Changes in Pore Structure of Coal Caused by CS2 Treatment and Its Methane Adsorption Response
The pore structure and gas adsorption are two key issues that affect the coal bed methane recovery process significantly. To change pore structure and gas adsorption, 5 coals with different ranks were treated by CS2 for 3 h using a Soxhlet extractor under ultrasonic oscillation conditions; the evolutions of pore structure and methane adsorption were examined using a high-pressure mercury intrusion porosimeter (MIP) with an AutoPore IV 9310 series mercury instrument. The results show that the cumulative pore volume and specific surface area (SSA) were increased after CS2 treatment, and the incremental micropore volume and SSA were increased and decreased before and after , respectively; the incremental big pore (greater than 10 nm in diameter) volumes were increased and SSA was decreased for all coals, and pore connectivity was improved. Methane adsorption capacity on coal before and after also was increased and decreased, respectively. There is a positive correlation between the changes in the micropore SSA and the Langmuir volume. It confirms that the changes in pore structure and methane adsorption capacity due to CS2 treatment are controlled by the rank, and the change in methane adsorption is impacted by the change of micropore SSA and suggests that the changes in pore structure are better for gas migration; the alteration in methane adsorption capacity is worse and better for methane recovery before and after . A conceptual mechanism of pore structure is proposed to explain methane adsorption capacity on CS2 treated coal around the .
Coal bed methane (CBM) is a serious risk (gas outburst) to safe production in coalmines and also is one of the most important energies in recent decades, but it is not easy to recover due to the low porosity and permeability of the coal seam reservoir. Previous studies have confirmed that coal has a dual pore system including primary porosity consisting of micropore, transitional pore, and mesopore in the coal matrix and second porosity composed of nonuniformly distributed macropore [1–4]. CBM is held to the internal specific surface area (SSA) of coal with absorbed state mainly, and some free gas exists in the natural fractures of coal and some gas dissolves in waters in coal [3–8]. Thus, the quantification of the pore structure is essential to estimating coal bed methane storage capacity and predicting gas production in reservoir conditions.
Various pore characterization techniques have been applied to quantify the pore structures from total porosity, pore volume, pore size distribution (PSD), SSA, and so on [2, 9–15]. These include low-pressure gas (N2/CO2) adsorption/desorption techniques [9–12], high-pressure mercury intrusion porosimetery (MIP) [12–14], ultra/small-angle neutron scattering (SANS) [15, 16], focused ion beam scanning electron microscopy , field emission scanning electron microscopy/transmission electron microscopy , nuclear magnetic resonance [19, 20], and X-ray computed tomography [17, 19, 20]. There are several types of pore classification systems employed for various pore scales in CBM research, including Hodot classification, the classification of International Union of Pure and Applied Chemistry (IUPAC), and Gan classification [21–23]. Hodot classification is accepted widely to describe the various scale pores of MIP.
Coal rank is the degree of the transformation of peat through lignite as well as subbituminous, bituminous, semianthracite to anthracite, and meta-anthracite coal, and the rank classes in terms of vitrinite reflectance (). Many studies have shown that the pore size distribution is affected by the coal rank and there is a U-shaped relationship between the pore size distribution and coal rank . A number of studies concluded that micropores and transitional pores are the primary space for CBM storage, whereas meso- and macropores are responsible for CBM migration [1, 25–31].
Methane adsorption in coal is controlled by internal factors, including coal rank, macerals (vitrinite, exinite, and inertinite), and pore size distribution [1, 26, 29, 30, 32], and influenced by external factors, such as temperature, pressure, and humidity [26, 33, 34]. The methane adsorption capacity in coal increases with coal rank and vitrinite content in macerals at high coal ranks, which is because vitrinite is more microporous with a higher SSA than inertinite is [35, 36]. The SSA of micropore has a positive relationship with CH4 adsorption capacity [37, 38].
