Methane Adsorption on Carbon Models of the Organic Matter of

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Methane adsorption on carbon models of the organic matter of organic-rich shales Jian Xiong, Xiangjun Liu, Lixi Liang, and Qun Zeng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03144 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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Energy & Fuels

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Methane adsorption on carbon models of the organic matter

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of organic-rich shales

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Jian Xiong1 Xiangjun Liu1* Lixi Liang1 Qun Zeng2

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1.State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University,

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Chengdu 610500, Sichuan 2. Institute of Chemical Materials, Engineering Physical Academy of China, Mianyang

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621999, Sichuan

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*Corresponding author: Xiangjun Liu, E-mail address: [email protected]

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Abstract: The organic matter of organic-rich shales has an important significance for the methane

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adsorption capacity on shales. The kerogen is simplified to ideal graphite, and oxygenated

10

functional groups are grafted onto graphite surfaces to obtain different O/C atomic ratios reflecting

11

varying maturation levels of kerogen. The adsorption behaviors of methane in the pores of

12

graphite with different O/C ratios were investigated by the grand canonical Monte Carlo (GCMC)

13

method. The results show that the isosteric heat of adsorption of methane is reduced with an

14

increase in the pore size or decrease in the O/C ratio. The methane adsorption capacity in

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micropores increases with an increasing pore size, whereas it decreases with an increasing pore

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size in mesopores. The methane adsorption capacity in pores with the same pore size decreases

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with decreasing O/C ratios. The proportion of the adsorbed gas in the pores decreases with

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increasing pressure with the same pore size or with an increasing pore size under the same

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pressure. Methane in the organic pores of the organic-rich shales is mainly in the adsorbed state

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when the pore size is less than 6 nm. The adsorption sites of methane gradually change from lower

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energy adsorption sites to higher ones with increasing temperature, leading to the reduction of the

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methane adsorption capacity. The water molecules in the pore affected by van der Waals forces,

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Coulombic forces and hydrogen bonding interactions are close to the oxygen-containing groups

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and occupy the adsorption space of methane molecules, leading to a decrease of the methane

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adsorption capacity. The reduction of the mole fraction of methane in the gas phase, change of

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adsorption sites of methane and decrease of the adsorption space of methane generate the methane

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adsorption capacity for the methane/carbon dioxide binary gas mixture adsorption system.

28 29

Keywords: organic matter; methane; graphite; O/C ratio; adsorbed gas; adsorption behavior;

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1 Introduction

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Shale gas, as one type of unconventional gas reservoir, is an important energy resource. In

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2013, the report released by the EIA indicated that the global technical recoverable reserves of

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shale gas totaled 220.73×1012 m3 1, indicating that globally the shale gas resource was abundant

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and has a huge potential for production. Compared with the conventional oil and gas reservoirs,

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ACS Paragon Plus Environment

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the modes of occurrence of the shale gas reservoirs mainly included the free state, adsorbed state

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and dissolved state. In 2002, Curtis 2 studied the characteristics of the shale gas reservoirs in the

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U.S.A, suggesting that adsorbed gas accounts for approximately 20-85% of the total gas content,

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and the adsorbed gas played an important role in the shale gas resource. Therefore, it is significant

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to investigate the methane adsorption capacity on shales for evaluation of the shale gas resource.

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In addition to the environmental factors, such as the pressure, temperature, moisture, etc., the

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physical and chemical properties (organic matter, mineral compositions, etc.) of organic-rich

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shales can impact the methane adsorption capacity on shales, indicating that the organic matter

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plays a significant role in the methane adsorption capacity on shales. For the shale gas reservoirs,

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it has also been shown that the organic matter mainly controls the gas adsorption 2-7. Therefore, the

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investigation of the methane adsorption capacity on the organic matter can be of important

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significance for evaluation of the shale gas resource.

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In an effort to better understand the methane adsorption on the organic matter from

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organic-rich shales, numerous researchers have made valuable contributions to the literature

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through laboratory studies in recent years. Zhang et al. 8, Rexer et al. 9, Guo et al. 10 and Liang et

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al.

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isolated from various formations based on isothermal adsorption experiments, and they discussed

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the influence of factors on the methane adsorption capacity, including temperature, pressure,

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moisture, etc. These researchers indicated that the methane adsorption capacity on different types

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of kerogen increased in the following order: type I < type II < type III kerogen, and the methane

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adsorption capacity on kerogen was more than those on pure minerals and shales. Furthermore,

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Rexer et al. 9, Guo et al. 10, Ross et al.12, Chen et al.13, Gou et al.14, Gasparik et al.15, Tan et al.16

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and Yang et al.17 conducted a series of isothermal adsorption experiments targeting the

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organic-rich shales from different formations to investigate the methane adsorption capacity on

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shales, and they discussed the influences of the TOC contents and maturation on the methane

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adsorption capacity using a statistical method. All of the research outlined above contrasted and

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compared the methane adsorption capacity on kerogen and organic-rich shales using the Langmuir

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parameters under equilibrium conditions based on the isothermal adsorption experiments, which

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were regarded as the results of the macroscopic adsorption behaviors and failed to deeply reflect

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the adsorption essence and microcosmic adsorption mechanism of the methane on organic matter.

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Furthermore, the methane adsorption capacity on organic matter has been obtained over a limited

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range of pressures and temperatures in these experiments.

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investigated the methane adsorption capacity on different types of kerogen (organic matter)

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As a theoretical research approach for investigating the adsorption properties of adsorbents,

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computer molecular simulation technology, including the grand canonical Monte Carlo (GCMC)

69

and molecular dynamics (MD) methods, has been used to investigate atomic-scale adsorption

2


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phenomena and properties between fluid molecules and the porous material in the past few years.

