Article pubs.acs.org/EF
Cite This: Energy Fuels XXXX, XXX, XXX-XXX
Influence of Fluid Exposure on Surface Chemistry and Pore-Fracture Morphology of Various Rank Coals: Implications for Methane Recovery and CO2 Storage Wei Li,† Hongfu Liu,† and Xiaoxia Song*,† †
Department of Earth Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China ABSTRACT: The surface chemistry and pore-fracture morphology of coals are critical to the process of CO2 sequestration in coal seams with enhanced coalbed methane (CH4) recovery (CO2-ECBM). To assess the influence of deionized water−CO2 mixture (DH2O−CO2) exposure on these properties, the interaction of DH2O−CO2 with three rank coals, i.e., sub-bituminous coal (SBC), high volatile bituminous coal (HVBC), and anthracite, was conducted on a dynamic supercritical fluid extraction system with a temperature of 45 °C and an equilibrium pressure of 12 MPa. Characterization methods including proximate analysis (PA), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), probe molecule (N2/ CO2) adsorption, and low-field nuclear magnetic resonance (NMR) were adopted to fully address the changes in surface functional groups and pore-fracture characteristics. The results indicate that the geochemical interaction occurs between the mineral matters and DH2O−CO2 as demonstrated by the change in the content of clays, carbonates, and sulfates in the coal matrix. DH2O−CO2 exposure also causes a decrease in the content of organic oxygen and carbon−oxygen functional groups, especially for COOH groups, but an increase in C−C/C-H species, and the impact is strengthened with the decreasing coal rank. The aforementioned aspects illustrate the reconfiguration of surface geometry and the chemical interaction between DH2O−CO2 and the oxygen-containing functional groups. In combination with the reduced volatile matter and organic sulfur groups, the results imply that the extraction effect and chemical interaction may contribute to the change in coal surface chemistry. DH2O− CO2 interaction degrades the accessibility of micropores of all the coals, whereas the reverse trend is found for macropores and fractures. DH2O−CO2 interaction facilitates the development of macropores and fractures and thus improves the permeability of coal seams, which can be attributed to the dissolution and mobilization of mineral matters by the acid water and the shrinkage of coal induced by the water loss. The variation of mesopores due to DH2O−CO2 exposure is strongly related to coal rank and surface chemistry. Specifically, the decreasing mesoporosity is found for SBC, whereas this trend is opposite for HVBC and anthracite. The relationship between adsorption pores (5 MPa) and higher temperature (∼330 K).51 In the current work, the pressure and temperature set for the interaction of DH2O− CO2 with coals are 12 MPa and 45 °C, respectively, which may favor the chemisorption of CO2 onto the coal defect sites as
well as the formation of new C−C bond. This could also explain the increasing intensity signal of C−C species of coal structure. The surface-bound water and CO2 or the imbibition of CO2 into coal structure would cause irreversible structure change and matrix swelling;57 thus, the reconfiguration of surface geometry is noted, and considerable change in the H
DOI: 10.1021/acs.energyfuels.7b02483 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 8 presents the DFT micropore size distributions (MIPDs) of coal samples before and after FE. For DN after FE
shape of the C (1s) curve is addressed in the present study. However, further studies are needed to demonstrate these deductions in the future. 3.5. Characteristics of Pore Structure from Gas Adsorption after DH2O−CO2 Exposure. 3.5.1. Effect of Fluid Exposure on Micropore Structure of Coals. Regarding the microporosity distribution of the raw coal samples, as shown in Figure 7 and Table 4, a polynomial-shaped trend can
Figure 7. Low-pressure CO2 adsorption isotherms at 273.15 K of coal samples: (a) DZY, (b) XG, and (c) DN.
