Experimental Investigation of the Geochemical Interactions between

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Experimental Investigation of the Geochemical Interactions between Supercritical CO2 and Shale: Implications for CO2 Storage in Gas-bearing Shale Formations Yi Pan, Dong Hui, Pingya Luo, Yan Zhang, Lei Sun, and Ke Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03074 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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Experimental Investigation of the Geochemical Interactions between Supercritical CO2 and Shale: Implications for CO2 Storage in Gas-bearing Shale Formations Yi Pan1, Dong Hui*1, Pingya Luo*1, Yan Zhang2, Lei Sun1, Ke Wang1

1

The State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest

Petroleum University, Chengdu, Sichuan 610500, China 2

Northwest Sichuan Mining District, Southwest Oil and Gas Field Company, PetroChina,

Jiangyou, Sichuan 621700, China

ACS Paragon Plus Environment

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ABSTRACT Interactions between injected CO2 and shale formation during the process of CO2 sequestration with enhancing shale gas recovery (CS-EGR) may alter the physical and chemical properties of the rock, affecting the efficiency of CO2 storage as well as CH4 production. To better understand these interaction-induced changes in shale properties, two shale samples selected from marine Longmaxi formation and terrestrial Chang-7 Member of Yanchang formation were first reacted with supercritical CO2 (scCO2) in a laboratory batch reactor at 80 ℃ and 15 MPa with different time intervals, then characterization methods were designed to access the geochemical changes including optical microscope (OM), X-ray diffraction (XRD), element analysis (EA), lowpressure gas adsorption (LPGA) and fourier transform infrared spectroscopy (FTIR). The results indicate that the nanopore structure system of the two shale samples was significantly changed after scCO2-shale interaction due to the scCO2-induced extraction of hydrocarbons, chemical reactions in minerals and swelling effect in clay minerals as well as organic matter. However, after exposure to scCO2 the variation trend of pore structure parameters between the marine Longmaxi and terrestrial Chang-7 sample was quite different, which was related to the huge discrepancies in terms of mineralogy and geochemical properties between them. For marine Longmaxi sample, the pore surface area and pore volume obviously decreased after a relatively short period of scCO2 treatment, whereas an opposite trend was observed in terrestrial Chang-7 sample after long-term scCO2 treatment. In addition, an obvious decrease in fractal dimensions for marine Longmaxi sample was also observed after scCO2 exposure, reflecting the degree of pore surface roughness and pore structure complexity were reduced, whereas the terrestrial Chang-7 sample exhibited an opposite trend. The results contribute to the understanding of the


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potential factors for the pore structure evolution during long-term CO2 storage and the possible effect on the CS-EGR process. 1. INTRODUCTION With increasing demand for energy and gradual depletion of conventional crude oil and gas, unconventional resources have attracted extensive attention.1 Shale gas, as a low carbon and efficient energy resource, accounts for 50% of the unconventional natural gas resources and is mainly distributed in North America, China, and Latin America.2, 3 China was estimated to have the world’s largest shale gas resource of approximately 31 trillion cubic meters and has the greatest potential for developing shale gas in the world.4 In order to reduce air pollutants and meet the demand of economic development, the Chinese government has planned to produce 30 billion cubic meters of shale gas per year by 2020.5 Currently, the development of the horizontal well and multi-stage hydraulic fracturing techniques makes it possible to exploit the commercial shale gas production from shale formations which are known for ultralow porosity and permeability. However, hydraulic fracturing will cause severe environmental problems such as a large amount of water consumption and the pollution of groundwater.6 In addition, the reinjection of the flow-back fracturing water into deep geologic formations is linked to trigger injection-induced earthquakes.7-9 Therefore, non-aqueous fracturing has been developed over the last several years. Supercritical CO2 (scCO2), as an environmentally friendly non-aqueous fracturing fluid, is a promising alternative to water for shale gas fracturing, which can fundamentally solve the environmental problems produced by the water fracturing.10 In addition, experimental and theoretical studies have demonstrated that CO2 possesses a stronger absorption capacity relative to CH4 in organic-rich shale.11, 12 It is believed that as long as the carbon dioxide is injected into the shale formation, more trapped methane will be released due to the


