Research ArticleResearch on Organic Nanopore Adsorption Mechanism andInfluencing Factors of Shale Oil Reservoirs

Yanfeng He,1 Guodong Qi,1 Xiangji Dou ,1 Run Duan,1 Nan Pan,1 Luyao Guo,1

and Zhengwu Tao2

1School of Petroleum Engineering, Changzhou University, Changzhou 213164, China2PetroChina Tarim Oilfield Company, Kulle, Xinjiang 841000, China

Correspondence should be addressed to Xiangji Dou; dxj@cczu.edu.cn

Received 21 October 2021; Accepted 16 November 2021; Published 1 December 2021

Academic Editor: Wei Yu

Copyright © 2021 Yanfeng He et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The adsorption properties of shale oil are of great significance to the development of shale oil resources. This study is aimed atunderstanding the microscopic adsorption mechanism of shale oil in organic nanopores. Thus, a molecular model of organicmicropore walls and multicomponent fluids of CO2, C4H10, C8H18, and C12H26 is constructed to investigate the adsorptionpattern of multicomponent fluids in organic nanopores under different temperature and pore size conditions. The quantity andheat of adsorption are simulated with the Monte Carlo method, which has been used in previous studies for single-or two-component fluids. The results demonstrate that the ability of CO2 to displace various alkanes is different. Specifically, medium-chain n-alkanes are slightly weaker than light alkanes in competitive sorption, and long-chain n-alkanes are less conducive tocompetitive sorption. The higher the CO2 sorption ratio, the more the sorption sites occupied by CO2. Thus, it is the bestreplacement for shale oil. The adsorption quantity of carbon dioxide, n-butane, and n-octane in organic nanopores firstincreases and then decreases as temperature rises. Meanwhile, the adsorption quantity of n-dodecane decreases firstly and thenincreases. With the increase in the pore size, the adsorption quantity of carbon dioxide, n-butane, and n-octane in organicnanopores increases while the adsorption quantity of n-dodecane first increases and then decreases. Besides, the model withlarger pore sizes is more sensitive to pressure changes in the adsorption of carbon dioxide and n-butane than the model withsmaller pore sizes. The heat of adsorption is CO2, C12H26, C8H18, and C4H10 in descending order. All are physical adsorption.Moreover, the adsorption quantity of all four components mixed fluid in the organic matter nanopores is positively correlatedwith the heat of adsorption.

1. Introduction

With the increasing demand for energy sources and thegradual depletion of conventional oil and gas resourcesaround the world, searching for another unconventionalalternative energy source is an inevitable trend for futureenergy development. Shale oil is a typical unconventionaloil and gas resource, an essential supplementary and alterna-tive energy source. Thus, countries around the world arebeginning to focus on shale oil exploration and extraction.Generally, most shale oil in an adsorbed or free state isstored in the shale, and a small amount of shale oil keeps adissolved (or miscible) state [1–3]. The characteristics ofshale oil stored in shale reservoirs with low porosity and

low permeability make it more difficult to exploit shale oil.Therefore, it is necessary to explore the adsorption anddesorption of shale oil in organic nanopores. Its laws willprovide theoretical guidance for shale oil extraction [4]. Inrecent years, molecular dynamic simulation techniques havepresented great potential for quantitative characterization ofadsorption resolution, interactions, and fluid transport innanomaterials. They have been gradually introduced intopetroleum research. Shale oil possesses a higher alkane con-tent compared with conventional crude oil. However, it is atypical multicomponent fluid whose component composi-tion is extremely complex [5]. Typically, shale oil understratigraphic conditions may contain short-chain n-alkanes(such as methane CH4), medium-chain n-alkanes (such as

HindawiGeofluidsVolume 2021, Article ID 6465193, 10 pageshttps://doi.org/10.1155/2021/6465193

n-hexane n-C6H14), and long-chain n-alkanes (such as n-dodecane n-C12H26), which are generally stored in organicnanopores in an adsorbed or free state. Shale oil is mainlycomposed of hydrocarbons, sulphur-containing compounds,nitrogen-containing compounds, etc. From the microscopicmechanism point of view, n-C12H26, as the main compoundcomponent of the oil phase of shale oil, can be a good substi-tute for shale oil at the nanoscale to simulate the competitivesorption of CO2 and shale oil. As the pore size of shale par-ent material is in the nanometer range, the permeabilitycoefficient of shale oil in shale parent material is extremelysmall. Besides, the macroscopic flow mechanism does notapply to the flow pattern of shale oil in the shale parentmaterial [6, 7]. Therefore, it significantly increases the diffi-culty of studying the adsorption and desorption of shale oilin nanopores.