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Some authors

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transport properties and structural properties of the methane in minerals pores, such as

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montmorillonite, illite and quartz, and discussed the influences of the pore size, pressure,

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temperature, moisture and carbon dioxide on the methane adsorption capacity. Furthermore, for

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the organic matter in the organic-rich shales, a simplified model, such as an ideal graphite-based

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simulation cell, was adopted to investigate the adsorption behaviors, transport properties and

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structural properties of the methane in carbon pores using the GCMC and MD methods by some

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researchers 25-28. In addition, Collell et al. 29, Zhang et al. 30 and Sui et al.31 studied the adsorption

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behaviors and transport properties of methane on simplified kerogen isolated from organic-rich

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shales using the GCMC and MD methods, but the influence of the maturation level of kerogen

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was not considered. This research indicates that the GCMC and MD methods have been proven to

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be effective methods to investigate adsorption behaviors of the adsorbate on the adsorbent and

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have obtained some knowledge on methane adsorption on organic matter. In those research studies,

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the total gas amount and excess adsorption amount on organic matter obtained from the simulation

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results could be used to study the methane adsorption capacity. The total gas amount could contain

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the following two parts: adsorbed gas amount and free gas amount. For the supercritical

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adsorption, the excess adsorption amount could not reflect the realistic adsorption amount and

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should be converted into the absolute adsorption amount.

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to show that the methane absolute adsorption amount on organic matter obtained from the

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simulation results and the proportion of the adsorbed gas have been well investigated.

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18-24

used the GCMC and MD methods to investigate the adsorption behaviors,

In previous research

11

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However, there is not enough reports

, the TOC content of kerogen isolated from the Lower Silurian

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Longmaxi Formation shale was found to be 78.5%, indicating that there was a higher carbon atom

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content in the kerogen. The composition and structure of the kerogen is complex 33, and there was

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no effective way to construct a realistic model of the molecular structure of kerogen 35-41

34

. Some

95

scholars

put forward many models of the chemical structure of kerogen by using different

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methods. These models only reflected the local structural characteristics of the molecular structure

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of kerogen, which could not explain its physical and chemical characteristics well, and thus, they

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have not been widely used

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simulation cell to study the adsorption behaviors and structural properties of the methane in pores,

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which may be related to the higher carbon atom contents of the kerogen. The maturation level of

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kerogen could also influence the adsorption behavior of methane. As kerogen evolves during

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maturation, the H/C and O/C atomic ratios are reduced. This process is reflected in the van

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Krevelen diagram 42, which can be used to distinguish the kerogen type based on the maturation.

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Therefore, in this work, to simplify the study, the kerogen would be simplified to ideal graphite,

34

. Additionally, many authors

25-28

used an ideal graphite-based

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and oxygenated functional groups are grafted onto graphite surfaces to obtain different O/C atomic

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ratios, which reflect varying maturation levels of kerogen43.

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The objective of this paper is to investigate the adsorption behavior of the methane on the

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organic matter of organic-rich shales by molecular simulation. The skeleton patterns of the slit-like

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organic pores were first built using a molecular simulation method. The adsorption behaviors of

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the methane in pores and the microcosmic adsorption mechanism of the methane in pores were

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investigated by the GCMC method. On this basis, the impacts of the pore size, temperature,

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moisture and carbon dioxide on the adsorption behaviors of the methane were investigated, and

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their mechanisms of interaction were discussed. The impact of the different O/C ratios on the

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adsorption behavior of the methane was also discussed. It was anticipated that our results might

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provide important theoretical and instructional significance for the exploration and development of

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shale gas reservoirs.

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2 Pore structure of kerogen

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In this study, the kerogen sample was isolated from the Lower Silurian Longmaxi Formation

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shales, and the detail information could be seen in Ref. 11. The TOC content of the kerogen sample

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is 78.5%, the equivalent vitrinite reflectance is about 2.8%, indicating it is over maturity. Based on

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the results of organic maceral analysis, the organic matter is dominated by type II kerogen, and

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there is small amount of type I kerogen. The experimental dates of the methane adsorption on

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kerogen under the different condition (without/with equilibrium water) are listed in Liang et al.11.

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On the basis, the pore structure characteristics of kerogen sample were obtained using the low

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pressure N2 adsorption analysis.

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The N2 adsorption-desorption isotherm of the kerogen is presented in Fig. 1. From Fig. 1, the

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N2 adsorption-desorption isotherm of the kerogen belongs to the type IV isotherm (isotherm with

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hysteresis loop) according to the International Union of Pure and Applied Chemistry (IUPAC)

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classification

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Furthermore, when the relative pressure is low, the adsorption branch of the isotherm is coincident

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with the desorption branch, whereas when the relative pressure increases, the adsorption branch

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and desorption branch of the isotherm would do not overlap, resulting in a hysteresis loop, which

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is consistent with the previous studies

44

. This conclusion is consistent with the previous studies on kerogen

47

45-46

45-46

.

. This may be related to the capillary condensation

. According to the IUPAC classification44, the hysteresis loop

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occurring within the mesopores

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shape of the kerogen may be both the characteristics of type H3 and type H2, which is usually

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associated with inkbottle-shaped pores and slit-shaped pores

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kerogen calculated from the N2 adsorption data using the Brunauer-Emmett-Teller (BET) method

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48

139

44

. The specific surface area of the

is 284.6 m2/g. The distribution of the specific surface area of the kerogen calculated according to both the

4


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NLDFT method (a) and the BJH method (b) are shown in Fig. 2. From Fig. 2, the pore size of

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kerogen mainly was less than 6nm, which is consistent with the previous studies to a certain extent

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45

143

Longmaxi Formation shales and the pore size mainly was less than 10nm.

. Xue et al.

144 145

45

found there were a large number of nanopores in kerogen isolated form the

Fig. 1 Low-pressure N2 adsorption-desorption isotherms of kerogen sample.

146 147 148

Fig. 2 Specific surface area distribution with pore size obtained from the N2 adsorption isotherm using the NLDFT

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3 Models and Methods

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3.1 Models

method (a) and the BJH method (b).