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displacement mechanism.13-15 Thus, the injection of CO2 into gas-bearing shale formation can have the dual benefits of displacing the shale gas as well as reducing the CO2 emissions, which is well known as the concept of carbon sequestration with enhanced gas recovery (CS-EGR).14, 16 Generally, the gas in shale is stored in form of free gas in the macropores and fractures, absorbed gas on the organic matter and clay minerals, and dissolved gas in kerogen and water. Hence the pore structure parameters, including pore surface area, pore volume, pore size distribution and the fractal dimension, play a critical role in the gas adsorption, release and flow behaviors. Recently, various methods are applied to characterize the microstructure of shale. These techniques, including low-pressure gas adsorption (LPGA),17 mercury injection capillary pressure (MICP),18 ultra-small-angle and small-angle neutron scattering (USANS /SANS),19, 20 and ultra-small-angle, small-angle, and wide-angle X-ray scattering (USAXS/SAXS/WAXS) 21, 22

were widely used to gain insight into the knowledge of the pore structure. As the scCO2 is injected into the shale reservoirs, chemical and physical interactions

between the shale and fluids within the nanopore system can affect the properties of the shale, which can, in turn, influence the fluids transportation and storage capacity. Previous studies paid more attention to the CO2-brine-caprock chemical interactions for a better understanding of the sealing integrity of the caprock and the possible changes in its integrity23-26, providing information about the geochemical reactions such as the mineral dissolution and precipitation process as well as the changes in the pore structure. Moreover, these chemical interactions also greatly influence the mechanical characteristic of the shale resulting in a decrease in uniaxial compressive strength and Young’s modulus.27, 28 Other work suggested that scCO2 could dissolve the shale surface matter and change the surface property leading to a decrease in shale surface tension and surface water wetting, which was thought to be beneficial to CO2 diffusion


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into the shale matrix.2 In addition, the swelling phenomenon of the organic matter and the clay minerals in shale caused by CO2 adsorption has also been widely studied by many researchers.2931

They advocated that the adsorption and absorption-induced swelling in shale could result in

some problems including a reduction in the permeability, closing the cracks and even changing the formation. On the other hand, as a supercritical fluid, scCO2 (TC = 31.8 ℃ and Pc =7.38 MPa) 32 , which possesses the special properties of both gas and liquid, is capable of extracting and mobilizing some higher molecular weight in shale and coal. Palma et al.33 successfully extracted n-aliphatic hydrocarbons from the ground samples of Marcellus shale using scCO2 at 80 ℃ and 21.7 MPa. Jiang et al. 34 reported that the shale gas seepage channels were widened since the organic matter in shale was extracted by scCO2. Zhang et al.35 investigated the influence of scCO2 on the pore structure of coal. They documented that after scCO2 treatment the seepageflow pores were promoted and compounds with weakly polar functional groups decreased significantly while the strongly polar functional groups showed a slight change. Furthermore, it has been proved that these scCO2-coal interactions will finally change the gases’ high-pressure adsorption capacity, which may influence the security of long-term CO2 storage.36 However, due to the complex organic and inorganic compositions together with the heterogeneous pore structure in shale, the fundamental knowledge of the interactions between scCO2 and gas-bearing shale is lacking and it is still not clear how well scCO2 can affect the mineral compositions and nanopore structure system in different sedimentary shale formations, which is of significance for the optimization of fracturing process and the prediction of the longterm effects of CO2 sequestration. The objective of this paper was to fully investigate and analyze the possible changes in two samples selected from the marine Longmaxi Formation and


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the terrestrial Chang-7 Member of Yanchang Formation with different scCO2 treatment intervals. Characterization methods such as X-ray diffraction, element analysis, low-pressure gases (CO2 and N2) adsorption and infrared spectroscopy were used to reveal the possible mechanisms behind the changes and the potential effects on the CS-EGR process. 2. EXPERIMENTAL SECTION 2.1. Samples Collection and Geological Backgrounds Shale gas resources are widely distributed in marine and terrestrial basins in China.37 Geographically, marine shale is mainly found in South China such as the Sichuan Basin, while terrestrial shale is primarily located in North China such as the Ordos Basin. Chronologically, marine shale was deposited in the Early Paleozoic period while the terrestrial shale was deposited in the Mesozoic-Cenozoic.38 Two core samples were selected from two different sedimentary facies, respectively, including the terrestrial shale in the Chang-7 Member of Upper Triassic Yanchang Formation from the southwest of Ordos Basin and marine shale in the Lower Silurian Longmaxi Formation from the southeast of Sichuan Basin as marked in Figure 1 (a). The simplified stratigraphic column of the Paleozoic Silurian Longmaxi Formation in Sichuan Basin and upper Triassic Yanchang Formation in Ordos Basin is shown in Figure 1(b) and Figure 1(c),39, 40 respectively. Deposited in bathyal-to-abyssal sea and anoxic environment, the Longmaxi formation is mainly carbonaceous black shale and siliceous black shale with a vast area of 128×103 km2. 41 The value of vitrinite reflectance (Ro) in Longmaxi shale is in the range of 1.8% to 3.3% with an average of 2.69%, indicating the marine shale is over mature and is in the gas generation window.39 Meanwhile, containing plentiful dark grey-grayish mudstone and black shale, the organic-rich shale in Chang-7 formation was deposited during the maximum lake extension in


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the Late Triassic with an area of 100×103 km2.42, 43 Shale from Chang-7 formation is characterized by a lower thermal maturity with the value of vitrinite reflectance ranging from 0.7% to 1.3%, demonstrating that the shale is within the oil window stage.38 The basic information concerning the two studied samples is listed in Table 1. In order to prevent undesired changes caused by atmospheric oxidation, the selected samples were carefully preserved in sealed containers full of helium (He).


