 °C 6 T 6 250 °C; 0.1 MPa < P 6 60 MPa. The program can accommodate any number of chemical species present in the liquid, gas, and solid phases. Temporal changes in reservoir rock porosity and permeability due to mineral dissolution and precipitation can modify fluid flow path characteristics. This feedback between flow and chemistry is considered in the code. The details of the simulator have been reported by Xu et al. (2006).

Table 6. Prior to the experiments, the solution was saturated with calcite, dolomite, siderite, and ankerite, but undersaturated with silicate minerals due to the minimum values of Si and Al in the simulation. The thermodynamic data for minerals, gases and aqueous species were obtained mostly from the EQ3/6V7.2b database of Wolery and Daveler (1992). For kinetically controlled mineral dissolution and precipitation, a general form of the rate law (Lasaga, 1984; Steefel and Lasaga, 1994) was used (Appendix A). The parameters used for the kinetic rate expressions were based on those given in previous studies (Table 7). The physical and chemical adsorption analysis results in a bulk specific surface area of the rock samples of 2 m2/g. The specific primary mineral surface area of the rhyolite was calculated according to

Ai ¼ A  W i

ð1Þ 2

where A is the mineral reactive-surface area (m /g), and Wi is the mineral volume fraction. The specific surface areas of the secondary minerals were obtained from previous works (Table 8). 5. Results and discussion

4.2. Model set-up

5.1. Fluid chemistry

Geochemical model runs were conducted for the rock sample reacting with CO2 and brine for 12 days at 160 °C and 35 MPa, which are the same conditions as those used in the experiments. The amount of minerals per kilogram of water was determined on the basis of a water/rock ratio of 5:1, and mineral abundances were expressed as percentages of total solids in the input of the simulator. The final mass proportions of single-phase CO2 and water at the start of the experiment were approximately 0.46 and 0.54, according to the equations used for calculating mass fractions of CO2 and water in CO2–H2O mixtures in the TOUGHREACT code. The initial major concentrations of aqueous components used in the simulation are given in Table 2. The concentrations of Si and Al used in the modeling were regarded as 1  108 mol/L (minimum value) because no dissolved silicate and aluminum salts were present in the distilled water used for preparation of the reaction solution. The primary minerals considered in the simulation were based on the rock samples in batch experiments (Table 6). Plagioclase was introduced in the model as a ratio of 9:1 mixture of albite and anorthite representative for An-component in plagioclase according to the major elements of the rhyolite composition shown in Table 4. Alkali feldspar was represented by K-feldspar because it is the main mineral in the feldspar group of the rhyolite according to geological data (Zhao, 2005). The Fe-bearing mineral was regarded as hematite in primary phases because Fe2O3 was identified in the chemical composition by XRF (Table 4), although the amount was too small to be detected by XRD. Dolomite, siderite, ankerite, dawsonite, smectite, and kaolinite are likely to form in the presence of CO2 and were specified as secondary minerals in the simulations (Table 6). The initial saturation states of minerals in the simulation prior to CO2 injection are also shown in

5.1.1. Reaction at 160 °C The dissolution of the injected CO2 increased the concentration + of H2CO3 (aq), HCO 3 and H (Fig. 5a), and decreased the pH value from 8.2 to 7.6 after 3 days. After 12 days, the pH increased to 7.78 as the increasing concentration of H+ interacted with the minerals. The HCO 3 content increased to 42.6 mmol/L after 12 days, which was caused by the dissolutions of native carbonate minerals and H2CO3. The concentration of Na+ in the solution did not show significant variations in the brine–CO2 experiments (Fig. 5a). The Ca2+ concentration increased at the beginning of the reaction, revealing the dissolution of calcite. Then, the Ca2+ content decreased, indicating the possible precipitation of calcium-bearing carbonate. The increase of Ca2+ content with time shows that the dissolution of native calcite, which is the main source of Ca2+. K+ concentration increased with time as the dissolution of K-feldspar (Fig. 5b). The increase of Si content is controlled by the dissolution of silicate minerals. Fe content increased slightly with time (Fig. 5c) due to the dissolution of iron-bearing minerals, as Fe2O3 was identified in the chemical composition by XRF. The Mg2+ concentration was lower than the initial value, revealing possible precipitation of magnesium-bearing minerals. As shown in Fig. 6, the concentrations of major ions dissolved into solution varied in CO2–brine and brine only. The concentrations of Ca2+ and Si in the CO2–brine experiment were significant higher than those in the CO2-free brine experiment. In solutions from reactions with CO2–brine, the concentrations of K and Fe were generally higher by 18% and 34%, respectively, and that of Mg was lower by 11%, compared with solutions from the sample reacted with CO2-free brine. The Na content in the different solutions did not show significant variation compared with the initial composition.