Those researches have confirmed the coal seam with low porosity and permeability, and the pore structures (PSDs, SSA, and pore volumes) are controlled by the coal rank and have an important impact on methane adsorption capacity; around , the evolution tends to change obviously. Only limited studies have quantified the alteration of the pore structure and gas adsorption. In our study, we will try to quantify the effect of the CS2 treatment effect on pore structure and methane adsorption changes with coal rank. Such changes have potential impacts on coal bed methane recovery.
2. Materials and Methods
2.1. Coal Samples
The coal samples were collected from undeformed Permian coal seams in Haitian coalmines located in eastern Yunnan Province and Tucheng, Yuni, Linyuan, and Jinjia located in Guizhou Province (Figure 1). All of the samples were wrapped in black polyethylene bags after collection before being transported to the laboratory for analysis. To eliminate the effects of the sedimentology and the parent material of coals, vitrinites were first separated and collected by hand and then pulverized for passage through a 35-mesh sieve. Finally, they were enriched using the heavy liquid floatation method until the vitrinite content was above 85% (Table 1). The samples were divided into 2 parts. One part was dried for mercury intrusion measurements and methane isothermal adsorption. The second part was treated by CS2 and then was dried for mercury intrusion measurements and methane isothermal adsorption.
2.2. CS2 Treatment
CS2 treatment was conducted using a Soxhlet extractor under ultrasonic oscillation conditions at room temperature using the following procedure with CS2. First, for each experiment, approximately 20 g of dry coal was placed into the flat container of the Soxhlet extractor. Then, approximately 600 mL of CS2 was added, and the extractor was sealed to begin treatment under ultrasonic oscillation. Each CS2 treatment was performed for approximately three hours. After the treatment, the liquid eluents were separated from coal, and the coal was dried at 80 ° C. The CS2 treatment was repeated until the mass of each treated coal was greater than 80 g.
2.3. Pore Structure Test
Measurements of the pore structures were conducted using a mercury intrusion porosimeter with an AutoPore IV 9310 series mercury instrument. For the mercury intrusion porosimetry measurements, a contact angle of 140° and a surface tension of 485 dyn/cm were assumed. All samples were dried at 80°C for at least three hours before the tests.
2.4. Methane Isothermal Adsorption Experiment
Methane isothermal adsorptions on samples were performed using a Laboratory Isotherm Adsorption System on an IS-100 adsorption system. Prior to measurement, the samples were crushed in accordance with the Chinese Standard GB/T 212-2001 and were dried at 80°C until the weight loss was less than 1%. The measurements were carried out at 30°C, and each isothermal adsorption curve includes six different pressures. Approximately 80 g of each sample was tested, and the adsorbate was 99.99% methane.
3. Results and Discussion
3.1. Pore Structure and Its Evolution
The pore distributions of samples are shown in Figure 2; the incremental macro- and mesopores have a very high percentage. It contradicts with the previous understandings [3, 37, 38]. It was regarded that there were so many gaps between sample particles in this study. Due to many gaps between sample particles, the pores that were over 10,000 nm in diameter were eliminated to reduce the interference on pore structure. Attributed to the lower limitation of 6 nm for AutoPore IV 9310, a modified Hodot classification was used as follows: micropores (6~10 nm), transitional pores (10~100 nm), mesopores (100~1000 nm), and macropores (1000~10,000 nm).
Previous studies have shown that the incremental macro- and micropores contribute the most to the cumulative pore volume and SSA, respectively [37, 38]. In our study, the cumulative pore volumes range from 0.0207 to 0.2332 cm3/g (Table 2), and the cumulative pore SSAs range from 4.7525 to 16.3162 m2/g (Table 3). The volumes from the incremental macropores and transitional pores account for over 65% of the cumulative volume, confirming that the big pores are the main contributors for the cumulative pore volume. The SSAs from the incremental transitional and micropores account for over 90% of the cumulative SSA, confirming that small pores are the main contributors to the cumulative pore SSA.
