Supercritical CO2-water-shale interactions and their effects on element mobilization and shale pore structure during stimulation

Accepted Manuscript Supercritical CO2-water-shale interactions and their effects on element mobilization and shale pore structure during stimulation

Xiangrong Luo, Xiaojuan Ren, Shuzhong Wang PII: DOI: Reference:

S0166-5162(18)30807-3 https://doi.org/10.1016/j.coal.2018.12.007 COGEL 3136

To appear in:

International Journal of Coal Geology

Received date: Revised date: Accepted date:

8 September 2018 18 December 2018 19 December 2018

Please cite this article as: Xiangrong Luo, Xiaojuan Ren, Shuzhong Wang , Supercritical CO2-water-shale interactions and their effects on element mobilization and shale pore structure during stimulation. Cogel (2018), https://doi.org/10.1016/j.coal.2018.12.007

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ACCEPTED MANUSCRIPT Supercritical CO2-water-shale Interactions and Their Effects on Element Mobilization and Shale Pore Structure during Stimulation Xiangrong Luo a, *, Xiaojuan Ren a, Shuzhong Wang b Engineering Research Center of Development and Management for Low

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a

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to Extra-Low Permeability Oil & Gas Reservoirs in West China, Ministry

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of Education, Shaanxi Key Laboratory of Advanced Stimulation Technology for Oil & Gas Reservoirs, School of Petroleum Engineering,

Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of

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b

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Xi'an Shiyou University, Xi'an, Shaanxi, 710065, China

Education, School of Energy and Power Engineering, Xi'an Jiaotong

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University, Xi'an, Shaanxi, 710049, China *Corresponding Address: No.18, East Section Second Dianzi Road, Xi'an,

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Shaanxi, 710065, China

*Corresponding Author. Tel.: +86 29 8838 2670

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*E-mail address: [email protected]

Abstract: There are many advantages to using supercritical carbon dioxide (ScCO2) fracturing technology to exploit shale gas reservoirs in China, including minimal damage to the environment or formation, and displacing methane (CH4) in the adsorbed state. When ScCO2 enters fractures in the formation, ScCO2-water-shale interactions may affect the

ACCEPTED MANUSCRIPT physicochemical properties of shale. In this study, a high-pressure reaction system was adopted to simulate ScCO2-water-shale interactions under ScCO2 stimulation conditions. The element mobilization and pore structure before and after the reaction were measured using ICP-MS, XRF.

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The results show that the major elements, including Ca, Mg, Na, K, and

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Al, exhibit varying degrees of mobilization after the interactions because

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of dissolution of carbonate and silicate minerals in shale samples. Compared with the major elements, trace elements have a lower mobility,

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quantified as less than 13.97%. The specific surface areas and pore

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volumes of two shale samples increase at different degrees after the reaction. The interactions have a more significant influence on the

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micropores. In addition, fractal features of the shale pore structure were analyzed. The fractal dimensions of the shale samples increase after the

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reaction, indicating that pore surface roughness increases, and pore structure morphology gradually transforms from regular to complex.

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Keywords: Supercritical CO2; shale gas; element mobilization; pore structure; fractal dimension 1. Introduction

The US has accelerated shale gas development in recent years. This not only changes the structure of supply and demand of natural gas in the United States but also has a significant impact on global energy supply (Weijermars, 2014; Eshkalak et al., 2014; Kim et al., 2017). Shale gas

ACCEPTED MANUSCRIPT resources in China show great potential with recoverable resources estimated to be 25.08×1012 m3. According to U.S. development experience, stimulating reservoir volume using slickwater is the key to success (Gu and Mohanty, 2014; Yuan et al., 2015). However, this kind of

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development technology consumes large amounts of water resources and

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may bring about environmental problems and formation damage

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(Ziemkiewicz et al., 2014; Luo et al., 2015; Luo et al., 2018). Compared with relatively abundant water resources in the United States, water is

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already scarce in China. Thus, it is difficult to meet the water demand for

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stimulating shale gas. Supercritical CO2 (ScCO2) fracturing is a new type of waterless stimulation method, which can be applied to the

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development of shale gas reservoirs with numerous advantages (Middleton et al., 2015; Zhong et al., 2015; Xian et al., 2015; Li et al.,

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2016). As supercritical CO2 is used to stimulate shale gas reservoirs, the carbonic acid that is highly reactive with some minerals in shale can be

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formed due to the interactions between supercritical CO2 and the subsurface water or slickwater (Jean et al., 2015; Yin et al., 2016). Under the conditions of interactions of supercritical CO2, water and shale, many physical and chemical properties of shale may be changed, including the element occurrence in shale, pore structure, adsorption properties, and mechanical properties (Massarotto et al., 2010; Gaus, 2010; Talman et al., 2013). Therefore, study of ScCO2-water-shale interactions under

ACCEPTED MANUSCRIPT simulating treatment conditions is important for ascertaining the stimulating mechanism of supercritical CO2; however, there is limited research on this topic. Previous studies have shown that carbonic acid, formed during CO2

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dissolution in water, can dissolve and mobilize major and trace elements

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in CO2 storage reservoirs (Lin et al., 2008; Rempel and Liebschner, 2011;

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Jean et al., 2015). Mobilization of elements is usually ascribed to the dissolution of the mineral matter in the shale. Under the interactions of

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the scCO2-water-rock, many kinds of minerals are dissolved, including