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The parameters of the graphite crystal cell are seen in Hassel et al.49. In this study, two typical

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oxygen-containing groups, –COOH and –OH, were selected, and the oxygen-functionalized

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graphene was modeled by attaching the functional groups in varying concentrations onto the

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exposed graphene carbon atoms 50-52. For a given surface oxygen content, the –COOH group to –

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OH group ratio was 1. On this basis, three different types of oxygen functionalized graphene were

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constructed. The O/C atomic ratios included 0.0625, 0.125 and 0.25, which were referred to as

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C16, C8 and C4, respectively. The crystal cell parameters of the graphite samples with different

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O/C ratios are presented in Table S1 (in the supporting data). According to the crystal unit cell, we

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built a simulation unit cell in a rectangular box with periodicity in the x and y directions.

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According to the N2 adsorption-desorption isotherm, the morphology of the pore shape in kerogen

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could be regarded as slit-shaped. On the basis of the simulation unit cell, we added a vacuum and

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z-direction between the inner planes of the two super-cell structures to build the slit-like pore. The

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length of the z direction is defined by the pore size of the super-cell structure and the vacuum. The

5


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pore size H is determined by the vacuum. In this manner, slit-like pores with different pore sizes

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would be obtained, including 1 nm, 1.5 nm, 2 nm, 3 nm, 4 nm, 6 nm, 8 nm, 10 nm. The schematic

166

representations of the slit-like pores are described in Fig. 3, and the parameters of the slit-like

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pores are shown in Table S1.

H 168 a C4

169 170

b C8

c C16

Fig. 3. Schematic representation of the slit-like graphite with pores having different O/C ratios ( carbon atom,

is an oxygen atom,

is a

is a hydrogen atom)

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The L-J parameters and charges of the sites in the unit cell of the graphite with different O/C

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ratios could be gained from the references 53-54, which are shown in Table 1. The potential model

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used for the methane molecule is from the TraPPE model 55, the water molecule used the SPC-E

174

force field 56 and the carbon dioxide molecule is simulated by using the EPM2 model 57. Notably,

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the O−H bond angle bending is modeled with a bending potential uanglebend(θ) = ½kθ(θ - θ0)2, where

176

θ is the current bond angle, θ0 is the equilibrium bond angle and kθ is the force constant.18,24 The

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O−H bond length is allowed to fluctuate according to the harmonic term ubendstretch(rij) = ½kr(rij -

178

r0)2, where kr represents the force constant, rij represents the current bond length and r0 represents

179

s the equilibrium bond length. 18,24. The L-J parameters and the charges of each atom in liquids are

180

presented in Table 1. In our simulation, the models of the graphite with different O/C ratios are

181

considered to be rigid bodies. The interactions contain the van der Waals and Coulombic forces in

182

the simulation. The van der Waals force is described by the L-J(12/6) potential model, and the

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comprehensive effect of the van der Waals force and Coulomb force is described by the following

184

potential model:

 σ 12  σ 6  qi q j ij ij E = ε ij   −    +  rij   rij   4πε 0 rij   185

where qi and

(1)

q j are the charges of atoms in the system, C; rij

is the distance between the

σij and εij

186

atoms i and j , nm; ε 0 is the dielectric constant, 8.854×10-12 F/m;

187 188

depth and L-J size, respectively. The L-J parameters are calculated by the standard Lorentz– Berthelot combining rules

σ ij = (σ ii + σ jj ) 2

ε ij = ε ii × ε jj 6


are the L-J well

(2)

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Energy & Fuels

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Table 1. L-J potential parameters and charges of the atoms. Atom Graphite

（

ε / kB ）/K

σ /nm

q/e

C

28

0.34

-

C（Graphite）

28

0.34

0.3

O（OH）

78.18

0.3166

-0.6

H（OH）

30

0.13

0.3

C（Graphite）

28

0.34

-0.06

C（COOH）

28

0.34

0.75

O（＝O）

78.18

0.3166

-0.5

O（－O－H）

78.18

0.3166

-0.55

H（－O－H）

30

0.13

0.36

C

148.10

0.3730

0.0

H

0.0

0.0

0.0

O

78.18

0.3166

-0.8476

H

0

0

0.4238

C

28.129

0.2757

0.6512

O

80.507

0.3033

-0.3256

Graphite + OH

Graphite + COOH

Methane

Water

Carbon dioxide

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3.2 Molecular simulation

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In this work, we use the GCMC method to investigate the adsorption behaviours of methane

192

in the silt-like pore. In the grand canonical ensemble, the chemical potential, the volume and the

193

temperature are the independent variables. Among them, the chemical potential is a function of the

194

fugacity instead of the pressure. In this research, the SRK state equation was adopted to calculate

195

the fugacity of the methane

196

temperatures and pressures are described in Figure S1 (in the supporting data). In our simulation,

197

the temperature ranges from 313K to 373K, and the maximum simulated pressure is 40MPa and

198

the simulation is under constant pressure point by point. Furthermore, the force field type choose

199

the Dreiding force field and the Coulomb force interaction and the van der Waals force interaction

200

are calculated by the Ewald & Group method and the Atom interaction-based method with the L-J

201

potential cutoff distance 1.55nm, respectively. In addition, the maximum load step in each

202

simulation is 3 × 106, the balance step and the process step are 1.5 × 106 and 1.5 × 106,

203

respectively. The related statistics is conducted by the later 1.5 × 106 kinds of configurations.

204

3.3 Absolute adsorption amount

58

. The fugacity coefficient of the methane at the different

205

For the supercritical adsorption, Gibbs proposed that the adsorbate molecule in the adsorbed

206

phase on the surface of the adsorbent can’t be served as the adsorption amount totally. And the

207

adsorbate molecules distributing in the adsorbed phase based on the gas phase density was 7


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208

independent with the gas/solid molecule inter-atomic forces

209

raised the concept of the excess adsorption amount:

nex = nab − ( ρ gVa ) 210

Where

Va

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32

. According to this view, Gibbs

( MS )

（3）

nex is the excess adsorption amount, mol/m2; nab is the absolute adsorption amount，

211

mol/m2,

is the adsorbed phase volume, cm3;

212

the surface area of the unit cell, m2;

213

calculated by the SRK equation 58 and the GCMC simulation in the empty box. The density of the

214

methane calculated using two methods are shown in Fig.4, indicating that there are a little

215

differences between two methods. Therefore, the SRK equation is an effective method to calculate

216

the density of the methane. In our simulation, the SRK equation would be adopted to calculate the

217

bulk gas density. The bulk density of the methane are presented in Figure S2 (in the supporting

218

data).