The sorption and desorption of shale gas in organic andin organic nanopores have been extensively studied by manyresearchers [8–10], while there are few studies on the sorp-tion and desorption of shale oil in organic and inorganicnanopores [11, 12]. Pathank et al. [13] discovered that car-bon dioxide can effectively replace shale gas adsorbed inshale cheesecake by investigating the adsorption-desorptionlaw of methane and carbon dioxide in the three systems ofmethane-carbon dioxide-kerogen. Wu and Liu [14]employed the GCMC method to simulate the effects of tem-perature, pressure, and gas molar fraction on the adsorptionand separation of CO/H2 mixture in carbon nanopores andobtained the optimal pore width for the separation of CO/H2. Zhou et al. [15] explored the effects of reservoir temper-ature, pressure conditions, and the molar fraction of CO2 inthe CO2/CH4 gas mixture on the separation of CO2/CH4 inslit-type pores in coal seams with the GCMC method. Withthe increase in pressure, the equilibrium separation coeffi-cient of CO2 relative to CH4, SCO2/CH4, initially increasesuntil it reaches the peak, then decreases, and lastly tends tobe constant at 20MPa. Besides, Chen et al. [16] discoveredthe adsorption characteristics of CH4 and C2H6 on illite athigh-temperature conditions (60°C, 90°C, and 120°C) byMonte Carlo method, revealing that the main influencingfactor of CH4 and C2H6 adsorption is the layer spacing,and the excess adsorption of C2H6 is twice as much as thatof CH4 at layer spacing above 2nm. Wang et al. [17] inves-tigated the storage pattern of straight-chain alkanes in slitpores formed by organic matter through molecular dynam-ics simulations, concluding that multiple alkane adsorptionlayers existed on the slit pore surface whose layer thicknessis approximately 0.42 nm. Moreover, Wang [18] establishedan initial model of scCO2 repelling shale oil in rock poresand performed the kinetic simulations to observe the micro-configuration diagrams of the oil repelling process and qual-itatively analyze the mechanism of scCO2 oil repelling. Theyrevealed the microaction mechanism of scCO2 repellingshale oil. Most of the shale oil adsorption simulation studieswere conducted with two components. There are few simu-lation experiments for exploring the adsorption and desorp-tion of multicomponent fluids such as CO2/CH4/C2H6 andsome polymeric alkanes under reservoir conditions. Manyresearchers have investigated the effects of temperature and

pore size and other conditions on the adsorption anddesorption of shale gas in organic nanopores and inorganicnanopores. However, the effect of temperature, pore size,component ratio, and other conditions on the adsorptionof organic nanopores in shale oil reservoirs is rarely studied.

Therefore, the Grand Canonical Monte Carlo (GCMC)and Molecular Dynamic (MD) methods are employed toresearch the effect of different molar ratios of CO2, C4H10,C8H18, and C12H26 on the competitive adsorption patternof each component with molecular dynamic simulations toaddress the above issues. The effect of formation tempera-ture on the adsorption of multicomponent fluids composedof CO2, C4H10, C8H18, and C12H26 in organic nanoporeswas investigated using the controlled variable method withthe same formation pressure and the unchanged pore space,at various temperatures of the formation [19]. Under thesame conditions of formation temperature and formationpressure, the size of organic nanopores in shale reservoirswas exchanged to explore the effect of the adsorption of mul-ticomponent fluids in organic nanopores, so as to analyzethe influence of changes of organic nanopores in shale oilreservoirs on the adsorption mechanism of shale reservoirs.