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Figure 1. Location map of the two studied samples (a), and the simplified stratigraphic column of the Paleozoic Silurian Longmaxi Formation in Sichuan Basin (b) and upper Triassic Yanchang Formation in Ordos Basin (c). Table 1. Basic Information of the Selected Samples


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sample

formation

location

TOC (%)

RO (%)

moisture content (%)

porosity (%)

Longmaxi

Longmaxi

Ordos Basin

3.81

2.5

1.7

3.4

Chang-7

Yanchang

Sichuan Basin

2.67

0.8

1.3

1.5

2.2. Supercritical CO2 Treatment Experiment The scCO2 treatment system mainly consisted of a carbon dioxide gas cylinder, a gas compression pump unit and a high-pressure reactor as shown schematically in Figure 2. CO2 (purity > 99.99%) was supplied from the Taiyu Industrial Gas Company, China. The highpressure reactor was produced by the Haian Oil Scientific Instrument Company, China. It can provide a maximum working pressure of 50 MPa and a maximum temperature of 300 ℃, respectively. The reaction pressure was set at 15 MPa based on the average pressure gradient (0.01 MPa/m) and the developed depth (about 1500 m) of the shale formations.44 Considering that a higher temperature can accelerate the reaction rate between the CO2 and shale, it may also better reflect the long-term physical and chemical changes that occurred in the formations.45 Thus, the experimental temperature was kept at 80 ℃, higher than the current reservoirs condition which was approximately 30-50 ℃ . Meanwhile, this temperature and pressure can ensure that the injected CO2 was in the supercritical fluid state. The two studied samples were saturated with scCO2 for different periods (10 days, 20 days and 30 days) in order to investigate the possible variation process. After reaching settled treatment days, the samples were carefully removed from the reactor for different characterizations. Note that the scCO2 treatment experiment was performed on the “as-received” shale samples without any wetting or drying process. The “as-received” water content may not accurately reflect the real situation of the water content in the shale formation because of additional water loss or an increase during the sampling process. In this paper the reaction pressure and temperature were set at fixed values,


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and our future studies will focus on the effect of various pressure and temperature conditions on CO2-shale interaction.

Figure 2. Schematic diagram of the scCO2 treatment system. 2.3. Characterization Methods The two bulk samples were broken and sieved to different meshes for various geochemical characterizations. Collected from the core fragments of the bulk samples, three subsamples were used for the surface morphology analysis. All the samples were divided into two groups. The first group was defined as the raw sample, and the other group was used for scCO2 treatment experiments, considered as the scCO2-treated sample. The characterization procedures and planned data points for each characterization are illustrated in Figure 3.


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Figure 3. Scheme of characterization procedures in this study. 2.3.1 Surface Morphology Analysis The raw surface morphology of the three subsamples (sample L-1, sample C-1 and sample C-2) was first studied using field-emission scanning electron microscopy (FE-SEM ,Quanta 450) equipped with energy dispersive spectroscopy (EDS). The FE-SEM method can provide highresolution images of the shale surface morphology, however, this method is not suitable for the analysis of the changes in shale surface nanostructure after scCO2 treatment since it requires a surface coating process for conductive purposes before each observation. And this coating may greatly influence the interactions between shale surface and scCO2, preventing possible reactions and giving rise to uncertainties in the characterization of possible variations on the same site with and without scCO2 treatment. Therefore, the raw surface topology of the three subsamples was also investigated by an optical microscope (Nikon Eclipse LV150N, Japan) which would not cause any damage to the raw samples during the observation process. Then, they were precisely re-located and re-observed using the same optical microscope after 30 days of scCO2 exposure.