To further study the changes in pore volume and SSA caused by CS2 treatment, and were defined by the following equations, respectively: where is the incremental pore volume change due to CS2 treatment, in cm3/g; and are the incremental pore volumes of the treated coal and the untreated coal, respectively, in cm3/g; is the change in the incremental SSA due to the treatment, in m2/g; and are the incremental SSAs of the treated coal and the untreated coal, respectively, in m2/g.
After CS2 treatment, the incremental pore volumes and SSAs changed substantially. As shown in Figures 3 and 4, and are positive and negative before and after , respectively; some of and are positive and the others are negative, demonstrating no obvious regularity. , , ΔSma, and all are positive. Those data indicate that after CS2 treatment, all incremental pore sizes are enlarged, the incremental micropore numbers before are increased, and the numbers of the incremental micropore and other pores are decreased in all samples.
Previous studies have shown that aliphatic groups containing oxygen link the multilayered stacks and have not yet been dissipated as volatiles at the early stage of coalification [37, 38]. It is compressed with coal rank increasing; the aliphatic groups containing oxygen link the multilayered stacks which began to dissipate and fill the pores and the micropores smaller, and then as maturation advances, oxygen-containing compounds and other volatiles are being lost. It indicates that the changes on pore structure due to CS2 treatment are controlled by the coal rank. The data indicate that the treatment of CS2 has enlarging effect on pore size and increasing effect on pore number. We concluded that the pore size-enlarging effect is the main result of the sample pore volumes, the pore number-increasing effect occurs in the micropores of sample SSAs before , and the evaluation of the pore structure due to CS2 treatment is controlled by the coal rank.
Hysteresis between the intrusion and extrusion can use the Ink Bottle theory  or connectivity model  to interpret. In these theories, pores in samples are rarely of a uniform shape and would be broken at all the throats between pore amounts of mercury trapped inside the sample. A network of pores is used to introduce an additional component considering the connection effects between the pores.
In this study, the hysteresises of the samples were reduced significantly by the treatment of CS2 (Figure 5). It could be explained using the Ink Bottle theory and/or connectivity model. Because of the treatment of CS2, small organic matters filled in open pores may be dissolved and flushed away to cause some incremental pore sizes to be enlarged and the number of incremental micropore to be increased, causing some closed and/or ink bottle pores with small organic matters to open and connect, indicating that CS2 treatment improves the connectivity of the pore; it is better for gas migration during the process of coalbed methane production.
3.2. Methane Isothermal Adsorption Capacity and Its Evolution
Excess methane isothermal adsorption was measured using the volumetric method, and the results are shown in Figure 6. The adsorbed volumes are expressed on a dry ash-free basis. The measured methane adsorption capacities of samples range from 14.99 to 28.87 cm3/g. Generally, the methane adsorption capacities of untreated coals increase with coal rank increasing, confirming that methane adsorption capacity on untreated coal is affected by the coal rank.
Based on the curves of the methane adsorption on samples, the Langmuir model can be used to describe the isothermal adsorption curves of the samples: where is the methane adsorption volume at equilibrium conditions, in cm3/g; is the equilibrium pressure, in MPa; is the Langmuir volume at standard state with a pressure of 101 kPa and a temperature of 273.15 K, in cm3/g; and is the Langmuir pressure, in MPa.
The Langmuir constants of the untreated and CS2-treated coals are shown in Table 4. To further study the changes in methane adsorption capacity that is due to CS2 treatment and the effect on coal bed methane recovery, and are defined using the following equations, respectively: where is the change in due to CS2 treatment, in cm3/g; and are the for CS2-treated coal and untreated coal, respectively, in cm3/g; is the change in due to the treatment, in MPa; and and are the for CS2-treated coal and untreated coal, respectively, MPa.
As shown in Figure 7, after CS2 treatment, the of Haitian coal and Tucheng coal is positive, while that of other coal samples is negative (Figure 7(a)). The of Haitian coal and Linyuan coal is positive and that of others is negative (Figure 7(b)). Thus, the treatment of CS2 increased and decreased the methane adsorption capacities in coals before and after , respectively, and there was no consistent trend for . The changes in the methane adsorption capacities due to CS2 treatment are controlled by the coal rank, which indicates that CS2 could be used for ECBM recovery for coals after from the view of methane adsorption, but not for low-rank coals.