2. Simulation System

2.1. Construction of the Molecular Model. Graphite is athree-dimensional structure, which was composed of carbonatoms. The first step to be taken in this paper is to select thegraphene single cell structure from the Materials Studiodatabase. Then, the single-layer graphene structure is con-structed using the build window of the software, and the gra-phene slit model is established by the build layer under theVisualizer task module. The vacuum layer between the twographene sheets is the pore space. Its thickness of 20Å,40Å, and 60Å is taken to build a 15a × 10b × c supercell gra-phene structure model, respectively. The X and Y directionsare set as a periodic structure while a, b, α, and β are fixed torepeatedly meet on infinite space to simulate the macro-scopic system, as illustrated in Figure 1. CO2, C4H10,C8H18, and C12H26 fluid molecular models are constructedby the Visualizer task module. Figure 2 presents the modelof each component fluid, where (a) is the carbon dioxidemolecular model, (b) is the n-butane molecular model, (c)is the n-octane molecular model, and (d) is the n-dodecanemolecular model.

2.2. Optimization of the Calculation Model. The graphenestructural model and the CO2, C4H10, C8H18, and C12H26molecular models need to be structurally optimized beforethe adsorption simulations of the molecules, so as to reachthe lowest point of potential energy, that is, energy minimi-zation. The resulting optimized models are used for molecu-lar simulations. Firstly, the geometry of the constructedmodels was optimized by selecting the smart option in thetask Geometry Optimization under the Molecular DynamicsForcite module to obtain a stable graphene slit structure.Afterward, the COMPASS force field was selected for theforce field; the atom-based summation method was usedfor the van der Waals forces; the rest of the settings were

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kept as the software defaults. The fluid models of CO2,C4H10, C8H18, and C12H26 were optimized in the same wayError! Reference source not found [20]. This is exhibited inFigure 2.

2.3. Potential Energy Model. In this paper, the simulations ofCO2, C4H10, C8H18, C12H26, and the C atoms in the graphitelayer and the molecule-molecule interactions are performedfor the L-J potential energy model [21], whose specificparameters are provided in Table 1. The van der Waalsforces between the covalently bonded molecules in the sys-tem are described by the potential energy function:

U ijð Þ = 4εijσijrij

!12

−σijrij

!6" #+ qqji

rij, ð1Þ

where: rij is the distance between particles i and j, qi is thecharged quantity of particle i, qj is the charged quantity ofparticle j, εij is the energy action parameter, and σij is thedimensional action parameter;

2.4. Simulation Program. The adsorption processes of car-bon dioxide, n-butane, n-octane, and n-dodecane in organicmatter nanopores were simulated to investigate the effects ofdifferent pore sizes, different temperatures, and differentmolar ratios of different fluids on the adsorption capacityof carbon dioxide, n-butane, n-octane, and n-dodecane inorganic matter nanopores. The components of fluids werecarbon dioxide, n-butane, n-octane, and n-dodecane inmolar ratios of 1 : 1 : 1 : 1, 4 : 1 : 1 : 1, and 1 : 4 : 1:1 to simulatethe competing adsorption patterns of different molar ratiosof the different components in the organic nanopores,respectively [22]. The simulated pore sizes range from 10Åto 60Å, and the simulated temperatures range from 333Kto 433K. The default temperature in all simulation scenariosis 373K, the pore sizes are 40Å, and the pressures are 0-30MPa, except for the temperature mechanism effect, whichis divided into 11 pressure points. Besides, the simulationsare developed separately.

The simulations are detailed as follows. Periodic bound-ary conditions are set in the X and Y directions of theorganic nanopores. The COMPASS force field is used forall calculations. Besides, the NVT system is used. It is a sys-tem in which the atomic number N , volume V , and temper-ature T are constants. The calculation is conducted using the

Fix Pressure task under the Sorption module. The totalnumber of production steps is 100,000, of which 10,000 areequilibration steps. The parameters such as van der Waalsforce, cut-off radius, and key tooth width are set as the sameas those used in the structural optimization.