Table 4. Micropore Parameters of Coal Samples
sample
state
DZY
raw state after exposure raw state after exposure raw state after exposure
XG
DN
DR specific surface area (m2/g)
DR pore volume (cm3/g)
DA average pore size (nm)
micropore capacity (cm3/g)
154.07 149.41
0.0617 0.0599
1.588 1.568
18.838 18.081
120.40 110.44
0.0483 0.0443
1.557 1.607
12.950 11.281
185.51 183.49
0.0743 0.0735
1.496 1.521
21.631 20.828
be observed between coal rank and micropore adsorption capacity, volume, and specific surface area, which has been reported in many studies.2,58 This trend is mainly attributed to the generation and decomposition of bitumen with the increasing maturity during coalification.59 Generally, as presented in Figure 7 and Table 4, FE causes a decrease in micropore capacity, volume, and specific surface area of all the samples, which is in good agreement with the change in the moisture content in Section 3.2. The results indicate that the accessibility of micropores of the test samples decreases. The microporosity is a critical parameter to the adsorption capacity.60,61 The decreasing microporosity means lower gas adsorption ability of the studied coals after FE. The test samples exhibit different decreasing degree of these parameters after FE. As listed in Table 4, the greatest reduction of these micropore parameters is about 8.27%−12.88% for lowrank XG, while DN shows the smallest reduction by 1.09%− 3.71%. Thus, the effect of FE on the alteration in micropore morphology is more pronounced in the low-rank coals.
Figure 8. Micropore size distributions of various coals: (a) DZY, (b) XG, and (c) DN.
(Figure 8c), a small decrease occurs in pore size of 0.45−0.55 nm, while a slight increase is documented in pores of 0.8−0.85 nm. The XG shows a multipeak distribution of pore size with four dominant pore peaks near 0.49, 0.58, 0.71, and 0.82 nm, respectively. In terms of XG after FE (Figure 8b), the I
DOI: 10.1021/acs.energyfuels.7b02483 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
probable reason to explain the decrease of microporosity in low-rank XG and DZY. However, the effect of fluid interaction on microporosity of HVBC XG is greater than that of SBC DZY. The difference between HVBC XG and SBC DZY may depend on the distribution of the oxygen-containing groups during coalification. Prinz and Littke56 found that, for the coal with Ro,max of approximately 0.8%, water and CO2 were mainly accommodated in micropores (0.5−0.8 nm) associated with functional groups. Therefore, the polar sites of XG with Ro,max values of 0.57%−0.73% may concentrate in the surface of micropores of 0.45−0.8 nm. The bonded CO2 and water on these polar sites will lead to the decrease of micropore volume in this pore size range. However, in terms of the lowest rank sample DZY with a Ro,max of 10 nm and thus leads to the noticeable growth of APS by 54%. Compared to DZY, XG shows an increase in mesopore and macropore volume over the whole pore size range (Figure 10b). However, a slight increase of pore volume occurs in pores of 30 nm, while only a small decrease is documented in mesopores from 4 to 30 nm for DN (Figure 10c). The various change in different pore size range leads to an unchanged total BJH pore volume. The reason for the dependence of low-pressure N2 PSDs of the test samples on DH2O−CO2 exposure is similar to that of microporosity distribution, which can be ascribed to rankdependent physicochemical properties of coal. Mastalerz et al.16 found that for vitrain with Ro,max = 0.57%, the mesoporosity with pore diameter of 4−10 nm was less developed, and pores >20 nm were more frequent after running high-pressure CO2 adsorption. They suggested that the mesopores ranging from 4 to 10 nm were the most favorable sites for CO2 adsorption and that the decrease of mesoporosity in this pore range contributed to the CO2 adsorption-induced coal matrix
Figure 9. Low-pressure N2 adsorption isotherms at 77 K of coal samples: (a) DZY, (b) XG, and (c) DN. K
DOI: 10.1021/acs.energyfuels.7b02483 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