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Furthermore, the high-resolution two-dimensional (2D) and three- dimensional (3D) morphology of both raw and CO2-treated samples were precisely recorded by the Extended Depth of Focus (EDF) module in the NIS-Element BR software equipped in the optical microscope system. 2.3.2. Element Analysis The carbon, hydrogen and nitrogen element contents of the untreated and scCO2-treated samples were tested by a Euro EA 300 element analyzer using the CHNS mode. An EA 300 element analyzer implements the chromatographic separation principle with a high sensitivity thermal conductivity detector to achieve an accurate analysis of C and H elements. The relative test error is less than 0.1%. 2.3.3. Mineral Composition Analysis Mineral alterations of the shale samples with and without scCO2 exposure were carried out on an X’Pert PRO MPD X-ray diffract meter at 40 kV and 30 mA with a Cu Kα radiation following the Chinese Oil and Gas Industry Standard SY/T5983-1994 and SY/T5163-1995. Prior to the XRD experiment, all the samples were milled to ultrafine particles and passed through a 200 mesh sieve. 2.3.4. Low-pressure N2 and CO2 Adsorption Low-pressure N2 and CO2 gas adsorption is a well-established approach for the characterization of the pore structural changes in shale samples before and after scCO2 treatment. According to the classification of the International Union of Pure and Applied Chemistry (IUPAC),46 the pores in shale are divided into three categories which are micropores (less than 2 nm), mesopores (between 2 and 50 nm) and macropores (larger than 50 nm).In general, the N2 gas adsorption method was used to evaluate the meso- and macropore structure and the CO2 was


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usually used to analyze the micropore structure. Both the N2 and CO2 adsorption isotherms were collected using an Autosorb IQ2 instrument produced by the American Quantachrome Instrument Corporation, adhering to Chinese National Standard GB/T19587-2004 and GB/T21650-2008. The relative pressure P/P0 (P is the gas vapor pressure and P0 is the saturation pressure of absorbent) was 0.003-0.03 for CO2 adsorption and 0.01-0.995 for N2 adsorption. According to previous study,47 different sample sizes affected the accuracy of the LPGA experiment and the particle sizes in the range of 60-140 mesh were recommended for both N2 and CO2 tests. Furthermore, the influence of the outgas temperature should also be considered since different outgas temperatures resulted in distinct results. It was suggested that low outgas temperatures were not suitable for the LPGA tests due to the release of absorbed moisture and possible hydrocarbon in micropores was not complete. As a result, the temperature of 250 ℃ was considered as a proper outgas temperature because it could maximize the results of LPGA while minimizing the changes in clay minerals or kerogen in shale.17, 48 For the N2 adsorption data, the surface area was interpreted by the multi-point Brunauer– Emmett–Teller theory (BET) equation 49, which was widely recognized as a standard method to evaluate the surface area of porous materials. On the basis of macroscopic and thermodynamic assumptions the Barrett–Joyner–Halenda (BJH)50 model was widely proposed as a standard pore size distribution (PSD) interpretation. However, it was found that this model underestimated the pore size distribution by up to 20-30% in particular for fine mesopores smaller than 10 nm.17, 46, 51

Alternatively, established on a different theoretical foundation, density functional theory (DFT)

is a more accurate method for the analysis of fine pores.46 Thus, both the BJH and the DFT model were considered for a comprehensive PSD analysis of the shale samples before and after


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the CO2 treatment. For CO2 adsorption data, the Dubinin–Radushkevich (D–R) model and the Dubinin–Astakhov (D–A) model were used for the special surface area and the pore volume analysis, respectively. In addition, the non-local density functional model (NLDFT) was considered for the PSD analysis. 2.3.5. Fourier Transform Infrared Spectrometry (FTIR) Analysis A Bruker Tensor FTIR spectrometer was employed to investigate the functional groups and chemical properties of two samples before and after 30 days of scCO2 treatment at the wavenumber ranging from 4000 cm-1 to 400 cm-1. 1 mg of the crushed shale samples (less than 200 mesh particle sizes) with 100 mg KBr were pressed into the pellet and then dried for 12 h in a vacuum oven to minimize the influence of the free water to the spectrum. 3. RESULTS AND DISCUSSION 3.1 Effect of scCO2 on the surface morphology The SEM images of the three raw subsamples are shown in Figure 4. From the figure, it can be seen that the surface morphology of the shale was quite complicated and heterogeneous. In addition, various substances were distributed irregularly on the shale surface. The analysis of their elements is listed in Table 2. It was found that these substances on the shale surface mainly contained C, O, Mg, Al, K, Ca and Si, indicating that the shale surface existed silicate minerals and carbonate minerals. The changes in the shale surface microstructure before and after 30 days of scCO2 exposure were observed with a metallurgical microscope as shown in Figures 5-7. After scCO2 treatment, some white and black substances disappeared (Figures 5-7, marked A), reflecting that scCO2 can effectively dissolve and extract the in-situ substances on the shale surface. Furthermore, the 3D figures also showed some changes in the roughness of the shale surface during the scCO2 treatment. For example, some of the sharp peaks became smooth after


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scCO2 (Figures 5-7, marked B), on the other hand, some smooth areas formed small peaks (Figures 5-7, marked C). These changes were related to the dissolution and extraction effect caused by scCO2.

