Experimental Study of the Reactions of Supercritical CO2 and

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Experimental study of the reactions of supercritical CO2 and minerals in high-rank coal under formation conditions Yi Du, Shuxun Sang, Wenfeng Wang, Shiqi Liu, Tian Wang, and Huihuang Fang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02650 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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Experimental study of the reactions of supercritical CO2 and minerals in high-rank coal under formation conditions Yi Dua, b, Shuxun Sanga, b∗, Wenfeng Wanga, b, Shiqi Liuc, Tian Wanga, b, Huihuang Fanga, b a. Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education, School of Mineral Resource and Geoscience, China University of Mining and Technology, Xuzhou 221116, China b. School of resources and geosciences, China University of Mining and Technology, Xuzhou 221116, China c. Low Carbon Energy Institute, China University of Mining and Technology, Xuzhou 221116, China

Abstracts: This study investigates the influence of supercritical CO2 (scCO2) injection on minerals in high-rank coal under the temperature, pressure, and hydrologic conditions of a deep coalbed. A typical high-rank coal reservoir in the Qinshui basin, the #3 coal seam, is the focus of this research. A coal–scCO2 geochemical reaction experiment is conducted to simulate the 2000 m burial depth of the coal seam. Field emission scanning electron microscopy is used to determine the locations of specific minerals and observe the effects of scCO2-H2O on these minerals at the micrometer scale. These results are combined with X-ray diffraction and inductively coupled plasma-atomic emission spectrometry and mass spectrometry analysis results, and the effects of the scCO2-H2O fluid on minerals in the high-rank coal over a short period are discussed. In addition, the influence on coal reservoir structure was studied based on intrusive mercury and liquid nitrogen adsorption experiment. The results suggest that instantaneous CO2 injection can provide a large amount of H+, and the initial ion release rate is high. Because of differences in mineral dissolution rates, scCO2-H2O has the strongest effect on calcite, followed by dolomite, aluminum hydroxide minerals, chlorite and albite; however, effects are not obvious for illite, kaolinite, and quartz. Due to the low mineral content of the coal and the short experimental period, independent secondary carbon sequestration minerals did not form. However, the surface of aluminum hydroxide minerals reached partial dissolution equilibrium, and new layered aluminum silicate minerals were generated. The dissolution of carbonate minerals, albite and chlorite increased the pore volume of the coal reservoir and improved the permeability of the samples. New layered aluminum silicate minerals and the chlorite with new occurrence increased the surface areas of samples after the reactions. After the reaction, porosity, pore volume and surface area of the sample were larger, which also confirmed the positive transformation effect of the mineral changes on the reservoir. Key words: scCO2; CO2-ECBM; mineral dissolution; mineral precipitation; reservoir property

1. Introduction Geological CO2 storage is an important topic of interest as it is expected to be an effective 1 / 21

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way of reducing carbon emissions. Deep salt aquifers, oil and gas reservoirs, deep unexploited coal seams, etc. are effective geological storage reservoirs 1. Geological CO2 storage-enhanced deep coalbed methane (CO2-ECBM) recovery is a promising technique that integrates the disposal of greenhouse gas emissions and the advantages of clean fossil energy development 2. Deep coal seams are widely distributed in China. However, compared with other countries, such as the United States, the coal level is significantly higher, and the permeability is generally low. CO2-ECBM processes in deep coal are more difficult and deserve special attention. Because the critical point of CO2 is 31.1 ºC and 7.38 MPa, CO2 is supercritical under the conditions of deep coal reservoirs 3. The coal reservoirs often contain groundwater that is formed by diagenesis, sedimentary associated actions, seepage and deep formations 4. In addition, coalbed gas well fracturing creates a large amount of water. There are three forms of water: bound water, free water and adsorption water. Therefore, a scCO2-H2O-coal geochemical reaction system is formed when CO2 is injected into a deep coal seam. The injected CO2 displaces the preexisting fluid in the pores and forms a mixed zone of CO2 and water. Then, CO2 dissolves in water to form carbonic acid at the gas-water interface 5. The acid water can react with the minerals in coal and change the composition of the coal. The scCO2-H2O-coal geochemical reaction system in coal reservoirs has attracted the attention of many scholars. From the migration of the main elements and trace elements, the minerals in coal have geochemical changes