ρg is

M is the molar mass of the gas, g/mol; S

is

the bulk gas density , g/cm3. The density could be

219 220

Fig.4 The density of the methane obtained from the GCMC simulation and the SRK equation (T=333K)

221

Fig. 5 exhibits the schematic representation of the excess adsorption amount and the absolute

222

adsorption amount, in which the area of the a represents the excess adsorption amount and the

223

total area of the a and b expresses the absolute adsorption amount. Making an assumption that the

224

total amount of the adsorbate in the adsorption system is N , corresponding to the total area of the

225

a, b, and c in Fig. 5, which is equal to the expression ( nab + ρgVg ). So ρ g Va + Vg

226

the total area of the b and c in Fig. 2. And then, the excess adsorption amount can be expressed as

227

follows:

(

nex =N − ρg (Va + Vg ) ( MS ) = N − ( ρgVp ) ( MS ) 228

Where N is the total amount of the gas, mol/m2;

)

stands for

（4）

Vg is the gas phase volume, g/cm3; Vp is

229

the free volume, g/cm3. The free volume in the pore can be determined by the method that Helium

230

could be used as a probe to obtain the volume 59. For each pore size and each temperature, the free

231

volume of the graphite with different O/C ratios pores would be calculated. In the simulation, the 8


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total amount of methane could directly obtained from the simulation results. Based on the free

233

volume in pore, the total amount of the gas can be converted into the excess adsorption amount

234

according to Eq.(4). The adsorbed phase volume could be calculated according to the region of

235

adsorbed phase (close to the pore wall, namely the area(0, z0)in Fig. 5) determined by the density

236

distribution of methane in pore. The detail information of the method can be seen in Ref.

237

Therefore, the excess adsorption amount can be converted into the absolute adsorption amount

238

according to Eq.(3). On the basis, the adsorbed gas amount and free gas amount in the simulation

239

system would be got, by which we can calculate the proportion of adsorbed gas in total amount of

240

gas.

241 242

24

.

Fig.5 The schematic representation of excess adsorption amount and absolute adsorption amount

243

4 Results and Discussion

244

4.1 The impact of pore size

245

The impact of the pore size on the methane adsorption in the pores of graphite with different

246

O/C ratios were investigated, and the temperature was 333 K. The excess adsorption capacity and

247

absolute adsorption capacity of methane in the pores of graphite with different O/C ratios are seen

248

in Figs. 6-8. It can be seen from Figs. 6-8 that there are similar laws regarding changes in the

249

excess adsorption capacity and absolute adsorption capacity of the methane in graphite with

250

different O/C ratios. The excess adsorption capacity and absolute adsorption capacity of methane

251

in micropores increased with increasing pore size. However, in mesopores, the excess adsorption

252

capacity and absolute adsorption capacity of methane gradually decreased with increasing pore

253

size. This may be due to the potential superimposed effect of the pore wall, which can

254

significantly affect the adsorption of the methane molecules in micropores, and the methane

255

adsorption capacity would be limited by the pore volume. That is to say, the pore volume increases

256

as the pore size increases, and thus, the methane adsorption capacity would increase. However, the

257

methane molecules in mesopores are mainly affected by the surface potential effect of the two

258

sides of the pore wall. With an increase of the pore size, the interactions between the methane

259

molecules and the pore wall decrease and the space for the movement of methane molecules

260

increases, which would reduce the force required to escape from the pore wall, and then, the

261

methane adsorption capacity would decrease with increasing pore size. This conclusion is 9


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262

inconsistent with previous studies 23, 28, indicating that the excess adsorption capacity of methane

263

in pores decreased with increasing pore size. This may be because the cutoff distance in our

264

simulation is larger.

265

Furthermore, from Figs. 6a-8a, the excess adsorption capacity of methane first increased with

266

falling pressure. That is to say, there exists a maximum value of the excess adsorption capacity

267

(nexc–max), and the corresponding pressure could be regarded as a maximum pressure (pmax). This

268

finding is in agreement with the results of previous studies on organic-rich shales

6, 9, 17

, which

22-24, 28, 30-31

269

were conducted by experiments. This observation was also reported in Ref.

270

were conducted by molecular simulations. The maximum value of the excess adsorption capacity

271

and its corresponding pressure under different pore sizes are presented in Table S2 (in the

272

supporting data). The pmax corresponding to the nexc–max was variable, and the pmax ranged from 10

273

MPa to 16 MPa. The nexc–max first increased and then decreased as the pore size increases. This

274

conclusion is in disagreement with the previous work on quartz

275

simulation, illustrating that the pmax was between 16 MPa and 18 MPa when the pore size ranged

276

from 1 nm to 20 nm. The finding is inconsistent to a certain extent with the previous research on

277

carbon 28 investigated by a molecular simulation, indicating that the pmax was between 6 MPa and

278

14 MPa when the pore size ranged from 1 nm to 9 nm. However, this conclusion is in line with

279

previous experimental studies on organic-rich shales

280

between 10 and 23 MPa, indicating that our simulation results are in agreement with the

281

experiments to a certain extent. In addition, we can note from Figs. 6b-8b that the absolute

282

adsorption capacity of methane increased with increasing the pressure, and it increased fast at first

283

and then slowly, which is in line with the previous experimental research on kerogen 6-7, 12-17

24

, which

investigated by molecular

6, 9, 17

, which indicate that the pmax was

8-11

or

284

organic-rich shales

. The difference in the absolute adsorption capacity of methane in 1.5

285

nm and 2 nm pores was smaller, and both values were larger than that in mesopores. However, the

286

absolute adsorption capacity of methane in mesopores decreased as the pore size increases.

