# Adsorption Characteristics of Alkanes with Different Carbon Numbers in Kaolinite Slits

Authors: Haitao Xue; Penglei Yan; Zhentao Dong; Rixin Zhao; Guozhi Ding; Jinliang Yan; Chunlei Li
Journal: Geofluids
 (2023)

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## Abstract

Shale oil is of interest for unconventional oil and gas exploration and development and has abundant geological reserves. Shale reservoirs contain numerous nanopores. Understanding the adsorption state of shale oil in the nanopores of shale is beneficial to improve the recovery of shale oil. In this study, the adsorption properties of shale oil in kaolinite slit pores were investigated by molecular dynamics simulation. In order to study the adsorption characteristics of shale oil with different components, a single-component model from n-C8 to n-C15 was established, and a mixed model of n-C8 and n-C15 with different mass ratios represented different crude oil components. The results show that the adsorption capacity per unit area increases with the increase of the alkane carbon number. The adsorption capacity of alkanes on the surface of silicon-oxygen tetrahedron is greater than that on the surface of aluminium-oxygen octahedron. The interaction force between kaolinite and alkane surface increases with the increase of alkane carbon number. The alkane adsorption capacity of silicon-oxygen tetrahedron is stronger than that of aluminium-oxygen octahedron. Competitive adsorption also exists between alkane molecules. Alkanes with higher carbon numbers are more easily adsorbed on the surface of kaolinite. Light alkanes are more likely to exist in free form than heavy alkanes. Based on molecular simulations, we studied the adsorption capacity of alkanes with different carbon numbers and calculated the adsorption capacity per unit area in the pores. It provides a theoretical basis for the calculation of shale oil geological reserves.


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## Body

## 1. Introduction

Shale oil widely exists in nanopores with a complex composition (polar and nonpolar components). The interaction between oil and rock is strong, making shale oil easily adsorbed on the pore wall [1–3]. At present, research on shale oil has made significant progress in experimental methods. Commonly used methods include capillary condensation theory and nuclear magnetic resonance. However, it is difficult to apply in the whole basin. The fluidity of shale oil is related to the occurrence mechanism and state in shale [4]. A large amount of shale oil exists in the nanopores in an adsorbed state. An insufficient understanding of the shale oil adsorption capacity and mechanism will affect the evaluation of shale oil mobility, thus affecting the exploration and development of shale oil. Clay minerals are an important component of shale reservoirs. The clay mineral content and composition of different minerals have an important impact on the brittleness, adsorption capacity, wettability, etc., of shale reservoirs [5–8]. Therefore, studying the adsorption mechanism and adsorption amounts of alkanes on the surface of clay minerals is of great help in improving the efficiency of shale oil exploration and development. In recent years, research on the adsorption mechanism of alkanes on pore walls has gradually increased. Fazelabdolabadi and Alizadeh-Mojarad [9] found through molecular dynamics simulation that the self-diffusion coefficient of molecules strongly adsorbed on the mineral surface decreased in a nanoconfined environment. Xiong et al. [10] found that with increasing temperature, the isosteric heat of methane decreased. The adsorption sites of methane molecules gradually changed from low-energy adsorption sites to high-energy adsorption sites, resulting in a decrease in methane adsorption capacity. Severson and Snurr [11] studied the effect of temperature and pore size on the adsorption of linear alkanes with different carbon numbers on activated carbon. They found that the larger the carbon number was, the easier the alkanes were adsorbed on the surface of activated carbon. These provide a basis for subsequent research but also have certain shortcomings. For example, it focuses on the influence of the environment on alkane adsorption but lacks research on the mechanism of action of the system itself.Molecular simulations are important for observing phenomena at the micro- and nanoscale. In particular, the unique nature of unconventional oil and gas reservoirs is that many adsorption phenomena in nanopores cannot be directly observed experimentally, or the cost of the experiment is high [12]. However, molecular simulations have the advantages of small cost and experimental reproducibility, which provide suitable conditions for our research at the microscale and nanoscale [13, 14].In recent years, molecular dynamics has also been applied to shale oil. Wang [15] used molecular simulation technology and found that with increasing shale pore size, the adsorption density of alkanes on the graphene surface gradually increased. Wu [16] used molecular dynamics simulation to find the adsorption density of alkanes on the graphene surface; this feature first increases and then remains the same as the aperture increases. Guo et al. [17] simulated the recovery of oil molecules by CO2 and N2 in dolomite slits by molecular dynamics. It is found that both CO2 molecules and N2 molecules have better oil displacement effects in the dolomite slits. However, the use of molecular simulation to study the adsorption characteristics of shale oil is still in the exploratory stage with currently many challenges. For example, the effect of alkane carbon number on fluid solidification is not clear. Additionally, the adsorption characteristics of alkanes on polar and nonpolar mineral surfaces and the competitive adsorption relationship between alkanes are also unclear [18–21]. This paper establishes different carbon number, alkane, and kaolinite slit models [22, 23]. Since kaolinite has two walls with different properties, polar and nonpolar, it is possible to study the adsorption of alkanes with different carbon numbers on polar and nonpolar surfaces. Through molecular dynamics simulation, the adsorption characteristics of alkanes with different carbon numbers on the surface of different properties and the competitive adsorption law between alkanes can be studied [24, 25].