6,7

. Using a scanning electron microscope, the

dissolution of carbonate minerals in can be found, which resulting in the fractures open and interconnected 8. The dissolution of the minerals in coal occurs over a short period after scCO2 injection, but precipitation is the long-term effect of scCO2-H2O-minerals. The process is similar to that of injecting CO2 into a deep brine reservoir

2, 9

. Many simulations and experimental studies have

found that minerals mainly undergo the following three changes after CO2 is injected into a deep brine reservoir: mineral dissolution 6, 9, the transformation of clay minerals 10, and the formation of new minerals 11. Although short-term experimental studies do not fully reflect the long-term effects on natural coal reservoirs after CO2 injection, they can provide a data framework and theoretical basis for long-term numerical simulations. The high-rank coal of the Qinshui basin is the focus of this research. Specifically, the reaction process of scCO2-H2O-coal under conditions at a depth of 2000 m (80 °C, 20 MPa) that only fewer scholars have studied it. The reactions are simulated using self-developed experimental instruments. To observe changes in the same minerals before and after the reactions, which few scholars have investigated in coal, scanning electron microscopy is 2 / 21


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used to identify and record the characteristic minerals. In addition, combined with a variety of analysis methods, such as X-ray diffraction (XRD) and ion chromatography-mass spectrometry, the effects and functional mechanisms of scCO2-H2O fluid on high-rank coal over a short reaction time are discussed based on variations in mineral compositions and ion concentrations.

2. Geological background The Qinshui basin is one of the most important exploration and development areas of coalbed methane in China and is the pilot test site for CO2-ECBM engineering. The Qinshui basin is located in southeastern Shanxi Province at 35°~38° north latitude and 112°00’~113°50’ east longitude

12

. The #3 coal seam of the Shanxi Formation exhibits a sheet-like distribution in the

entire region, with a thickness of 0.35~7.84 m and a deep burial depth 200~5000 m. It is the main coal seam of the Shanxi Formation and the focus of this study. The #3 coal is mainly anthracite with a high vitrinite reflectance and low ash, volatile matter and water contents. The vitrinite content of the coal ranges from 45%-70%, and the inertinite content is 20%~36%. The coal lithotypes are mainly semi-bright coal and semi-dull coal. Basic information of samples is shown in Table 1. Table 1. Basic information of samples Buried depth

Ro,max,

Vitrinite

Inertinite

Moisture

Ash

Volatile

Fixed carbon

(m)

(%)

Vol.%

Vol.%

(wt.%)

(wt.%)

(wt.%)

(wt.%)

Location Xinjing Colliery

585

3.64

70.70

29.30

1.66

10.02

10.1

80.89

Sihe Colliery

346

3.33

79.30

20.70

1.48

13.12

6.32

81.39

Bofang Colliery

273

2.83

71.72

28.28

2.05

9.4

9.86

81.67

Fig. 1 Sampling locations and the Qinshui Basin

3. Experiments and tests 3.1 Samples 3 / 21


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The experimental samples of the 3# coal were collected from Bofang mine, Sihe mine and Xinjing mine in the Qinshui basin. The samples are mainly anthracite with high metamorphism, as shown in Table 1. According to the X-ray analysis, the mineral content of the coal is 10.85%~16.98%, and the major minerals include clay, sodium feldspar, quartz and carbonate minerals. Due to the limits of the XRD test, dolomite was not detected in the samples from the Bofang and Xinjing mines, and aluminum hydroxide minerals were not detected in the samples from the Sihe and Xinjing mines. However, small amounts of these minerals can be observed using a field emission scanning electron microscope (FE-SEM). Table 2. Mineral composition of samples before and after reaction Aluminum Total

Kaolinite

Illite

Chlorite

Albite

Calcite

Dolomite

Quartz

Rutile

(%)

(%)

(%)

(%)

(%)

(%)

(%)

(%)

(%)

Sample ID

hydroxide minerals (%)

XJ-1

11.78

8.54

0.31

/

1.43

0.48

/

1.01

/

/

XJ-2

11.28

8.48

0.31

/

1.33

0.15

/

1.00

/

/

SH-1

15.61

2.30

8.17

/

1.16

1.92

0.24

1.82

/

/

SH-2

14.41

2.31

8.16

/

1.13

0.94

0.15

1.85

/

/

BF-1

9.63

3.41

3.29

0.39

0.81

0.40

/

0.74

0.21

0.38

BF-2

9.27

3.42

3.25

0.31

0.68

0.19

/

0.73

0.22

0.47

Notes: "/" indicates that the mineral has not been detected by XRD. XJ-1, BF-1, SH-1 are the prereaction samples, and XJ-2, BF-2, SH-2 are the postreaction samples.