3. Study of Competitive Adsorption Patterns

3.1. Competitive Adsorption Laws for the Same Molar Ratio.The main component of shale oil contains some long-chainn-alkanes. Currently, a large number of studies have beenconducted to enhance the recovery of shale oil by injectingcarbon dioxide into the shale reservoir. Since different typesof fluids have different adsorption capacities, the grandcanonical Monte Carlo method is used to simulate the com-petitive adsorption of multicomponent mixed fluids with thesame molar ratio in the pores of organic shale. The compet-ing sorption patterns of carbon dioxide, n-butane, n-octane,and n-dodecane in organic nanopores were explained from amicroscopic perspective. Besides, the competitive adsorptionof a mixture of carbon dioxide, n-butane, n-octane, and n-dodecane in the same molar ratio (carbon dioxide : n −butane : n − octane : n − dodecane = 1 : 1 : 1 : 1) in organicnanopores with a pore size of 40Å was simulated at 373K.The adsorption quantity of the multicomponent fluids wasobtained for pressure variations from 1 to 30MPa. Theadsorption isotherm curves for a molar ratio of 1 : 1 : 1 : 1 areillustrated in Figure 3. Additionally, the adsorption configura-tion for a multicomponent fluid mixed with the same molarratio is exhibited in Figure 4.

As observed in Figure 3, the adsorption of carbon diox-ide, n-butane, n-octane, and n- dodecane in the pores oforganic matter all rapidly increased with pressure beforethe low-pressure stage, followed by a slow rise. The fourfluids reached saturation at approximately 10MPa, and car-bon dioxide reached saturation before n-butane, n-octane,and n-dodecane [23]. The sorption of carbon dioxide isapproximately five times greater than that of n-butane whilethe difference in sorption between n-octane and n-dodecaneis not significant, with n-octane slightly higher than n-dodecane. There was a significant reduction in theadsorption of the three-component fluids compared to theadsorption of n-butane, n-octane, and n-dodecane as singlecomponents in the pores of the organic matter.

The adsorption configuration diagram demonstrates aclear adsorption layer on the organic wall. Besides, theadsorption pattern of different components of the fluid onthe organic wall is different, reflecting that CO2 has differentabilities to displace various types of alkanes [24, 25]. A largenumber of long-chain n-alkanes were adsorbed on the gra-phene wall, such as CO2 and n-butane. Regarding themedium-chain n-alkanes, n-octane (green in the adsorptionconfiguration diagram) is more apparently adsorbed inaggregates on the organic wall. Due to the strength of theadsorption capacity, the medium-chain n-alkanes areslightly weaker than the light alkanes in competitive adsorp-tion. Moreover, most of the adsorption sites on the organicwall are occupied by CO2 and n-butane. The competitiveadsorption capacity of long-chain n-alkanes is the weakest,

CC

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Figure 1: Organic matter pore model.

3Geofluids

and the adsorption quantities are the smallest among themulticomponent fluids. This reveals that long-chain n-alkanes are not conducive to competitive adsorption whileshale oil is composed mostly of medium-chain and long-chain n-alkanes. Additionally, it is extremely difficult todevelop shale oil. Therefore, this multicomponent competi-tive adsorption model well explains the competitive adsorp-tion law of CO2, n-butane, n-octane, and n-dodecane inorganic nanopores at the microscopic level. Experimentalstudies of shale oil are limited by the difficulty of observingthe microscopic processes of CO2-repelled shale oil, andthe microscopic mechanisms of CO2-repelled shale oil innanopores are unclear. Therefore, it is important to simulatethe process of CO2 repelling shale oil through the competi-tive adsorption between CO2 and multicomponent toachieve the guidance of studying the microscopic mecha-

nism of CO2 repelling shale oil in nanopores, and this hasimportant practical significance to guide the CO2 extractionof shale oil.

