Pore structure characterization and its effect on methane adsorption in shale kerogen

Wang, Tian-Yu; Tian, Shou-Ceng; Liu, Qing-Ling; Li, Gen-Sheng; Sheng, Mao; Ren, Wen-**; Zhang, Pan-Pan

doi:10.1007/s12182-020-00528-9

Pore structure characterization and its effect on methane adsorption in shale kerogen

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Published: 12 November 2020

Volume 18, pages 565–578, (2021)
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Petroleum Science

Pore structure characterization and its effect on methane adsorption in shale kerogen

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Tian-Yu Wang^1,2,
Shou-Ceng Tian^1,2,
Qing-Ling Liu³,
Gen-Sheng Li¹,
Mao Sheng^1,2,
Wen-** Ren^1,4 &
…
Pan-Pan Zhang^1,2

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Abstract

Pore structure characterization and its effect on methane adsorption on shale kerogen are crucial to understanding the fundamental mechanisms of gas storage, transport, and reserves evaluation. In this study, we use 3D scanning confocal microscopy, scanning electron microscopy (SEM), X-ray nano-computed tomography (nano-CT), and low-pressure N₂ adsorption analysis to analyze the pore structures of the shale. Additionally, the adsorption behavior of methane on shales with different pore structures is investigated by molecular simulations. The results show that the SEM image of the shale sample obviously displays four different pore shapes, including slit pore, square pore, triangle pore, and circle pore. The average coordination number is 4.21 and the distribution of coordination numbers demonstrates that pores in the shale have high connectivity. Compared with the adsorption capacity of methane on triangle pores, the adsorption capacity on slit pore, square pore, and circle pore are reduced by 9.86%, 8.55%, and 6.12%, respectively. With increasing pressure, these acute wedges fill in a manner different from the right or obtuse angles in the other pores. This study offers a quantitative understanding of the effect of pore structure on methane adsorption in the shale and provides better insight into the evaluation of gas storage in geologic shale reservoirs.

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1 Introduction

The extraction of gas from shale is regarded as an energy game-changer (Hughes 2013). As a transition fuel, shale gas offsets reducing of conventional gas production (Vidic et al. 2013). Horizontal drilling coupled with multi-stage hydraulic fracturing makes it possible to extract hydrocarbons from shale reservoirs (Ren et al. 2016; Shi et al. 2019). Gas in shale reservoirs mainly exists in three forms: free gas, adsorbed gas, and dissolved gas. Particularly, about 20%‒85% of the total gas-in-place is adsorbed gas (Curtis 2002). In shale gas production, the desorption of adsorbed gas can compensate for the loss of free gas, which is a significant reason that shale gas maintains long-term stable production (Wang et al. 2019a). Therefore, understanding the adsorption of hydrocarbons in shale is essential for reliable resource assessment and shale gas recovery.

In general, pores in shale are much smaller than those in conventional sandstone reservoirs (Loucks et al. 2012; Shen et al. 2019). Pore structure characterization has attracted great attention. Nitrogen adsorption tests are used to measure pore size distribution in shale samples (Feng et al. 2018; Groen et al. 2003). Also, the non-local-density functional theory is also used to characterize the pore structures in shale media (Wei et al. Full size image

3.2 Analysis of pore characteristics

The size distribution and shape of shale pores and throats determine the reserves and transmission capacity of shale gas. Figure 2a plots the nano-CT image of a shale block with a diameter of 38.4 μm. The pore size distribution and shape factor are determined using the attribute extraction method. The red part in Fig. 2b represents the pores and throats. The largest maximal balls are identified as pores, while the smallest balls are throats (Dong and Blunt 2009). The pore network is simplified using the ball-and-stick model, and the parameters of pores and throats are obtained. The statistical sizes of pores and throats include their inscribed radius and shape factors. The statistical comparisons of the inscribed radius, shape factors of pores and throats are plotted in Fig. 2c-2d, respectively. The radii of most of the pores and throats are less than a micron, including the organic pores and inorganic pores in shale media. The peak values of the radii of pores and throats are 8.22 × 10⁻⁸ m and 1.46 × 10⁻⁷ m, respectively. The throat is a continuous channel between the nanopores and a key parameter of shale transport capacity.

The connectivity of the pore network is expressed by coordination numbers. The coordination numbers of pores and throats are shown in Fig. 3. The average coordination number is 4.21 and the distribution of coordination numbers demonstrates that the pores in shale samples have high connectivity (Zhang et al. 2015). The presence of pores with high connectivity may be attributed to the existence of more tortuous paths that join a large number of pores together (Al-Kharusi and Blunt 2007).