3.2 Geochemical simulation experiment (1) Experimental samples The experimental samples were crushed to form a 200-mesh powder for XRD tests (the test results are shown in Table 1), and 4-8mm particles for intrusive mercury and liquid nitrogen adsorption experiment. Additionally, samples were cut into 1×1×1 cm3 coal blocks with smooth surfaces and polished using an argon ion polisher for FE-SEM tests. The remaining samples after testing and the original FE-SEM samples were used in geochemical reaction experiments. These samples totaled approximately 150 g. To accelerate the reaction process, the solid-to-liquid ratio of the reaction was increased by adding 600 mL of ionized water. (2) Laboratory equipment A self-developed experimental instrument, which is shown in Figure 2, was used to perform scCO2-H2O-coal geochemical experiments. The sample cell has an internal volume of 1.2L. The pressure and temperature limits of the system are 30MPa and 200°C respectively.

4 / 21


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Fig. 2 Illustration of the scCO2-H2O geochemical reaction equipment

(3) Experimental conditions The experiments simulated the coal reservoir conditions at a depth of 2000 m. Based on the geothermal gradient and pressure gradient of the Qinshui basin

13

, the experimental temperature

was calculated as approximately 80°C, and the experimental pressure was determined to be approximately 20 MPa. This pressure was created using CO2 gas. (4) Experimental process Samples were placed into the autoclave, which was filled with CO2 after vacuum treatment. During the experiment, the temperature and pressure were continuously monitored, and the reaction system was stable (shown in Fig.3). Water samples (20 mL) were taken once every 24 h, and 0.5 mL of nitric acid was added to prevent ion precipitation, which can affect the determination of cationic concentration tests using ICP-AES and ICP-MS methods. After ten days, samples were removed and placed in a vacuum drying oven to dry at 50°C for 24 hours.

Fig. 3 Changes in temperature and pressure during the experiments

3.3 FE-SEM localization analysis method An FE-SEM was used to observe the changes in the same mineral before and after reactions at the micron-to-nanometer scale. Because minerals in coal are generally fine, positioning is difficult; therefore, photos were taken at low magnification in back-scattering mode with the help of AMICS software. The locations of characteristic minerals were recorded, for example the BF sample (Fig.4). Then, the characteristic minerals were gradually magnified, and the secondary 5 / 21


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electron pattern was used to observe the details of the mineral surface. AMICS is an innovative software platform for automatically identifying and quantifying minerals and their synthesis phases. After set up the sample scan area in the software, the image can be automatically taken and automatic spliced with 200 times magnification under back-scattering mode. The X-ray energy spectrometer is automatically used to determine the composition of the mineral at every 4µm. However, for clay minerals (especially kaolinite, feldspar and illite), due to its small size and complex composition, its analysis is not very accurate.

Fig. 4 FE-SEM localization analysis schematic diagram of BF sample

3.4 Test and analysis (1) XRD To investigate the formation of new minerals after the geochemical reaction experiment, XRD analysis was performed on the samples before and after the reaction. The Bruker D8 X-ray diffractometer located at the Analysis and Test Center of China University of Mining and Technology was used for the analysis. The test angle ranged from approximately 3-65°, and Jade 5 software was used to analyze the XRD experimental results. (2) FE-SEM Argon ion polishing was completed using the Technoorg SC-1000 argon ion polishing system, and FE-SEM observations were completed with the Sigma 300 Zeiss FE-SEM and Bruker Inca energy dispersive spectrometer (EDS) in the laboratory of the Guizhou Coalfield Geology Bureau. It is important to note that prereaction SEM samples should not be coated with gold because it has a protective effect on the surface of the sample and will impede the effect of scCO2-H2O on the surface of coal. However, due to the poor surface flatness of the samples after the reaction, charges are not easily transmitted, and the mineral surface is notably discharged; therefore, 6 / 21


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samples must be sprayed with gold. (3) ICP-AES/MS The concentration of cationic ions in water samples was tested using the 96–750-type inductively coupled plasma-atomic emission spectrometer (ICP-AES) and the inductively coupled plasma-mass spectrometer (ICP-MS) at the Regional Geological Survey Center of Hebei Province. Additionally, according to the measured ion concentrations, based on the principle of charge conservation, the module of “EQUILIBRIUM PHASES” of PHREEQC-3 (version 3.3.7) software 12

, was used to calculate the pH value of the solution and the saturation index of the minerals

during the experiment. (4) Mercury intrusion and N2 isothermal adsorption High pressure mercury injection experiment was carried out with the AutoPore IV 9500 in Guizhou coal geology bureau. The low temperature N2 isothermal adsorption experiment was carried out by the Quantachrome autosorb-1 physical adsorbator in Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process of China University of Mining and Technology. The pore classification adopts the scheme of International Union of Pure and Applied Chemistry (IUPAC), which means the micropore diameter is less than 2nm, the mesoporous diameter is 2 - 50nm, and the macropore diameter is greater than 50nm 13.

