Article pubs.acs.org/EF
Nanopore Structural Characteristics and Their Impact on Methane Adsorption and Diffusion in Low to Medium Tectonically Deformed Coals: Case Study in the Huaibei Coal Field Song Yu,*,†,‡ Jiang Bo,†,‡ and Liu Jie-gang†,‡ †
China University of Mining and Technology, Xuzhou 221116, China Key Laboratory of Coalbed Methane Resource & Reservoir Formation Process, Ministry of Education, Xuzhou 221008, China
‡
ABSTRACT: Coupled with isothermal adsorption and the Steele potential function, the characteristics of nanopores and their impact on methane adsorption and diffusion in low- to medium-rank tectonically deformed coals (TDCs) were revealed by highpressure mercury intrusion and low-pressure N2/CO2 gas adsorption. The specific surface area (SSA) of low to medium TDCs is mainly provided by micropores (50 nm, 99.68−99.91%). The fractal characteristics of nanopores can be divided into four groups, i.e., D1 (>100 nm), D2 (8 nm), and D4 ( D2, indicating that the heterogeneity of seepage pores is stronger than that of adsorption pores. For scaly coals, D2 ≈ D1, demonstrating the close heterogeneity and connectivity in adsorption and seepage pores, which are beneficial for coalbed methane (CBM) desorption and diffusion. However, D2 > D1 for wrinkle and mylonitic coals, indicating a stronger heterogeneity in adsorption pores than seepage pores, especially for mylonitic coals. D4 gradually increases with the enhancement of tectonic deformation, and D3 shows a sharp increase in wrinkle coals. D2, D4, and SSA (1000 nm diameter occurs. However, the proportion of pores in the size range of 100−1000 nm contributing to the PV (35.32−40.04%) increases in TDCs, which have developed through ductile deformation (Figure 5). The increase in the proportion of pores of 100−1000 nm diameter and the decrease in that of pores of >1000 nm diameter during the brittle, brittle−ductile, and ductile deformation stages indicate that the promotion effect of tectonic deformation on the macropore PV gradually increases along with higher levels of tectonic deformation. The mylonitic coals have a lower total macropore PV than scaly and wrinkle coals, while the PV of 50−100 nm pores is higher than those of the latter two, which further demonstrates that the transformation of mylonitization on the pore structure has the effect of decreasing the pore diameter scale. 4.2. Characteristics of Nanopores. 4.2.1. PS. All of the 17 samples have adsorption loops, indicating that the pore shape is generally in an open state.56 The adsorption curves of the samples are quite steep at pressures close to the saturated vapor pressure, and the desorption curves have a sharp inflection point for the TDCs at a relative pressure of approximately 0.5.
(7)
where V is the gas adsorption volume at the balance pressure p, V0 is the gas volume of monolayer adsorption, p0 is the saturated vapor pressure, A is the slope of the ln(V/V0) vs ln(ln(p0/p)) logarithmic curve, and C is a constant. Then the fractal dimension is A + 3.
4. RESULTS AND DISCUSSION 4.1. Characteristics of Macropores. The CCs of the primary coals were (1.62−3.52) × 10−10 (with an average of 2.34 × 10−10 m2/N) and are higher than the values of Li et al. ((2.50−3.13) × 10−10 m2/N),49 Guo et al. ((0.93−2.74) × 10−10 m2/N),50 Toda and Toyoda ((0.7−2.3) × 10−10 m2/ N),58 and Spitzer ((1.72−2.09) × 10−10 m2/N).59 Even considering the mercury compressibility (0.4 × 10−10 m2/N), the CCs of the primary coals are significantly high, which may be due to the coal micromechanism. The microhardness of the inertinite group is higher than those of the vitrinite groups,60 and thus, the samples with a high vitrinite group content have relatively higher elastic behavior, resulting in high CCs for the vitrain. On the other hand, plenty of micropores in the samples used here can also lead to high CCs just as Schuyer et al.61 and Nelson et al.62 reported. The CC range for primary coals is significantly lower than those for cataclastic ((1.52−6.12) × 10−10 m2/N, 3.45 × 10−10 m2/N on average) and schistose ((2.24−5.79) × 10−10 m2/N, 4.31 × 10−10 m2/N on average) coals. For scaly and wrinkle coals, the CCs were (2.70−8.71) × 10−10 m2/N (5.66 × 10−10 m2/N on average) and (9.24−13.25) × 10−10 m2/N (11.26 × 10−10 m2/N on average), respectively. The CC increases with the enhancement of the tectonic deformation (Figure 3). It has been shown that the coal compressibility increases with increasing micropore volume.49,50 Also the micropore volume increases with the enhancement of the tectonic deformation, which could lead to increasing compressibility. The micropore volume is discussed in section 4.2.3. It can be understood that the compressibility evaluated above was based on the microporous coal matrix, instead of the true solid volume, since high-pressure stages (P > 20 MPa) will cause a relatively high extent of compression. The observed mercury volume and corresponding fractal dimensions should be corrected using the CCs, and the corrections for fractal dimensions are discussed in the fractal analysis (see section 4.3.1). D
DOI: 10.1021/acs.energyfuels.7b00512 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 4. HPMI PSD (a) and injection and withdrawal curves (b) in primary coals and TDCs.