Figure 4. Typical SEM images of the surface morphology of the raw shale samples Table 2. Analysis of the Mineral Elements in the Raw Shale Surface.

mineral element content (%) sample

formation

L-1

Longmaxi

C-1 C-2

C

O

Mg

Al

Si

S

K

Ca

Fe

19.99

28.64

0.41

6.47

35.22

0.56

3.92

3.79

0.99

Yanchang

7.64

25.33

2.09

10.30

49.84

-

1.77

3.03

-

Yanchang

21.11

21.95

2.36

19.01

28.78

-

2.91

1.05

3.89


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Figure 5. Surface morphology of Longmaxi shale before and after CO2 exposure. (Sample L-1)

Figure 6. Surface morphology of Chang-7 shale before and after CO2 exposure. (Sample C-1)


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Figure 7. Surface morphology of Chang-7 shale before and after CO2 exposure. (Sample C-2) 3.2. Effect of scCO2 on the Mineral Compositions The mineral compositions of two studied shale samples before and after scCO2 treatment are reported in Table 3. It can be seen that both of the two raw shale samples consisted of six mineral phases which were quartz, clay minerals, feldspar, calcite, dolomite and pyrite. However, the content of the different minerals in the two raw samples was distinct. The Chang-7 sample was characterized by a high content of clay minerals followed by quartz and feldspar, whereas the Longmaxi sample was highest in quartz with relatively minor amounts of other minerals. Due to the lack of enough amount of materials the specific kind of clay minerals in the samples was unknown. However, previous literatures have widely reported that the clay minerals were composed of illite, chlorite, kaolinite, and mixed-layer illite/smectite in both Longmaxi Formation shales and Yanchang Formation shales.39, 52


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Table 3. Mineral Compositions of the Samples before and after scCO2 Treatment

sample

treatment time (day)

Longmaxi

Chang-7

mineral compositions (%) quartz SiO2

calcite CaCO3

dolomite CaMg(CO3)2

feldspar K(AlSi3O8)/ Ca(Al2Si2O8)

pyrite FeS2

clay

0

62.39

3.16

4.76

5.41

4.27

19.99

10

57.05

2.91

5.39

7.25

2.34

25.04

20

54.02

2.63

3.67

10.64

2.21

26.82

30

52.68

2.84

3.95

17.67

2.33

20.52

0

23.26

3.01

2.97

20.69

2.56

47.49

10

20.87

2.97

4.03

21.71

1.41

49.01

20

21.06

3.31

4.09

22.84

1.96

46.73

30

24.26

1.19

1.08

22.29

3.77

47.39

Note: clay minerals are the total amount of illite, kaolinite, chlorite and mixed-layer illite/smectite. From Table 3, some variations in the mineral compositions were observed after exposure to scCO2. The slight changes were attributed to the sample heterogeneity and experimental uncertainty. It was believed that the XRD analysis may not accurately detect the small changes that occurred in the minerals during the scCO2 treatment and the sample heterogeneity may lead to variations up to 5 %.53, 54 Furthermore, the complicated chemical reactions induced by the scCO2 were also responsible for the interpretations of these changes. It is believed that the carbonic acid, which is formed by the dissolution of CO2 in water, can react with minerals in terms of dissolving and changing their compositions.55 When the CO2 diffuses into the nanopore system of shale, it will dissolve into the water film, leading to a series of reversible chemical reactions, as listed follows: 28, 54, 56 CO2 + H2O ↔ H2CO3 ↔ H++ HCO 3

(1)


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Quartz + 4H+ ↔ Si4+ + H2O

(2)

Alkali feldspar + 4H+ ↔ K+ + Al3+ + 3SiO2 + 2H2O

(3)

Plagioclase + 8H+ ↔ Ca2+ + 2Al3+ + 2SiO2 +4H2O

(4)

Illite + 8H+ ↔ 0.6K+ + 2.3Al3+ + 0.25Mg+ + 3.5SiO2 +5H2O

(5)

Chlorite + 16H+ ↔ 2.3Al3+ + 5Fe2+ + 3SiO2 + 12H2O

(6)

Calcite + 2H+ ↔ Ca2+ + CO2 + H2O

(7)

Dolomite +4H+ ↔ Ca2+ + Mg2+ + 2CO2 + 2H2O

(8)