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calcite, dolomite, plagioclase, illite, chlorite, magnesite, feldspar, and siderite (Alemu et al., 2011; Wellman et al., 2003; Wang et al., 2013;

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Rathnaweera et al., 2016). In addition, dissolution of the mineral matter leads to a physical change of the shale. Several studies have shown that

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under ScCO2-H2O treatments, the pore structure of coal changed as a result of the dissolution of mineral matter in the coal (Kumar and Shankar,

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2000; Karacan, 2007; Ozdemir, 2016). According to Liu et al. (2010), the ScCO2-H2O-coal interactions increase the porosity and total pore volume in coal under the conditions of CO2 geological sequestration. Dry ScCO2 also affects physical properties of rock. During an experimental study on the interactions of shale-ScCO2, Yin et al. (2016) observed that the micropores were most affected, and the specific surface area decreased after the interactions.

ACCEPTED MANUSCRIPT This study focused on the key problems of ScCO2-water-shale interactions, such as mineral element mobilization, and microscopic pore structure changes. In our study, shale samples from Qaidam Basin and Ordos Basin in China were collected and used to experimentally study

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ScCO2-water-shale interactions under supercritical CO2 stimulation

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conditions. The element mobilization, changes in pore structure, and

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mineral composition after the reaction were discussed in depth. The mineralogy and fractal theory were also used to analyze the mechanisms

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of ScCO2-water-shale interactions with respect to element mobilization

2. Experimental section

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2.1. Samples and materials

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and pore structure.

In this study, shale samples were obtained from Qaidam Basin and

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Ordos Basin in China. Jurassic source rocks are mainly distributed in the northern margin of Qaidam Basin, and the Lower and Middle Jurassic

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dark and carbonaceous shales are developed. The shale samples used in our experimental work were obtained from the Middle Jurassic Dameigou formation at a depth of approximately 1307 m. The Mesozoic Triassic Yanchang formation, mainly distributed in the south of Ordos Basin, consists of high-quality lacustrine source rocks with dark shale. Several sets of source rocks from Chang 10 to Chang 1 are developed, among which Chang 7 and Chang 9 are very important Mesozoic source rocks.

ACCEPTED MANUSCRIPT Chang 7, which is located in the southeastern Ordos Basin, was sampled at a depth of 1053 m. Samples were then sealed and transported to the laboratory. In the laboratory shale samples were crushed to grains ranging from 0.6 to 0.9 mm in size and divided into two by sieving method. One

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was used to interact with ScCO 2 and water; another one was used to

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perform the element and pore structure test before interaction. A mass of

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each sample is approximately 150 g. The shale samples from Qaidam Basin and Ordos Basin were labeled as CS1# shale and FS1# shale,

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respectively. Ultrapure water (resistivity 18 mΩ·cm at 25°C) and

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food-grade CO2 with a purity higher than 99.999% were used in the experiments.

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2.2. Experiment of supercritical CO2-water-shale interactions 2.2.1. Experimental setup

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The experimental setup (Fig. 1) consisted of a vacuum-pumping system, compression and gas injection system, high-pressure reaction

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system, and data collection system. In the compression and gas injection system, CO2 was compressed and injected into the reactor using an ISCO pump. The 500-mL volume high-pressure reaction system consists of a constant temperature oven with an accuracy of ±0.1°C and a high-pressure reactor made from hardening alloy materials that are resistant to acid and alkali. There is a removable sample cell in the reactor with a stainless steel strainer having a mesh size of 25 μm. A pressure

ACCEPTED MANUSCRIPT transmitter and a resistance temperature detector (RTD) are connected to a computer to monitor and record the changes in pressure and temperature. The system temperature can reach 200°C, and the pressure can reach 30 MPa. In addition, there is some auxiliary equipment,

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including an X-ray diffractometer (XRD) used to test mineral

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composition, inductively coupled plasma mass spectrometer (ICP-MS)

analyzer.

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2.2.2. Experimental parameter design

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used to test trace elements, and specific surface area and pore size

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The critical temperature and pressure of CO2 were 31.1°C and 7.38 MPa, respectively. The surface tension of CO2 under a supercritical state

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is zero. Therefore, supercritical CO2 can enter any space greater than the supercritical CO2 molecular size and interact with minerals in the shale.

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The properties of CO2 near the critical point are adjustable and mutational. For example, the viscosity, diffusion coefficient, and density will vary as

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a function of temperature and pressure. Therefore, the temperature and pressure in the experiment will have an important effect on physicochemical properties of shale. Experiments were performed at 50°C and 10 MPa to simulate the interactions of ScCO2-water-shale under practical treatment conditions and to ensure relative safety and low energy consumption. The reaction time also has a certain influence on the results of the

ACCEPTED MANUSCRIPT experiment. At early times, the reaction will be incomplete; however, at later times, the reaction may be complete. Previous studies have shown that equilibrium will be reached at later times. Thus, the experiment time was set to 96 hours. In this study, a 150-g shale sample was placed in the

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reactor at three different times, and the experiments were carried out

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under the same conditions to ensure that the shale samples were in full

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contact with the supercritical CO2 and water. For example, 350 g of ultrapure water and a 50-g shale sample were simultaneously placed in

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2.2.3. Experimental procedures

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the reactor to form a water/rock ratio of 7:1.