In the previous section, it has been shown that the shale samples have complicated pore structures. In this section, we are primarily concerned with the quantitative analysis of pore structure parameters. N₂ adsorption isotherms are used to measure the specific surface area, pore volume, average pore diameter, and micro-pore volume of four shale samples. N₂ adsorption isotherms of the four shale samples can be seen in Fig. 4. The adsorption and desorption isotherms are almost overlapped under relatively low pressures. However, with increasing relative pressure, the adsorption and desorption isotherms do not overlap, which results in a hysteresis loop. This is because the capillary condensation occurs within the meso-pores (Sing 1985). N₂ adsorption/desorption isotherms with hysteresis loops indicate that the shale samples contain both meso-pores and macro-pores (**ong et al. 2015). Table 1 displays the pore structure parameters calculated by the Brunauer–Emmett–Teller method. The specific surface area is 9.83–22.84 m²/g with an average value of 14.03 m²/g, and the pore volume ranges from 1.34 × 10⁻² to 2.27 × 10⁻² cm³/g with an average value of 1.61 × 10⁻² cm³/g. The average pore diameter and micro-pore volume range from 3.97 to 5.44 nm with an average value of 4.785 nm and from 3.19 × 10⁻⁴ to 4.59 × 10⁻³ cm³/g with an average value of 1.81 × 10⁻³ cm³/g, respectively. Some previously published experimental data (Sun et al. 2016; Yang et al. 2016) are also included in Fig. 5 and it can be seen that the specific surface area positively correlates with the pore volume (Fig. 5a). In contrast, the average pore size is in a negative correlation to the pore volume and specific surface area (Fig. 5b, c), which is consistent with previous studies (Chalmers et al. 2012). In addition, the pore shape is also an important factor that affects the adsorption capacity of methane on shale, which can be observed directly from the SEM image, as shown in Fig. 6. The SEM image of the shale sample obviously displays four different pore shapes, including slit pore, square pore, triangle pore, and circle pore. We use GCMC simulations to study the adsorption behavior of methane on shale with these four pore shapes.

Table 1 Pore structure parameters of shale samples

Full size table

4 Molecular simulation

4.1 Models and methodology

The kerogen model is built by Ungerer et al. (2014) based on the experimental data (Kelemen et al. 2007), as shown in Fig. 7a. The composition of the kerogen is C₂₅₁H₃₈₅O₁₃N₇S₃. It is typical of kerogen deposited in excellent conditions of preservation, such as anoxic lacustrine environments, and it is classified as immature based on its hydrocarbon generation (Ungerer et al. 2014). The parameters of the molecular model match analytical data well with reasonable accuracy, as indicated in Table S1 (see electronic supplementary material). We use 10 kerogen molecules to build a kerogen cell, as shown in Fig. 7b. In our previous work, we have validated the molecular model with the experimental data from the literature (Wang et al. 2018b). The density is 1.097 g/cm³, which is in agreement with the measured result ranging from 1.0 g/cm³ to 1.15 g/cm³ (Mastalerz et al. 2012b). In this work, we modify the kerogen model with different pore shapes compatible with SEM images. Figure 8 plots the kerogen models with different pore shapes. Different pore shapes have the same pore volume, which means the matrix volumes are the same. The diameter of the circle pore is 2 nm. The volume of the circle pore is 3.9825π nm³, and thus, the surface areas of the slit pore, square pore, triangle pore, and the circle pore are 38.0038, 28.2351, 27.7895, and 25.0220 nm², respectively, as shown in Table 2. The adsorption isotherms of methane on shale kerogen are simulated by the GCMC method, and the COMPASS (Sun 1998) force field is adopted for all simulations. For the nonbonding interactions, we use the Ewald method and the Lennard-Jones 9-6 potential to describe the electrostatic potential and the vdW interaction, respectively. In the GCMC simulation, the chemical potential is a function of fugacity. The fugacity of methane is calculated by the Peng-Robinson equation (Poling et al. 2001).

Table 2 Pore structure parameters of shale samples

Full size table

4.2 Adsorption isotherms of methane on shale

Four molecular models are constructed to study the effect of pore shapes on methane adsorption on shale kerogen. The adsorption isotherms of methane on the slit pore, square pore, triangle pore, and the circle pore of shale are calculated to study the effects of the pore structure. The absolute adsorption capacities and excess adsorption capacities are plotted in Fig. 9. The excess adsorption capacities are calculated by the following equation (Wang et al. 2018a):

$$n_{\text{e}} = n_{\text{a}} - \rho V_{\text{a}} /M$$

(1)

where n_a is the absolute adsorption capacity, mol/kg; n_e is the excess adsorption capacity, mol/kg; ρ is the equilibrium density of methane, kg/m³; V_a is the adsorption volume, m³/kg; M is the molar mass of methane, kg/mol.