## 2. Methodology

### 2.1. Model

Kaolinite cells without isomorphic replacement were selected as the research object. The chemical equation of kaolinite is Al2Si2O5(OH)4. The initial atomic position refers to AMSCD (American Mineralogist Crystal Structure Database), and its original lattice parameter is a=5.154 Å, b=8.942 Å, c=7.391 Å, α=91.926°, β=105.046°, and γ=89.797°. The lattice parameters α, β, and γ were set to 90° to facilitate the construction of the subsequent “oil-water–rock” model. Finally, using the GENCONF command to expand the unit cell into three layers in the direction of the crystal z-axis, a kaolinite model with a size of 6.18×6.25×4.9 nm3 was obtained. Kaolinite has two surfaces with different properties: silicon-oxygen tetrahedron (T) and aluminium-oxygen octahedron (O) (Figure 1). It can be used to study the effect of polar and nonpolar walls on alkane adsorption.Figure 1
Unit cell structure model of kaolinite.This paper uses Avogadro software to build an alkane model (Figure2). The molecular configuration is optimized, and then, the CGenFF tool is used to convert it into a molecular model file and a force field file suitable for the CHARMM force field. The size of the alkane box is 6.18×6.25×8 nm3. The number of molecules in the box should be equal to the actual density under the conditions of simulated temperature and pressure. It can be calculated using Equation (1). Since the adsorption density curve is to be calculated, the control variables need to be controlled. Each alkane component in the mixed component box is of the same mass. The density of each component and the number of molecules in the model are shown in Table 1.
(1)N=MNAρV.Figure 2
(a) Molecular structure model of alkanes with different carbon numbers and model of kaolinite wall surface; (b) model of kaolinite alkanes (red boxes indicate periodic boundaries).
(a)(b)Table 1
Alkane-kaolinite molecular model parameters.
Type of modelAlkaneDensity (g/cm3)Molar mass (g/mol)Number of moleculesSingle-component alkane-kaoliniten-C80.70114.231146n-C90.72128.261051n-C100.73142.29955n-C110.74156.31881n-C120.75170.33822n-C130.76184.36762n-C140.76198.39716n-C150.77212.41674Mixed component alkane-kaoliniten-C8/n-C9/n-C10/n-C11/n-C12/n-C13/n-C14/n-C15——747n-C8, n-C15——800——920——1035N is the number of inserted molecules. M is the relative molecular mass of the inserted molecules (g/mol). NA is Avogadro’s constant. ρ is the density of the inserted alkanes at normal temperature and pressure (g/cm3). V is the volume of the inserted area (nm3).


### 2.2. Force Field

The ClayFF force field was selected for the simulation of kaolinite. The Charmm36 force field was used to simulate the alkane molecule [26–29]. The ClayFF force field and Charmm36 force field were used to simulate the adsorption of organic molecules on the quartz surface. The adsorption simulation results obtained were consistent with the quantum mechanics ab initio molecular simulation results and the X-ray diffraction experimental results. It showed that the ClayFF force field and the Charmm36 force field have an excellent effect together [30, 31].


### 2.3. Simulation Parameter

This paper uses Gromacs software to perform molecular dynamics simulations [32, 33]. The software was initially designed for complex proteins and other differentiated molecules. Nevertheless, Gromacs software is faster than other molecular simulation programs in calculating nonbonding interactions. Therefore, it is often studied in the simulation of nonbiological systems. The electrostatic force model was used as the PME model. The van der Waals force radius represents the range of intermolecular forces. The van der Waals force radius selected in this paper was 1.4 nm. The simulation process is as follows: first, minimize the initial model’s energy to eliminate overlapping and too close molecules. Then, the model is relaxed for 50 ps; the parameter settings are as follows: NPT ensemble, the temperature is 298 K, pressure is 100 bar, the temperature control method is V-rescale, the pressure control method is Berendsen, and XYZ periodic boundary is used. Finally, a 20 ns molecular dynamics simulation is performed on the model. The parameters were set as follows: NPT ensemble, temperature, 323 K, pressure, 100 bar, Nose–Hoover for temperature and pressure control, and XYZ periodic boundary.