Table 6 Initial volume fractions and saturation indices (SI) of primary and possible secondary mineral in the simulation. Mineral

Volume fractions (%)

SI

Mineral

Volume fractions (%)

SI

Calcite Quartz K-feldspar Albite Anorthite Hematite Kaolinite

9.6 62.4 18.4 8.1 0.9 0.4 0

2.11 5.58 19.06 18.93 19.48 8.95 19.49

Ca-smectite Na-smectite Illite Dolomite Siderite Dawsonite Ankerite

0 0 0 0 0 0 0

26.90 26.95 25.59 3.10 3.34 6.38 5.41

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J. Na et al. / Applied Geochemistry 59 (2015) 166–177 Table 7 Parameters for kinetic rate constants of minerals. Mineral

Quartz Calcite K-feldspar Illite Kaolinite Albite Anorthite Hematite Siderite Dolomite Ankerite Dawsonite Na-smectite Ca-smectite

Kinetic rate constants

Source of kinetic rate data

Neutral mechanism

Acid mechanism

k25 (mol/m2/s)

k25 (mol/m2/s)

E (kJ/mol)

n(H+)

6.02  103 8.71  1011 1.05  1011 4.89  1012

14.4 51.7 23.6 65.9

1.0 0.5 0.34 0.777

12

6.31  10 3.02  1017 8.91  1018

94.1 58.9 17.9

0.823 0.4 0.472

3.16  1014 1.05  1011 6.45  104 6.46  104 6.46  104 6.46  104 1.05  1011 1.05  1011

23.6 23.6 36.1 36.1 36.1 36.1 23.6 23.6

1.41 0.34 0.5 0.5 0.5 0.5 0.34 0.34

3.02  1017 3.02  1017

58.9 58.9

0.4 0.4

14

1.02  10 1.05  108 9.05  1012 1.66  1013 6.92  1014 9.12  1013 7.58  1014 2.51  1015 1.26  109 2.95  108 1.26  109 1.26  109 1.66  1013 1.66  1013

E (kJ/mol) 87.6 23.5 51.7 35 22.2 69.7 17.80 66.20 62.76 52.2 62.76 62.76 35 35

Base mechanism k25 (mol/m2/s)

E (kJ/mol)

n(H+) Labus and Bujok (2011) Xu et al. (2014) Xu et al. (2014) Xu et al. (2014) Xu et al. (2014) Labus and Bujok (2011) Xu et al. (2014) Zhang et al. (2009) Xu et al. (2014) Xu et al. (2014) Xu et al. (2014) Xu et al. (2014) Xu et al. (2014) Xu et al. (2014)

Table 8 Specific surface area data. Mineral Calcite Quartz K-feldspar Albite Anorthite Hematite Kaolinite

A (m2/g) 2 1.38 0.40 0.177 0.02 0.0082 115.6  104

Source Calculation Calculation Calculation Calculation Calculation Calculation Ketzer et al. (2009)

Mineral Ca-smectite Na-smectite Illite Dolomite Siderite Dawsonite Ankerite

A (m2/g)

Source 4

106.0  10 106.0  104 106.0  104 9.8 9.8 9.8 9.8

Fig. 5. Brine chemistry as a function of reaction time at 160 °C for the rock samples reacted with CO2–brine.

Ketzer et al. (2009) Ketzer et al. (2009) Ketzer et al. (2009) Xu et al. (2014) Xu et al. (2014) Xu et al. (2014) Xu et al. (2014)

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5.2. Changes in solid phase

Fig. 6. Brine chemistry reacted with CO2–brine and CO2-free brine for 12 days.

5.1.2. Reaction at different temperatures Experimental results for various temperature cases show that higher temperatures resulted in smaller releases of Ca2+ from the dissolution of calcite (Fig. 7a). K+ and Na+ concentrations increased in the three samples with an increase in temperature, although dissolution of albite is an exothermic reaction (Fig. 7a and b). This result could be attributed to the effect of the evaporation of brine water into the CO2 plume, which concentrates salt in the brine under high temperatures (Borgia et al., 2012). Fig. 7c clearly demonstrates that the concentrations of Fe and Mg were lower at higher temperatures, which was accompanied by greater precipitation of Mg- and Fe-bearing minerals.