Taken the absolute and excess adsorption isotherms of the slit pore as an example. At 30 MPa, the absolute adsorption capacities are 7.21, 6.82, 6.28, and 5.89 mmol/g at 298, 328, 358, and 388 K, respectively. This indicates that the absolute methane adsorption capacity increases with increasing pressure, while decreases with increasing temperature. This is because the adsorption of methane on shale is an exothermic process (Hao et al. 2013). The absolute adsorption increases sharply at low pressures. However, with increasing pressure, the absolute adsorption increases slowly. This indicates that as the pressure decreases, the methane desorption increases. In the shale gas production, the loss of production due to decreased reservoir pressure is compensated by desorption of adsorbed gas, which may be one of the reasons that the shale gas maintains a long-term stable production (Wang et al. 2018b). For excess adsorption of methane, with increasing pressure, the excess adsorption increases, and then decreases gradually to converge. The adsorption capacities of methane on other shapes of shale pores also follow this trend.

4.3 Analysis of pore structure effects

The pore structure affects the methane adsorption on shale. In this section, we study the effect of pore shape on methane adsorption on shale nano-pores. Figure 10 depicts the absolute and excess adsorption of methane on different pores of kerogen. The absolute adsorption at 30 MPa on triangle pore is 7.93 mmol/g at 298 K, and that on the slit pore, square pore, and the circle pore are 7.21, 7.30, and 7.46 mmol/g, respectively. The adsorption capacity of methane on the triangle pore is larger than that on the slit pore, square pore, and the circle pore. This phenomenon is also observed for carbon pore structures (Song et al. 2018). Specifically, compared with the adsorption capacity of methane on the triangle pore, the adsorption capacity of methane on the slit pore, square pore, and the circle pore are reduced by 9.86%, 8.55%, and 6.12%, respectively. The adsorption capacity of methane on the triangle pore is larger than that on the slit pore, square pore, and the circle pore. Figure 11 depicts the distributions of methane molecules in four different pores at 1 MPa and 388 K. Methane molecules distribute randomly in slit pores, as shown in Fig. 11a. Particularly, Fig. 11b, c indicates that methane molecules prefer to adsorb near the corner. In Fig. 11d, we can see that methane molecules adsorb on the surface of the circle pore. The main difference between the triangle pore and other shapes is that the presence of acute angles which form wedges along the pore length. With increasing pressure, the acute wedges fill in a manner different from the right or obtuse angles found in the other pores (Malanoski and van Swol 2002b). It is also presented by Malanoski and van Swol (2002a) that more adsorbent adsorbed near the acute angle corner, which results in the reduction of variation in the curvature of the pore film. Figure 12 depicts the methane distributions in triangle pores at different pressures. It can be noticed that methane molecules adsorb in both the kerogen matrix and triangle pores. Particularly, Fig. 12b indicates that methane molecules prefer to adsorb near the corner at low pressures. With the pressure increases, the methane adsorption in the triangle pore increases, and there are more methane molecules adsorbed in the center area of triangle pores. The excess adsorption isotherms simulated by the Song et al. (2018) show a notable difference for different pore structures. This is because different pore structures have different pore volumes. The pore volume amplifies the difference between absolute adsorption and excess adsorption, and the effect causes a significant decrease in the excess adsorption of circle and square pores. In the present work, because the effect of the pore volume is eliminated, the excess adsorption on shales with different pore structures have slight different values. As the temperature increases, the differences between the adsorption of various shapes decrease gradually. When the gas temperature rises to 388 K, the adsorption capacities of methane on the slit pore, square pore, and the circle pore are reduced by 4.23%, 7.61%, and 4.64%, respectively. This can be explained by the fact that the effect of temperature is more prominent with high adsorption capacity.