The experiment proceeded as follows: Prior to the experiments, shale

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samples were repeatedly cleaned using ultrapure water and were then dried in a vacuum drying oven at 40°C for 48 hours. Then, valve 3 was

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closed and the shale samples and the ultrapure water were poured into the reactor. The vacuum pump was opened for 20–30 min before injecting

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CO2 to remove the effect of oxygen on the reaction. Then, CO2 was injected into the intermediate container and the reactor by opening the corresponding valves connected to the gas cylinder. After the pressure in the system stabilized, the ISCO pump was opened to compress CO2 in the reactor, and at the same time, the reactor was heated. Temperature and pressure changes were observed on the computer in real time. After the pressure and temperature reached the required values, valve 2, valve 1,

ACCEPTED MANUSCRIPT and the ISCO pump were closed. During the reaction, the temperature and pressure of the reactor were monitored in real time. When the deviation of pressure and temperature exceeded 0.1 MPa and 1°C, respectively, they were regulated to the required parameters. For example,

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when the system pressure decreased, valve 2, valve 1, and the ISCO

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pump were opened to compress CO 2, thus the system pressure increased.

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Valve 2, valve 1, and the ISCO pump were closed until the system pressure reached the required value and remained unchanged. After the

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reaction, the heating system of the reactor was closed; then, water was

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discharged from the reactor into the leachate tank by opening valve 3 and valve 4. The elements in water were analyzed using an iCAP 6000

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inductively coupled plasma atomic emission spectrometer (ICP-AES). Gas was discharged from the reactor through a vent by opening the

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exhaust valve below the reactor. Finally, the shale samples were taken out of the reactor and dried using a vacuum drying oven at 40°C for 48 hours.

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The major elements comprising the shale samples were analyzed using a Venus200 X-ray fluorescence (XRF) spectrometer, and the trace elements were analyzed using an ICP-MS. Mineral composition was evaluated using a D/max-2500 X-ray diffractometer. The specific surface area and pore size distribution of shale samples were measured using N2 adsorption at 77 K in a fully-automatic V-Sorb 2800P specific surface area and pore size analyzer.

ACCEPTED MANUSCRIPT 2.2.4. FHH fractal theory The pore surface of a natural porous medium is not strictly smooth, but very rough, with fractal characteristics. The fractal dimension, D, is an important parameter to identify structural complexity and surface

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roughness of rock with a value range of 2–3. The fractal dimension of a

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flat smooth surface is 2, and the fractal dimension of an irregular and

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rough surface is 3. Many methods have been used to obtain the fractal dimension, including gas adsorption, scanning electron microscopy

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(SEM), and small angle X-ray scattering. Among these, the gas

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adsorption method is very simple and effective and is also commonly used. FHH theory is often used to predict the fractal dimension (Pfeifer et

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al. 1989). According to the tested nitrogen adsorption data, the fractal dimension, D, can be obtained based on the FHH model (Cai et al., 2013;

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Yin et al. 2016):

P0 ) P

(1)

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ln V  C  ( D  3) ln(ln

where V is the adsorption volume of liquid nitrogen under balanced pressure, mL; P0 is the gas saturation pressure, Pa; and C is a constant. According to the experimental data of low temperature liquid nitrogen adsorption, lnV and ln[ln(P0/P)] are used as the ordinate and abscissa, respectively, and then the curve is fitted, and the slope, λ, can be obtained. According to D=3+λ, the fractal dimension, D, can be calculated. 3. Experimental results

ACCEPTED MANUSCRIPT 3.1. Blank experiment The sample cell and reactor used in the experiments can cause interference to elements in water, so in this study the blank experiment was conducted after the experiments on ScCO 2-water-shale interactions.

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There were no shale samples in sample cell in the blank experiment. In

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ideal conditions the contents of elements in water should be close to zero

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after the blank experiment. The test results of major and trace elements in water after the blank experiment are shown in Table 1. It can be seen that

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the content of major and trace elements is all below detection limit. This

the experimental results.

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fully shows that the materials of reactor and sample cell have no effect on

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3.2. Characteristics of element mobilization Carbonic acid is formed after CO2 is dissolved in water, and the

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mineral matter is dissolved and mobilized from the shale. This is the essence of ScCO2-water-shale interactions under supercritical CO2

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stimulation conditions. Element mobilization can be studied by observing changes in the content of the major and trace elements in the shale before and after the reaction. The corresponding ion concentration change in aqueous solution can support the element mobilization characteristics from another perspective. The element mobility, η, was introduced to more accurately reflect the relative mobilization characteristics of the elements in shale that resulted from the ScCO2-water-shale interactions.

ACCEPTED MANUSCRIPT The formula for quantifying element mobility is as follows:   (Cb - Ca ) / Cb 100%

(2)

where η is element mobility, %; Cb is the element content in shale before the reaction, μg/g; and Ca is the element content in shale after the reaction,

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μg/g. If the mobility calculation results in a positive element mobility,

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then the element has been mobilized.

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3.2.1. Major element mobilization

The test results of the major elements in shale samples before and after

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the reaction are presented in Fig 2. Fig 3 shows the test results of the

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major elements in the water sample after the reaction. As shown in Fig 2, the major elements all exhibit a positive mobility, indicating that the

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major elements in shale samples have varying degrees of mobilization after ScCO2-water-shale interactions. Overall, for CS1# and FS1# shale

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samples, Ca, Mg, Na, K, and Al show high mobility, whereas the mobility of Fe, Si, P, and Mn is low, with a maximum of 3.69%. The mobility of

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the major elements varies substantially for the shale samples. In CS1# shale samples the mobility of Ca is only 6.73%, whereas the mobility of Ca reached 38.15% in FS1# shale samples. The difference in the mobility of the major elements in shale samples is closely related to the mineral composition and occurrence of elements. As shown in Fig 3, Ca, Na, K, and Mg are enriched in the water sample, which is basically consistent with the test results of the major elements in shale samples.