3.2. Competitive Adsorption Patterns for Different MolarRatios. The competing adsorption of mixed component fluidsof carbon dioxide, n-butane, n-octane, and n-dodecane in fourdifferent molar ratios (carbon dioxide : n − butane : n −octane : n − dodecane = 4 : 1 : 1 : 1, 1 : 4 : 1, 1 : 1 : 4 : 1 and1 : 1:1 : 4) in organic matter nanopores with a pore size of40Å was simulated using the Monte Carlo method. Adsorp-tion ratio curves were obtained for multicomponent fluids atpressure variations from 1 to 30MPa. The simulated temper-ature was 373K, and the simulated pressure was 0.1-30MPa.Figures 5–7 exhibit the adsorption ratio curves of carbon diox-ide and n-butane, carbon dioxide and n-octane, and carbondioxide and n-dodecane, respectively.

These curves demonstrate that the sorption ratios of car-bon dioxide to n-butane, n-octane, and n-dodecane all firstincrease and then tend to equilibrate. Overall, the increasein the adsorption ratio indicates that carbon dioxide consis-tently occupies more adsorption sites in competition with n-butane, n-octane, and n-dodecane in the adsorption process.The larger the proportion of carbon dioxide in the fluid mix-ture, the more the adsorption sites occupied by carbon diox-ide and the relatively less the n-butane, n-octane, and n-dodecane adsorbed in the organic matter pores. The effectof the different ratios of molarity on the adsorption ratio isstill significant. As reflected from the adsorption ratiocurves, the adsorption ratios of carbon dioxide to n-butane,n-octane, and n-dodecane mixed ratios are 4 : 1, 1 : 1, and1 : 4, respectively. The adsorption ratio when carbon dioxideis 4 molar ratio is greater than the adsorption ratio when n-butane, n-octane, and n-dodecane are 4 molar ratio to 1molar ratio. This indicates that the higher the carbon dioxideadsorption ratio, the more carbon dioxide. Moreover, thereplacement effect on shale oil is best and contributes moreto significant CO2 sequestration. Simultaneously, the abilityof adsorption of crude oil itself is different under strati-graphic conditions owing to different components. Themagnitude of the sorption ratio of CO2 to n-butane, n-octane, and n-dodecane reflects the sorption rate of eachcomponent. Furthermore, the 4 : 1 mixture of CO2 to n-butane, n-octane, and n-dodecane leads to a large difference

(a) (b)

(c) (d)

Figure 2: Fluid molecular model.

Table 1: L-J potential energy parameter table.

Site σ/nm (ε/KB)/K

CO2 0.363 242.0

C(CNT) 0.350 35.26

C4H10 0.525 350.5

C8H18 0.652 594.0

C12H26 0.785 715

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Figure 3: Isothermal adsorption curves for mixed fluid componentsof 1 : 1 : 1 : 1.

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in sorption between the components. This implies that thereplacement capacity of CO2 for the different componentsis not the same [26].

4. Effect of Temperature onCompetitive Adsorption

The effect of temperature on the adsorption of multicompo-nent fluid mixtures was investigated with the adsorptionquantity and adsorption configurations of carbon dioxide[27], n-butane, n-octane, and n-dodecane in organic matternanopores at a mixture ratio of 1 : 1 : 1 : 1 as an example. Thesimulated temperature interval is 333-433K, the pore widthis 40Å, and the simulated pressure is 0.1-30MPa. The giantregular Monte Carlo method is employed for simulation.The isothermal adsorption curves of carbon dioxide, n-butane, n-octane, and n-dodecane under different tempera-ture conditions are presented in Figures 8–11, respectively.

As indicated in the graph above, the adsorption quantityof carbon dioxide, n-butane, and n-octane in the organicnanopores of the multicomponent fluids first increases andthen decreases with temperature, reaching a maximum forall three components at 373K. As the temperature increases,the adsorption quantity decreases because the increase intemperature and pressure at the beginning of the simulationcauses n-butane to add more kinetic energy and move morevigorously. Meanwhile, the chance of collision between

C C

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Figure 4: Adsorption configurations for mixed fluid components of 1 : 1 : 1:1.

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Figure 5: Carbon dioxide to n-butane adsorption ratio curve.

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Figure 6: Carbon dioxide to n-octane adsorption ratio curve.

