Adsorption Characteristics of Illite and Kerogen Oil Phase: Thermodynamics Experiments

Abstract

In order to study the adsorption process and adsorption characteristics of shale oil at the macro scale, the isothermal adsorption experiments of illite and kerogen on a heptadecane (oil phase) solution were carried out by infrared spectrophotometry and gas chromatography–mass spectrometry. Based on the adsorption isotherm model and adsorption thermodynamic model, the characteristics of heptadecane adsorbed by illite and kerogen at different temperatures and oily solution concentrations were studied. The experimental results show that the concentration and temperature of the alkane solution help to enhance the adsorption and increase the saturated adsorption capacity. The difference is that the concentration will have a certain effect on the adsorption rate, while the temperature will not. Based on the three adsorption isotherm models, it was found that Langmuir and Freundlich were more suitable for describing the adsorption process of the heptadecane solution by illite and kerogen, and the adsorption characteristics of heptadecane molecules at different temperatures and adsorbents were evaluated. Heating leads to an increase in the collision efficiency between adsorbate molecules and adsorbents, thereby accelerating the migration rate of alkanes. Therefore, increasing temperature helps to enhance the adsorption capacity of rocks and increase the saturated adsorption capacity of minerals. The research results clarify the adsorption characteristics of shale oil heavy components from the macro level and fill the research gap in the application of solid–liquid isothermal adsorption physical experiments on the adsorption and occurrence of shale oil.

1. Introduction

At present, mankind is facing an iteration of new and old energy replacement, and the oil and gas industry worldwide has gradually extended to unconventional fields [1,2,3,4]. With the support of new theories and technology, it is difficult to directly measure the amount of oil and gas adsorption and evaluate the adsorption process by means of laboratory experiments [5,6,7], numerical simulation and molecular simulation. An isothermal adsorption experiment can effectively make up for this deficiency, providing important technical support for the study of oil and gas adsorption and migration, and is of great significance to oil and gas exploration and development [8,9].

By combining the isothermal adsorption of shale–water vapor with the theoretical model, Dong et al. described the capillary phenomenon and multi-molecular layer adsorption behavior of shale in the process of water vapor adsorption and found that the adsorption was a spontaneous, exothermic and entropy-reducing process [10]. Huang et al. quantitatively analyzed the effect of water molecules on methane adsorption in shale, clarified the mechanism of water molecules on the methane adsorption rate under different pressure gradients, and deepened the understanding of underground supercritical adsorption [11]. Based on this, Wang carried out research on the adsorption mechanism of methane in shale under supercritical conditions, analyzed the main controlling factors of methane adsorption in shale by thermodynamic model, and clarified the thermodynamic characteristics of methane adsorption under supercritical conditions [12]. In order to improve oil and gas recovery, Abdulkareem et al. carried out experiments on shale adsorption and desorption of CO₂ based on the gravimetric method and used the numerical equilibrium isotherm model to clarify the influence of physical properties of shale reservoirs on the adsorption rate [13]. Tian carried out experimental and theoretical research on the adsorption of methane, ethane pure component gas and its binary mixture in shale. The multi-temperature Langmuir–Freundlich adsorption equation was selected to characterize the adsorption difference of oil and gas components in shale. Based on the isothermal adsorption method, a comprehensive evaluation model of shale gas content was established [14]. By studying the adsorption law of shale in the Fuxian area of Ordos Basin, Chen et al. established a four-parameter Langmuir expansion model for accurately predicting the amount of methane adsorption, which provided a new method for the quantitative calculation of oil and gas adsorption [15]. Since then, many scholars have carried out a lot of research on the process and characteristics of oil and gas adsorption by combining isothermal adsorption experiments with mathematical evaluation models [16,17,18,19,20].