### 2.4. Data Processing Method

To calculate the mass density distribution of alkanes in the slit pores of kaolinite. First, divide the nanopores intoN (N=400) units parallel to the kaolinite surface and define the following function:
(2)LnZi,j=1n−1Δz<zi<nΔz,LnZi,j=0others.For thenth unit, the mass density of each unit from the time step JN (JN=10 ns) to JM (JM=20 ns) was divided into
(3)ρm=1SΔzJM−JN+1∑j=JNJM∑i=1NLnzi,j∙mi.In the above equation,S is the surface area of the kaolinite. N is the number of all atoms in the fluid in the simulation system. mi represents the molar mass of atom i. ρm divided by Avogadro’s constant can be converted to gram per cubic centimeter.The adsorption capacity of alkanes can be obtained by integrating the density curve:(4)madAl=∫l1l2Smodel∙ρoildl,madSi=∫l3l4Smodel∙ρoildl.Among them,madAl is the adsorption amount of aluminium-oxygen octahedron in milligrams. madSi is the adsorption amount of silicon-oxygen tetrahedron, in milligrams. l1 is the starting position of the density curve on the surface of the aluminium-oxygen octahedron. l2 is the end position of the density curve at the aluminium-oxygen octahedron. l3 is the position where the free phase and the adsorbed phase of the density curve are separated by the silicon-oxygen tetrahedron. l4 is the position where the adsorption curve ends at the silicon-oxygen tetrahedron. Their units are all nanometers (ρoil, l1, l2, l3, and l4 can all be obtained from Figure 3).Figure 3
Adsorption density curve of n-C8 adsorption free phase division.When calculating the recoverable amount of shale oil, an important parameter is the adsorption capacity per unit area (adsorption capacity). The amount of adsorption per unit area is related to the adsorption quality, adsorption density, and surface area of kaolinite, as Equation (5) and Equation (6) show.
(5)cadAl=madAlSadAl=∫l1l2Smodel∙ρoildlSadAl,(6)cadSi=madSiSadSi=∫l3l4Smodel∙ρoildlSadSi.


## 2.1. Model

Kaolinite cells without isomorphic replacement were selected as the research object. The chemical equation of kaolinite is Al2Si2O5(OH)4. The initial atomic position refers to AMSCD (American Mineralogist Crystal Structure Database), and its original lattice parameter is a=5.154 Å, b=8.942 Å, c=7.391 Å, α=91.926°, β=105.046°, and γ=89.797°. The lattice parameters α, β, and γ were set to 90° to facilitate the construction of the subsequent “oil-water–rock” model. Finally, using the GENCONF command to expand the unit cell into three layers in the direction of the crystal z-axis, a kaolinite model with a size of 6.18×6.25×4.9 nm3 was obtained. Kaolinite has two surfaces with different properties: silicon-oxygen tetrahedron (T) and aluminium-oxygen octahedron (O) (Figure 1). It can be used to study the effect of polar and nonpolar walls on alkane adsorption.Figure 1
Unit cell structure model of kaolinite.This paper uses Avogadro software to build an alkane model (Figure2). The molecular configuration is optimized, and then, the CGenFF tool is used to convert it into a molecular model file and a force field file suitable for the CHARMM force field. The size of the alkane box is 6.18×6.25×8 nm3. The number of molecules in the box should be equal to the actual density under the conditions of simulated temperature and pressure. It can be calculated using Equation (1). Since the adsorption density curve is to be calculated, the control variables need to be controlled. Each alkane component in the mixed component box is of the same mass. The density of each component and the number of molecules in the model are shown in Table 1.
(1)N=MNAρV.Figure 2
(a) Molecular structure model of alkanes with different carbon numbers and model of kaolinite wall surface; (b) model of kaolinite alkanes (red boxes indicate periodic boundaries).
(a)(b)Table 1
Alkane-kaolinite molecular model parameters.
Type of modelAlkaneDensity (g/cm3)Molar mass (g/mol)Number of moleculesSingle-component alkane-kaoliniten-C80.70114.231146n-C90.72128.261051n-C100.73142.29955n-C110.74156.31881n-C120.75170.33822n-C130.76184.36762n-C140.76198.39716n-C150.77212.41674Mixed component alkane-kaoliniten-C8/n-C9/n-C10/n-C11/n-C12/n-C13/n-C14/n-C15——747n-C8, n-C15——800——920——1035N is the number of inserted molecules. M is the relative molecular mass of the inserted molecules (g/mol). NA is Avogadro’s constant. ρ is the density of the inserted alkanes at normal temperature and pressure (g/cm3). V is the volume of the inserted area (nm3).


## 2.2. Force Field

The ClayFF force field was selected for the simulation of kaolinite. The Charmm36 force field was used to simulate the alkane molecule [26–29]. The ClayFF force field and Charmm36 force field were used to simulate the adsorption of organic molecules on the quartz surface. The adsorption simulation results obtained were consistent with the quantum mechanics ab initio molecular simulation results and the X-ray diffraction experimental results. It showed that the ClayFF force field and the Charmm36 force field have an excellent effect together [30, 31].