LPN2GA of the samples can be divided into three types (H1, H2, and H3). The H1 type is for primary and cataclastic coals. At a relative pressure of 0.4−1.0, the hysteresis loops are very narrow. The adsorption−desorption curves are stable at relative pressures below 0.8, and then the curves ascend significantly and rapidly as the relative pressure approaches 1 (Figure 6a,b). The hysteresis loops of this type are very small, corresponding to parallel plate pores with all sides open.65 The H2 type is for scaly and schistose coals. It is distinguished from H1 by a sharp inflection point on the desorption curve at a relative pressure of approximately 0.5 (Figure 6c,d), which is a reflection of the complicated pore system.2,56,65 At a low relative pressure, the adsorption branch coincides with the desorption branch, indicating that the morphology of pores with a small pore diameter is impermeable with one side almost closed, i.e., type II pores.63,64 At a relative pressure of >0.42 to 0.50, corresponding to pores with larger diameters, a significant adsorption loop is formed and open type I pores begin to appear. The H3 type is for the wrinkle and mylonitic coals and has a sharp inflection point at a relative pressure of 0.5 (Figure 6e,f). Unlike H2, the desorption branch decreased rapidly at a relative pressure slightly below the inflection point, indicating the existence of plenty of fine bottleneck pores.
Figure 5. Macropore volume distribution in primary coals and different TDCs.
Thus, the general morphology of the hysteresis loops is close to the B type proposed by De Boer,63 featuring the characteristics of the H4 type and the H3 type recommended by IUPAC.45,64 These associations indicate the existence of irregular nanopores. The changes in the hysteresis loops indicate that the tectonic deformation affected the nanopore structure.2−4,40 On the basis of the features of the hysteresis loops and the division scheme proposed by De Boer and IUPAC, the hysteresis loops of the
Figure 6. Hysteresis loop types and nanoporous shape in primary coals and TDCs. E
DOI: 10.1021/acs.energyfuels.7b00512 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 7. SSA of pores of 1.7−210 nm diameter (a), incremental and cumulative SSA plots for the primary coals and TDCs (b−f), and percentage of pores of 1.7−7.0, 7.0−43, and 43−210 nm diameter (g).
pressure. Thus, the PS is strongly closed, resulting in wide hysteresis loops, demonstrating the existence of a seepage barrier generated from the fine flask pores with an ink-bottle shape (Figure 6e,f). 4.2.2. PSD of Macro- and Mesopores. LP-N2GA provides data on the BET SSA and BJH meso-PV. The total SSA of 1.7− 210 nm increases significantly with the intensity of tectonic deformation, especially in wrinkle coals, indicating that ductile deformation has a more significant effect on the nanopore SSA than brittle deformation (Figure 7a). The total SSA of cataclastic coals is predominately distributed between 7.08 and 42.95 nm pores and is similar to that of primary coals (Figure 7b,c). The SSA of 1.7−210 nm pores in schistose coals (1.1183 m2/g on average) is close to that of scaly coals (0.9573 m2/g), both of which are higher than that of cataclastic coals (0.3658 m2/g) (Figure 7d,e). The N2 adsorption capacity of schistose coals is smaller than that of scaly coals. In comparison, the peak value of SSA in schistose coals (0.045 m2/g) is lower than that of scaly coals (0.072 m2/ g), because of a lack of pores between 10 and 40 nm (Figure 7d). The N2 adsorption capacity and total SSA (2.9225 m2/g) of wrinkle coals show a sharper increase than those of the other TDCs (Figure 7f), and the SSA distribution is consistent with that of schistose coals (Figure 7d,f). The cumulative SSA is obviously distributed in three segments (1.7−7, 7−43, and 43− 210 nm). More than half (46.61−72.91%) of the SSA is provided by pores of 114 nm, the cumulative SSA increases slightly with further increasing pore diameter. The majority (92.37−96.63%) of the SSA is provided by pores of 10 nm diameter correspond to parallel plate pores. Pores in TDCs of these kinds are semiclosed (Figure 6c,d). The PS of pores of 100 nm) and a high-pressure range (adsorption pores, D2, L < 100 nm) (Figure 9a−f). D2 has a wide distribution (2.289− 2.971), which on one hand indicates significant differences in discontinuities and roughness in different TDCs and on the other hand may be related to the difference in compression effects of different