Based on the existing theoretical and experimental techniques, the research objects of isothermal adsorption physics experiments are mostly limited to the solid–gas two-phase state, and few experts carry out solid–liquid isothermal adsorption experiments for shale oil (oil phase) [21,22,23,24,25]. Yu et al. carried out solid–liquid (oil phase) isothermal adsorption experiments on shale for the first time. The adsorption model was used to analyze the adsorption characteristics of shale to diesel oil. It was clear that the adsorption of shale to diesel oil under natural conditions conforms to the Langmuir model and tends toward monolayer adsorption [26].

The Beibu Gulf Basin in the South China Sea is an important area for the exploration and development of shale oil in China. Studying the adsorption characteristics of shale oil in this area indicated that it is conducive to promoting the increase of oil and gas reserves and production in China and even the world [27,28,29,30]. Based on the phase mass spectrometry of shale oil-saturated hydrocarbons in the study area and the XRD experiment of the reservoir, researchers learned that the heavy component of saturated hydrocarbons is mainly heptadecane, illite is the main component of soil minerals in shale and kerogen is also an important part of shale. Therefore, the solid–liquid isothermal adsorption experiments of the illite and kerogen oil phase (heptadecane) were carried out, which is helpful in the investigation of the adsorption law and characteristics of shale oil, to clarify the reaction mechanism of alkane concentration on the adsorption process, fill the research gap of solid–liquid (oil phase) isothermal adsorption experiments in oil and gas exploration and provide theoretical support for the occurrence mechanism of shale oil.

2. Illite and Kerogen Oil Phase Adsorption Experiment

Tetrachloroethylene has stable physical and chemical properties and has a strong ability to dissolve oils and fats. Therefore, it is often used as an organic solvent and desiccant. Using tetrachloroethylene, the oily solution of heptadecane was configured. Based on infrared spectrophotometry and gas chromatography–mass spectrometry, the adsorption details of heptadecane by illite and kerogen were investigated [31,32]. The adsorption characteristics of oil molecules adsorbed by illite and kerogen were analyzed by adsorption thermodynamics [33,34].

2.1. Samples

The isothermal adsorption experiments were procured (

Table 1. Experimental samples of solid–liquid (oil phase) isothermal adsorption.

Sample Kind		Purity /Specification	Sample Source
Adsorbent	Illite	75–100 μm	Dehang Mineral Products Co., Ltd.
	Kerogen	90.51%	Shale core debris in Weixinan Sag, Beibuwan Basin
Adsorbate	Heptadecane	AR	Chengdu Cologne Chemicals Co., Ltd.
	Tetrachloroethylene	AR	Chengdu Cologne Chemicals Co., Ltd.

). By screening and separation, illite powder with a particle size of 75–100 μm was obtained. After impurities were removed in the muffle furnace, high-purity illite powder was produced. The illite samples were heated at high temperatures, developing a darkening color and a reddish-brown tinge (Figure 1a). As a macromolecular organic substance, kerogen does not have a fixed molecular structure. Therefore, after acidic treatment, alkaline treatment and other steps, the high-purity kerogen sample from the Beibu Gulf Basin was prepared, and its average firing vector was 90.51% (Figure 1a).

According to the different functions of the adsorbent, it was divided into an alkane solution for plotting the standard curve and for measuring the adsorption capacity. Using analytical balance, 2.500 g heptadecane was accurately weighed in a 250 mL volumetric flask and dissolved in tetrachloroethylene to obtain a 250 mL solution of 10,000 mg/L alkane tetrachloroethylene. Based on the infrared spectrophotometry and GC–MS method, the stepwise dilution method was used to configure the solution for the marking and determination of adsorption experiments (

Table 2. Configuration of oily alkane solution of n-heptadecane and tetrachloroethylene.

Usage of the Experiment	Experiment Methods	Concentration (mg/L)
Drawing of standard curve	Infrared spectrophotometry	20	40	60	80	100	-	-	-
	GC–MS	10	10	10	10	10	-	-	-
Determination of adsorption capacity	Infrared spectrophotometry	50	100	200	500	1000	2000	5000	8000
	GC–MS	50	100	200	500	1000	2000	5000	10,000
Volume (Ml)		10	10	10	10	10	10	10	10

).