5.2.1. Reaction of solid phase at 160 °C Calcite surfaces showed significant differences in their reactions in CO2-free (Fig. 8a) and CO2–brine (Fig. 8b) systems. The development of corroded ridges on the originally smooth surface shows the dissolution of calcite after CO2 injection, whereas the smooth surface of calcite in the CO2–free system was similar to the rock samples before the reaction. The pyramid-shaped ridges are the products of anisotropic dissolution of the calcite. However, this dissolution morphology was temporary because the dissolution of the crystal face in the next layer would have begun after the complete dissolution of the corroded ridges. For K-feldspar, dissolution occurred in both the CO2-free and CO2–brine systems. The surface of K-feldspar in the CO2-free experiments exhibited dissolution textures (Fig. 8c). That in the CO2–brine experiment showed corrosion pits and grooves (Fig. 8d), indicating more dissolution than that in the CO2-free experiments, which is consistent with the increase in concentration of K (Fig. 6). The dissolution of calcite and K-feldspar can be written as

CaCO3 þ CO2 þ H2 O ! Ca2þ þ 2HCO3 2KAlSi3 O8 þ 3H2 O þ 2CO2 ! 2Kþ þAl2 Si2 O5 ðOHÞ4 þ 2HCO3 þ 4SiO2 The dissolution of quartz was observed through dissolution pits and textures on the surface (Fig. 8e and f) in both the CO2-free and CO2–brine systems. The dissolution occurred in the presence of cations, which weakened the Si–O bond by forming a complex surface and thus enhanced the dissolution rate of the quartz (Dove and Crerar, 1990).

Fig. 7. Brine chemistry as a function of reaction temperature for rock samples reacted with the CO2–brine system.

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Corroded

(a) Smooth surface of calcite in the brine only experiment

(b) Dissolution of calcite in the brine–CO 2 experiment

(c) Dissolution of K-feldspar in the brine only experiment

(d) Dissolution of K-feldspar in the brine–CO2 experiment

(e) Dissolution of quartz in the brine only experiment

(f) Dissolution of quartz in the brine–CO 2 experiment

K-feldspa

Albite

Dissolution terraces

(g) Albite surface in the brine only experiment

(h) Dissolution of albite in the brine–CO 2 experiment

(i) Dissolution terraces of albite in the brine–CO2 experiment

Calcite

(j) Secondary calcite in the brine–CO2 experiment

(k) Secondary ankerite in the brine–CO2 experiment

Fig. 8. Scanning electron microscopy (SEM) images of minerals after 12 days at 160 °C with brine only and CO2–brine.

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Few dissolution textures of albite were noted (Fig. 8g) in the absence of CO2. After the CO2–brine experiment, however, the albite surface showed both dissolution textures and pits (Fig. 8h). Moreover, dissolution terraces were apparent in the surface topography of the albite (Fig. 8i) in the CO2–brine experiment, indicating that dissolution was heterogeneous after CO2 injection and was controlled by the following reactions:

2NaAlSi3 O8 þ 3H2 O þ 2CO2 ! 2Naþ þAl2 Si2 O5 ðOHÞ4 þ 2HCO3 þ 4SiO2 + The increase of pH and HCO 3 due to the interaction of H and minerals is favorable to the formation of secondary carbonate minerals in the brine–CO2 experiment, whereas carbonate minerals did not precipitate in the brine–rock experiment. A small amount of secondary diamond and hexagonal calcite was present on the surface of rock samples with the primary enrichment of Ca2+ mainly because of the violent dissolution of calcite (Fig. 8j). This re-precipitation of calcite after its dissolution was also observed in batch experiments of Ketzer et al. (2009) and Alemu et al. (2011) at high pressures (11–15 MPa) and high temperatures (200–250 °C). Hence, the CO2 consumed by the dissolution of primary calcite is released when the calcite finally precipitates. Moreover, Ca2+ released from dissolution of silicate minerals could react with dissolved CO2 to be trapped permanently. The dissolution of the An component in plagioclase supplied Ca2+ for the formation of new calcite. However, because the experiment period was insufficient and the calcium in the chemical composition of rhyolite was rare prior to the reaction (Table 4), the amount of Ca2+ released was too small to affect the precipitation of calcite. A type of massive, approximately hexagonal mineral precipitated, which is likely to be ankerite composed of Ca, Mg, C, and Fe according to Energy Dispersive Spectroscopy (EDS) analysis (Fig. 8k). Carbonate precipitation of ankerite was also observed in batch experiments performed by Kirste et al. (2004) at 95 °C and 10 MPa, when the dissolution of chamosite in the experiment provided sufficient Fe2+ and Mg2+ in the reaction solution. Batch experiments performed by Kaszuba et al. (2005) also showed that carbonate precipitation of magnesite and siderite occurred in sandstone shale systems (200 °C, 20 MPa); however, those minerals were not detected by SEM in our experiments. The precipitation of secondary calcite and ankerite showed that the injected CO2 could be sequestered by the precipitation of secondary carbonate minerals in a CO2-EGS reservoir. Secondary clay minerals such as kaolinite, illite, and smectite were absent in SEM analysis. As indicated in Fig. 9, minerals of rhyolite after reaction revealed a significant alteration in XRD analysis. The analysis revealed smaller X-ray peaks diagnostic of quartz after both CO2– brine and brine only experiments as compared with those prior to the experiment. This result is in agreement with the dissolution of quartz detected by SEM in those experiments. The visual