As previously stated in XPS analysis, water will preferentially adsorb over CO2 to these oxygenated polar sites. This also can be evidenced by the research of Sakurovs and Lavrencic,74 who discovered that water prevented CO2 from effectively wetting the low-rank coals with high oxygen even at high pressure. Hence, for the lowest rank DZY after FE, the changing characteristics of mesoporosity may be remarkably affected by water than by CO2. In addition, Day et al.19 and Fry et al.65 disclosed that for SBC the maximum moisture-induced swelling was roughly equal to the dry maximum CO2-induced swelling. The report of Hayashi et al.80 and Mares et al.81 confirmed that the bonded water or adsorbed water were mainly located in pores with size of 2−10 nm for the lower rank coals. From the aforementioned analyses, the great decline in mesoporosity ranging between 2 to 10 nm of DZY is the result of the resultant swelling effect. As a result, as shown in Figure 10a, some smaller mesopores are enlarged into larger mesopores and macropores. In addition, because the decrease of VM in DZY occurs, the opening of some pores due to supercritical CO2 extraction may contribute to the enlargement of pores with size above 10 nm.26 The entrainment of partial MM by FE may also account for the increasing larger mesopores and macropores previously blocked by MM. For XG, the increment in entire N2 pore volume can be ascribed to decrease and expansion of micropores owing to matrix swelling, and the reason has been fully discussed in section 3.5.1. For anthracite DN, CO2−coal interaction is dominated in H2O/CO2/coal systems, and the rearrangement of mesopores is mainly the reflection of CO2 exposure. The change in mesopore size distributions of DN (Figure 10c) demonstrates the viewpoint of Mastalerz et al.,82 who suggested that the mesoporosity of 4−10 nm decreased as a result of CO2-induced coal matrix swelling, and the subsequent post-adsorption shrinkage increased pores >20 nm. Additionally, a portion of mesopore walls of anthracite is composed of molecular orientation domains,56 and some residual functional groups like C=O groups, as revealed by FTIR and XPS, may situate within these mesopore walls in high-rank coals.71 When CO2 and water adsorb on these residual functional groups, the larger mesopores may transform into smaller mesopores and thus lead to the increasing BET surface. 3.6. Characteristics of Pore Structure from NMR after DH2O−CO2 Exposure. 3.6.1. NMR T2 Spectra Distributions of Raw Coals. The NMR transverse relaxation time (T2) spectra distributions at water-saturated conditions and irreducible water situations can be used to characterize pore type, pore connectivity, porosity, and PSDs. The T2 spectrum distribution provides information about PSDs with T2 ranging from 0.01 to 10000 ms. Shorter relaxation time corresponds to smaller pores and longer time corresponds to larger pores.83 For coals, the relaxation time at T2 < 10 ms, 10 ms < T2 < 100 ms, and T2 > 100 ms represents adsorption pores (0.1 μm) (P2 peak), and fractures (P3 peak),84 respectively. The macropores and fractures can be classified into seepage pores.85 The spectra area and peak types (unimodal, bimodal or multimodal) of T2 distributions at Sw and Sir reflect the pore development, pore volume, and pore connectivity.83,84 The NMR T2 spectra at the Sw of coals before and after FE are presented in Figure 11. Figure 12 presents the comparison of T2 spectra at Sw and Sir after FE. Table 6 lists NMR parameters related to pore property. The T2 spectrum distribution of DZY at Sw (Figure 12a1) shows two distinct peaks P1 and P2 but a slight peak at P3,
Figure 10. Differential meso- and macropore size distributions of various coals: (a) DZY, (b) XG, and (c) DN.
swelling. This result is in good agreement with the change in the mesoporosity of DZY presented in our work. However, in our case, both the supercritical CO2 and water interact with coal surfaces. Consequently, the competitive adsorption on low-rank coal surface sites between CO2 and water plays an important role in the variability of mesoporosity. For the lowrank coal with Ro,max of 10 ms (Figure 11a), the amplitude corresponding to macropore and fracture becomes higher, and the spectrum area at Sw increased by 1.5 times (Table 6), indicating an increase in seepage pore volume. The enlarged P2 and P3 peak means that new macropores and fractures are generated or some pores merge into supermacropores and fracture, which directly improves the pore-fracture connection and gas permeability (9.452 mD). The increasing accessible macroporosity and fracture coincide with the observation of Massarotto et al.,17 and Liu et al.,29 who suggested that the MM originally sealed pore space was dissolved and mobilized by acid water after water−CO2 exposure, and consequently new macropores and cracks were generated. Additionally, the matrix shrinkage due to water loss during CO2 sequestration19,64 could also account for the increasing pore volume. However, as presented in Figure 11a and Table 6, the spectrum becomes narrower and the spectrum area at Sw for T2 < 10 ms decreases. This variation suggests that the adsorption pore volume decreases, which is
Figure 11. NMR T2 spectra distributions at Sw of various coals: (a) DZY, (b) XG, and (c) DN.