ACCEPTED MANUSCRIPT 3.2.2. Trace element mobilization Fig 4 and Fig 5 show the test results of the trace elements in shale samples before and after the reaction and the trace elements in the water sample after the reaction, respectively. As shown in Fig 4, the trace

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elements in the shale samples also exhibit positive mobility. Compared

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with the major elements, the mobility of the trace elements is relatively

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low. The mobility of the trace elements in CS1# shale samples is 0.57– 13.97% versus 0.62–10.04% in FS1# shale samples. For CS1# and FS1#

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shale samples, the trace elements that have a relatively strong

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mobilization ability are mainly Sr, Zn, and Co, followed by Ba, Ni, and Cr. The mobility of Se, V, and Li is low, with a maximum value of 3.78%.

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As shown in Fig 5, Sr, Zn, Co, and Ba are enriched in the water, which is similar to the test results of trace elements in shale samples. This also

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confirms the mobilization characteristics of the trace elements in shale samples from another perspective.

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3.3. Pore structural changes 3.3.1. N2 adsorption isotherms The N2 adsorption isotherms of shale samples before and after the reaction are shown in Fig 6. The adsorption and desorption curves are close to the X axis at low pressure (P/Po<0.1), indicating that the interaction force between the nitrogen gas and shale is weak and the adsorption capacity is very small. However, the curves rise sharply at

ACCEPTED MANUSCRIPT high pressure (P/Po=0.9–1). This phenomenon shows that the pores of the shale samples are mainly micropores and mesopores (<50 nm), while there is a small percentage of macropores (>50 nm) (Sing 1985). According to the classification of BDDT (Brunauer et al., 1940), the N2

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adsorption isotherms of shale samples can be classified as category II,

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implying multilayer adsorption. The adsorption and desorption curves do

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not coincide and have a hysteresis loop. This shows that capillary condensation occurred in the tested shale samples (Gregg and Sing, 1992). P/Po

is

0.4–0.99,

there

are

hysteresis

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When

loops

in

the

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adsorption-desorption isotherms; when P/Po is less than 0.40, the adsorption and desorption curves almost overlap. The hysteresis loops

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appear with increasing P/Po, implying that open pores exist in the shale (Yin et al., 2016). The shape of the hysteresis loop between the

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adsorption and desorption branches can provide information about the shale pore structure (Sing, 1985; Chalmers et al., 2012). According to the

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classification of the International Union of Pure and Applied Chemistry (IUPAC) (Brunauer et al., 1940), the hysteresis loop of CS1# shale can be classified as category H2, indicating that ink-bottle pores or parallel plates pores are developed (Sing, 1985; Yang et al., 2014). The hysteresis loop of FS1# shale can be classified as category H3, indicating that wedge pores developed from loose stacking of the sheet particles. The closed position of the hysteresis loop occurs at approximately P/Po=0.4.

ACCEPTED MANUSCRIPT This shows smaller mesopores are developed in shale samples. As shown in Fig 6, the hysteresis loop shape of shale samples before and after the reaction has only a slight difference. It can be concluded that ScCO2-water-shale interactions have a slight influence on the pore shape

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of the tested shale samples. In addition, Fig 6 shows that the nitrogen

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adsorption capacity of the shale samples after the reaction is greater than

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before the reaction. Therefore, the ScCO2-water-shale interactions can enhance the nitrogen adsorption ability of shale samples. This is closely

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related to increased specific surface area after the reaction (Venaruzzo et

3.3.2. Specific surface area

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al., 2002; Ji et al., 2012).

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The specific surface area (SSA) of the samples before and after reaction is presented in Table 2. Two SSAs containing BET and BJH

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were obtained in this study, with the SSA increasing after the reaction. The SSAs of the pore size of the shale samples during each stage were

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obtained based on the BJH method to analyze SSA changes in depth. Fig 7 and Fig 8 show the SSA and SSA percentages of pore size of the shale samples during each stage before and after the reaction, respectively. For CS1# shale, mesopores have the highest SSA percentages before and after the reaction, followed by micropores and macropores. The SSAs of the mesopores and micropores increase after the reaction, whereas the SSA of the macropores decreases. The SSA percentages of pore size

ACCEPTED MANUSCRIPT during each stage are not consistent before and after the reaction. The SSA percentages of the mesopores and macropores decrease after the reaction, whereas the SSA percentages of micropores increase. For FS1# shale, the mesopores are the main contributors to the SSA before and

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after the reaction, followed by micropores and macropores. The SSA of

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the pore size during each stage increases after the reaction, with the SSA

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increase of the micropores being the most substantial. The SSAs percentages of the mesopores and macropores slightly decrease after the

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reaction, whereas the SSA percentages of the micropores substantially

3.3.3. Pore size distribution

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increase.