Fig. 9. X-ray diffraction pattern for unreacted rhyolite and that reacted for 12 days with brine and CO2–brine at 160 °C. Q: quartz; F: feldspar; A: ankerite; C: calcite.

appearance of secondary calcite and ankerite in the CO2–brine experiment detected in the SEM images was also confirmed by the presence of their XRD peaks shown in Fig. 9. However, these peaks were not evident for rhyolite in the pre-experiment and the brine–rhyolite experiment. In batch experiments, acid-induced reactions between mineral formation in the rock and in CO2–brine were clearly observed after CO2 injection. The mineral alteration in host rocks after CO2 injection affects the flow property in the reservoir, which is composed of matrix and fractures representing the main flow paths for the fluid. The injected supercritical CO2 quickly fills these fractures and displaces the water from the geological formation to retard reactions between formation water and rocks (Fig. 10). Calcite is initially present in the fractures and generally dissolves due to the low pH after CO2 injection. The reaction rate for calcite is higher than that for other minerals such as quartz and Al-silicates. Thus, the strong dissolution of natural calcite could increase the porosity and permeability of fractures and can then affect the growth and longevity of the reservoir. Moreover, the short period of geochemical interaction in fractures prevents Ca2+ released from native calcite from reacting with HCO 3 precipitates at a low pH. Nevertheless, the matrix remained in mixed two-phase CO2–water conditions, and mineral transformations continued to occur for a long period with pH buffered by reactions including aqueous complexation and mineral alteration. The precipitation of calcite and ankerite in our experiments was expected to trap amounts of CO2 in the matrix, which is favorable for CO2 storage. 5.2.2. Reaction of solid phases at 170 °C The process for calcite dissolution is as follows: corrosion pits ? corroded cone ? corroded ridges ? complete dissolution of corroded ridges (Meng et al., 2013). The dense corrosion pits on the surface of calcite that reacted at 170 °C (Fig. 11a in Appendix B) were early appearances of corroded ridges in the processes of calcite dissolution at 160 °C (Fig. 8b). The higher temperature resulted in a smaller dissolution of calcite, which is in agreement with that shown in Fig. 7a. Secondary diamond calcite and ankerite were present on the surfaces of rhyolite samples (Fig. 11b and c in Appendix B). Moreover, the development of crystal structure was better than that in the samples at 160 °C, which is consistent with the decrease in concentration of Mg and Fe (Fig. 7c), because precipitation occurs easier at a higher temperature. Formation temperature plays a crucial role in the chemical interaction between fluid and rock. Under the same pressure conditions, higher temperatures relate to lower solubility of calcite in the formation water. Hence, higher temperature would inhibit the dissolution of nature calcite in fractures. As a result, the potential alteration of fracture permeability/porosity due to the interaction of CO2, brine, and rock appears to be lower, which is favorable for the stability of the system during the operation period. Alternatively, higher temperatures relate to higher precipitation rates of carbonate minerals such as calcite, ankerite, siderite, and dolomite, which facilitate CO2 mineral trapping. The precipitation of carbonate in batch experiments are also possibly related to the dissolution of primary Fe-bearing minerals and calcite. Nevertheless, this effect does not decrease the porosity and permeability of fractures, which would not block the fluid movement mainly because reactions between fluid and rock stop after the fractures are quickly filled with CO2. However, Borgia et al. (2012) found that the higher temperature could enhance salt precipitation in fractures, which clogs the flow system of the reservoir. Salt precipitation was absent in our experiments because the temperature and NaCl concentration of the brine in the Yingcheng Formation are significantly less than that in the simulations reported by Borgia et al. (2012).