It is interesting to note the different variation patterns in quartz content between the two shale samples during the process of scCO2 treatment. For the Longmaxi sample, the content of quartz constantly decreased from 62.39% to 52.68% with increasing time. However, the quartz content in the Chang-7 sample showed a slight decrease from 23.26% to 20.87% for the initial 10 days of treatment while gradually increasing to 24.26% after 30 days treatment. In general, the increased quartz content results from the dissolution of other minerals (such as feldspar and clay minerals), while the drop behavior is attributed to the dissolution of quartz itself, as shown in Equations (2)-(6). Nevertheless, similar experimental research performed by Yin et al.44 demonstrated that the content of quartz increased for all shale samples, instead of a decrease trend after scCO2 treatment. That may be explained by the different reaction conditions. The temperature of 40 ℃ was selected as their experimental condition, while in our study the reaction temperature was set at 80 ℃. Previous literature reported that higher temperature could speed up the rates of silicate-CO2 reactions by the increase in relevant chemical kinetic parameters.45, 57 Rathnaweera et al.56 studied the interactions between scCO2 and sandstone in saline aquifers at 40 ℃ over a duration of 1.5 years and found a significant quartz dissolution behavior after long-


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term CO2 treatment. They suggested that the corrosive behavior of the quartz could not be observed over a short period of time or under low temperature condition. The clay content in two samples, meanwhile, experienced first an increase and then a decrease process and finally showed slight variations compared to the raw samples after 30 days of scCO2 treatment. In addition, as one of the brittle minerals, the feldspar content in two samples increased significantly with increasing time. Similar phenomenon was also observed by Busch et al.58 who documented that the feldspar content increased from 0.04 to 0.09 g/g within durations ranging from 2 days to 32 days at 15 MPa and 50 ℃. Changes in the carbonate minerals (calcite and dolomite) content are of great significance for the study of long-term CO2 sequestration. Mineral trapping, which is one of the most important CO2 storage mechanisms, sequesters CO2 in the form of carbonate minerals through chemical reactions with the magnesium and calcium ions provided by silicate minerals.55 Based on the reaction kinetics, it seems that the most immediate reaction may occur in the carbonate minerals.57 The dissolution and precipitation process of calcite, as well as the dolomite, were also observed with different scCO2 exposure periods, indicating that there were complex chemical reactions in the mineral trapping process and further research was required. 3.3. Effect of scCO2 on the Element Content Table 4 shows the content of carbon, hydrogen and nitrogen elements before and after scCO2 treatment. The carbon content was 7.04% and 4.99% in the raw Longmaxi and Chang-7 shale sample, respectively, which was the total amount of the carbon element including organic carbon and inorganic carbon in the shale sample. From the table, we can observe that there was a decrease in carbon and hydrogen content for the two samples (especially for the Longmaxi sample) after scCO2 treatment for 10 days while they slightly increased with increasing time.


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These variations were presumed to be the results of extraction of organic matter as well as the chemical reactions in shale minerals caused by scCO2. In addition, no significant changes in nitrogen content was found after scCO2 saturation. That can be explained by the limited dissolving capacity of scCO2 for the polar macromolecules. The organic-rich shale is generally considered as a source rock owing to the abundant organic matter content. The kerogen, which contains several types of elements including carbon, hydrogen, nitrogen and sulfur, accounts for 80% to 90% of the total organic matter. As a macromolecular polymer with different polar functional groups, the structure and composition of kerogen is very complex and, still has no fixed expression.59 Although previous studies have indicated that scCO2 could mobilize a wide range of hydrocarbons from the organic-rich shale, the results confirmed that scCO2 had a limited impact on the kerogen due to its macromolecular structure and strong polarity. Table 4. Content of C, H and N Elements in Shale before and after scCO2 Treatment sample

Longmaxi

Chang-7

treatment time

carbon

hydrogen

nitrogen

(day)

(%)

(%)

(%)

0

7.044

1.181

0.935

10

6.386

1.087

0.964

20

6.450

1.050

0.901

30

6.425

1.016

0.944

0

4.736

0.507

0.15

10

4.599

0.428

0.16

20

4.947

0.464

0.12

30

4.986

0.467

0.14

3.4. Effect of scCO2 on the Pore Structure System 3.4.1. Adsorption Isotherms


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Low-pressure CO2 and N2 adsorption isotherms for the two shale samples are shown in Figure 8 and Figure 9, respectively, demonstrating obvious changes in adsorption before and after scCO2 treatment. From the figures, the raw Longmaxi sample showed the highest CO2 and N2 adsorption quantity while the adsorption quantity decreased significantly after exposure to scCO2. Conversely, although the raw Chang-7 sample exhibited the least amount of CO2 adsorption, the adsorption ability gradually increased with increasing time. The changes in adsorption caused by scCO2 were attributed to variations of the surface area and pore volume in nanopores, which will be discussed later in this paper.

