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Environmental and Carbon Dioxide Issues 2
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Adsorption mechanism of CO/CH in kaolinite clay: insight from molecular simulation Wenning Zhou, Haobo Wang, Yuying Yan, and Xunliang Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00539 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019
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Energy & Fuels
Adsorption mechanism of CO2/CH4 in kaolinite clay: insight from molecular simulation Wenning Zhou,*,†,‡ Haobo Wang,† Yuying Yan,§ and Xunliang Liu†,‡ †School
of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China
‡Beijing
Key Laboratory of Energy Conservation and Emission Reduction for Metallurgical Industry, Beijing 100083, China
§Fluids
& Thermal Engineering Research Group, Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, UK
KEYWORDS: Adsorption mechanism; Kaolinite clay; Supercritical carbon dioxide; Methane; Molecular simulations
ABSTRACT: Understanding the adsorption mechanism of CO2/CH4 in kaolinite clay is essential for the carbon dioxide geological sequestration and enhanced gas recovery in shale reservoirs. In the present work, the grand canonical Monte Carlo simulations were employed to investigate the mechanism of competitive adsorption of CO2/CH4 in kaolinite clay. The effects of pore size (1-6 nm), pressure (0.1-30 MPa), temperature (298-378 K) and moisture content (0-0.122 g/cm3) on
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the adsorption behaviors of pure component and CO2/CH4 mixture were explored in depth. Specifically, two adsorption layers, i.e., strong and weak adsorption layer, in kaolinite slit-like micropore under high pressure condition have been observed. It was found that pore size and pressure have great effects on gas adsorption mechanism in kaolinite. The two adsorption mechanisms, including monolayer adsorption and micropore-filling under high pressure or small pore size conditions were discussed. In addition, simulation results showed that CO2 has much stronger adsorption ability than CH4 in kaolinite. The adsorption capacity of CH4 was significantly suppressed in the presence of CO2, especially in strong adsorption layer. An adsorption selectivity over 7 has been found in strong adsorption layer. Temperature and moisture content have great influences on the adsorption capacity and adsorption selectivity. However, the influences have different scales in strong and weak adsorption layers. It is expected the obtained results could provide insights into the adsorption mechanism of CO2/CH4 and offer fundamental data for CS-EGR project in kaolinite clay.
1. INTRODUCTION In recent years, the exploration and development of shale gas have received extensive attention owing to its low pollution, abundant reserves and the promising potential for CO2 storage.1,
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However, the ultra-low permeability of shale reservoir results in huge difficulty in the development of shale gas. In addition to hydraulic fracturing, the use of supercritical CO2 (SC-CO2) has recently been proposed as a promising technology to enhance shale gas recovery
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due to its unique characteristics such as low viscosity, high diffusivity and near zero surface tension.3, 4 Meanwhile, the sequestration of CO2 in subsurface reservoirs with the simultaneous recovery of shale gas has been considered as the most attractive scenario among various carbon capture, utilization and storage (CCUS) technologies.5, 6 Methane, the main component of shale gas, stored in shale matrix mainly as free gas in fractures and pores, adsorbed gas in the organic matter and clay minerals, and dissolved gas in the formation fluids. Among these three forms, the adsorbed gas could account for 20-85% of total gas-in-place in shale reservoirs.7 Therefore, the adsorption/desorption behaviors of CO2/CH4 are critical for the long-term sustainability of shale gas production as well as the CO2 sequestration performance in shale reservoirs. Some researchers have suggested that the total organic carbon (TOC) content mainly contributes the CH4 adsorption capacity in shale reservoirs.8, 9 While a few studies indicate that clay minerals can also significantly affect the adsorption properties of shale due to their large number of micropores and mesopores with high surface areas.10-12 According to shale component analysis, the amount of clay minerals could take up to 54.6-73.5% of shale reservoir in the Ordos Basin, China. Kaolinite is one of the most abundant components in clay minerals.13 Thus, the molecular knowledge of CO2-clay and CH4-clay interactions as well as competitive adsorption mechanism of CO2/CH4 in kaolinite is of great importance for CO2 sequestration and enhanced gas recovery (CS-EGR) in shale reservoirs. A number of experimental investigations have been carried out on gas adsorption in shale organic matter and clay minerals.14 Jiang et al.15 performed SEM experiments of SC-CO2