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175

Fig. 10. Schematic diagram of geochemical processes of a CO2-EGS.

5.3. Simulation of lab experiment Chemical interaction of CO2 with saline formation water caused an increase in HCO 3 concentration and a substantial decrease in pH in the numerical simulations. The simulated values of pH and HCO 3 differed significantly from the measured values (Fig. 12a and b in Appendix B). This occurred because CO2 escaped from the reactor when the samples were collected in the room environment with normal temperature and pressure. In the simulation, the lowered pH induced the dissolution of primary minerals and precipitation of secondary minerals, influencing aqueous concentrations. The concentrations of Na+, Ca2+, Mg2+, Fe2+, and K+ were generally consistent with the measured values (Fig. 12c and d in Appendix B). The predicted Si concentrations were higher than the experimental data. K-feldspar and quartz appeared to be undersaturated, when albite was also undersaturated at the beginning of the research, which is consistent to the SEM result from the experiments as discussed above (Fig. 13a in Appendix B). When the primary calcite dissolved, the calcite was undersaturated early in response to the pH drop, which is in agreement with the SEM results. The system was supersaturated with respect to kaolinite, smectite, and albite after its initial undersaturation, although no significant amount of these minerals was observed by SEM (Fig. 13b in Appendix B). This occurred likely because of the slow precipitation rate of clay minerals and albite. In the simulation, the system was supersaturated with respect to ankerite after 12 days, which agrees well with SEM and XRD results. Dolomite and dawsonite were undersaturated, indicating agreement with the experiments. The geochemical modeling was partly successful in reproducing the experiment, which offers a detailed view of the interaction mechanisms of the chemical processes occurring in a CO2-EGS system. However, the simulation did not always match the experimental results well (Alemu et al., 2011; Wigand et al., 2008). These discrepancies may be caused by the following factors: (1) problems with the solubility and kinetic data for minerals used in the simulations; (2) impurities (such as solid solution) in the minerals and possible formation of compounds that may have affected the solubility of the remaining mineral components; (3) precipitation of clay minerals and albite did not occur in our experiments although this may occur in longer-term experiments or in fields. 6. Comparison with field observations Few mineralogical descriptions of the alteration of rock in the conditions of high-pressure CO2 are available in the previous literatures. The following examples are given for comparison with the experiments presented in this paper. Giolito et al. (2007) reported on mineral alteration in the Bagnore and Piancastagnaio geothermal fields at Monte Amiata, Italy, in which CO2-rich fluids is a two-phase mixture of up to 15 wt% gas content were present in the produced steam, and the alteration displayed unusual mineral compositions. Vein deposits, which are

