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Environmental and Carbon Dioxide Issues
Influence of supercritical CO2 Exposure on CH4 and CO2 Adsorption behaviors of shale: Implications for CO2 Sequestration Junping Zhou, Shuang Xie, Yongdong Jiang, Xuefu Xian, Qili Liu, Zhaohui Lu, and Qiao Lyu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00551 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018
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Influence of supercritical CO2 Exposure on CH4 and CO2 Adsorption behaviors of shale: Implications for CO2 Sequestration 3
Junping Zhou1,2*, Shuang Xie1,2, Yongdong Jiang1,2, Xuefu Xian1,2, Qili Liu1,2, Zhaohui Lu , Qiao Lyu4 1. State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing, 400044, China 2. College of Resources and Environmental science, Chongqing University, Chongqing ,400044, China 3. Key Laboratory of Shale Gas Exploration, Ministry of Land and Resources, Chongqing Institute of Geology and Mineral Resources, Chongqing, 400042, China 4. School of Geosciences and Info-Physics, Central South University, Changsha, 410012, China *Corresponding authors. E-mail address:
[email protected] (Junping Zhou).
Abstract: The interaction of shale-CO2 during the CO2 sequestration and enhanced shale gas recovery process has significant influence on the adsorption properties of shale. In this study, the influence of supercritical CO2 (ScCO2) exposure on CH4 and CO2 adsorption behavior of shale was studied. The pore structure and functional groups of different shale samples before and after ScCO2 (P=8MPa, 12MPa, 16MPa; T=35°C) exposure were measured by low-pressure nitrogen adsorption and Fourier Transform Infrared Spectroscopy (FTIR) method respectively. Moreover, CH4 and CO2 adsorption isotherms of shale samples before and after ScCO2 exposure were also obtained to reveal the mechanism of the influence of ScCO2 exposure on CH4 and CO2 adsorption behaviors of shale. The results indicated that after treated by ScCO2, the adsorption capacity of CO2 and CH4 in all the tested samples were decreased. The main reason for the decreases is the changes of pore structures, mineral composi-
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tions and surface chemistry in shale caused by ScCO2 exposure as confirmed by the low-pressure nitrogen adsorption and FTIR analysis results. According to the results of low-pressure nitrogen adsorption-desorption analysis, the total specific surface area, fractal dimensions of pore structure, specific surface area and pore volume of micropores in shale were decreased, while the total pore volume and average pore size in shale were increased after exposed to ScCO2. FTIR analysis indicated that the content of silicate, carbonate and aromatic hydrocarbon in shale were decreased. The changes of pore structure and surface chemistry in shale are also related to the treatment pressure of ScCO2 and types of shale. After treated by ScCO2, the selectivity ratios of CO2 over CH4 ( α CO2
CH 4
) were also changed
with a variety of trend. Keywords: shale gas; adsorption; pore structure; CO2 enhanced shale gas recovery; FTIR analysis; CO2 sequestration
1 Introduction With the rising concerns on global warming, mitigation of CO2 emissions into the atmosphere has become an urgent issue. To overcome this challenge, CO2 sequestration in geological formations is considered as one of the most promising options.1 In previous work, researchers are mainly focused on conventional geological formation, such as deep saline aquifers, depleted gas and oil reservoirs and unminable coal seams for CO2 storage, each of them manifests different trapping mechanisms.1-7 Recently, with the booming development of shale gas in china, shale formation has been becoming an attractive target for long-term CO2 sequestration in china as it offers the following advantages: (1) Shale gas reservoir, with huge resource reserves and widely distribution in china, can provide large 2
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scale capacity for CO2 storage.8-12 The recoverable shale gas resources of China are about 25.08×1012 m3, which are widely distributed in marine and continental basin.13 Marine and continental shale sedimentary area is about 30×106m2 and 28×106m2 respectively, thus, it is easily to achieve source-sink matching for CO2, as shale gas reservoirs are generally located nearby the large point sources of CO2 emission. Therefore, the cost of the CO2 transport in carbon dioxide capture and storage(CCS) project will be reduced greatly. (2) Adsorption capacity of CO2 in shale is greater than that of CH4, and CO2 preferentially adsorbs over CH4 in shale.14-20 Thus, the injection of CO2 into shale formations is not only sequestering CO2 permanently, but also enhancing shale gas recovery simultaneously, which may further reduce the cost of CO2 sequestration. (3) Shale, as a function of its traditionally low porosity and permeability, also typically functions as both the “source rock” and the “reservoir rock”. 