4. Results and discussions 4.1 Total mineral composition changes According to the XRD test data (Table 1) of the same samples before and after the reaction, the total mineral contents all decreased after the reaction. The change in the clay mineral composition was small, but the change in the chlorite concentration was large. The quartz and rutile concentrations remained essentially unchanged, while calcite exhibited the largest decrease, followed by dolomite and albite. Additionally, the aluminum hydroxide minerals increased. But as XRD is only a semi-quantitative method for minerals in rocks, a small content change of minerals is likely to be measurement error. Therefore, XRD data can only be used as a reference. 4.2 Changes in the fluid chemical composition 4.2.1. Change in pH CO2 is a "reactive gas" that can dissolve in water to form an acidic fluid. CO2 is soluble in water to form carbonic acid (Equ 1), and carbonic acid quickly decomposes to form bicarbonate ions (Equ 2). The associated increase in acidity dissolves soluble minerals

14

. Based on the

simulation results of PHREEQC software, at the moment the CO2 is injected into the ionized water, the pH of the solution is 3.77. As shown in Fig. 5, the pH of the solution increased sharply 7 / 21


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after CO2 injection under the conditions at 2000 m. This result suggests that a large number of ions underwent exchange reactions with H+. CO2(g)+H2O⇋H2CO3 (1) H2CO3⇋H++HCO-3

(2)

Fig. 5 In situ pH (calculated with PHREEQC software) changes versus the reaction time

4.2.2. Cationic concentration changes Because K, Na, Ca and Mg are analyzed using the same test method, namely, IPC-MS, they exhibited similar fluctuations in BF, SH, and XJ samples. In addition, the sample concentrations notably changed (increased or decreased) on some days; for example, the last sampling of BF. These changes were not necessarily caused by intense ion dissolution or precipitation and may have been caused by sample testing or sampling error. Therefore, we discuss the overall ion dissolution trend in Fig. 6. After CO2 injection, the concentration of Ca in solution increased rapidly and then slowly increased. The Si ion concentration continued to rise, and the concentration of K ions fluctuated near approximately 4 mg/L. The Fe ion concentration first increased rapidly and then stabilized before eventually exhibiting a slight decline. Notably, the Fe ions in the BF sample decreased most rapidly and obviously. However, the XRD and SEM analyses did not indicate iron-bearing mineral production; therefore, the cause of Fe ion decline requires further study. Moreover, the concentration of Al ions initially increased and then slightly decreased. Based on the declines in the Fe and Al ion concentrations, we can speculate that the samples may generate aluminosilicate minerals or transitional products with iron. Due to the continuous dissolution of quartz in the solution, with a slightly higher dissolution rate than clay minerals, Si ions do not display a decreasing trend.

8 / 21


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Fig. 6 Changes in the ion concentrations of the solution versus the reaction time

4.2.3. Saturation index changes After CO2 injection, the pH increases with the continuous dissolution of ions, and the saturation index (SI) of minerals in the solution gradually increases (Fig. 7). However, the solution is undersaturated with respect to all minerals, namely, SI < 0. Aluminum hydroxide mineral displays the highest SI value, which is close to 0, followed by carbonate minerals and quartz. Kaolinite has the highest SI value of the clay minerals, and chlorite has the lowest. If the reaction time is sufficiently long, the solution may be more likely to be saturated to aluminum hydroxide minerals and carbonate minerals. This condition is conducive to precipitation and effective carbon sequestration.