287

288 289 290

Fig. 6. The excess adsorption isotherms of methane (a) and absolute adsorption isotherms of methane (b) in different sized C4 pores.

10


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291 292 293


294 295 296 297


Chen et al.

23

suggested that the absolute adsorption capacity of methane from both

298

simulations and experiments should be expressed per unit of specific surface area to make the

299

simulation results comparable with the experimental results. According to our previous research11,

300

Fig. 9 presents the absolute adsorption isotherms of methane from both simulation results and

301

experimental results. From Fig. 9, it can be seen that there were both similarities and differences

302

between the simulation and experimental results. The simulation results of methane adsorption in

303

the C4 pores as well as C8 pores and the experimental results of methane adsorption on kerogen

304

matched well, whereas the simulation results of the methane adsorption in C16 pores and the

305

experimental results of the methane adsorption on kerogen did not match perfectly. The methane

306

adsorption capacity on kerogen was between the methane adsorption capacities in C4 and C8

307

pores with pore sizes of 2 nm and those in C4 and C8 pores with pore sizes of 10 nm. That is to

308

say, the methane adsorption capacity in pores of a specific pore size would be approximately equal

309

to an equivalent pore size on kerogen. We can note from Fig. 9 that the equivalent pore size of the

310

C4 is approximately 10 nm, and the equivalent pore size of the C8 is smaller than that of the C4,

311

which could be between 4 nm and 6 nm. This may be because the absolute adsorption capacity of

312

methane in the C4 pores is larger than that in C8 pores. Therefore, in this respect, the simulation

313

results are reasonable. 11


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

314 315 316

Fig. 9. The absolute adsorption isotherms of methane from both simulation results and experimental results (a: C4,

317

According to Eq. (3) and Eq. (4), the adsorbed gas amount in the simulation system would be

318

obtained, by which we can calculate the proportion of adsorbed gas in the total amount of gas. The

319

relationships between the proportion of the adsorbed gas in the total amount and pressure in

320

graphite with different O/C ratios are shown in Fig. 10. The proportion of the adsorbed gas

321

decreased as the pore size or the pressure increases, which suggested that the proportion in lower

322

pressure or smaller pores was larger than that in higher pressure or larger pores. Additionally, the

323

proportion in pores of graphite with different O/C ratios under the same pore size reduced fast at

324

first and then slowly with increasing pressure. At the same time, we can note that the proportion of

325

the adsorbed gas reduced with decreasing the O/C ratio under the same pressure and pore size.

326

Furthermore, when the pore size was more than 6 nm, the proportions in C4, C8 and C16 pores

327

under a pressure of 20 MPa were 33.40%, 32.57% and 30.15%, respectively, and the proportions

328

in C4, C8 and C16 pores under a pressure of 40 MPa were reduced to 23.16%, 22.27% and

329

21.02%, respectively. This conclusion suggested that the proportions of the adsorbed gas under

330

higher pressure in graphite with different O/C ratios were lower, which indicated that the methane

331

in pores existed mainly as free gas. According to the results of low-pressure N2 adsorption

332

experiments, we found that the pore size of the kerogen samples was mainly less than 6 nm, that is

333

to say, the methane in the organic pores of the organic-rich shales could be mainly in the adsorbed

334

state.

335 336 337

b: C8, and c: C16)

Fig. 10. Relationship between the proportion of adsorbed gas in the total amount and pressure in pores with different pore sizes (a: C4, b: C8, c: C16).

338

The average isosteric heats of adsorption of methane in the pores of graphite with different

339

O/C ratios with different pore sizes are presented in Fig. 11. It can be seen from Fig. 11 that the

340

isosteric heat of adsorption of methane in pores decreased fast at first and then slowly with 12


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341

increasing pore size, indicating that the isosteric heat of adsorption of methane in smaller pores

342

was larger than that in larger pores. This finding is in line with the previous research studies on

343

illite

344

isosteric heat of adsorption of methane in C4, C8 and C16 pores corresponding to a pore size of 1

345

nm were 19.65kJ/mol,18.58 kJ/mol and 16.74 kJ/mol, respectively, and the average isosteric heats

346

of adsorption of methane in C4, C8 and C16 pores corresponding to a pore size of 20 nm were

347

reduced to 9.64 kJ/mol, 9.12 kJ/mol and 8.51 kJ/mol, respectively. At the same time, with the

348

same pore size, the isosteric heat of adsorption of methane in graphite with different O/C ratios

349

decreased as the O/C ratio decreased according to the order C4 > C8 > C16. This order is

350

consistent with the order of the methane adsorption capacity in the pores of graphite with different

351

O/C ratios. In other words, the isosteric heat of adsorption of methane could reflect the methane

352

adsorption capacity to a certain extent.

353

23

or quartz

24

, which were conducted by molecular simulation. In our simulations, the

The isosteric heats of adsorption on different types of kerogen (type I, type II and type III) 8

354

obtained from experimental data

355

There were deviations between the simulated results and the experimental results, but they also

356

show similarities to a certain extent. This may be because there are differences between research

357

objects. The pore size of the kerogen in experiments is a continuous distribution with variable pore

358

size distributions, and the isosteric heat of adsorption of methane obtained from the experimental

359

results reflects the synthetic results of the continuous pores. However, in simulations, the pore

360

skeletons of the graphite with different O/C ratios are a single pore size, and the isosteric heat of

361

adsorption of methane obtained from the simulation results reflects the results of the single pore

362

size and changes when the pore size changes. Furthermore, the isosteric heat of adsorption on

363

different types of kerogen decreased as the O/C ratio decreases (type I < type II < type III), which

364

is in agreement with the simulation result in this study. In addition, the isosteric heat of adsorption

365

of methane in the pores of graphite with different O/C ratios with different pore sizes was less than

366

42 kJ/mol, demonstrating that the adsorption of the methane was physical adsorption. This

367

conclusion is in agreement with previous works 6-17 using experimental investigations.

368 369 370

were 10.3 kJ/mol, 21.9 kJ/mol and 28 kJ/mol, respectively.