## 2.3. Simulation Parameter

This paper uses Gromacs software to perform molecular dynamics simulations [32, 33]. The software was initially designed for complex proteins and other differentiated molecules. Nevertheless, Gromacs software is faster than other molecular simulation programs in calculating nonbonding interactions. Therefore, it is often studied in the simulation of nonbiological systems. The electrostatic force model was used as the PME model. The van der Waals force radius represents the range of intermolecular forces. The van der Waals force radius selected in this paper was 1.4 nm. The simulation process is as follows: first, minimize the initial model’s energy to eliminate overlapping and too close molecules. Then, the model is relaxed for 50 ps; the parameter settings are as follows: NPT ensemble, the temperature is 298 K, pressure is 100 bar, the temperature control method is V-rescale, the pressure control method is Berendsen, and XYZ periodic boundary is used. Finally, a 20 ns molecular dynamics simulation is performed on the model. The parameters were set as follows: NPT ensemble, temperature, 323 K, pressure, 100 bar, Nose–Hoover for temperature and pressure control, and XYZ periodic boundary.


## 2.4. Data Processing Method

To calculate the mass density distribution of alkanes in the slit pores of kaolinite. First, divide the nanopores intoN (N=400) units parallel to the kaolinite surface and define the following function:
(2)LnZi,j=1n−1Δz<zi<nΔz,LnZi,j=0others.For thenth unit, the mass density of each unit from the time step JN (JN=10 ns) to JM (JM=20 ns) was divided into
(3)ρm=1SΔzJM−JN+1∑j=JNJM∑i=1NLnzi,j∙mi.In the above equation,S is the surface area of the kaolinite. N is the number of all atoms in the fluid in the simulation system. mi represents the molar mass of atom i. ρm divided by Avogadro’s constant can be converted to gram per cubic centimeter.The adsorption capacity of alkanes can be obtained by integrating the density curve:(4)madAl=∫l1l2Smodel∙ρoildl,madSi=∫l3l4Smodel∙ρoildl.Among them,madAl is the adsorption amount of aluminium-oxygen octahedron in milligrams. madSi is the adsorption amount of silicon-oxygen tetrahedron, in milligrams. l1 is the starting position of the density curve on the surface of the aluminium-oxygen octahedron. l2 is the end position of the density curve at the aluminium-oxygen octahedron. l3 is the position where the free phase and the adsorbed phase of the density curve are separated by the silicon-oxygen tetrahedron. l4 is the position where the adsorption curve ends at the silicon-oxygen tetrahedron. Their units are all nanometers (ρoil, l1, l2, l3, and l4 can all be obtained from Figure 3).Figure 3
Adsorption density curve of n-C8 adsorption free phase division.When calculating the recoverable amount of shale oil, an important parameter is the adsorption capacity per unit area (adsorption capacity). The amount of adsorption per unit area is related to the adsorption quality, adsorption density, and surface area of kaolinite, as Equation (5) and Equation (6) show.
(5)cadAl=madAlSadAl=∫l1l2Smodel∙ρoildlSadAl,(6)cadSi=madSiSadSi=∫l3l4Smodel∙ρoildlSadSi.