9 / 21


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Fig. 7 Changes in the saturation index of the solution versus the reaction time

4.3 Variations in the same mineral assemblages 4.3.1. Carbonate minerals The SEM results suggest that calcite and dolomite are co-existent and fill coal rock cracks 15 or plant cells (Fig. 8a). By the effects of pressure and weak acidic fluid, the surfaces of these minerals develop microfractures and a small amount of round, oval, and square dissolution pores (Fig. 8c, e). Due to ScCO2-H2O effects, the calcite on the surface of the samples dissolves and becomes almost invisible (Fig. 8b). Additionally, the inner calcite dissolves, resulting in corrosion cones (Fig. 8d) and new dissolution crystal faces (Fig. 8f). The surface of the carbonate region is mainly dolomite (Fig. 8d), and there are only rhomboid pores on the dolomite surface (Fig. 8d). Compared with calcite, dolomite is less soluble. The significant carbonate mineral dissolution is consistent with XRD results (Table 1). In the weak acid system of ScCO2-H2O, calcite (Equ 3, 4)

16

and dolomite (Equ 5, 6)

easily dissolved and have large dissolution rates. Sonnenthal and Spycher

18

17

are

conducted mineral

dissolution reactions under the conditions of 60℃ and pH = 4. They found that the reaction rate of calcite ranged from 10-5-10-3 mol·m-2 ·sec-1 and that of dolomite ranged from 10-7-10-5 mol·m-2·sec-1. CaCO3+H+→Ca2++ HCO-3

(3)

CaCO3+H2CO3→Ca2++2HCO-3

(4)

Ca(Fe0.7Mg0.3)(CO3)2+2H+→Ca2++0.7Fe2++0.3Mg2++2HCO-3

(5)

CaMg(CO3)2+2H+→Ca2++Mg2++2HCO-3

(6)

The dissolution process of calcite is the first to generate dissolution pits and dissolution zones. Then, dissolution crystal cones are formed, followed by new crystal surfaces

19

. The dissolution

crystal cones (Fig. 8d) of the internal calcite (Fig. 8d) and the newly dissolved crystal surfaces (Fig. 8f) were observed via SEM. Notably, the degree of the dissolution of calcite is extremely high. Previous experiments and geological examples

20, 21

confirm that the solubility of calcite is

larger than that of dolomite, and the solubility of calcite increases due to the presence of Mg. Therefore, after CO2 injection, calcite is more soluble than iron dolomite. The dissolution of carbonate minerals is beneficial to increase the reservoir space of free gas, while the releasing of large amount of Ca, Mg and Fe ions is in favor of the long-term carbon fixation. 10 / 21


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Fig. 8 Changes in carbonates before and after the reactions (a: carbonates in XJ sample before the reaction; b: carbonates in XJ sample after the reaction; c: dolomite dissolved pores in XJ sample before the reaction; d: dolomite dissolved pores in XJ sample after the reaction; e: calcite microcracks and pores in BF sample before the reaction; f: internal calcite in the dissolution macro pore in BF sample after the reaction)

4.3.2. Aluminosilicate minerals In the original sample, albite often co-exists with kaolinite (Fig. 8a) and fills in plant cells. Due to the severity of discharge, the associated occurrence morphology is unclear. After the reaction, the albite tends to dissolve along the grain surface (perpendicular to the surface of the observation). With this process, the distances between albite crystal faces increase, and a corrosion fracture can be observed on the surface (Fig. 9b). The chlorite in coal often fills in fractures as bands (Fig. 9c). A comparison of the SEM images before and after the reaction shows that the fractures in contact between the coal and the 11 / 21


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minerals widen. However, this change does not justify the chlorite corrosion. Instead, this may have been caused by the plasticizing effect of ScCO2 on coal. Notably, chlorite edge extrudes the surface of the sample, and chlorite forms large layered bending sheets (Fig. 9d). Overall, the changes in the sample composition confirm the changes in its chemical composition. The dissolution of chlorite can be seen clearly in the Fig. 9e and f. Kaolinite and illite in coal fill in plant cells as aggregates. The occurrence of kaolinite is difficult to observe due to the severity of the associated discharge. Illite can be observed as large curved thin slices. A comparison of SEM images before and after the reactions suggests that kaolinite does not seem to have changed (Fig. 9g, h), but the bending degree of illite layers is comparably larger, and the intercrystalline fractures are pronounced. However, in general, ScCO2-H2O has a weak effect on kaolinite and illite.