2.2. Experimental Methods and Steps

In order to investigate the occurrence characteristics of shale oil, the solid–liquid isothermal adsorption experiments of the alkane solution (oil phase) in illite and kerogen were carried out to study the effect of the solution concentration on the whole adsorption process. The combination of the isothermal adsorption results with the adsorption isotherm model and adsorption thermodynamic model could describe the adsorption process of alkanes in depth, which was helpful for investigating the reaction mechanism of adsorption behavior to the solution concentration and adsorption temperature and to clarify the adsorption process and characteristics of heptadecane in illite and kerogen [35,36].

First, 10 mL of alkane solution with different concentrations was taken; second, 0.2 g of illite powder and kerogen powder were added; finally, after sealing, it was placed on a constant temperature water bath shaker and oscillated for 24 h. Since kerogen is extremely difficult to separate from the solution, the mixture of kerogen and alkane solution needed to be left standing and overnight. Then, the supernatant was separated from the illite and kerogen powder by a 2.5 mL syringe and 0.45 μm filter head, and the filtrate was obtained. According to the difference between the solution concentration before and after adsorption, the influence of the adsorption process was investigated (Figure 1b).

The difference in the adsorption of the alkane solution (oil phase) by illite and kerogen was studied at 25 °C, 50 °C and 60 °C under atmospheric pressure. The specific experimental methods were the same as above. The experimental results were helpful in understanding the effect of temperature on adsorption.

In the experiment, the concentration of the oil phase solution before and after adsorption was measured by an infrared oil analyzer and GC-MS, and the relationship between the adsorption amount and oil phase concentration and adsorption temperature was determined [37]. Based on the calculation of isothermal adsorption capacity, the isothermal adsorption curve was drawn, and the adsorption isotherm model and adsorption thermodynamic model were used to explore the adsorption characteristics of heptadecane on the surface and pores of illite and kerogen.

$$\mathrm{Q}=\frac{\mathrm{V}\times ({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})}{\mathrm{m}}$$

(1)

In the formula, $\mathrm{m}$ is the shale mass, g; ${\mathrm{C}}_0$ is the concentration of the solution before adsorption, mg/L; ${\mathrm{C}}_{\mathrm{e}}$ is the solution concentration after adsorption, mg/L; $\mathrm{V}$ is the volume of solution, L; $\mathrm{Q}$ is the adsorption capacity, mg/g.

Based on the thermodynamic relationship, according to the equilibrium constant, the standard Gibbs free energy change (${∆\mathrm{G}}^{\mathsf{\theta }}$) is calculated to obtain [38,39].

$$∆{\mathrm{G}}^{\mathsf{\theta }}=-\mathrm{R}\mathrm{T}\mathrm{l}\mathrm{n}{\mathrm{K}}^{\mathsf{\theta }}$$

(2)

The standard enthalpy change (${∆\mathrm{H}}^{\mathsf{\theta }}$) and standard entropy change (${∆\mathrm{S}}^{\mathsf{\theta }}$) of relevant thermodynamic parameters were calculated by the van’t Hoff equation and Gibbs–Helmholtz equation.

$$∆{\mathrm{G}}^{\mathsf{\theta }}=∆{\mathrm{H}}^{\mathsf{\theta }}-\mathrm{T}·∆{\mathrm{S}}^{\mathsf{\theta }}$$

(3)

$$\ln {\mathrm{K}}^{\mathsf{\theta }}=\frac{∆{\mathrm{S}}^{\mathsf{\theta }}}{\mathrm{R}}-\frac{∆{\mathrm{H}}^{\mathsf{\theta }}}{\mathrm{R}·\mathrm{T}}$$

(4)

where, $∆{\mathrm{G}}^{\mathsf{\theta }}$ is the standard Gibbs free energy change in the adsorption process, J/mol; $K^{\theta }$ is the equilibrium constant of the adsorption process, and the dimension is 1; $R$ is the molar gas constant, 8.314 J/(mol·K); $T$ is the thermodynamic temperature in the adsorption process, K; ${∆S}^{\theta }$ is the standard entropy change, J/(mol·K); ${∆\mathrm{H}}^{\mathsf{\theta }}$ is the standard enthalpy change, J/mol.