3.3. Effects of Pore Structure Changes on Methane Adsorption Capacity
The effect of pore structure changes due to CS2 treatment on the methane adsorption capacities are shown in Figure 8. increased when increased, and there is no relation to , , , and . Previous studies have shown that the methane adsorption capacity in coal is primarily controlled by the magnitude of the pore SSA, and the pore SSA is controlled by the coal rank and influenced by the maceral composition [37, 40]. In this study, the relation of to and shows that the methane adsorption capacity is closely related to the micropore SSA, demonstrating that the pore number is increased for coal before and the pore size is enlarged in all diameters for all coals due to CS2 treatment. It also confirms that the changes of pore structures and methane adsorption capacities due to CS2 treatment are controlled by the coal rank.
3.4. A Conceptual Pore Structure Evolution Model with CS2 Treatment
As previously mentioned, the discrepancy in the sense of the change in pore structure may be attributed to changes in the size and connectivity of the micron-scale inaccessible pores. Lee’s study has provided evidence that the amount of normal alkanes drops at the early stage of coalification, then fill in pores and cause the size of small pores to be reduced; as maturation advances, more methane is generated and normal alkanes are lost . Therefore, there are some differences in pore structure changes for CS2-treated coal before and after . A conceptual model for pore structure evolution resulting from CS2 treatment is proposed, as shown in Figure 9. Due to CS2 treatment, some closed and or ink bottle pores filled with small organic molecules of coal open and connect before , and the amount of micropores which can be measured by MIP was increased (Figure 9(a)); the pore sizes were enlarged in all coals (Figures 9(a) and 9(b)), and the connectivity of the pores was improved (Figure 9(c)). However, further studies will be needed to confirm this hypothesis for the changes in pore structures of coal caused by CS2 treatment.
Studies have shown that there is a positive relation between micropore SSA and methane adsorption [1, 26, 29, 32]. In this study, after CS2 treatment, the change in methane adsorption capacity on coal before and after is increased and decreased, which confirms the changes in pore structure due to CS2 treatment.
Alteration of the pore structure for 5 sets of untreated and CS2-treated coals was characterized by MIP, and methane adsorption capacity was estimated by the volumetric method. Based on these results, several conclusions are summarized as below: (1).After CS2 treatment, the cumulative pore volume and SSA were increased, and the incremental micropore volume and SSA were increased and decreased before and after , respectively; the incremental big pore (greater than 10 nm in diameter) volumes were increased and SSA were decreased for all coals, and pore connectivity was improved. The pore number is increased for coal before , and the pore size is enlarged in all diameters for all coals due to CS2 treatment; the changes are controlled by the coal rank. The change in pore structure is better for gas migration.(2).Methane adsorption isotherms of CS2 treatment coals followed the Langmuir model. After CS2 treatment, the methane adsorption capacity on coal is increased and decreased before and after , which indicates that the change in methane adsorption capacity due to CS2 treatment is controlled by the coal rank, and the solvent of CS2 can be used to enhance coal bed methane recovery for the coal after . The alteration in methane adsorption capacity is worse and better for methane recovery before and after .(3).There is a positive relationship between and , while there is no obvious relation with and other , confirming that the pore number is increased for coal before and the pore size is enlarged in all diameters for all coals due to CS2 treatment.(4).A conceptual mechanism of the effect of CS2 treatment coal is proposed which is consistent with the observed evolution of the pore structure and methane adsorption response.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The authors would like to thank various organizations for the financial support. This work was jointly supported by the National Natural Science Foundation of China (no. 41572138) and the research fund of Jiangsu Key Laboratory of Coal-based CO2 Capture and Geological Storage (China University of Mining and Technology) (no. 2017A04). The authors thank the editor and reviewers for their detailed comments and suggestions.
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