## 3. Results

### 3.1. Single-Component Simulation

Figure4 shows the density curve of single-component alkanes n-C8, n-C9, n-C10, n-C11, n-C12, n-C13, n-C14, and n-C15 after simulation. From Figure 4(a), at the positions of 1.5 nm-3.5 nm and 7 nm-9.5 nm, the density curve had obvious peaks. The density continuously decreased from the pore surface to the centre of the pore. There was alkane adsorption on the surface of the aluminium-oxygen octahedron of the kaolinite slit and the surface of the silicon-oxygen tetrahedron. On the surfaces of kaolinite with different properties, the same carbon alkane had the same number of adsorption layers. The density peaks of alkanes on the surface of silicon-oxygen tetrahedron are almost all larger than those on the surface of aluminium-oxygen octahedron. The silicon-oxygen tetrahedron is a nonpolar surface. The aluminium octahedron is a polar surface. Therefore, the adsorption between the nonpolar surface and alkane molecules is stronger than that of the polar surface.Figure 4
(a) Density curve of n-C8 alkanes; (b) density curve of n-C11 alkanes; (c) density curve of n-C12 alkanes; (d) density curve of n-C15 alkanes.
(a)(b)(c)(d)The number of adsorption layers tends to increase gradually with increasing carbon number. The n-C8 density curve showed that the number of adsorbed layers was three layers, and the wave peak of the fourth layer was less obvious than that of the first three layers. However, as the carbon number increased, the peak amplitude of the fourth layer increased. When the density curve of n-C11 clearly showed four peaks, it indicated that there were four layers of adsorption. Meanwhile, in the density curve of n-C12, the fifth peak also began to take shape. In the density curve of n-C15, the fifth peak was more obvious, but it was weaker than the first four peaks. As the carbon number of alkanes increases, the number of adsorption layers and the total adsorption thickness gradually increase.Since the peaks appearing near the two surfaces of the kaolinite in the density curve were symmetrical, we took the peaks of the aluminium-oxygen octahedron as an example and named them from left to right: the first adsorption layer, the second adsorption layer, the third adsorption layer, the fourth adsorption layer, and the fifth adsorption layer. They are shown in Figure5. The data of each adsorption layer are shown in Table 2.Figure 5
Schematic diagram of the adsorption layer division on the n-C15 density curve.Table 2
Peak table of different adsorption layers of different carbon number alkanes∗.
TypePeak density (kg/m3)No. 1No. 2No. 3No. 4No. 5n-C81376.631424.76860.20889.93741.57729.56////n-C91489.201498.96911.25933.62758.43750.37////n-C101523.481556.84946.21959.21775.03781.10////n-C111602.341578.93987.42987.08804.42804.97736.90742.73//n-C121650.341664.011023.931051.34829.60827.04751.08765.95//n-C131689.351729.091060.551086.77843.99853.25783.96782.94//n-C141805.681811.961091.761099.38856.30867.97782.45774.31//n-C151733.781835.251099.021126.57875.19891.80794.94793.07751.63759.04∗Tips: for the same adsorption layer, the data on the left are the peak value of the adsorption layer at the aluminium-oxygen octahedron, and the right is the peak value of the adsorption layer at the silicon oxygen tetrahedron.For alkanes with the same carbon number (using n-C8 as an example), the density curve of the first adsorption layer has the largest adsorption density peak. The adsorption density peaks on the two surfaces of the kaolinite reached 1376 kg/m3 and 1424.76 kg/m3, respectively. The adsorption effect was obvious, and the molecules presented a regular distribution. The peak density of the second adsorption layer and the third adsorption layer gradually decreased. This indicates that the adsorption influence between the alkane and the kaolinite surface was gradually weakened, and the force between the alkane molecules was gradually strengthened. Until the adsorption effect completely disappeared, the density of the alkane did not change. The arrangement of the alkane molecule presents a disordered distribution. For the same adsorption layer, the peak density increased as the carbon number increased for alkanes with different carbon numbers, as shown in Figure 6. This result showed that with the increase in carbon number, the adsorption effect of alkanes and kaolinite surfaces becomes increasingly stronger.Figure 6
(a) Peak change curve of alkanes with different carbon numbers in the first adsorption layer; (b) peak change curve of alkanes with different carbon numbers in the second adsorption layer; (c) peak change curve of alkanes with different carbon numbers in the third adsorption layer; (d) peak change curve of alkanes with different carbon numbers in the fourth adsorption layer.
(a)(b)(c)(d)The change in the amount of adsorption per unit area also reflected that as the carbon number increased, the adsorption capacity of alkanes on the surface of kaolinite became stronger, as shown in Table3. Regardless of whether it was on the aluminium-oxygen octahedral surface or the silicon-oxygen tetrahedral surface of kaolinite, the adsorption amount per unit area increased with increasing carbon number. For example, the average adsorption capacity of n-C8 was 908.77 kg/m2. The average adsorption capacity of n-C11 was 1217.54 kg/m2. The average adsorption capacity of n-C15 was also able to reach 1607.30 kg/m2, which was close to three times that of n-C8.Table 3
Adsorption capacity per unit area of single-component alkanes with different carbon numbers.
AlkanesAdsorption capacity-polar surface (kg/m2)Adsorption capacity-nonpolar surface (kg/m2)The average amount of adsorption (kg/m2)n-C8898.16919.38908.77n-C9912.43920.56916.50n-C10946.07950.37948.22n-C111198.561236.511217.54n-C121223.671258.891241.28n-C131274.661280.911277.79n-C141323.841330.061326.95n-C151596.301618.301607.30