3. Experimental Results

The adsorption isotherm refers to the relationship curve between the concentration of the solute in the two phases when the solute molecules reach the adsorption equilibrium on the two-phase interface at a certain temperature, which can provide deeper information on the interaction between the adsorbate and the adsorbent. The adsorption capacity of a given adsorbate can be determined by adsorption isotherms, and adsorption equilibrium is one of the most important factors in the study of the adsorbent–adsorbate system. By plotting the isothermal adsorption curves of illite and kerogen to the alkane solution and fitting the adsorption data with the isothermal adsorption mathematical model, the adsorption site information on the surface of the adsorbent and the adsorption characteristics of the adsorption system can be obtained. The commonly used mathematical models include the Langmuir, Freundlich and Temkin adsorption isotherm models [40].

Obviously, with the increase of the solution concentration, the saturated adsorption capacity of heptadecane in illite, however, the slope of the adsorption isotherm seems to become smaller, which means that the adsorption rate is negatively correlated with the solution concentration. According to the isothermal adsorption results, the adsorption rate (expressed by the slope of the adsorption isotherm) is almost not affected by temperature, but it is not difficult to see that the heating significantly increases the saturated adsorption capacity of the heptadecane solution. Therefore, heating is conducive to the adsorption reaction (Figure 2a).

Based on the isothermal adsorption experiment, the isotherms and fitting models of heptadecane adsorbed by kerogen were also plotted. Similarly, the adsorption concentration and temperature help to increase the saturated adsorption capacity of heptadecane by kerogen. However, as the concentration increases, the adsorption rate (slope of the adsorption isotherm) gradually decreases. The adsorption trend of heptadecane in illite is consistent with that in kerogen (Figure 3a).

In addition, the adsorption isotherms of heptadecane were linearly fitted to analyze the adsorption characteristics of heptadecane in illite and kerogen (Figure 2b–d and Figure 3b–d).

4. Analysis and Discussion

The isothermal adsorption simulation and physical isothermal adsorption experiments based on molecular dynamics usually need to be described by mathematical models. The fitting degree is used to judge the scientificity and rigor of the adsorption process. According to the fitting correlation of different models and the applicable scope of adsorption models, the adsorption behavior and adsorption characteristics of adsorbates in adsorbents are clarified [41].

4.1. Evaluation of Adsorption Isotherm Model

The isothermal adsorption results of the illite and kerogen heptadecane solutions show that with the increase of alkane concentration, the equilibrium adsorption capacity of illite and kerogen to heptadecane also increases and, finally, tends to be stable. At three temperature gradients of 25 °C, 50 °C and 60 °C, the Langmuir, Freundlich and Temkin model equation fitting adsorption results and related fitting parameters of illite and kerogen adsorbing the heptadecane solution were calculated (

Table 3. Related parameters of adsorption isotherm model of illite and kerogen to heptadecane solution.