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Figure 7: Carbon dioxide to n-dodecane adsorption ratio curve.

5Geofluids

carbon dioxide, n-butane, n-octane, and organic matternanopore walls increases, as well as the adsorption quantity.As the temperature and pressure continue to increase, the n-butane molecules gain more kinetic energy and thus tend toadsorb away from the organic nanopore structure, and theadsorption volume decreases.

The temperature-dependent adsorption pattern of n-dodecane in organic matter nanopores is opposite to thatof carbon dioxide, n-butane, and n-octane. Figures 4 and 5reveals that the amount of n-dodecane adsorbed firstdecreases and then increases with increasing temperature,and the maximum adsorption of n-dodecane was discoveredat a temperature of 433K, ascribed to the competitionbetween the multicomponent mixture of carbon dioxide,n-butane, and n-octane. As the temperature increased to373K, the molecular movement of carbon dioxide, n-butane,

and n-octane intensified, and the adsorption quantityreached its maximum [28]. When the temperature reaches373K, n-dodecane as a long-chain n-alkane does not inten-sify the movement of n-dodecane molecules, and the chanceof collision between n-dodecane and organic matter nano-pore walls does not increase, contributing to the reductionof adsorption quantity. Therefore, the maximum adsorptionquantity of n-dodecane was achieved at a temperature of433K.

The temperature dependence of the adsorption of themixed fluids competing for adsorption in the organic nano-pores can be compared with that of the single-componentfluids, demonstrating that the effect of temperature on theadsorption of the multicomponent fluids in the organicnanopores is weaker than that in the pure components n-butane, n-octane, and n-dodecane.

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Figure 8: Isothermal adsorption curves for carbon dioxide atdifferent temperatures.

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Figure 11: Isothermal adsorption curves for n-dodecane atdifferent temperatures.

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As revealed from the graph above, the sorption ratios ofcarbon dioxide to n-butane, n-octane, and n-dodecane alltend to first increase and then decrease with the increasingtemperature increases. The ratio eventually stabilizes. Thiscan be explained that n-butane is a long-chain alkane, andmolecular movement is more intense when the temperatureincreases. The sorption ratio of carbon dioxide to n-dodecane was less affected by temperature, and the effectof temperature change on n-dodecane in the fluid mixturewas negligible. This suggests that short-chain alkanes aremore susceptible to temperature influence compared tolong-chain alkanes, and high temperature is more conduciveto the desorption of alkanes and more beneficial to thereplacement effect of shale oil.

5. Effect of Pore Size onCompetitive Adsorption

The effect of pore size on the adsorption of multicomponentfluid mixtures was investigated by analyzing the adsorptionand adsorption configurations in the nanopores of organicmatter under a 1 : 1 : 1 : 1 mixture ratio of carbon dioxide,n-butane, n-octane, and n-dodecane [29]. The majority ofshale oil storage environments are found in shale mesoporesranging in size from approximately 10Å to 60Å, which con-tains three intervals of pore size, including micropore, meso-pore and macropore, and the range of pores in this intervalallows for effective analysis of the effect of pore size onorganic nanopore adsorption in shale reservoirs. The iso-thermal adsorption curves of carbon dioxide, n-butane, n-octane, and n-dodecane under different pore sizes areprovided in Figures 12–15, respectively. Figures 16 and 17present the adsorption configurations for pore sizes of 10Åand 60Å, respectively.

As indicated in Figures 12–15, the adsorption of car-bon dioxide, n-butane, and n-octane in multicomponentfluids in the organic matter pore model at six pore sizesfrom smallest to largest is 10Å, 20Å, 30Å, 40Å, 50Å,and60Å. This implies that the adsorption of carbon diox-

ide, n-butane, and n-octane by organic matter nanoporesincreases with the increase in pore size. However, theadsorption quantity of n-dodecane in multicomponentfluids in six pore sizes of organic matter pore models fromsmallest to largest is 10Å, 60Å, 50Å, 40Å, 30Å, and 20Å.The adsorption quantity of n-dodecane first increased andthen decreased with the increasing pore size. Besides, thepore size had a greater influence on the adsorption offluids in the organic structure. The variation of carbondioxide, n-butane, and n-octane adsorption with tempera-ture and pressure is the smallest in the 10Å organicmatter nanopore model. The variation of carbon dioxide,n-butane, and n-octane adsorption with temperature andpressure is the largest in the 60Å organic matter nanoporemodel. Thus, it can be assumed that competitive adsorp-tion has no greater effect on the increase in the adsorptionquantity of carbon dioxide, n-butane, and n-octane withthe increasing pore size.