Figure 8. CO2 adsorption isotherms of raw and scCO2-treated shale samples for (a) marine Longmaxi shale and (b) terrestrial Chang-7 shale. According to the definition of IUPAC,46 adsorption isotherms can be divided into six different types. The CO2 adsorption isotherms obviously belonged to Type I, indicating mircoporous solids, while the N2 adsorption isotherms were of Type II, which was related to the multi-layer adsorption. In fact, whether samples were raw or scCO2-treated both the N2 and CO2 isotherms showed similar adsorption isotherm shapes.


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Figure 9. N2 adsorption (filled symbols) and desorption (open symbols) isotherms of raw and scCO2-treated shale samples for (a) marine Longmaxi shale and (b) terrestrial Chang-7 shale. Furthermore, the reversed S-shaped N2 adsorption isotherms could be divided into a low relative pressure segment (P/P0 = 0.05~0.45) and a high relative pressure stage (P/P0 = 0.45~0.995). The adsorption curves showed a slow and stable rise in the low relative pressure stage, reflecting the nitrogen molecule adsorbed on the shale surface as being in the monomolecular state in the micropores. For the high relative pressure segment, the slope of the curves was higher than the previous stage, which was indicative of the multi-layer adsorption in the mesopores. In addition, due to the presence of the macropores, the curves rose rapidly in terms of relative pressure between 0.9 and 1, dominated by the capillary condensation mechanism. In addition, the adsorption-desorption isotherms of N2 before and after scCO2 treatment exhibited a clear hysteresis loop as illustrated in Figure 9. As a typical characteristic of the microporous material, the hysteresis loop is defined as the detachment of the desorption branch from the adsorption branch due to the presence of the capillary condensation and evaporation in the mesopores. Meanwhile, the “forced closure” phenomenon of desorption


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branch at the relative pressure of 0.45 was also observed. This process was known as the Tensile Strength Effect (TSE) which resulted from the instability of the hemispherical meniscus during the desorption process in pores with critical diameter of approximately 4 nm.60, 61 Generally, the hysteresis loop can be correlated with the pore shapes. According to the classification of IUPAC,46 there are five pore types in microporous material, which are designated H1 to H5. It appears that both of the marine Longmaxi sample and the terrestrial Chang-7 sample were similar to the H3 type, which was related to the silt- or plate shaped pore. And this slit-shaped pore has been widely observed by many researchers not only in marine shales, 41, 62 but also in terrestrial shales.63 From Figure 9, it was clear that the hysteresis loops in the raw and scCO2treated samples exhibited slight shape discrepancies, indicating that the scCO2 had a limited impact on the pore shape, which was in accordance with the conclusion reported by previous studies.44, 64 3.4.2. Surface Area and Pore Volume The pore structure parameters determined by the CO2 and N2 adsorption after different scCO2 exposure periods are shown in Table 5. It can be seen that for marine Longmaxi sample the D-R surface (SDR) of 24.740 m2/g and BET surface (SBET) of 20.669 m2/g were much higher than those in terrestrial Chang-7 sample. These discrepancies have been linked to the different sedimentary environments.37, 38 Meanwhile, a wide variation in pore structure parameters after scCO2 exposure was also observed. These scCO2-induced variations significantly exceeded the variations caused by the experimental error (3%-4%). Therefore, it required a careful interpretation of these changes. For the initial 10 days of scCO2 treatment, the SD-R and SBET for Longmaxi sample significantly decreased by 69.19% (from 24.740 m2/g to 10.621 m2/g) and 48.96% (from 20.669


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m2/g to 10.550 m2/g) respectively, reflecting that some miropores or smaller mesopores were transformed into larger mesopores or macropores. On the other hand, there was a slight change in the pore structure parameters for Chang-7 sample. It appears that compared to the terrestrial Chang-7 sample the initial stage of scCO2 treatment had a more significant impact on the pore structure of the marine Longmaxi sample, especially on the mircopores. Generally, the changes in pore structure caused by scCO2 may be related to three possible interpretations as shown in Figure 10: (1) organic matter extraction mechanism. (2) dissolution and precipitation of the minerals. (3) adsorption-induced expansion mechanism. According to previous results of the EA and XRD analysis in this paper, the extraction effect caused by scCO2 may be mainly responsible for the relevant changes at this stage. As confirmed above, scCO2 can dissolve some organic matter, especially the soluble hydrocarbon which fills in the nanopore medium. Furthermore, previous literature also documented that the over-mature marine Longmaxi shale was dominated by the organic matter pores which made major contribution to the pore surface and pore volume, while as to the less mature terrestrial Chang-7 shale, which was characterized by the interlayer pores, it was the clay minerals together with the organic matter that jointly contributed to the surface area and the pore volume.38 Thus we deduced that the dissolving extraction mechanism at the initial scCO2 treatment stage was more effective for the organic-rich marine Longmaxi sample, leading to the obvious changes in the pore structure parameters.