Fig. 11. The average isosteric heats of adsorption of methane in pores with different pore sizes.

The potential energy distribution of methane in the pores of graphite with different O/C ratios

13


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371

can be directly obtained from the simulation results. The potential energy distribution curves of

372

methane under different pressures (pore size of 4 nm) are shown in Fig. 12. From Fig. 12, the

373

potential energy distribution curves of methane gradually moved to the left as the pressure

374

increases, and the most probable potential energy between methane and graphite with different

375

O/C ratios gradually decreased with increasing pressure. This conclusion suggests that the

376

adsorption sites of the methane molecules in pores gradually change from higher energy

377

absorption sites to lower energy ones with an increase of the pressure. The adsorption state of the

378

methane in the pores of graphite with different O/C ratios under high pressure is more stable than

379

that under low pressure. The potential energy distribution curves of methane with different pore

380

sizes under a pressure of 20 MPa are shown in Fig. 13. From Fig. 13, the potential energy

381

distribution curves of methane gradually moved to the right with increasing pore size, and the

382

most probable potential energy between methane and graphite with different O/C ratios gradually

383

decreased as the pore size increases, indicating that the adsorption sites of methane in the pores of

384

graphite with different O/C ratios gradually change from lower energy absorption sites to higher

385

energy ones with increasing pore size. That is to say, the adsorption capacity of methane in

386

micropores is stronger than that in macropores.

387 388 389

Fig. 12. The potential energy distribution curves of methane under different pressures (pore size of 4 nm) (a: C4, b:

390 391 392

Fig. 13. The potential energy distribution curves of methane with different pore sizes (pressure of 20 MPa) (a: C4,

393

4.2 The impact of temperature

C8, c: C16).

b: C8, c: C16).

394

The impact of temperature on the methane adsorption in the pores of graphite with different

395

O/C ratios was investigated with a pore size of 4 nm. The absolute adsorption capacities of

396

methane in the pores of graphite with different O/C ratios are seen in Fig. 14. It can be seen in Fig.

397

14 that the absolute adsorption capacity of methane in the pores decreased as the temperature 14


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398

increases under the same pressure, indicating that higher temperature restricted the methane

399

adsorption. This may be because the methane adsorption in the pores of graphite with different

400

O/C ratios is physical adsorption. With an increase of the temperature, the thermal motion of the

401

methane molecules would increase, leading to an increase of the mean kinetic energy of the

402

methane molecules, which would make the force to escape from the pore walls of graphite with

403

different O/C ratios easier, thus causing the reduction of the methane adsorption capacity. This

404

finding is in agreement with a previous study on kerogen 8 conducted through experiments. This

405

observation was also reported in Ref.

406

suggesting that the methane adsorption capacity on carbon, simplified kerogen or minerals

407

(montmorillonite and quartz) declined as the temperature increases. This finding indicates that the

408

influence of the temperature on the methane adsorption capacity on carbon, simplified kerogen or

409

minerals has the same tendency for variance. Fig. 15 (the average isosteric heats of adsorption of

410

methane in the pores of graphite with different O/C ratios under different temperatures) also

411

illustrates this conclusion. From Fig. 15, the average isosteric heats of methane decreased when

412

the temperature increased. This finding illustrated that the interactions between methane molecules

413

and graphite with different O/C ratios reduced as the temperature increases, leading to the

414

reduction of the methane adsorption capacity.

22-24, 28, 30-31

investigated by molecular simulation,

415 416

Fig. 14. The absolute adsorption isotherms of methane under different temperatures (a: C4, b: C8, c: C16).

417 418

Fig. 15. The average isosteric heats of adsorption of methane under different temperatures.

419

Fig. 16 describes the potential energy distribution curves of methane under different

420

temperatures. From Fig. 16, we can observe that the potential energy distribution curves of

421

methane gradually moved to the right when the temperature increased, and the most probable

422

potential energy of methane and graphite with different O/C ratios gradually increased as the

423

temperature increases. This finding suggests that the adsorption sites of the methane molecules in 15


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

424

the pores of graphite with different O/C ratios gradually change from lower energy adsorption

425

sites to higher energy ones with increasing temperatures, thus causing a reduction of the methane

426

adsorption capacity.

427 428 429

Fig. 16. The potential energy distribution curves of methane under different temperatures (pressure of 20 MPa) (a:

430

4.3 The impact of moisture

C4, b: C8, c: C16).

431

The impact of moisture on methane adsorption in the pores of graphite with different O/C

432

ratios was investigated, including the use of three different water contents (wt% = the mass of the

433

water molecules/the simulation cell mass). The pore size was 4 nm, and the temperature was 333

434

K in the simulation. In addition, we need to first determine the adsorption sites of water molecules

435

in graphite pores with different O/C ratios, and the annealing simulation method would be adopted

436

in this simulation. At the same time, the free volumes of the pores of graphite with different O/C

437

ratios need to be re-calculated under the condition of water. The distributions of the different water

438

contents in pores can be found in Figures S3-S6 (in the supporting data). Figures S3-S5 describe

439

the distributions of the different water contents in the pores of graphite with different O/C ratios,

440

and Figure S6 presents the distributions of the different water contents in graphite without

441

oxygen-containing group pores. From Figures S3-S6, there were both similarities and differences

442

in the distributions of water molecules in graphite with/without oxygen-containing group pores.

443

The water molecules were mainly close to the surface of the pore walls and occupied the area near

444

the pore wall of the graphite with/without oxygen-containing group pores. The water molecules

445

accumulated in the graphite with/without oxygen-containing group pores instead of being

446

decentralized. This is probably because there is not only the Coulombic force interaction but also

447

the van der Waals force interaction between water molecules and graphite with/without

448

oxygen-containing groups, resulting in the aggregation of water molecules in the area near the

449

pore walls of the graphite with/without oxygen-containing group pores.