22-27 Thus, it can serve as a perfect sealing lithology, which is very beneficial to CO2 sequestration as it can minimize the risk of CO2 leakage. Overall, CO2 storage in shale formations could enhance shale gas recovery, reduce leakage risks and save storage costs. So far, numerous works have been conducted to study the adsorption behavior of CH4 and CO2 on shale under the simulated geological conditions. The results show that the CO2 adsorption capacity in shale is higher than that of CH4. For CO2/CH4 mixtures adsorption in shale, CO2 also preferentially adsorbs over CH4 in the competitive adsorption process. The competition adsorption behaviors of CO2/CH4 may be related to the pore structure and mineral composition of shale.20, 22, 28-29 For CO2 enhanced shale gas recovery(CO2-ESGR) or CO2 sequestration in shale formations, it should be noted that the burial depth of shale gas reservoirs in China generally ranges from 2000 to 5000 m, where the temperature and pressure are far over the critical points of CO2 (Tc = 31.05°C, Pc 3
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=7.38MPa).30, 31 With the injection of CO2 into shale gas reservoir, ScCO2-shale interaction will cause the changes of pore structure and mineral composition in shale, which may influence the CH4/CO2 adsorption behaviors in shale. 32-36 Yin et al reported that the extraction or dissolution effect of ScCO2 and CO2 adsorption-induced swelling caused the pore structural and mineral composition changes in shale.32 Ao et al also documented that the ScCO2 exposure caused the physical structural and chemical properties variation in shale.33 Pan et al found that due to the discrepancies in terms of mineralogy and geochemical properties between different types of shale, the variation tendency of pore structure parameters caused by ScCO2-shale interactions for different shale samples are quite different.34 The above research results indicate that complex ScCO2-shale interactions may influence the adsorption behaviors of shale, but few research efforts have been focused on this area so far. While, accurate evaluation of the CH4 and CO2 adsorption capacity is of great importance to predict CO2 storage potential, as well as shale gas recovery efficiency of the target reservoirs. Thus, the aim of this work is to study the effects of ScCO2 exposure on the CH4 and CO2 adsorption behaviors in shale. To address this issue, pore structure characterization, FTIR properties (functional groups) and CH4, CO2 adsorption isotherms were obtained for the identical shale sample before and after ScCO2 exposure at different CO2 pressures. Furthermore, some information about the possible mechanism behind the influence of ScCO2 exposure on the adsorption behaviors of shale was provided.
2 Experimental Section 2.1 Sample preparation Four shale samples used in this work were collected from the Longmaxi and Wufeng formation located in the Sichuan Basin. The area is the most promising region of shale gas exploration in China. 4
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Lithologies of the Wufeng-Longmaxi shale are primarily composed of carbonaceous shale, siliceous shale, silty shale, calcareous shale and ordinary shale.37-39 All the collected samples were crushed and sieved to generate shale particles with a size of 150-250 µm for ScCO2-shale interaction and adsorption experiments. To prevent undesirable physical and chemical changes due to atmospheric oxidation, samples were carefully preserved in sealed containers full of helium (He). Prior to the experiments, Total organic carbon (TOC) and X-ray diffraction (XRD) analysis of all the tested shale samples were performed, the results are shown in Table 1. Table 1 Mineralogical composition (%) and TOC content of shale samples Mineralogical composition (%)
Sample
TOC number
Quartz
Carbonate
Clay Minerals
Others
LMX
53.3
15.5
19.5
9.3
4.27
WF1
49.2
28.8
18.7
3.7
3.89
WF2
55.1
10.5
20.6
13.8
4.51
WF3
55.8
11.5
26.9
5.6
4.49
Note: Clay minerals are the total amount of kaolinite, montmorillonite, illite and chlorite; carbonate minerals are the total amount of calcite and dolomite; other minerals are the total amount of potassium feldspar, plagioclase, pyrite, etc. 2.2 Exposure of shale to ScCO2 The CO2 exposure experiments were conducted in the sample cell of the high-pressure adsorption apparatus which is also used in gas adsorption experiments. The experimental apparatus comprises a gas cylinder to supply CO2, an ISCO (260D) syringe pump (Teledyne ISCO, USA) to inject CO2, and a reference cell and a sample cell, the main scheme diagram of the entire experimental system is shown in Figure 1. During the exposure processes, the sample temperature was controlled by a ther5
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mostatic oil bath, which can control the temperature with an accuracy of ±0.1℃. In this work, to ensures the CO2 reaching its supercritical state, the exposure temperature was set to a constant value of 35°C, the pressure was set to 8, 12, and 16MPa that represents the supercritical CO2 states with different conditions, respectively. The exposure time was set to more than 7 days to ensure sufficient shale-CO2 interaction. All the tested shale samples were collected for pore structure characterization, FTIR analysis and adsorption experiments before and after the ScCO2 exposure.