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Figure 10. Schematic illustration of the three possible mechanisms that were responsible for the changes in the pore structure. Raw sample (a) and scCO2-treated sample (b). However, this conclusion was not fully applicable to the changes in pore structure parameters in the next scCO2-shale interaction stage since limited organic matter moieties may be extracted in a brief period. 33 As illustrated in Table 5, with increasing intervals from 10 days to 30 days, the meso- and macropore structure parameters in Longmaxi sample and Chang-7 sample showed quite different variation trends. The SBET for Longmaxi sample kept on showing a decrease of 42.91% (from 10.550 m2/g to 6.023 m2/g), while for Chang-7 sample it exhibited an opposite increasing trend with the value of SBET increased by 94.09% (from 7.008 m2/g to 13.602 m2/g). One possible explanation for this phenomenon was the complex minerals dissolution and precipitation mechanism caused by scCO2. The intraparticle pores, which are defined as the pores between the brittle minerals (such as quartz, feldspar and carbonate


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minerals) in the range of 50-200 nm, are generally thought to be in a chemically unstable state.65 Taking into consideration the fact that these brittle minerals also contribute to the pore surface area and pore volume, it is expected to relate the changes in meso- and maropore structural parameters with the variations in mineral compositions. According to the comparative analysis from the Table 3 and Table 5, we can observe that the quartz may have a weak positive effect on SBET and VBJH. This phenomenon was consistent with the studies conducted by Yang et al.66 and Huang et al.52 to some extent , nevertheless, in their work this relationship was observed by statistical method based on the experimental analysis of several raw shale samples. Anyhow, one undeniable fact is that the large quantity of mesopores is preserved due to the protection of the brittle minerals.61, 67 Therefore, it appears that the dissolution and precipitation of the brittle minerals as well as the clay minerals will collapse or enlarge the original intraparticle pores, leading to variations in meso- and macropore structure parameters. On the other hand, theoretical studies have proved that CO2 adsorption will cause the expansion of organic matter and clay minerals, which will further change the pore morphology. For organic matter, CO2 adsorption will reduce the surface potential followed by the swelling mechanism to increase the surface area 68

, while the expansion of the clay minerals results from the intercalation of CO2 in their

interlayer region.69 Hence, it seems that for Chang-7 sample the huge content of the clay minerals may play a critical role in the increase of SBET. This conclusion was partly supported by the experimental data reported by Lahann et al.70 and Yin et al.44, in which there was a similar increase in SBET for the clay-rich shale while an opposite decrease in SBET for clay-poor shale after the scCO2 treatment.


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Table 5. Pore Structure Parameters before and after scCO2 Treatment treatment sample

Longmaxi

Chang-7

CO2

N2

time (day)

SD-R (m2/g)

VD-A (ml/g)

SBET (m2/g)

VBJH (ml/g)

0

24.740

0.031

20.669

0.041

5

10.621

0.005

10.550

0.037

15

10.893

0.005

6.280

0.030

30

11.586

0.005

6.023

0.029

0

11.094

0.006

8.834

0.038

5

10.812

0.006

7.008

0.027

15

12.413

0.006

9.520

0.033

30

15.461

0.009

13.602

0.045

In contrast to the complex changes in meso- and macropores, the mircopore structure parameters of both Longmaxi and Chang-7 sample revealed the same increasing trend with increasing intervals from 10 days to 30 days. Analysis from Table 5 demonstrated that the SDR of Longmaxi and Chang-7 samples increased by 9.08 % (from 10.621 m2/g to 11.586 m2/g) and 42.99 % (from 10.812 m2/g to 15.461 m2/g), respectively, indicating that the amount of miropores increased. An alternative explanation for the uniform increase was because the swelling effect became a dominated factor for the changes in mircopores. Ross et al. 71 reported that the clay minerals developed huge amounts of interparticle pores with an effective radius in the range of 1- 2 nm. Therefore, we deduced that compared to Longmaxi sample the significant increase of SDR in Chang-7 sample was also related to the large content of the clay minerals. However, due to the complexity and heterogeneity of the shale composition further studies are still required to clarify this phenomenon. 3.4.3. Pore Size Distribution


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The pore size distribution is defined as the distribution of the pore volume or surface area in relation to the pore size and these distribution curves can be applied to obtain some qualitative information such as the pore size range and dominant pore size. Figure 11 presents the PSDs of the micropores determined by CO2 adsorption using the DFT method before and after scCO2 exposure. Meso- and macropore size distributions using the DFT and BJH methods with the adsorption branch of N2 isotherms are illustrated in Figure 12 and Figure 13, respectively. In particular, the DFT analyses primarily concentrated on the smaller mesopores (

Experimental Investigation of the Geochemical Interactions between

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