450

Compared with the distributions of water molecules in graphite without pores containing

451

oxygen groups (Fig. 17), we could note that the oxygen atoms of most of the water molecules

452

were close or pointed toward the oxygen-containing groups instead of the carbon atoms on the

453

surface of graphite pores with different O/C ratios or the hydrogen atoms of the surrounding water

454

molecules, and the more obvious this phenomenon was, the smaller the O/C ratio was. The reason

16


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455

why this phenomenon happened is that there are the oxygen-containing groups –COOH and –OH

456

on the surface of graphite with different O/C ratios, and the water molecules would point toward

457

those oxygen-containing groups because of hydrogen bonding interactions

458

indicate that the water molecules in the pores of graphite with different O/C ratios could be

459

affected by the combined van der Waals force, Coulombic force and hydrogen bonding

460

interactions, and they occupy the area near the pore wall in the form of aggregates, thus occupying

461

the methane adsorption space.

60

. These findings

462 a graphite

463 464

b C4

c C8

d C16

Fig. 17. A comparison of the distribution of water molecules in graphite with/without oxygen-containing groups under a water content of 8.0 wt%.

465

The absolute adsorption isotherms of methane in the pores of graphite with different O/C

466

ratios under different water contents are presented Fig. 18. From Fig. 18, it can be seen that the

467

absolute adsorption isotherm of methane moved downward when the water content increased at

468

the same temperature and pressure. That is to say, the absolute adsorption capacity of methane

469

decreased as the water content increases, illustrating that water molecules were not conducive to

470

methane adsorption in the pores of graphite with different O/C ratios at the same temperature and

471

pressure. This may be because the water molecules occupied the adsorption space of the methane

472

molecules. This conclusion is in accordance with previous works 18-19, 22, 24, 28, 30 investigated using

473

molecular simulation, which suggested that the water greatly reduced the methane adsorption

474

capacity in carbon, simplified kerogen or mineral pores. This finding is also in agreement with the

475

previous research

476

conducive to methane adsorption on kerogen. Fig. 19 presents the comparison of the absolute

477

adsorption isotherms of methane from both simulation and experimental results. As seen in Fig. 19,

478

there are similar laws regarding changes in the absolute adsorption capacity of the methane from

479

both simulation and experimental results, and the simulation results of the methane adsorption in

480

the C8 pores could be comparable with the experimental results of the methane adsorption on

481

kerogen under equilibrium water. The absolute adsorption capacity of methane on kerogen with

482

equilibrium water is less than that in C8 pores under a water content of 8.0 wt%, and this may be

483

because the water content of the kerogen sample under equilibrium water is approximately 14

484

wt%.

11

investigated by experiments, indicating that the water molecules were not

17


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

485 486 487

488 489

Fig. 18. The absolute adsorption isotherms of methane under different water contents at 333 K (a: C4, b: C8, c: C16).

Fig. 19. The absolute adsorption isotherms of methane from both simulation and experimental results.

490

The potential energy distribution curves of methane under different water contents are

491

illustrated in Fig. 20. It can be seen from Fig. 20 that there are no obvious changes or little

492

changes to the peak of the potential energy distribution curve with increasing water contents,

493

indicating that the relationship between the methane molecules and water molecules in the pores

494

of graphite with different O/C ratios was not a relationship related to competition for the

495

adsorption site but a relationship related to competition for adsorption space. In other words, water

496

molecules in the pores of graphite with different O/C ratios did not occupy the adsorption sites of

497

methane molecules on the surface of pore walls, and water molecules in the pores of graphite with

498

different O/C ratios might occupy the adsorption space of methane molecules. Therefore, the

499

water molecules would occupy the adsorption space of the methane molecules in the pores of

500

graphite with different O/C ratios, which would decrease the adsorption space of methane

501

molecules and then lead to the reduction of the methane adsorption capacity. When the hydrophilic

502

fracturing fluid is adopted during the process of SRV fracturing, a large number of the fracturing

503

fluid losses and the water molecules enter into the nano-scale pores in the shale formation because

504

of the capillary effect 11, 61-62. The water molecules would occupy the adsorption space of methane

505

molecules, leading to an accelerated desorption velocity of the methane molecules absorbing on

506

the surface of the shales to a certain extent.

18


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507 508 509 510

Fig. 20. The potential energy distribution curves of methane under different water contents at 333 K (pressure of 20 MPa) (a: C4, b: C8, c: C16).

4.4 The impact of carbon dioxide

511

The carbon dioxide was chosen to investigate the impacts of the gas compositions on the

512

adsorption behaviors of methane in graphite with different O/C ratios pores. And the pore size was

513

4nm and the temperature was 333K in simulation. The absolute adsorption isotherms of methane

514

in the pores of graphite with different O/C ratios under different mole fraction of carbon dioxide

515

are shown in Fig. 21. From Fig. 21, the absolute adsorption capacity of the methane decreased

516

when the mole fraction of carbon dioxide increased or the mole fraction of method decreased

517

under the same temperature and pressure. This finding tells us that the lower the mole fraction of

518

methane in the gas phase is, the less the methane adsorption capacity in the pores of graphite with

519

different O/C ratios is.

520 521 522

Fig. 21 The absolute adsorption isotherms of methane in pores under different mole fractions of carbon dioxide at 333K (a: C4, b: C8, c: C16)

523

Fig. 22 presents the potential energy distribution curves of methane and carbon dioxide under

524

different mole fractions of the carbon dioxide. From Fig. 22, the most probable potential energy

525

between methane and graphite with different O/C ratios under different mole fractions of carbon

526

dioxide was greater than that between carbon dioxide and graphite with different O/C ratios,

527

which indicated that the potential energy distribution of methane was different with that of carbon

528

dioxide. That is to say, the adsorption sites of carbon dioxide molecules in the pores of graphite

529

with different O/C ratios located in lower energy adsorption sites, whereas the adsorption sites of

530

methane molecules in the pores of graphite with different O/C ratios lied in higher energy

531

adsorption sites. Therefore, the adsorbed states of carbon dioxide molecules in the pores of

532

graphite with different O/C ratios were more stable than that of methane molecules. At the same

533

time, it can be seen from Fig. 22 that the interactions between carbon dioxide molecules and 19


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

534

methane molecules could alter the potential energy distribution curves. And the potential energy

535

distribution curves of methane gradually moved to the right with increasing the mole fraction of

536

carbon dioxide, and the most probable potential energy between methane and graphite with

537

different O/C ratios increased with increasing mole fraction of carbon dioxide. The conclusions

538

indicate that the adsorption sites of methane molecules in the pores of graphite with different O/C

539

ratios gradually change from lower energy absorption sites to higher ones as mole fraction of

540

carbon dioxide increases, and then causing the decrease of the methane adsorption capacity. This

541

also illustrate that carbon dioxide molecules in the pores of graphite with different O/C ratios

542

bring about change of the adsorption sites of methane molecules and reduction of the adsorption

543

space of methane molecules. Therefore, the carbon dioxide adsorption capacity in the pores of

544

graphite with different O/C ratios was larger than methane.