### 3.2. Mixed Alkane Simulation

Four composite component alkane models are established to study the adsorption characteristics of mixed component alkanes. The model types are as follows: (1) n-C8, n-C9, n-C10, n-C11, n-C12, n-C13, n-C14, and n-C15; (2) n-C8, n-C15 mass ratio 1 : 3; (3) n-C8, n-C15 mass ratio 1 : 1; and (4) n-C8, n-C15 mass ratio 3 : 1.In model (1), the density curve and adsorption capacity obtained after 20 ns of molecular dynamics simulation are shown in Figure7 and Table 4. From the figure, we can see that there is an evident adsorption phenomenon near the kaolinite wall (at 2 nm and 10 nm). The higher the carbon number is, the higher the adsorption density peak near the kaolinite wall surface. The lower the carbon number is, the lower the density of the first adsorption layer. This shows that all kinds of alkanes in the mixed components are affected by other kinds of alkanes to varying degrees. The greater the carbon number is, the stronger the force between the alkane and the kaolinite wall surface, and the easier it is to adsorb on the kaolinite surface. There is a competitive adsorption relationship between alkanes.Figure 7
Density curve of multicomponent alkanes.Table 4
Adsorption capacity per unit area of each component of mixed alkanes.
AlkanesAdsorption capacity polar surface (kg/m2)Adsorption capacity nonpolar surface (kg/m2)Average adsorption capacity (kg/m2)n-C8137.80154.70146.25n-C9133.34164.67149.01n-C10137.56158.19147.88n-C11179.29177.78178.54n-C12180.01136.53158.27n-C13164.43200.27182.35n-C14200.87194.47197.67n-C15218.711127.04172.88Total adsorption1352.011313.65—Models (2)–(4) are three contrast models with different mass ratios established to study the competitive adsorption law between high-carbon alkanes and low-carbon alkanes. After the same 20 ns molecular simulation, the density distribution curve is shown in Figure8. In Figure 8(a), the mass ratio of n-C8 : n-C15 is 1 : 3. The n-C15 component alkane occupies the main space in all adsorption layers, and the n-C8 component alkane density has only a small fluctuation. In Figure 8(b), the mass ratio of n-C8 : n-C15 is 1 : 1, and the n-C8 component alkane has a more obvious adsorption layer. Although the quality of the two alkanes is the same, the adsorption density peak of n-C15 is higher on the kaolinite wall. In Figure 8(c), the mass ratio of n-C8 : n-C15 is 3 : 1. Although the overall density of the n-C8 component alkanes is greater than that of the n-C15 component, the adsorption density peak of the n-C15 component alkanes in the first adsorption layer is equal to that of the n-C8 component. These three sets of comparative models illustrate a competitive adsorption relationship between alkanes. The greater the carbon number of the alkane is, the stronger the interaction between the alkane and the wall, and it will be preferentially adsorbed on the kaolinite wall.Figure 8
Different n-C8 and n-C15 mass ratio density distributions: (a) C8 : C15 mass ratio 1 : 3; (b) C8 : C15 mass ratio 1 : 1; (c) C8 : C15 mass ratio 3 : 1.
(a)(b)(c)The system after the simulation is sliced and observed through VMD software, as shown in Figure9. The alkane presents a regular arrangement in the adsorption layer and shows an irregular distribution in the nonadsorption layer of the system. In addition, the n-C8 component alkane molecules are arranged more disorderly than the n-C15 component alkane molecules. Why would such a phenomenon happen? The main reason may be that the carbon chain of n-C15 alkane molecules is longer than that of n-C8 molecules. The n-C15 alkane molecules are more likely to be arranged in an orderly state under the adsorption of the kaolinite wall. This is because the n-C15 component alkane is closer to the solid state, and the molecular arrangement is more orderly. The n-C8 component alkane is closer to the gaseous state, and the molecules are more active, showing a disordered distribution state.Figure 9
Slices of the n-C8 : n-C15 component mass ratio of the 1 : 1 system: (a) slices from 2 nm to 2.6 nm; (b) slices from 5.8 nm to 6.4 nm; (c) slices from 9 nm to 9.6 nm.
(a)(b)(c)