To quantify the effect of the water content on adsorption capacities of methane on different shaped pores, methane adsorption at 30 MPa is simulated, as shown in Fig. 13a. Compared with the results of previous research (Gasparik et al. 2013; Zhang et al. 2014; Zhao et al. 2017), water contents in moisture kerogen models are 0.6, 1.2, 1.8 and 2.4 wt%, respectively. The methane adsorption on all different pores decreases with increasing water content. Taken the methane adsorption in the triangle pore as an example, in 0.6, 1.2, 1.8, and 2.4 wt% moisture kerogen models, the methane adsorption capacities are 7.58, 7.31, 6.89, and 6.67 mmol/g, respectively, at 30 MPa. Compared with the adsorption capacity of methane on the kerogen model without water, the adsorption capacities on the moisture kerogen models reduce by 4.22%, 7.62%, 12.94%, and 15.67%, respectively. The pore shapes also affect the adsorption capacity of methane on moisture kerogen models. The adsorption of methane on the triangle pores is less affected by water content than other pore shapes. For different pore shapes, the methane adsorption capacities decrease by about 18.47%, 17.20%, 15.74%, and 17.40%, respectively on the slit pore, square pore, triangle pore, and circle pore models with a water content of 2.4 wt% at 388 K. This is because the triangle pores have the largest methane adsorption capacity without the water content. Wang et al. did simulations (Wang et al. 2018b) to investigate the effects of water content on methane adsorption and concluded that water molecules adsorb in the kerogen matrix and decrease the pore volume.

5 Conclusions

A series of experimental techniques, such as SEM, 3D scanning confocal microscopy, X-ray nano-CT, and low-pressure N₂ adsorption analysis, are performed on the shale samples to characterize pore structures. Additionally, the adsorption behavior of methane on shale with different pore structures are studied by GCMC simulations. Through the experimental and simulation studies, major findings are summarized as follows.

(1)
The distinction between the organic matter and inorganic matter is obvious, which indicates that shale is highly heterogeneous at the micro-scale. Carbon is a major component of the organic matter of shale, while the inorganic matter surface is rich in oxygen. In addition, four different pores are observed on the SEM image of the shale sample, including slit pore, square pore, triangle pore, and circle pore.
(2)
The radii of most of the pores and throats are less than a micron, including the organic pores and inorganic pores in shale media. The peak values of the radii of pores and throats are 8.22 × 10⁻⁸ m and 1.46 × 10⁻⁷ m, respectively. The average coordination number is 4.21 and the distribution of coordination numbers demonstrates that there are some pores with high connectivity.
(3)
Compared with the adsorption capacity of methane on triangle pores, the adsorption capacity of methane on slit pore, square pore, and circle pore is reduced by 9.86%, 8.55%, and 6.12%, respectively. With increasing pressure, these acute wedges fill in a manner different from the right or obtuse angles found in the other pores. As the temperature increases, the differences between the adsorption on various structures decrease gradually. When the gas temperature rises to 388 K, the adsorption capacity of methane on the slit pore, square pore, and the circle pore is reduced by 4.23%, 7.61%, and 4.64%, respectively.
(4)
Methane adsorption on all different pores decreases with increasing water content. The adsorption of methane adsorption on triangle pores is less affected by the water content than other pore shapes.

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Acknowledgements

The authors acknowledge financial support from the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (No. 51821092), the General Projects of the Natural Science Foundation of China (No. 51674275) and the Foundation of State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Bei**g (No. PRP/open-2003). Tianyu Wang acknowledges the China Scholarship Council for financial support during his visit to Harvard University.

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State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Bei**g), Bei**g, 102249, China
Tian-Yu Wang, Shou-Ceng Tian, Gen-Sheng Li, Mao Sheng, Wen-** Ren & Pan-Pan Zhang
Harvard SEAS-CUPB Joint Laboratory on Petroleum Science, 29 Oxford Street, Cambridge, MA, 02138, USA
Tian-Yu Wang, Shou-Ceng Tian, Mao Sheng & Pan-Pan Zhang
China National Oil and Gas Exploration and Development Company Ltd, Bei**g, 100034, China
Qing-Ling Liu
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, 610500, China
Wen-** Ren

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Tian-Yu Wang
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Shou-Ceng Tian
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Qing-Ling Liu
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Gen-Sheng Li
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Mao Sheng
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Wen-** Ren
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Pan-Pan Zhang
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Correspondence to Shou-Ceng Tian.

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Wang, TY., Tian, SC., Liu, QL. et al. Pore structure characterization and its effect on methane adsorption in shale kerogen. Pet. Sci. 18, 565–578 (2021). https://doi.org/10.1007/s12182-020-00528-9

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Received: 03 February 2020
Accepted: 22 October 2020
Published: 12 November 2020
Issue Date: April 2021
DOI: https://doi.org/10.1007/s12182-020-00528-9

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Pore structure characterization and its effect on methane adsorption in shale kerogen

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