indicating well-developed adsorption pores and macropores but a poor fracture system. The macrolithotype of DZY is mainly attritus (Figure 2a), which determines the low frequency of the fracture86 to form the P3 peak. The high NMR porosity (15.76%) and high permeability (7.788 mD) resulted from well-developed pores with T2 < 100 ms may be related to relatively high inertinite content (Table 6).87 The wide distributions of P1 and P2 suggest multiple pore types M
DOI: 10.1021/acs.energyfuels.7b02483 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 12. Comparison of NMR T2 spectra distributions at Sw and Sir of various coals before and after fluid exposure: (a1 and b1) DZY, (a2 and b2) XG, and (a3 and b3) DN.
Table 6. Permeability and Information of NMR Analysis ST (×103 ms) sample
state
KHe (mD)
10 ms
φNMR (%)
φI (%)
φF (%)
DZY
raw state after exposure raw state after exposure raw state after exposure
7.788 9.452 0.074 2.209 1.008 1.036
2.840 2.766 0.857 0.918 0.876 1.013
0.482 0.724 0.051 0.170 0.018 0.036
15.76 19.75 1.70 4.42 2.81 5.09
13.28 15.72 1.64 3.90 2.65 4.58
2.49 4.03 0.05 0.53 0.16 0.51
XG DN
a KHe: helium permeability. ST: spectrum area of NMR T2 distribution at Sw conditions. φNMR: NMR total porosity. φI: bound fluid porosity. φF: free fluid porosity.
Sir; viz., more free fluid can be expelled from macropores and fractures during centrifugal process coincided with the improved pore connectivity, while the opposite trend is recorded at T2 < 2.5 ms, and the amplitude of irreducible water situation is beyond the amplitude of water-saturated situation. This phenomenon may be a result of pendular ring.91
consistent with the results obtained from low-pressure nitrogen adsorption method. The gap between P1 and P2 becomes wider, which reveals that the connectivity of adsorption pores and seepage pores is lower. The decreasing connectivity can be evidenced by the T2 distribution at Sir (Figure 12b1). Greater spectrum deviation at T2 > 2.5 ms is observed between Sw and N
DOI: 10.1021/acs.energyfuels.7b02483 Energy Fuels XXXX, XXX, XXX−XXX
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water and to loss of MM in T2 range of 11−100 ms. These mobilized MM may be trapped in fractures, including microfractures caused by matrix shrinkage, which leads to the decreasing P3 peak. After centrifuging, the T2 spectrum becomes unimodal and the spectrum deviation with T2 less than 4 ms between Sw and Sir is larger than that of raw coal samples (Figure 12b3). The connectivity between adsorption pores and seepage pores is elevated, which is beneficial for CH4 recovery during CO2 sequestration process. Although the pore connectivity is promoted, the decreasing fracture porosity results in a little increase in gas permeability (Table 6). The change in T2 spectra distributions within H2O/CO2/coal interaction highlight the need to consider the flow path or flow network of core plug in which fluid or acid water transport.67 3.6.4. Effect of Fluid Exposure on NMR Porosity of Coals. As displayed in Table 6, FE causes an increase in NMR porosity (φNMR), BVI porosity (φI), and FVI porosity (φP). However, the amplitude of variation in φNMR, φI, and φP is different among the coals with different rank. The increasing order of the porosity is HVBC XG, anthracite DN, and SBC DZY. The HVBC XG has the greatest improvement of more than 130% in porosity from its original porosity, while the improvement in porosity of SBC DZY is less than 65%. The magnitude of the change in porosity of anthracite DN is in the middle and increases by 72.46% - 222.31%. In addition, φI differs from φP in the increasing trend, which discloses the alteration of the pore-fracture systems imposed by FE is an asynchronous work at different scale. The increasing amplification of φP is three to six times as large as that of φI, which means that the influence of FE on φP is greater than that on φI. The maceral composition may account for the above phenomenon. The primary coal composition of both XG and DN is vitrinite, while inertinite is dominated in DZY. The growth rates of the three types of porosity in particular for φP seem to increase with the decreasing inertinite content. The aforementioned analyses indicate that the vitrinite is more sensitive to CO2−H2O than inertinite at the macropore scale (>0.1 μm), which depends on the observation of Massarotto et al.17 Our results underscore the significance of considering the lithotype composition during CO2-ECBM process.63
Figure 13. Example of calculation of φI porosity and φF porosity with measurements at Sw and Sir.