12 / 21


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Fig. 9 Changes in aluminum silicates before and after the reactions (a: albite in SH sample before the reaction; b: albite microfracture along the crystal surface in SH sample after the reaction; c: chlorite in BF sample before the reaction; d: surface morphology of chlorite changes in BF sample after the reaction; e: further amplification of the rectangular area in figure c; f: further amplification of the rectangular area in figure d which shows the dissolution of chlorite; g: kaolinite and illite in SH sample before the reaction; h: kaolinite and illite in SH sample after the reaction)

Many studies have shown that sodium feldspar, kaolinite, illite, chlorite and other aluminosilicates can dissolve due to the actions of weak acids (Equs. 7-10) 22-25, but the associated reaction rate is much lower than that of carbonate minerals. Black, et al.

26

concluded that at the

conditions of 60 °C and pH = 4, the albite reaction rate is approximately 10-10 mol·m-2·sec-1, and that of clay ranges from 10-14~10-10 mol·m-2·sec-1. Specifically, the reaction rates of kaolinite and illite are low, and those of chlorite and montmorillonite are high. Because the pH of the reaction solution is less than 5, the Al in the solution is mainly in the form of Al3+ or Al(OH)2+, and Si mainly exists in the form of H4SiO4 27. NaAlSi3O8+4H+→Na++Al3++2H4SiO4

(7)

Al2Si2O5(OH)4+6H+→2Al3++2H4SiO4+H2O

(8)

K0.75Al1.75[Si3.5Al0.5O10](OH)2+14H+→0.75K++2.25Al3++3.5H4SiO4+2H2O Ⅱ 0.1

Ⅱ 0.1

Mg4.9Al0.7 Fe Fe [Si3.5Al0.5]O10(OH)8+2H

(9)

+

→4.9Mg+1.2Al3++0.1Fe2++0.1Fe3++3.5H4SiO4+4H2O······(10) Although most experimental studies have shown that the dissolution of potassium and sodium feldspar is accompanied by the formation of kaolinite and quartz

28, 29

. However, the

experimental results suggest that the dissolution of albite did not directly produce kaolinite (Fig. 9a, b), possibly due to the short reaction time. Due to the co-existence of albite and kaolinite in the original samples, the diagenetic evolution stage of kaolinite is transformed by dissolved albite (Fig. 9a); therefore, if the reaction time is sufficiently long, kaolinite entirely replaces albite. Chlorite mainly forms in weak acid–weak alkali environments, but kaolinite and illite form in weak acid environments

30

. Therefore, under acidic conditions, the dissolution rate and

dissolubility of chlorite are larger than those of kaolinite and illite

26

, and dissolution effects are

more obvious. 13 / 21


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During the reaction, the Mg-O and Fe-O bonds of chlorite are broken by actions involving H+. These actions destroy the forces between the original octahedral layers and hydrogen bonds, resulting in the curvature of the layers. Compared with the original morphology, the surface area increases. Among the aluminosilicates, chlorite has the great significance to the sequestration of CO2, which can provide a large amount of Mg, Fe ions to the solution and is in favor of the long-term carbon fixation. In addition, it also has some effect on reservoir transformation (pore volume, specific surface area and permeability). 4.3.3. Oxide minerals Quartz is widely distributed in coal. Self-generated quartz with a massive structure, which is formed by the precipitation of peat bog solution, can be seen in the samples from the study area. However, the changes in quartz grains before and after the reactions are not obvious (Fig. 10). Many studies have shown that quartz can dissolve in acidic solution (Equ. 11) 31, but has a low dissolution rate 26. As the temperature and pH increase, the dissolution rate increases 32. Zhu, et al. 33

al.

found that quartz exhibited obvious corrosion pits in an scCO2-H2O system at 200 °C. Chen, et 32

conducted hydrothermal experiments with quartz and found that the quality of quartz

decreased at temperatures greater than 200 °C under acidic conditions. Therefore, due to the low experimental temperature, the effect of scCO2-H2O on quartz is weak. SiO2+2H2O→H2SiO3·H2O

(11)

Fig. 10 Slight changes of quartz in XJ samples before and after the reactions

4.3.4. Aluminum hydroxide minerals and new minerals The aluminum hydroxide minerals in coal fill in plant cells as aggregate and symbiotically exists with clay (Fig. 11a). After the reactions, the crystal shape of aluminum hydroxide minerals did not change significantly, while the mineral edge produced new minerals (Fig. 11b). Based on the SEM results, the new minerals are mainly composed of Al, Si and O (Table 3). Additionally, aluminum hydroxide minerals readily reacted with H+ and released Al3+ and Al(OH)2+ in water. Al3+ and Al(OH)2+ combined with large amounts of H4SiO4 and cations in solution and formed 14 / 21


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new unknown aluminosilicate precipitates, leading to a decrease in the Al content of the solution (Fig. 7a). Bi, et al.