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Figure 12: Isothermal adsorption lines for carbon dioxide atdifferent pore sizes.

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Figure 13: Isothermal adsorption curves for n-butane at differentpore sizes.

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Figure 14: Isothermal adsorption lines for n-octane at differentpore sizes.

7Geofluids

It can be concluded that the model with larger pore sizehas a greater effect on the adsorption quantity of carbondioxide, n-butane, and n-octane than the model with smallerpore size in the adsorption quantity of carbon dioxide, n-butane, and n-octane adsorbed that is more sensitive to pres-sure changes [30]. The pore size was in the range of20Å~40Å when the pore size was a large variation in theadsorption of n-dodecane. It can be hypothesized that thecompetitive adsorption of long-chain alkanes in multicom-ponent fluids is gainful at 20Å~30Å of organic matternanopores, and the smaller pore size models are more sensi-tive to pressure changes in the adsorption of n-dodecanethan the larger pore size models.

The graph of Figure 18 suggests that the sorption ratio ofcarbon dioxide to n-butane, n-octane, and n-dodecane in themixed fraction significantly decreases as the pore sizeincreases. The sorption ratio of carbon dioxide to n-butanechanges most significantly, which has a small increase inpore size from 10Å to 20Å, a significant decrease in the sec-tion from 20Å to 30Å and then a steady decrease. Theadsorption ratio of n-octane and n-dodecane decreases grad-ually with increasing pore size, and there is no process ofincreasing the adsorption ratio first, which is quite differentfrom n-butane. As the adsorption ratio decreases, the

decrease in carbon dioxide sorption sites increases in n-butane, n-octane, and n-dodecane sorption sites. Therefore,the larger the pore size, the lower the sorption ratio, themore favorable the competition between carbon dioxideand the multicomponent fluid adsorption, and the morefavorable to CO2 replacement of shale oil in shale stor-age [31].

6. Heat of Absorption

The heat of adsorption is the heat released by the adsorptionof adsorbent molecules on the adsorbent. It is an essentialthermodynamic property of the adsorption process, whichreflects the adsorption capacity of the adsorbent on theadsorbent and the nature of adsorption. The heat of adsorp-tion is a crucial thermodynamic parameter distinguishingchemisorption from physical adsorption. The heat ofadsorption is greater than 42 kJ/mol for chemisorption andless than 42 kJ/mol for physical adsorption [32]. Simulta-neously, Figure 19 illustrates the molecular simulation ofthe heat of adsorption with pressure and temperature forthe same proportion of a mixture of carbon dioxide, n-butane, n-octane, and n-dodecane in organic nanopores ata pore size width of 40Å, a temperature of 333-433K, andpressure of 10MPa.

The graph above indicates that the heat of adsorption isCO2, C12H26, C8H18, and C4H10 in descending order. Theheat of adsorption of carbon dioxide, n-butane, n-octane,and n-dodecane by organic nanopores is less than 42 kJ/

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Figure 15: Isothermal adsorption curves for n-dodecane atdifferent pore sizes.

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Figure 16: Adsorption configuration for a 10Å pore size.

B

AC

Figure 17: Adsorption configuration for a 60Å pore size.

0

10

20

30

40

50

10 20 30 40 50 60

Ads

orpt

ion

ratio

Pore size (Å)

CO2: C4H10CO2: C8H18CO2: C12H26

Figure 18: Adsorption ratio of methane to each component atdifferent temperatures.