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treatment on shale samples from the Longmaxi formation in the Sichuan Basin, China. Their results showed that clay minerals in the shale released crystal water with the grains becoming small after SC-CO2 treatment. New pore microstructures with enhanced connectivity were formed in the shale matrix, which is favorable to the seepage of shale gas. Ross and Bustin16 compared CH4 adsorption capacities of various clay minerals in shale reservoirs in the Western Canadian Sedimentary Basin (WCSB). They reported that clay minerals are capable of adsorbing gas to their internal structure. Specifically, kaolinite has larger CH4 adsorption capacity than that of illite and montmorillonite on a moisture equilibrated basis, but the opposite trend was found under dry condition. Ji et al.17 focused on the main controls of methane adsorption capacity in clay-rich rocks. Their results indicated that clay type affects CH4 adsorption capacity and the presence of moisture can greatly reduce gas-sorption capacity. High-pressure (up to 18.0 MPa) adsorption of methane on various clay minerals was examined in the work.18 The obtained adsorption capacities (6.01 cm3/g, 3.88 cm3/g and 2.22 cm3/g for montmorillonite, kaolinite and illite, respectively) indicate high contribution of clay minerals to CH4 adsorption in shale. Heller and Zoback19 conducted adsorption experiments of CH4 and CO2 in pure carbon, pure clay mineral and various shale samples. Their results demonstrated the adsorptive capacities of CO2 are approximately 1.5 times, 4 times greater than that of CH4 in illite and kaolinite clay, respectively. However, the pressure for CO2 adsorption in their measurements was limited to 50-800 psi. These abovementioned experimental studies found important relationships between clay mineral type and gas adsorption and also had shown that the role of clay minerals in the adsorption of gas cannot be neglected. However, the experimental investigations can only
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quantitatively compare the gas adsorption capacities and the adsorption mechanisms of pure CH4 and CO2/CH4 mixture remain unclear, especially under high pressure conditions. In addition, the measured gas adsorption capacities could be inaccurately due to the complex structure of shale formations and limitations of experiment.20 Compared with experimental studies, molecular simulations have advantages to explore the microscale interactions under a wide range of conditions.21-23 Thus, molecular simulation technique could be a suitable tool to study gas adsorption mechanism in clay minerals. Zhang et al.24, 25 conducted molecular simulations to examine the effects of pore size and water content on CH4 adsorption in kaolinite. They found that the absolute adsorption capacities of CH4 on kaolinite surfaces for different layer distances follow the Langmuir isotherm and significantly reduce in the presence of water. The adsorption capacities of different gases in kaolinite were investigated by using the grand canonical Monte Carlo method (GCMC).26 The results indicated the adsorption capacities follow the trend CO2>CH4>N2. Competitive adsorption behaviors of methane and carbon dioxide in montmorillonite have been studied by a few researchers.27-29 The preferential adsorption of CO2 over CH4 was reported and the effects of pressure, pore size and water content on the competitive adsorption behaviors were discussed. Lee et al.30 employed ab initio molecular dynamics simulations to study methane adsorption mechanism from a H2O/CH4/CO2 mixture on Ca−Montmorillonite. The information on adsorption energetics, speciation, and structural and thermodynamic properties at molecular level were obtained. Jin and Firoozabadi31-33 focused on methane and carbon dioxide adsorption behaviors in slit pores of
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clay by using the Monte Carlo simulations. They claimed that the adsorption of CO2 is dominated by clay surface area and forms a strong adsorption layer at low pressure in nanosized clay pore. As pressure increases, the adsorption of CO2 is further enhanced and a second adsorption layer forms. Multilayer adsorption of CO2 and CH4 were also observed due to the nanoconfinement effect in clay pore.28,
34, 35
Although much work has been conducted, an
in-depth investigation of the adsorption mechanism of CO2/CH4 mixture and its influencing factors in kaolinite clay is still missing. In the present study, the grand canonical Monte Carlo method was adopted to explore the adsorption mechanism of CO2/CH4 in slit-like kaolinite pore. The effects of pore size, pressure, temperature and moisture content on the adsorption mechanism and competitive adsorption performances of CO2/CH4 were examined and discussed. It is expected that the results could provide molecular-level insights on the adsorption mechanism of CO2/CH4 in clay and lay foundation on the CS-EGR project in shale reservoirs. 2. MODELS AND METHODOLOGY 2.1 Models Kaolinite, a typical 1:1 type clay mineral, was used in this study to represent clay mineral in shale matrix. The 1:1 layer structure consists of a tetrahedral sheet and an octahedral sheet linked by common oxygen atoms parallel to the (0 0 1) sheet. The kaolinite crystal cell applied in this work was experimentally determined by Bish and Von Dreele.36, 37 The unit cell parameters are
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as follows: a=5.1535 Å, b=8.9419 Å, c=7.3906 Å, as can be seen in Fig. 1(a). To consider the periodicity and basal spacing of natural kaolinite, a supercell structure containing 4 × 4 unit cells along x and y directions was established as the basic sorbent layer which can minimize the finite size effect. Then two layers were stacked parallel to make up a slit-like kaolinite pore by sandwiching a vacuum slab. The width of the slab in the z direction is defined as the pore size of the kaolinite pore, as shown in Fig. 1(b). Therefore, the size of the studied kaolinite model is 2.0614 nm × 3.5768 nm in the x-y plane. According to the pore characteristics analysis, micropores (i.e.,