34

conducted hydrothermal experiments on aluminum hydroxide minerals at

different temperatures and pH levels and found that when the reaction temperature was greater than 160°C, the morphology changed, and at greater than 210°C, the phase changed. Therefore, the reaction temperature was not high enough to change the crystal structure and crystal phase.

Fig. 11 Aluminum hydroxide minerals and the new minerals (a: flat surface of aluminum hydroxide minerals in BF sample before the reaction; b: new minerals generation on aluminum hydroxide minerals and new fracture generation)

4.4 Dissolution and precipitation of minerals 4.4.1. Dissolution of minerals and ion release The pH of the deionized water rapidly decreased to 3.77 at the moment when CO2 was injected, leading to the rapid dissolution of the initial minerals and an increase in the ion release rate. Under high temperature and pressure conditions, scCO2 can enter small pores and continue to react, but by this time, the ion release rate decreases. Equs. 1–11 show that the release of K ions mainly comes from illite during the reactions, while Na ions mainly come from albite; Ca ions mainly come from calcite and dolomite; Fe and Mg mainly come from iron dolomite and chlorite; Al mainly comes from aluminosilicate and aluminum hydroxide minerals; and Si mainly comes from aluminum silicate and quartz. When CO2 was injected 24 hours later, the concentration of Ca ions was the largest in the Bofang, Sihe and Xinjing solutions. The dissolution of calcite was highest (Fig. 8), and this result was consistent with the XRD and SEM results. However, the concentration of Na ions was higher than those of Mg and Fe ions (Fig. 7). Although more albite was observed than dolomite, the dissolution rate of dolomite was 103~105 times that of albite 26. Assuming that the Na ions in the solution were all derived from albite, the Mg ions are mainly from dolomite. The solubilities of albite and dolomite were calculated for the SH sample (as an example), based on Equ 13.

∗

100%

(13) 15 / 21


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In the formula, X is the mineral solubility (%); is the ion concentration (mol/L); is the volume of solution (L); is the total mineral quantity (mol) in the solid sample; i is the ion type; and j is the mineral phase. The dissolution rate of sodium feldspar is 15%, and the dissolution rate of dolomite is 13.8%. This finding contradicts the results of the previous studies and does not agree with the SEM results. In fact, the ionic release rates of aluminum silicate minerals such as feldspar and clay are sequential under acidic conditions. Due to the low activation energy, the layer ions (K and Na) are first released by H+ actions

35, 22

. The concentrations of Na ions in the sample solutions are three

orders of magnitudes higher than Al and Si ion concentrations, indicating that Na ions are initially released from albite. Therefore, the solubility of dolomite is greater than that of albite. 4.4.2. Secondary mineral precipitation and partial dissolution equilibrium The dissolution of calcite, dolomite, and chlorite can provide Ca, Fe, and Mg cations for the formation of secondary carbonates. However, secondary carbonate minerals do not precipitate at an SI of less than 0 because their contents in coal range from only 9.63 -15.61%, and the reaction time is short; therefore, it is difficult to reach solubility equilibrium. Scholars have observed the formation of secondary carbonates in long-term experiments (weeks, months, 1 year, etc.) using sandstone and mudstone

5, 36

. Moreover, the SI value of aluminum hydroxide minerals is the

highest, indicating that secondary aluminum hydroxide minerals may be more likely generated compared to other minerals as the reaction progresses. Although no secondary carbon sequestration minerals were generated, new layered aluminosilicate minerals were generated on the surface of aluminum hydroxide minerals after scCO2-H2O reactions. However, the SI values of aluminum silicate minerals are very low. Additionally, no new mineral precipitation should occur as the solutions are unsaturated to aluminum silicate minerals. However, the SI is calculated according to the concentration of the entire solution, and precipitation occurs in specific local regions. Due to the lack of stirring devices in the experimental apparatus, the ions released by the mineral dissolution cannot be rapidly and uniformly released into the entire solution but are concentrated near the mineral. Because aluminum hydroxide minerals (Al(OH)3) readily reacts with H+, a large number of Al3+ ions concentrate near the mineral in the solution. Additionally, combined with H4SiO4 and Fe ions already in solution and because of the low solubility of aluminum silicate, new aluminum silicate minerals are generated on the surface of aluminum hydroxide minerals. 4.5 The effect of mineral reactions on the reservoir structure The results showed that the porosity and total pore volume of the sample were increased after the test of mercury injection (Table 3). The porosity and pore volume of SH sample increased 16 / 21