Adsorbent	Temperature /°C	$\mathbf{Langmuir} \mathbf{Model}$ $\frac{{\mathbf{C}}_{\mathbf{e}}}{{\mathbf{Q}}_{\mathbf{e}}}=\frac{{\mathbf{C}}_{\mathbf{e}}}{{\mathbf{Q}}_{\mathbf{m}}}+\frac{1}{{{\mathbf{K}}_{\mathbf{L}}\mathbf{Q}}_{\mathbf{m}}}$			$\mathbf{Freundlich} \mathbf{Model}$ ${\mathbf{l}\mathbf{n}\mathbf{Q}}_{\mathbf{e}}={\mathbf{l}\mathbf{n}\mathbf{K}}_{\mathbf{F}}+\frac{1}{\mathbf{n}}\mathbf{l}\mathbf{n}{\mathbf{C}}_{\mathbf{e}}$			$\mathbf{Temkin} \mathbf{Model}$ ${\mathbf{Q}}_{\mathbf{e}}=\mathbf{A}\mathbf{ln}{\mathbf{C}}_{\mathbf{e}}+\mathbf{B}$
		${\mathbf{Q}}_{\mathbf{m}}$	KL	${\mathbf{R}}^2$	KF	n	${\mathbf{R}}^2$	A	$\mathbf{B}$	${\mathbf{R}}^2$
Illite	25	y = 0.05742x + 67.86			y = 0.6792x − 2.9563			y = 2.263x − 7.163
		17.415	0.000849	0.981	0.0520	1.472	0.973	2.263	−7.163	0.824
	50	y = 0.04855x + 84.86			y = 0.6535x − 2.8480			y = 2.3124x − 7.419
		20.597	0.00572	0.903	0.0579	1.530	0.990	2.312	−7.419	0.716
	60	y = 0.04375x + 65.95			y = 0.6271x − 2.486			y = 2.5725x − 7.747
		22.857	0.000663	0.912	0.0832	1.594	0.981	2.572	−7.747	0.698
Kerogen	25	y = 0.03764x + 56.88			y = 0.6986x − 2.7716			y = 3.767x − 13.972
		26.567	0.00066	0.973	0.062	1.431	0.966	3.767	−13.972	0.857
	50	y = 0.03456x + 36.58			y = 0.6569x − 2.2089			y = 4.228x − 14.469
		28.935	0.00945	0.982	0.109	1.522	0.966	4.228	−14.469	0.879
	60	y = 0.03249x + 29.86			y = 0.6407x − 1.9662			y = 4.610x − 15.50
		30.778	0.00108	0.987	0.139	1.560	0.963	4.610	−15.50	0.894

).

The mathematical model was used to evaluate the adsorption of the alkane solution by illite and kerogen, respectively. The results show that the effect of temperature on the adsorption process is reflected in the molecular adsorption layer, which in turn affects the change of adsorption capacity. The calculation results of different adsorption models are different. At 25 °C, 50 °C and 60 °C, the highest fitting degrees of the three evaluation models for illite adsorption alkane solution are 0.981, 0.990 and 0.981. It is considered that the Langmuir isothermal adsorption model is more suitable for describing the adsorption process at 25 °C, while the Freundlich isothermal adsorption model is more suitable for describing the adsorption process at 50 °C and 60 °C. Based on the isothermal adsorption mathematical evaluation model, the application scope and description characteristics of the model are evaluated. The adsorption process at 25 °C is mainly single-molecule adsorption, forming a single-molecule adsorption layer. The adsorption process at 50 °C and 60 °C is mainly multi-layer adsorption, and the adsorption heat and affinity do not need to be evenly distributed on the heterogeneous surface [42,43].

Differently, the highest fitting degree of the isothermal adsorption model to evaluate the adsorption of the alkane solution by kerogen was calculated by the Langmuir theory. The fitting degree of the isothermal adsorption evaluation at 25 °C, 50 °C and 60 °C was 0.973, 0.982 and 0.987, respectively. The fitting analysis showed that the adsorption of the alkane solution by kerogen was mainly single-molecule adsorption, forming a single-molecule adsorption layer.

The SEM imaging of organic-rich shale shows that it is characterized by rich clay minerals (illite) and organic matter (kerogen) and has a large number of illite pores and kerogen pores, which provide a large amount of storage space for shale oil (Figure 4a). After the above adsorption experiments and analysis calculations, the occurrence characteristics of the adsorbed heptadecane are clearly presented. Heptadecane forms a single-molecule adsorption layer in the pores of kerogen (Figure 4b), while heptadecane forms a single-molecule adsorption layer in the pores of illite, and there is a phenomenon of multi-layer adsorption of heptadecane molecules (Figure 4c).