TDCs, as shown in Figure 3.49,50 Note that D2 is obtained within the high-pressure range when both porefilling and coal compressibility occur during mercury injection. According to the average CCs of different TDCs, the compression quantity of primary coals is 0.0008 cm3/g, suggesting that the coal volume decreases by 0.008% when the pressure is above 413 MPa. The compression quantities of cataclastic and schistose coals are 0.0012 and 0.0015 cm3/g, respectively, accounting for 0.01−0.017% compression in the coal sample volume, and those of wrinkle and mylonitic coals are 0.0019 and 0.0039 cm3/g, respectively, accounting for 0.58−1.7% compression. The coal matrix compression quantities have effects on the fractal results, especially for the wrinkle and mylonitic coals. Therefore, when using eq 6 to calculate D2, the observed cumulative mercury amount should be corrected on the basis of the CCs. The cumulative mercury injection amount, after calibration, can be expressed as
(10)
where a and b are constants. Since the absolute pore volume at P0 is unknown, eq 10 can be rewritten as Vp − Vp = b(pi D2 − p0 D2 ) i
0
(11)
To obtain the corrected D2, the corrected data were input into eq 11. The fitting was performed in the whole pressure range (P0 to Pmax) from the intermediate pressure point (Pi) to the maximum pressure point (Pmax). It was found that the D2 value obtained depends on the starting point of Pi. The D2 values obtained in all cases are between 2 and 3, which illustrates the power law relationship with the fractal pore surface. The eventually stabilized value at the maximum pressure is taken as the corrected D2. D2 and D1 in primary coals are 2.33−2.71 (2.47 on average) and 2.80−2.83 (2.81 on average), which may indicate that the pore morphologies of seepage pores are more complicated than those of adsorption pores. D1 values in primary coals are lower than those in TDCs; therefore, the SSAs and adsorption capacities are lower compared to those of the TDCs. D2 and D1 in brittle TDCs (i.e., cataclastic and schistose coals) are higher than those in primary coals, indicating that brittle deformation promotes the nanopores’ heterogeneities, especially for seepage pores. With an increase in the intensity of brittle deformation, D1 increases significantly and D2 increases slightly (Figure 9g,h), which is related to the widely developed fractures in brittle deformed TDCs.35 From HPMI results, cataclastic and schistose coals have the highest D1 and lack pores of 100−1000 nm size. Therefore, adsorption pores in cataclastic and schistose coals have a high SSA proportion. The isothermal adsorption results also show that schistose coals have a greater methane adsorption capacity and can be high-quality CBM reservoirs, which is consistent with other results.2,38,40 Compared with those in brittle deformed TDCs, D2 in scaly coals increases and D1 decreases, demonstrating that the seepage pores and adsorption pores have similar heterogeneities and connectivities. This is consistent with the relatively equal developments in pores of 10−100, 100−1000, and >1000 nm diameter as inferred by HPMI. The similar heterogeneities of adsorption and seepage pores in scaly coals is beneficial for the gas desorption and diffusion in coal reservoirs. With greater intensity of ductile deformation, D2 and D1 gradually increase and decrease, respectively, in wrinkle and mylonitic coals (Figure 9g,h). This may indicate that strong ductile deformation could have a strong transformational effect on H
DOI: 10.1021/acs.energyfuels.7b00512 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 11. Isothermal adsorption curve types of tectonically deformed coals.
Figure 12. Relationship between fractal dimensions D2 (a), D3 (b), and D4 (c) and maximum adsorption capacity.
nanopores, particularly for the adsorption pores. D2 is relatively high in wrinkle and mylonitic coals, demonstrating the strong heterogeneity in adsorption pores. D2 is significantly higher than D1 in mylonitic coals, and the heterogeneity of adsorption pores is significantly greater than that of seepage pores, leading to a high adsorption capacity. However, CH4 has difficulty in desorbing and diffusing in the micropores of mylonitic coals because of the effect of the abundance of ink-bottle-type pores. 4.3.2. D3 and D4. The FHH fractal curves have a demarcation point at p0/p = 0.5. Thus, the pore diameter can be divided into two stages, i.e., D3 (p0/p < 0.5, 8−100 nm) and D4 (p0/p > 0.5,