545

In a word, with the increase of mole fraction of carbon dioxide in the methane/carbon dioxide

546

binary gas mixture, the mole fraction of methane would decrease, the adsorption sites of methane

547

molecules would vary and adsorption space of methane molecules would reduce, these above

548

three may comprehensively bring about the decrease of the methane adsorption capacity in the

549

pores of graphite with different O/C ratios. The result could provide a theoretical basis for

550

feasibility, to a certain extent, that the methane in shale gas reservoirs can be displaced by carbon

551

dioxide, which would make the methane recovery rate improved.

552 553 554

Fig. 22. The potential energy distribution curves of methane and carbon dioxide under different mole fractions of

555

5 Conclusions

carbon dioxide at 333K (pressure of 20MPa) (a: C4, b: C8, c: C16)

556

In this paper, the adsorption behavior of methane in the pores of graphite with different O/C

557

ratios was investigated by the GCMC method. The impacts of the pore size, temperature, moisture

558

and carbon dioxide on methane adsorption in pores were discussed, and the impact of the different

559

O/C ratios on methane adsorption in pores was also discussed. The following conclusions can be

560

drawn:

561 562

(1)With the increasing the pore size or decreasing O/C ratio, the isosteric heats of adsorption of methane in the pores of graphite with different O/C ratios are reduced.

563

(2)With an increase of the pressure or decrease of the pore size, the adsorption sites of

564

methane molecules gradually change from higher energy adsorption sites to lower ones, leading to

20


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Energy & Fuels

565

an increase of the methane adsorption capacity. The methane adsorption capacity in micropores

566

increases with an increasing pore size, whereas it decreases with an increasing pore size in

567

mesopores. The methane adsorption capacity in pores decreases with decreasing O/C ratios. The

568

proportion of the adsorbed gas in pores decreases with increasing pressure for the same pore size

569

and decreases with increasing pore size under the same pressure.

570

(3)With an increase of temperature, the isosteric heat of adsorption of methane in pores

571

decreases, and the adsorption sites of methane gradually change from lower energy adsorption

572

sites to higher energy ones, leading to the reduction of the methane adsorption capacity. The water

573

molecules in pores affected by the van der Waals forces, Coulombic forces and hydrogen bonding

574

interactions are close to the oxygen-containing groups and take up the adsorption space of

575

methane molecules, leading to a decrease of the methane adsorption capacity.

576

(4) For the methane/carbon dioxide binary gas mixture adsorption system, the carbon dioxide

577

adsorption capacity in pores is larger than that of methane. The reduction of the mole fraction of

578

methane in the gas phase, change of the adsorption sites of methane and decrease of the adsorption

579

space of methane lead to the methane adsorption capacity.

580

Acknowledgments:

581

This research was supported by the National Natural Science Foundation of China (NSFC) (Grant

582

No. 41602155) and the United Fund Project of National Natural Science Foundation of China

583

(Grant No. U1262209), and the Young scholars development fund of SWPU (No. 201599010137).

584

Reference

585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603

1 U.S. Energy Information Administration (EIA). Technically Recoverable Shale Oil and Shale Gas Resources: An Assessment of 137 Shale Formations in 41 Countries Outside the United States. http://www.eia.gov/analysis/studies/worldshalegas. 2013-9-24 2 Curtis J B. 2002. Fractured shale gas systems. AAPG Bulletin, 86(11):1921-1938. 3 Ross, D. J., Bustin, R. M. Shale gas potential of the lower jurassic gordondale member, northeastern British Columbia, Canada. Bulletin of Canadian Petroleum Geology, 2007, 55(1), 51-75. 4 Chalmers, G.R., Bustin, R.M. Lower Cretaceous gas shales in northeastern British Columbia, Part I: geological controls on methane sorption capacity. Bulletin of Canadian petroleum geology, 2008, 56(1), 1-21. 5 Chalmers, G.R., Ross, D.J., Bustin, R.M. Geological controls on matrix permeability of Devonian Gas Shales in the Horn River and Liard basins, northeastern British Columbia, Canada. International Journal of Coal Geology, 2012, 103, 120-131. 6 Gasparik, M., Bertier, P., Gensterblum, Y., Ghanizadeh, A., Krooss, B. M., Littke, R. Geological controls on the methane storage capacity in organic-rich shales. International Journal of Coal Geology, 2014, 123, 34-51. 7 Li, J., Yan, X., Wang, W., Zhang, Y., Yin, J., Lu, S., Yan, Y. Key factors controlling the gas adsorption capacity of shale: A study based on parallel experiments. Applied Geochemistry, 2015, 58, 88-96. 8 Zhang T., Ellis G. S., Ruppel S. C., Milliken K., Yang R.. Effect of organic-matter type and thermal maturity on methane adsorption in shale-gas systems. Organic Geochemistry, 2012, 47, 120-131. 9 Rexer T.F., Mathia E.J., Aplin A.C., Thomas K.M.. High-pressure methane adsorption and characterization of pores in Posidonia shales and isolated kerogens. Energy & Fuels, 2014, 28(5), 2886-2901.

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Methane Adsorption on Carbon Models of the Organic Matter of

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