4.2. Thermodynamic Evaluation of Adsorption

Based on the adsorption isotherm model and related parameters of illite and kerogen on the heptadecane solution (

Table 4. Thermodynamic parameters of adsorption of heptadecane by illite and kerogen.

Sample	Temperature (K)	ΔHθ (kJ/mol)	ΔSθ (J/(mol·K))	ΔGθ (kJ/mol)
Illite	298.15	114.049	324.869	18.145
	323.15			7.654
	333.15			6.887
Kerogen	298.15	11.663	−21.757	18.146
	323.15			18.711
	333.15			18.899

), the best adsorption model of illite and kerogen on heptadecane at different temperatures was selected.

The calculation shows that the equilibrium adsorption capacity of illite and kerogen to heptadecane increases with the increase of temperature (Figure 5a). This may be because the increase of temperature increases the collision efficiency between the adsorbate molecule and the adsorbent and accelerates the diffusion transmission rate of the heptadecane molecule into the micropore channel inside the matrix rock, indicating that the adsorption process of illite and kerogen to heptadecane is an endothermic reaction. The increase of temperature is beneficial to the adsorption reaction and enhances the adsorption capacity of the matrix rock [44].

The adsorption experiment data of illite and kerogen on heptadecane were analyzed, and the thermodynamic curves (Figure 5b) were drawn with 1/T as the abscissa and the ordinate as the ordinate. Combined with the adsorption thermodynamic parameters of illite and kerogen on heptadecane at different temperatures, it was found that the standard enthalpy change in the adsorption process was always positive, indicating that the adsorption of illite and kerogen on heptadecane is an exothermic process [45]. The increase of temperature was beneficial to the adsorption reaction, which was consistent with the results of previous adsorption isotherm experiments. At the same time, the standard entropy change of illite to heptadecane is positive, indicating that the degree of confusion is increasing during the whole adsorption reaction. This analysis is consistent with the research results of X et al. It is considered that the adsorption characteristics of rock surface are mainly spontaneous and exothermic, which is beneficial for improving the recovery of the reservoir [46]. However, the standard entropy change of kerogen to heptadecane is negative, indicating that the degree of confusion during the entire adsorption reaction is continuously reduced. The entropy changes of heptadecane adsorbed by the two matrices show the opposite phenomenon, which may be due to the huge difference in the crystal structure of illite and kerogen. Illite with a crystal cell structure and kerogen with an amorphous cell structure have different adsorption sites and different hydrophilic and lipophilic properties [42,43,47].

5. Conclusions

The adsorption process of the heptadecane solution (oil phase) by illite and kerogen solid powder was systematically studied by an adsorption isotherm model and thermodynamic formula, and the adsorption characteristics at different concentrations and temperatures were investigated. By investigating the effects of experimental samples, solution concentration and adsorption temperature on the adsorption process, the differences and laws of oil phase adsorption of shale components were clarified.

With the increase of concentration and temperature, the saturated adsorption capacity of illite and kerogen to heptadecane is significantly increased. In the low concentration stage, the adsorption rate is faster with the increase of concentration. As the concentration continues to increase, the adsorption rate slows down. Differently, the adsorption rate is almost unaffected by temperature. In the adsorption process of heptadecane by illite, at 25 °C, the adsorption characteristics are mainly monolayer adsorption, but at 50 °C and 60 °C, the adsorption characteristics are mainly multi-layer adsorption. In contrast, at 25 °C, 50 °C and 60 °C, heptadecane formed a single-molecule adsorption layer in the kerogen slit.

In addition, the adsorption process of heptadecane by illite and kerogen is an endothermic reaction. Heating can improve the collision efficiency between adsorbate molecules and adsorbents, which leads to the increase of the adsorption capacity of alkane molecules on rocks, thus enhancing the adsorption capacity of rocks. The degree of confusion of the adsorption system of illite and kerogen shows two distinct trends, probably because the adsorption sites of illite and kerogen have different hydrophilic properties and lipophilic properties.