encountered at high temperature (160–350 °C) and pressure (up to 20 MPa), includes abundant carbonates mainly composed of ankerite. The type of these carbonates agreed with those found in our batch experiments and numerical simulations, i.e., the main deposition is ankerite. Mineralogical and groundwater changes caused by magmatic CO2 invading a gas reservoir in a lithic sandstone of the Pretty Hill Formation in the Ladbroke Grove gas field in South Australia were reported by Watson et al. (2004). Field observations consistent with our results include dissolution of feldspars, reduction of calcite, and an increase in the quantity of ankerite. Ré et al. (2014) compared generalized secondary mineralogy found in natural geothermal systems developed in silicic rocks between 100 °C and 300 °C including temperature ranges in batch experiments (150–170 °C) up to 30 MPa. Secondary calcite is common in CO2-rich hydrothermal systems such as that in the Ohaaki-Broadlands, which is consistent with the significant precipitation of calcite detected by SEM after reaction in our batch experiments. Mixed-layer clays and quartz are commonly deposited as secondary minerals in active geothermal systems, although these minerals were not detected in the experiments as secondary minerals likely because of the short duration of the experiments and the slow reaction rates of clay minerals and quartz. Moreover, the initial K+ concentration of the starting fluid may have played a negative role in the dissolution of K-feldspars, which would have limited the extent of its alteration to kaolinite. Wallrock alteration involving CO2 metasomatism in mesothermal gold deposits can interpret the nature and sequence of concurrent hydrogen and carbonate metasomatism, which is an analog to the CO2-EGS hosted rock (Apps and Pruess, 2011). Han (2013) investigated wallrock mineralization characteristics of Tuanjiegou gold deposit in the Songliao Basin. According to geological data, carbonate veins formed in the stage of gold ore-forming were observed in altered granite porphyry (Han, 2013). Moreover, fluid inclusions of the gold deposit showed average ore-forming temperature and pressure are 166 °C and 28 MPa, respectively. These conditions are similar to that of the above batch experiments. The carbonate veins included calcite and ankerite, which is consistent with the precipitation of secondary carbonate minerals in our batch experiments. The fluid inclusions of Haigou gold deposit in the Songliao Basin showed that the mineralization temperature was 160–240 °C, which is close to experimental temperature. Moreover, the CO2-rich fluids reached 5 wt% gas content. In the SEM analysis of wallrock granite samples, the results were consistent with those in our experiments including corrosion of the K-feldspars (Fig. 14a in Appendix B) and secondary carbonate mineral (Fig. 14b in Appendix B). Moreover, the precipitation of albite and smectite agreed with above simulations, in which albite (Fig. 14c in Appendix B) and clay minerals (Fig. 14d in Appendix B) were supersaturated in the late period of the reaction. 7. Conclusions Experiments and numerical simulations with and without CO2 were performed to evaluate the geochemical processes among

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supercritical CO2, brine, and host rock under the conditions of the Yingcheng Formation as a CO2-based EGS reservoir in the Songliao Basin. The following conclusions can be drawn on the basis of this study: 1. The addition of supercritical CO2 to a brine–rock system of the reservoir would change the system from rock–fluid reactions and then cause mineral alterations, which would likely change the fluid flow properties. Supercritical CO2 induced the dissolution of primary rock minerals such as calcite and feldspar. Secondary calcite and ankerite precipitated due to the sufficient availability of cations such as rich Ca2+, Fe2+, and Mg2+ in the initial reaction solution and via dissolution of primary Fe-bearing minerals and calcite. Therefore, both sustained geothermal energy recovery and CO2 geological storage may be achieved. 2. Numerical simulations were generally consistent with laboratory experiments, which provide a tool for scaling the time up for long period of time. This type of integrated study can constrain thermodynamic (solubility) and kinetic data of minerals used in the model and can validate the geochemical model that will be used in the future for long-term reservoir performance. 3. The limited information currently available for mineral alteration in high-temperature natural CO2 reservoirs is generally consistent with our lab experiments and numerical simulations. The range of problems concerning the chemical processes in CO2-based EGS systems is quite broad. The present results are specific to the conditions and parameters considered. The study can give useful insight into the dynamic evolution of geochemical processes and may provide information for future design and optimization of such a CO2-EGS system. Acknowledgments This work was supported by Chinese Ministry of Science and Technology 863 Program (No. 2012AA052801), by the National Natural Science Foundation of China (Grant No. 41272254 and 41402205). Appendix A Precipitation and dissolution of minerals are kinetically controlled. The kinetic laws used here are derived from transition state theory (Lasaga, 1984). Effective reaction rates can be expressed through the following general equation:

  l n Q rate ¼ Am km 1  K

ðA:1Þ

where Am is the specific surface area, km is the kinetic rate constant, Q is the ion activity product, K is the equilibrium constant for the specific mineral–water reaction, and l and n are two constants which depend on experimental data, which are usually but not always taken equal to 1. For many minerals, the kinetic rate constant k can be summed from three different mechanisms (Lasaga et al., 1994; Palandri and Kharaka, 2004):

 nu   Ea 1 1 nu H þ k25 k ¼ k25 exp  T 298:15 R "  # EHa 1 1 OH  exp  anHH þ k25 T 298:15 R "  # EOH 1 1 a OH  exp  anOH T 298:15 R

ðA:2Þ

where superscripts or subscripts nu, H, and OH indicate neutral, acid and basic mechanisms, respectively; Ea is the activation energy, k25 is the rate constant at 25 °C, R is gas constant, T is absolute temperature, a is the activity of the species; and n is power term (constant). Notice that parameters h and g (see Eq. (A.1)) are assumed the same for each mechanism.

Appendix B. Supplementary material Supplementary data (Figs. 11–14) associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. apgeochem.2015.04.018.

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