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Pore size and pore volume were obtained using the N2 adsorption isotherms. Based on these data, the pore size distribution curves of shale

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samples before and after the reaction are shown in Fig 9. At the same time, the pore volumes and pore volumes percentages of the shale

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samples during each stage before and after the reaction are shown in Fig 10 and Fig 11, respectively. The distribution curve shapes of differential pore volumes and accumulation pore volumes, before and after the reaction, are similar. However, they do not overlap, and there are different degrees of fluctuation. The shale samples have a broad pore size distribution with a large amount of micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm). The mesopores are the main contributors

ACCEPTED MANUSCRIPT to the pore volumes. The pore volumes for CS1# shale were 0.0113 cm3/g and 0.01405 cm3/g, before and after the reaction, respectively. After the reaction, the pore volumes of the micropores and mesopores increase, whereas the pore volumes of the macropores decrease. Only the pore

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volumes percentages of the micropores increase. These results are in line

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with the SSA changes mentioned above. For FS1# shale, the pore

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volumes before and after the reaction are 0.0117 cm3/g and 0.0131 cm3/g, respectively. The pore volumes for each pore size increase during all

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stages, and only the pore volume percentages of the micropores increase

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after the reaction. This shows that under supercritical CO2 stimulation for shale gas reservoirs, ScCO2-water-shale interactions have a strong

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influence on the micropores of shale. The increase in the micropores results in an increase in the SSA and total pore volume. Therefore,

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ScCO2-water-shale interactions under supercritical CO2 stimulation can enhance the gas adsorption capacity of shale.

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3.4. Mineral composition

Table 3 shows the mineral composition changes of shale samples before and after the reaction. The two shale samples have different mineral compositions. The CS1# shale sample mainly contains kaolinite, illite, and quartz; before the reaction the kaolinite content reached 53.3%. The FS1# shale sample mainly contains illite, quartz, and plagioclase; before reaction, the illite content reached 46.5%. The shale minerals vary

ACCEPTED MANUSCRIPT in composition before and after the reaction, and the contents of all minerals increase or decrease to varying degrees. In CS1# shale, the kaolinite and quartz contents change substantially, and the illite, quartz and dolomite contents change substantially in FS1# shale. Wang et al.

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(2016) investigated and quantified reactions over time between CO2, cap

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rocks, and brine associated with selected cap rocks of the No. 3 coalbed

and

SEM

analysis

revealed

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of Qinshui Basin in China. For lithic sandstone after the reaction, XRD similar

changes

in

mineralogical

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compositions. Shale minerals can be classified into three categories:

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siliceous minerals including quartz, and feldspar; clay minerals including kaolinite, illite, chlorite, and montmorillonite; and carbonate minerals

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including calcite and dolomite. Fig 12 shows the composition changes of three types of shale minerals before and after the reaction. The clay

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mineral contents of two shale samples are high and can reach 50%, whereas the contents of the siliceous minerals and carbonate minerals are

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low. These are the typical characteristics of continental shale gas reservoirs in China. After the reaction, the two shale samples exhibit similar changes in the mineral composition. The contents of siliceous minerals increase, whereas the contents of clay minerals and carbonate minerals decrease. Previous researchers showed similar results, including Yin et al. (2016), who studied the physical and structural changes in shale after 30 days of ScCO2 exposure (T= 40°C, P=16 MPa). The contents of

ACCEPTED MANUSCRIPT siliceous minerals, clay minerals and carbonate minerals showed similar changes. The dissolution of carbonate and silicate minerals resulted in the mobilization of mineral elements and pore structural changes in the shale

4. Discussion

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4.1. Mobilization and occurrence mode of elements

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samples (Yin et al., 2016).

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An element is the most basic unit of minerals. Under the interactions of ScCO2-water-shale, the major and trace elements in shale are dissolved

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and mobilized, and the mobilization characteristics are controlled by the

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occurrence modes of elements in the mineral. The major elements, including Si, Al, K, Na, Mg, P, Ca, and Fe, form carbonate, silicate, oxide,

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hydroxide, sulfide, and sulfate (Dai et al., 2005; Cai et al., 2015). For example, Al often occurs in shale in the form of the aluminosilicate (e.g.,

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clay mineral, feldspar) and hydroxide, whereas Ca exists mainly in the form of carbonate (e.g., calcite, dolomite) and sulfate. In this study, the

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elements Ca and Mg, which exist mainly in carbonate minerals, could be mobilized from the shale. The elements Na, K, and Al, which exist mainly in clay and feldspar minerals, could also be mobilized from the shale. The elements such as Si that occur in quartz and clay minerals were not readily mobilized. Minerals in shale are the main carriers of trace elements. Due to their differences in nature, the different trace elements occur in a variety of

ACCEPTED MANUSCRIPT shale minerals in different forms. Previous studies have shown that the trace elements Be, Cr, Cs, Ga, Li, Ti, V, Ni, and Sc have a strong affinity for clay minerals. The trace elements Co, Ni, Pb, Zn, As, Se, and Mo have a strong affinity for sulfide minerals. The trace elements Sr, Zn, Co,

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and Ba have a strong affinity for carbonate minerals. The elements Ti, Cu,

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Pb, V, Zn, Cd, Ta, Cr, and Sn are mainly related to the oxide minerals

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(Finkleman, 1980; Kuhn et al., 1980; Palmer and Filby, 1984; Littke, 1987; Swaine, 1990; Alastuey et al., 2001; Dai, 2002). In this study, the

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trace elements Sr, Zn, Co and Ba, which mainly exist in carbonate and

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sulfide minerals, were more easily mobilized. The trace elements Cr and Ni related to the clay minerals were also somewhat mobilized. However,

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the trace elements V and Be related to the clay minerals exhibited slight mobilization. It is important to note that Sr, Zn, Co and Ba are pollutants

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and may enter the groundwater system, leading to damage in the environment (Finkelman, 1994; Swaine, 1995). Thus, under the

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conditions of ScCO2 stimulation or geological sequestration in shale gas reservoirs, it is necessary to monitor the underground water below operating regions.