Figure 1. Schematic of the experimental system used for ScCO2 exposure and gas adsorption. 2.3 CH4 and CO2 adsorption experiments in shale before and after ScCO2 exposure Volumetric method was used to measure the adsorption isotherms of CH4 and CO2 on shales before and after ScCO2 exposure. The experiments of gas adsorption were also performed in the high-pressure volumetric analyzer, as shown in Figure 1. For each sample, 100–150g of pulverized powder sample with particle size of 150-250 µm was used for the CH4 and CO2 adsorption measurements. Prior to adsorption measurements, all the samples before and after ScCO2 exposure were entirely degassed by a vacuum pump system, then CH4 and CO2 adsorption measurements were per6
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formed on the identical sample. All the samples are exposed to CO2 in the pressure of 8MPa at first, then followed by 12MPa and 16MPa respectively. At each end of exposure, CH4 adsorption measurement was performed at first, as the CH4 adsorption experiment completed, the sample is pumped to vacuum again, then CO2 adsorption measurement was performed again. The temperatures of adsorption experiments were identical to the exposure temperature (T=35°C). 2.4 Pore structure and functional groups characterization 2.4.1 Low pressure N2 adsorption analysis Low-pressure nitrogen adsorption method was applied to analyze the pore structure of shale. The N2 adsorption-desorption isotherms of raw and CO2 treated shale samples at a temperature of 77 K and at a relative pressure (p/p0) range of 0.005~0.995 were collected by using a Micrometritics ASAP 2020 system. The pore structure parameters in terms of specific surface area (SSA), total pore volume (TPV), and pore size distribution (PSD) were obtained using Brunauer-Emmett-Teller (BET) model, Barrett-Joyner-Halenda (BJH) and density functional theory(DFT) model, respectively.40, 41 2.4.2 FTIR analysis FTIR analysis performed in a MAGNA- IR550 spectrometer (Nicolet Corp., USA) was used to determine the chemical properties of shales before and after ScCO2 exposure. Samples for FTIR analysis were prepared by the potassium bromide (KBr) pellet method. The shale samples and the dried KBr were fully pulverized and mixed at a mass ratio of 1:50. The spectrums were obtained with 32 scans at a resolution of 4 cm−1.
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3 Results and discussions 3.1 Pore structure characterization 3.1.1 Low pressure N2 adsorption-desorption isotherms The nitrogen adsorption-desorption isotherms of all the tested shale samples before and after ScCO2 exposure are shown in Figure 2. According to the classification of International Union of Pure and Applied Chemistry (IUPAC), the N2 adsorption isotherms of all the samples can be classified as type IV, which indicates that the pore in shale are mainly composed of parallel plate pores.42, 43 LMX
16
12
WF1
untreated adsorption-desorption 8MPa CO2 treated adsorption-desorption
12
Adsorbed Volume (cm /g, STP)
12MPa CO2 treated adsorption-desorption
3
3
Adsorbed Volume (cm /g, STP)
16
8
untreated adsorption-desorption 8MPa CO2 treated adsorption-desorption
4
12MPa CO2 treated adsorption-desorption 16MPa CO2 treated adsorption-desorption 0 0.0
0.2
0.4
0.6
0.8
16MPa CO2 treated adsorption-desorption
8
4
0 0.0
1.0
0.2
Relative Pressure (p/p0) 24 24
0.4 0.6 Relative Pressure(p/p0)
0.8
1.0
WF3
WF2 20
Adsorbed Volume (cm /g, STP)
20
12 8 untreated adsorption-desorption 8MPa CO2 treated adsorption-desorption
4
12 8
untreated adsorption-desorption 8MPa CO2 treated adsorption-desorption 12MPa CO2 treated adsorption-desorption
4
16MPa CO2 treated adsorption-desorption
12MPa CO2 treated adsorption-desorption 16MPa CO2 treated adsorption-desorption
0 0.0
16
3
16
3
Adsorbed Volume (cm /g, STP)
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0.2
0.4 0.6 Relative Pressure(p/p0)
0.8
0 0.0
1.0
0.2
0.4 0.6 Relative Pressure(p/p0)
0.8
1.0
Figure 2. Low temperature nitrogen adsorption-desorption isotherms of shale samples before and after ScCO2 exposure As seen in Figure 2, there are no differences in the shape of the hysteresis loops of isotherms for untreated and ScCO2 treated samples. It can be concluded that the influences of shale-ScCO2 interaction on the pore shape of all the tested shale samples are negligible. While, Figure 2 shows that the 8
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adsorption amount of N2 in ScCO2-treated sample is lower than that in untreated sample. That is, the N2 adsorption capacity of shale is decreased by ScCO2 treatment. The main reason is that the adsorption capacity is mainly related to the amount of micropores and mesopores, which are widely developed in the organic matter. As part of the organic matter in shale can be dissolved by ScCO2, then the numbers of micro- and mesopores in shale were decreased after ScCO2 exposure, as shown below, leading to a decrease of the adsorption capacity in shale.
3.1.3 Pore structure parameter analysis According to the IUPAC classification,42 the pore size of materials can be divided into three types: micropore (pore size