8 Geofluids

mol, which is physical adsorption. The equivalent heat ofadsorption of carbon dioxide, n-butane, n-octane, and n-dodecane increases with the increasing pressure at the sametemperature. The equivalent heat of adsorption of n-dodecane first decreases and then increases as the tempera-ture increases. It is positively correlated with the amount ofn-dodecane mixed component fluid adsorbed in the organicmatter nanopores. Carbon dioxide, n-butane, and n-octaneall first increase and then decrease in the equivalent heat ofadsorption with the increasing temperature. This is posi-tively associated with the amount of carbon dioxide, n-butane, and n-octane adsorbed in the organic nanopores,that is in that the increase in temperature leads to anincrease in the kinetic energy of carbon dioxide, n-butane,and n-octane seeds, making it easier for them to escape fromthe surface of the organic nanopore model, resulting in adecrease in the heat of adsorption.

7. Conclusion

(1) In simulations of competitive adsorption of the samemolar ratios of CO2, n-butane, n-octane, and n-dodecane, the adsorption patterns of the differentcomponents of the fluids on the pore surfaces ofthe organic matter and the ability of CO2 to displacethe various alkanes were different. Medium-chainalkanes are slightly weaker than short-chain alkanesin competitive adsorption while long-chain alkanesare less favorable to competitive adsorption

(2) In the multicomponent fluid competition adsorptionlaw, the adsorption quantity of each component inthe pores of organic matter increases with pressure.Besides, the adsorption quantity changes signifi-cantly when each component is adsorbed due tothe different molar ratios. The sorption ratio of car-bon dioxide at 4 molar ratios is greater than that ofn-butane, n-octane, and n-dodecane at 4 molarratios versus 1 molar ratio. This demonstrates thatthe higher the carbon dioxide sorption ratio, themore the carbon dioxide present, and the more thesorption sites occupied by the carbon dioxide. More-

over, it is the best replacement for shale oil and con-tributes more to the massive sequestration of carbondioxide. When CO2 is mixed with n-butane, n-octane, and n-dodecane in a 4 : 1 ratio, the differencein sorption between the components is large, andCO2 does not have the same replacement capacityfor different components

(3) The adsorption of carbon dioxide, n-butane, and n-octane in the organic matter nanopores of the multi-component fluids first increased and then decreasedwith temperature, reaching a maximum at a temper-ature of 373K. The adsorption of n-dodecane firstdecreased and then increased until the maximumadsorption of n-dodecane was reached at a tempera-ture of 433K

(4) The adsorption of carbon dioxide, n-butane, and n-octane by organic nanopores increases as the poresize increases. Meanwhile, the adsorption occupancyof n-dodecane in the competitive adsorption of mul-ticomponent fluids first increases and then decreases.The larger the pore size, the lower the adsorptionratio. This is more conducive to the competitiveadsorption between carbon dioxide and multicom-ponent fluids and the replacement of shale oil inshale storage by carbon dioxide

(5) The heat of adsorption is CO2, C12H26, C8H18, andC4H10 in descending order. As the temperatureincreases, the equivalent heat of adsorption of car-bon dioxide, n-butane, and n-octane firstly increasesand then decreases while that of n-dodecane firstlydecreases and then increases. Moreover, there is apositive correlation between the adsorption amountof all four components in organic nanopores

Data Availability

All data included in this study are available upon request bycontact with the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Acknowledgments

The authors would like to appreciate the financial support ofthe National Natural Science Foundation of China [No.52004038], funding name: Study on the Microscopic PhaseBehavior and Migration Mechanism of CO2 and Multicom-ponent Alkanes in Shale Dynamical Nanopore. The authorsalso would like to appreciate the support of the General Pro-ject of Natural Science Research in Colleges and Universitiesof Jiangsu Province [No. 20KJB440003], study on carbondioxide-methane transport mechanism of coal seam nano-pores with multiple inducement deformation fields.

0

20

40

60

80

100

333 353 373 393 413 433

Hea

t of a

bsor

ptio

n

Temperature (K)

CO2 C4H10C8H18 C12H26

Figure 19: Variation curve of heat of adsorption with temperature.

9Geofluids

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10 Geofluids