4.2. Modes of element mobilization According to the combination state of the elements and minerals in shale, the modes of element mobilization can be divided into two categories: mechanical mobilization

and

dissolved

mobilization.

ACCEPTED MANUSCRIPT Dissolved mobilization refers to minerals in shale that react with formation water in the case of CO2 injection, resulting in the dissolution and mobilization of the elements. Dissolved mobilization is generally regarded as the main form and carrier of element mobilization. However,

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not all minerals can react under the conditions of ScCO2-water. Mineral

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dissolution mainly depends on the solubility of minerals. In general, the

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solubility order of minerals is calcite > dolomite > plagioclase feldspar > k-feldspar > quartz. The solubility of silicate minerals is much lower than

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that of carbonate minerals (Wellman et al., 2003; Tang et al., 2016).

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According to this logic, the mobilization of the elements in the form of carbonate should be more obvious than that in the form of silicate.

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However, in this study, Al related to the clay minerals showed significant mobilization, even its mobility is higher than that of Ca and Mg in CS1#

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shale. In fact, the solubility of minerals is related to the chemical composition of the mineral itself; in addition, it is also related to

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temperature and pressure of the formation water, the amount of CO2 injected, and the pH value. The reactions between ScCO2-water and minerals in shale mainly involve hydrolysis, carbonatation and cation exchange (Alemu et al., 2011; Liu et al., 2012; Wang et al., 2013; Ozdemir 2016; Wang et al., 2016). The essence of hydrolysis is that H+ or OH- is ionized by water, and then enter the mineral crystal lattice, replacing a cation or anion,

ACCEPTED MANUSCRIPT respectively, resulting in mineral decomposition and element release. Under certain conditions, the hydrolysis of silicate minerals can also occur, and the cations of alkali metals and alkaline earth metals and OH-

ScCO2-water-shale

into

the water.

interactions,

the

Under the reactions

conditions

between

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will be mobilized

of

minerals,

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carbonate ions, and bicarbonate ions are common as part of the

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carbonatation reaction. The carbonatation reaction results in part or total dissolution of minerals. Cation exchange mainly occurs in clay minerals;

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when Na+ containing clay contacts free Ca2+, the clay will release Na+ and

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will adsorb Ca2+. Through several kinds of reactions, most major elements in shale can be dissolved and mobilized. At the same time, some

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trace elements related to minerals are also mobilized. 4.3. Effect of pore structural changes on adsorption

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The mineral matter in shale can be dissolved and mobilized with ScCO2-water treatment, leading to pore structural changes. Thus the

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adsorption capacity of shale can be affected. For example, Massarotto et al. (2010) found that after treatment with ScCO 2 and H2O, mineral matter is dissolved and mobilized, leading to increased high pressure CO2 excess adsorption. Ozdemir (2016) showed that the adsorption capacity of CO 2 on the acid treated coals was higher than both the base treated and untreated coals. Based on the above studies on pore structural changes, it can be concluded that ScCO2-water-shale interactions have a certain

ACCEPTED MANUSCRIPT modification effect on the pore structure of shale. After reaction, the specific surface areas and pore volumes of two shale samples increase at different degrees; in particular, micropore volumes have a substantial improvement. Liu et al. (2010) investigated changes in pore structure of

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anthracite coal associated with CO 2 sequestration process and also found

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that interaction of coal with ScCO 2–H2O increases total pore volume

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most importantly in the micro-pore range. Previous studies have found that CO2 and methane (CH4 ) adsorption capacity, with respect to shale,

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exhibit a large difference, and CO2 can exchange CH4 gas under

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adsorption (Busch et al., 2003; Weniger et al., 2010; Chareonsuppanimit et al., 2012; Liu et al., 2013; Luo et al., 2015). The pore structure has an

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important influence on the CO2 and CH4 adsorption capacity with respect to shale. Through our previous study, the specific surface areas or

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micropore volumes played an active role in preferential sorption of CO2; preferential sorption of CO2 was more obvious for shale with a large

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micropore volume or specific surface area (Luo et al., 2015). In the process of supercritical CO2 fracturing of shale gas reservoirs, the specific surface areas and micropore volumes of shale increase due to ScCO2-water-shale interactions. When injecting CO2 to enhance shale gas recovery after fracturing, the CH4 in the state of adsorption will be effectively displaced by CO2 due to the preferential adsorption, and shale gas recovery will be improved (Schepers et al., 2009; Godec et al., 2013;

ACCEPTED MANUSCRIPT Sun et al., 2013). Therefore, the pore structural changes caused by ScCO2-water-shale interactions in ScCO2 fracturing of shale gas reservoirs improve shale gas recovery. In addition, CO2 sequestration in the exhausted shale gas reservoirs also involves ScCO2-water-shale

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interactions, which result in increases in the SSA and pore volumes of

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shale. The CO2 adsorption capacity with respect to shale shows a positive

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correlation with the SSA and pore volumes (Ross and Bustin, 2007; Ross and Bustin, 2009; Voskuilen et al., 2012; Luo et al., 2015). Therefore,

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this CO2 adsorption capacity will be enhanced after the interactions. This

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is advantageous for the geological sequestration of CO2. 4.4. Fractal analysis of pore structure

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Shale has a complicated pore structure and a rough surface; therefore, the fractal dimension can be used to describe the complex pore structure.