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most because of the more carbonate minerals. Fig. 12 also shows that the incremental pore volume of the samples have increased after the reaction. The BET specific surface area, measured by liquid nitrogen adsorption (Table 3), also increased after the reaction. These are also consistent with the research of Liu 36, 37. Although the supercritical CO2 can extract organic matter in coal, but can only extract trace the small molecule functional groups. The effects on the pore size distribution, pore volume and specific surface area are small 38, therefore the changes of pores are mainly caused by the change of mineral composition in the experiments. Table 3 Pore parameters of the samples before and after the reaction Total pore volume (ml/g)

Porosity (%)

BET specific surface area (m2/g)

XJ-1

0.03460

4.29610

0.11000

XJ-2

0.03600

4.88340

0.19830

Sample ID

SH-1

0.03250

4.22150

1.35790

SH-2

0.07130

9.37930

1.73880

BF-1

0.03450

4.33190

0.15360

BF-2

0.03910

5.39730

0.40700

Fig. 12 Incremental pore volume of the samples before and after the reaction

Figure 8 shows that calcite and dolomite in plant cells undergo considerable corrosion, even exhibiting complete corrosion. This process forms a large number of pores, and the pore size and volume increase. Additionally, more pores become connected, and the permeability increases. As albite dissolution occurs, corrosion fractures form on the observation surface (Fig. 9b). These fractures are perpendicular to the observation plane and parallel to the dissolution of the crystal surface. Therefore, sodium feldspar dissolution also increased the permeability of the sample. In addition, the contact gaps between the coal matrix and the chlorite layers in cracks become larger after the reaction (Fig. 9d). This change may be caused by chlorite dissolution or the plasticizing of scCO2, and the increase in fractures increases the permeability of the samples. Therefore, the dissolution of carbonate, feldspar and chlorite, which are affected by ScCO2-H2O, can cause the pore volume and permeability to increase. After the reaction, chlorite edges extrude the surfaces of the samples, and these edges form large layered bending sheets (Fig. 9d) with specific surface areas larger than those of the original samples. Moreover, new layered aluminum silicate minerals are generated on the surface of aluminum hydroxide minerals due to its nanoscale interlayer structure, which results in the 17 / 21


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formation of a large number of nanoscale pores. These new minerals have larger specific surface areas than aluminum hydroxide minerals

5. Conclusion To study the influences of CO2 and water on minerals in coal under in situ reservoir conditions, self-developed experimental instruments were used in the laboratory to simulate the reactions of a scCO2-H2O-coal system under the conditions at a depth of 2000 m (80℃ and 20 MPa). The following conclusions were drawn from this study: (1) CO2 injection can cause the pH of deionized water to reach 3.77. The high activity of H+ increases the ion release rate at the beginning of the experiment. As the reaction continues, H+ is gradually consumed, and the ion release rate gradually decreases. (2) Due to the different mineral dissolution rates, the degree of dissolution varies greatly. scCO2-H2O causes considerable calcite dissolution and exposes internal calcite. This process is accompanied by the presence of crystal cones and new crystal surfaces. Additionally, the surface of dolomite exhibits many diamond-shaped corrosion pits. The chlorite is partially eroded and has a morphological change. Albite is readily dissolved along the surface of the crystal, and chlorite erodes and changes in appearance. The dissolution of illite, kaolinite and quartz were not obvious. (3) Due to low mineral content in the coal and the short experimental time, independent secondary carbon sequestration minerals did not form; however, the surface of aluminum hydroxide minerals reached partial dissolution equilibrium, and new layered aluminum silicate minerals were generated. (4) The dissolution of carbonate minerals, albite and chlorite increased the pore volume of the coal reservoir and improved the permeability of the samples. Moreover, the new formation of layered aluminum silicate minerals and the new occurrence of chlorite increased the surface areas of samples after the reactions. After the reaction, porosity, pore volume and surface area of the sample were larger, which also confirmed the positive transformation effect of the mineral changes on the reservoir.

Acknowledgement This work was supported by the National Natural Science Foundation of China (41330638, 41402135, 41272154), and the Scientific Research Foundation of Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education (China University of Mining and Technology) (2016-004).

Corresponding author E-Mail address: [email protected] (Yi Du); [email protected] (Shuxun Sang)

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Experimental Study of the Reactions of Supercritical CO2 and

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