The pendular ring is related to pore-wall chemistry, pore throat to pore body ratio, and pore connectivity. As previously stated, some adsorbed water and CO2 on the oxygen-containing functional groups sites are hard to remove. As a result, the bonded CO2 and water may make the pore throat become narrower as well as increase the pore-to-throat ratio, which accelerates the pore-shielding effect and increases the difficulty of water removal from smaller pores. Therefore, the spectrum area of Sir at T2 < 2.5 ms is greater than that of Sw. The degraded connectivity between adsorption pores and seepage pores is not favorable for CH4 desorption and CO2 diffusion. For XG, all the three peaks enlarge, and the spectrum becomes wider (Figure 11b), indicating a growth in total pore volume and enlargement in PSDs. The P2 and P3 show a greater increase and the peaks move toward a greater T2 value, meaning that the more and new macropore-fracture systems are formed which greatly improves the permeability (2.209 mD). XG undergoes the largest increment in both macropores and cracks among the three samples, which is associated with maceral composition and the accessibility of MM in pore space to fluid. The SEM images in the work of Massarotto et al.17 showed that structural change or new cracks were more easier to take place in vitrinite-rich coals than that in inertinite-rich coals after exposure to CO2−H2O. This observation is consistent with the result of this study that the vitrinite-rich sample XG shows greater expansion in P2 and P3 peaks compared to inertinite-rich sample DZY. The MM in pores and fractures of XG may be more flushed away by water among the samples, which may be another reason to explain the different response in the evolution of macropores and fractures. After centrifuging, the T2 spectrum shows that all of water saturated in fractures is removed, while a part of water in macropores is moveable and water in adsorption pores is still remained (Figure 12b2). The result suggests that FE may merely improve the number of adsorption pores but could not effectively enhance the connection of adsorption pores and macropores. For DN after FE, a new bimodal T2 distribution is observed (Figure 11c). The P3 peak almost disappears, while a new P2 peak is generated and a broader P1 peak is registered. The enlarged P1 peak with T2 toward from 0.86 to 1.0 ms indicates that more adsorption pores are formed and the APS is augmented, which corroborates greater APS from the lowpressure nitrogen adsorption. The created P2 peak may be attributed to the matrix shrinkage due to the progressive loss of
4. IMPLICATIONS FOR COALBED METHANE RECOVERY AND CO2 STORAGE Based on our study, DH2O−CO2 exposure induces the decrease of both microporosity and the oxygen-containing functional groups. However, FE improves the development of macropores and fractures and enhances pore connectivity of macropores and fractures of all the coals. The effect of DH2O− CO2 exposure on the mesopore depends on coal rank and coal surface functional groups distribution. The adsorption and desorption behavior of CH4 and CO2 in various rank coals will show different response to these changes after contact with fluid. For HVBC and anthracite after FE, the decline in microporosity and oxygenated groups may decrease CO2 adsorption capacity. However, the increasing mesopore specific area and adsorption-pore volume (