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In this study, the fractal dimension is used to study the structure and surface characteristics of shale samples before and after the reaction.

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According to N2 adsorption data, the fractal dimension, D, based on the FHH model is shown in Table 4. Since the N2 adsorption-desorption isotherms separate after the relative pressure P/Po>0.40, when the relative pressure P/Po is greater than 0.40, the adsorption data are used to calculate the fractal dimension. As shown in Table 4, the fractal dimensions of shale before and after the reaction are 2.518–2.781, and all the correlation coefficients are greater than 0.96, indicating that shale

ACCEPTED MANUSCRIPT samples before and after the reaction have strong fractal characteristics. Fig 13 shows the FHH curves of shale samples before and after the reaction. After the reaction, the fractal dimensions of two shale samples increase, suggesting that the ScCO2-water-shale interactions change the

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pore morphology of shale. The increases in the fractal dimensions of the

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shale samples show that the pore surface roughness increases, and the

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pore structure morphology gradually transforms from regular to complex (Pfeifer and Avnir, 1983). This is because the clay minerals and minerals

are

dissolved

under

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carbonate

the

conditions

of

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ScCO2-water-shale interactions, resulting in an increase in the number of micropores. Thus, the complexity of the pore structure increases, leading

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to increases in the fractal dimensions. Yin et al. (2016) studied the effect of ScCO2 on the pore structure of shale and found that a decrease in

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micropores could decrease the fractal dimension. Previous studies have shown that the fractal dimension shows a certain

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correlation with SSA and TPV. In this study, the correlations were also analyzed. As shown in Fig 14–16, there is a direct trend of an increase in the fractal dimension for the sample with BET SSA, similar to previous research results (Li et al., 2016; Yang et al., 2016, Liu et al., 2015). The results of previous studies were inconsistent in the correlation between the fractal dimension and mineral content. For example, Hu et al. (2016) found that the fractal dimension showed a positive correlation with the

ACCEPTED MANUSCRIPT quartz content and a negative correlation with the clay mineral content. By contrast, Yang et al. (2014) found no discernable correlation between the clay mineral content and fractal dimension. In this study, there are no obvious correlations between the fractal dimension and TPV, clay

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mineral content, and silicate content. This shows that minerals might have

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a crucial impact on the fractal features of shale. The fractal dimension, a

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comprehensive parameter, used to describe the pore structure of shale, could be affected by the sedimentary environment of shale gas reservoirs

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and mineral genesis. Therefore, in future research, it is necessary to

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collect more types of shale for fractal analysis. 5. Conclusions

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Experiments on ScCO2-water-shale interactions were conducted at 10 MPa and 50°C over 96 hours. The experimental results show that

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hydrolysis and carbonatation of the minerals are mainly responsible for mobilization of the elements from the shale samples of Qaidam Basin and

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Ordos Basin in China. Mobilization of the major and trace elements in shale is controlled by the occurrence mode of elements. The elements Ca and Mg that exist mainly in the form of carbonate minerals can be readily mobilized. The elements Na, K, and Al that exist mainly in clay and feldspar minerals can be mobilized. The trace elements Sr, Zn, Co and Ba that mainly exist in carbonate and sulfide minerals are more easily mobilized than other trace elements. Under the conditions of ScCO2

ACCEPTED MANUSCRIPT stimulation or geological sequestration in shale gas reservoirs, the underground water below the operating regions must be monitored because trace elements such as Sr, Zn, Co, and Ba can be mobilized and potentially contaminate groundwater.

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ScCO2-water-shale interactions have a certain modification effect on

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the pore structure of shale. The hysteresis loop shape of shale samples

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before and after the reaction differed only slightly, indicating that ScCO2-water-shale interactions have a slight influence on the pore shape

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of the tested shale samples. After the reaction, the SSA and pore volumes

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of the two shale samples increase. The micropore volumes also increase substantially. These responses are beneficial for ESGR and CO2

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geological sequestration. The fractal dimensions of shale before and after the reaction are 2.519–2.782, and all the correlation coefficients are

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higher than 0.96. The dissolution of the clay minerals and carbonate minerals results in an increase in the number of micropores. Thus, the

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fractal dimensions of the two shale samples increase, suggesting that pore structures transform from regular to complex. A trend of the fractal dimension increasing with growing BET SSA is observed. When simulating ScCO2-water-shale interactions, shale samples are crushed to grains to enhance the hydrodynamic conditions and increase the mineral reaction surface and dissolution. However, study results may not be consistent with actual reservoir conditions, and the interactions of

ACCEPTED MANUSCRIPT ScCO2-water-shale need to be further examined. In addition, in this study, we conducted the interaction experiments at constant temperature and pressure. Different temperatures and pressures may have an impact on the ScCO2-water-shale interactions.

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Acknowledgements

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This work was funded by the National Natural Science Foundation of

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China (Grant No. 51741407), the Project of the Shaanxi Province Science and Technology Program (Grant No. 2015KTCL01-08) and the National

acknowledge the Shaanxi Key

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2016ZX05050006). The authors

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Major Research Program for Science and Technology of China (Grant No.