## 3.1. Single-Component Simulation

Figure4 shows the density curve of single-component alkanes n-C8, n-C9, n-C10, n-C11, n-C12, n-C13, n-C14, and n-C15 after simulation. From Figure 4(a), at the positions of 1.5 nm-3.5 nm and 7 nm-9.5 nm, the density curve had obvious peaks. The density continuously decreased from the pore surface to the centre of the pore. There was alkane adsorption on the surface of the aluminium-oxygen octahedron of the kaolinite slit and the surface of the silicon-oxygen tetrahedron. On the surfaces of kaolinite with different properties, the same carbon alkane had the same number of adsorption layers. The density peaks of alkanes on the surface of silicon-oxygen tetrahedron are almost all larger than those on the surface of aluminium-oxygen octahedron. The silicon-oxygen tetrahedron is a nonpolar surface. The aluminium octahedron is a polar surface. Therefore, the adsorption between the nonpolar surface and alkane molecules is stronger than that of the polar surface.Figure 4
(a) Density curve of n-C8 alkanes; (b) density curve of n-C11 alkanes; (c) density curve of n-C12 alkanes; (d) density curve of n-C15 alkanes.
(a)(b)(c)(d)The number of adsorption layers tends to increase gradually with increasing carbon number. The n-C8 density curve showed that the number of adsorbed layers was three layers, and the wave peak of the fourth layer was less obvious than that of the first three layers. However, as the carbon number increased, the peak amplitude of the fourth layer increased. When the density curve of n-C11 clearly showed four peaks, it indicated that there were four layers of adsorption. Meanwhile, in the density curve of n-C12, the fifth peak also began to take shape. In the density curve of n-C15, the fifth peak was more obvious, but it was weaker than the first four peaks. As the carbon number of alkanes increases, the number of adsorption layers and the total adsorption thickness gradually increase.Since the peaks appearing near the two surfaces of the kaolinite in the density curve were symmetrical, we took the peaks of the aluminium-oxygen octahedron as an example and named them from left to right: the first adsorption layer, the second adsorption layer, the third adsorption layer, the fourth adsorption layer, and the fifth adsorption layer. They are shown in Figure5. The data of each adsorption layer are shown in Table 2.Figure 5
Schematic diagram of the adsorption layer division on the n-C15 density curve.Table 2
Peak table of different adsorption layers of different carbon number alkanes∗.
TypePeak density (kg/m3)No. 1No. 2No. 3No. 4No. 5n-C81376.631424.76860.20889.93741.57729.56////n-C91489.201498.96911.25933.62758.43750.37////n-C101523.481556.84946.21959.21775.03781.10////n-C111602.341578.93987.42987.08804.42804.97736.90742.73//n-C121650.341664.011023.931051.34829.60827.04751.08765.95//n-C131689.351729.091060.551086.77843.99853.25783.96782.94//n-C141805.681811.961091.761099.38856.30867.97782.45774.31//n-C151733.781835.251099.021126.57875.19891.80794.94793.07751.63759.04∗Tips: for the same adsorption layer, the data on the left are the peak value of the adsorption layer at the aluminium-oxygen octahedron, and the right is the peak value of the adsorption layer at the silicon oxygen tetrahedron.For alkanes with the same carbon number (using n-C8 as an example), the density curve of the first adsorption layer has the largest adsorption density peak. The adsorption density peaks on the two surfaces of the kaolinite reached 1376 kg/m3 and 1424.76 kg/m3, respectively. The adsorption effect was obvious, and the molecules presented a regular distribution. The peak density of the second adsorption layer and the third adsorption layer gradually decreased. This indicates that the adsorption influence between the alkane and the kaolinite surface was gradually weakened, and the force between the alkane molecules was gradually strengthened. Until the adsorption effect completely disappeared, the density of the alkane did not change. The arrangement of the alkane molecule presents a disordered distribution. For the same adsorption layer, the peak density increased as the carbon number increased for alkanes with different carbon numbers, as shown in Figure 6. This result showed that with the increase in carbon number, the adsorption effect of alkanes and kaolinite surfaces becomes increasingly stronger.Figure 6
(a) Peak change curve of alkanes with different carbon numbers in the first adsorption layer; (b) peak change curve of alkanes with different carbon numbers in the second adsorption layer; (c) peak change curve of alkanes with different carbon numbers in the third adsorption layer; (d) peak change curve of alkanes with different carbon numbers in the fourth adsorption layer.
(a)(b)(c)(d)The change in the amount of adsorption per unit area also reflected that as the carbon number increased, the adsorption capacity of alkanes on the surface of kaolinite became stronger, as shown in Table3. Regardless of whether it was on the aluminium-oxygen octahedral surface or the silicon-oxygen tetrahedral surface of kaolinite, the adsorption amount per unit area increased with increasing carbon number. For example, the average adsorption capacity of n-C8 was 908.77 kg/m2. The average adsorption capacity of n-C11 was 1217.54 kg/m2. The average adsorption capacity of n-C15 was also able to reach 1607.30 kg/m2, which was close to three times that of n-C8.Table 3
Adsorption capacity per unit area of single-component alkanes with different carbon numbers.
AlkanesAdsorption capacity-polar surface (kg/m2)Adsorption capacity-nonpolar surface (kg/m2)The average amount of adsorption (kg/m2)n-C8898.16919.38908.77n-C9912.43920.56916.50n-C10946.07950.37948.22n-C111198.561236.511217.54n-C121223.671258.891241.28n-C131274.661280.911277.79n-C141323.841330.061326.95n-C151596.301618.301607.30


## 3.2. Mixed Alkane Simulation

Four composite component alkane models are established to study the adsorption characteristics of mixed component alkanes. The model types are as follows: (1) n-C8, n-C9, n-C10, n-C11, n-C12, n-C13, n-C14, and n-C15; (2) n-C8, n-C15 mass ratio 1 : 3; (3) n-C8, n-C15 mass ratio 1 : 1; and (4) n-C8, n-C15 mass ratio 3 : 1.In model (1), the density curve and adsorption capacity obtained after 20 ns of molecular dynamics simulation are shown in Figure7 and Table 4. From the figure, we can see that there is an evident adsorption phenomenon near the kaolinite wall (at 2 nm and 10 nm). The higher the carbon number is, the higher the adsorption density peak near the kaolinite wall surface. The lower the carbon number is, the lower the density of the first adsorption layer. This shows that all kinds of alkanes in the mixed components are affected by other kinds of alkanes to varying degrees. The greater the carbon number is, the stronger the force between the alkane and the kaolinite wall surface, and the easier it is to adsorb on the kaolinite surface. There is a competitive adsorption relationship between alkanes.Figure 7
Density curve of multicomponent alkanes.Table 4
Adsorption capacity per unit area of each component of mixed alkanes.
AlkanesAdsorption capacity polar surface (kg/m2)Adsorption capacity nonpolar surface (kg/m2)Average adsorption capacity (kg/m2)n-C8137.80154.70146.25n-C9133.34164.67149.01n-C10137.56158.19147.88n-C11179.29177.78178.54n-C12180.01136.53158.27n-C13164.43200.27182.35n-C14200.87194.47197.67n-C15218.711127.04172.88Total adsorption1352.011313.65—Models (2)–(4) are three contrast models with different mass ratios established to study the competitive adsorption law between high-carbon alkanes and low-carbon alkanes. After the same 20 ns molecular simulation, the density distribution curve is shown in Figure8. In Figure 8(a), the mass ratio of n-C8 : n-C15 is 1 : 3. The n-C15 component alkane occupies the main space in all adsorption layers, and the n-C8 component alkane density has only a small fluctuation. In Figure 8(b), the mass ratio of n-C8 : n-C15 is 1 : 1, and the n-C8 component alkane has a more obvious adsorption layer. Although the quality of the two alkanes is the same, the adsorption density peak of n-C15 is higher on the kaolinite wall. In Figure 8(c), the mass ratio of n-C8 : n-C15 is 3 : 1. Although the overall density of the n-C8 component alkanes is greater than that of the n-C15 component, the adsorption density peak of the n-C15 component alkanes in the first adsorption layer is equal to that of the n-C8 component. These three sets of comparative models illustrate a competitive adsorption relationship between alkanes. The greater the carbon number of the alkane is, the stronger the interaction between the alkane and the wall, and it will be preferentially adsorbed on the kaolinite wall.Figure 8
Different n-C8 and n-C15 mass ratio density distributions: (a) C8 : C15 mass ratio 1 : 3; (b) C8 : C15 mass ratio 1 : 1; (c) C8 : C15 mass ratio 3 : 1.
(a)(b)(c)The system after the simulation is sliced and observed through VMD software, as shown in Figure9. The alkane presents a regular arrangement in the adsorption layer and shows an irregular distribution in the nonadsorption layer of the system. In addition, the n-C8 component alkane molecules are arranged more disorderly than the n-C15 component alkane molecules. Why would such a phenomenon happen? The main reason may be that the carbon chain of n-C15 alkane molecules is longer than that of n-C8 molecules. The n-C15 alkane molecules are more likely to be arranged in an orderly state under the adsorption of the kaolinite wall. This is because the n-C15 component alkane is closer to the solid state, and the molecular arrangement is more orderly. The n-C8 component alkane is closer to the gaseous state, and the molecules are more active, showing a disordered distribution state.Figure 9
Slices of the n-C8 : n-C15 component mass ratio of the 1 : 1 system: (a) slices from 2 nm to 2.6 nm; (b) slices from 5.8 nm to 6.4 nm; (c) slices from 9 nm to 9.6 nm.
(a)(b)(c)


## 4. Conclusions

(1)
In the single-component model, the peak adsorption density of alkanes on the surface of kaolinite silica tetrahedron is higher than that on the surface of aluminium-oxygen octahedron. The adsorption per unit area near the surface of the silicon-oxygen tetrahedron is 4~37 kg/m2 higher than that near the aluminium-oxygen octahedron. The alkane adsorption capacity of silicon-oxygen tetrahedron is stronger than that of aluminium-oxygen octahedron(2)
In the mixed model, there is a competitive adsorption relationship between alkanes with different carbon numbers. Alkanes with larger carbon numbers are more easily adsorbed on the surface of kaolinite(3)
Under actual geological conditions, shale oil with high heavy alkane content has poorer fluidity. The presence of nonpolar minerals reduces the fluidity of shale oil. There are many nonpolar minerals in the formation, and shale oil is easily adsorbed on the pore wall. Therefore, in actual formations, there are less nonpolar minerals and areas with higher content of light alkanes are more conducive to the migration of shale oil(4)
By changing the mass ratio of each component of the model, the adsorption density of different crude oils in the pores can be calculated, and then, the adsorption amount per unit area can be calculated. This can provide a certain basis for the overall resource reserve assessment



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*Source: 6397404-2023-03-08.xml*