Laboratory of Advanced Stimulation Technology for Oil & Gas

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Reservoirs, School of Petroleum Engineering, Xi'an Shiyou University,

Appendix Table 1

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for supporting the research.

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Test results of major and trace elements after the blank experiment. Major element content (μg/L)

Sample

Blank sample

Fe

Al

Ca

Mg

Si

K

Na

Mn

P










Trace element content (μg/L) Sample

Blank sample

Se

Cr

Co

Ni

Zn

V

Sr

Ba

Li










DL: detection limit. Table 2 Pore structure parameters of the shale samples before and after the reaction. Sample

State

2

BET SSA (m /g)

2

BJH SSA (m /g)

3

TPV (cm /g)

ACCEPTED MANUSCRIPT CS1#

FS1#

Before reaction

6.462

2.993

0.0113

After reaction

13.900

6.066

0.0141

Before reaction

2.719

1.531

0.0117

After reaction

3.403

1.736

0.0131

BJH SSA: BJH Adsorption cumulative surface area of pores; TPV: total pore volume. Table 3

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Mineral composition of shale samples before and after the reaction. M ineral composition (wt.%) State

FS1#

Chlorite

Before reaction

53.3

21.5

N/D

After reaction

46.1

19.3

N/D

Before reaction

N/D

46.5

8.8

After reaction

N/D

41.3

Quartz

Dolomite

Others

N/D

N/D

3

32.3

N/D

N/D

2.3

22.2

13.6

5.7

3.2

16.8

2.5

1.6

22.2

Plagioclase

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Illite

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CS1#

Kaolinite

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Sample

9.4

28.4

N/D: not detected. Others refer to K-feldspar for CS1# shale; Others contain calcite and hematite

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in FS1# shale.

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Table 4 Fractal dimensions of the shale samples before and after the reaction, derived from the FHH model. State

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Sample CS1#

FS1#

Fitting equation

D

R2

Before reaction

y=-0.337x+0.878

2.663

0.973

After reaction

y=-0.219x+1.628

2.781

0.965

Before reaction

y=-0.482x-0.062

2.518

0.991

After reaction

y=-0.446x+0.180

2.554

0.986

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ACCEPTED MANUSCRIPT Vent V5 Pressure Meter

Pressure Meter Oven ( T=const ) Computer

Reducing Valve

Vacuum Pump V1

V2

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Vent Intermediate Container

Storage Pot

CO2

CO2

ICP-AES Analysis

Pressure Transmitter

Needle Valve

Reactor

Leachate Tank

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CO2

Shale Sample

Constant Pressure Pump

V4

High pressure reaction system

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High Pressure Steel Cylinders

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V3

Vent

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a)

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Fig. 1. Schematic of experimental setup used for supercritical CO2 -water-shale interactions.

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b)

Fig. 2. Content and mobility of the major elements in CS1# shale (a) and FS1# shale (b) before and after the reaction.

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Fig. 3. Content and average content of major elements in the water sample.

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a)

b)

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Fig. 4. Content and mobility of the trace elements in CS1# shale (a) and FS1# shale (b) before and

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after the reaction.

Fig. 5. Content of trace elements in the water sample.

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a)

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Fig. 6. Isotherm results of N2 sorption and desorption for shale samples. (a) CS1# shale and (b) FS1# shale

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Fig. 7. Specific surface areas of the pore size of shale samples during each stage before and after the reaction. (a) CS1# shale and (b) FS1# shale

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Fig. 8. Specific surface area percentages of pore size of shale samples during each stage before and after the reaction. (a) CS1# shale and (b) FS1# shale

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Fig. 9. Pore size distribution of shale samples before and after the reaction. (a) CS1# shale and (b) FS1# shale

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Fig. 10. Pore volumes, before and after the reaction, by range of pore sizes of each stage for (a) CS1# shale and (b) FS1# shale.

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Fig. 11. Pore volume percentages, before and after the reaction, for ranges of pore sizes of shale samples during each stage: (a) CS1# shale and (b) FS1# shale.

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Fig. 12. Mineral contents of three categories of shale samples before and after the reaction.

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Fig. 13. lnV vs. lnln (P o /P) of N2 adsorption at low temperature for CS1# and FS1# shale samples before and after the reaction. (a) CS1# shale before the reaction, (b) CS1# shale after the reaction, (c) FS1# shale before the reaction and (d) FS1# shale after the reaction.

Fig. 14. Fractal dimensions vs. BET specific surface areas for CS1# and FS1# shale samples before and after the reaction. (“A” and “B” in the horizontal axis represent “after the reaction” and “before the reaction” respectively, e.g., CS1#-B: CS1# shale before the reaction)

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Fig. 15. Fractal dimensions vs. total pore volumes for CS1# and FS1# shale samples before and

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after the reaction.

Fig. 16. Fractal dimens ions vs. mineral composition for CS1# and FS1# shale samples before and after the reaction.

ACCEPTED MANUSCRIPT Highlights

ScCO2 -water-shale interactions under the stimulation conditions were studied

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The major and trace elements in shale indicate varying degrees of mobilization

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Both of the specific surface areas and pore volumes increase after reaction

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The interactions have a more significant influence on the micropores

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Fractal theory was used to analyze pore structure changes of shale

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