Characterization of Whole-Aperture Pore Structure and Its Effect on

Feb 19, 2018 - samples were obtained by regression fitting.10,18. 4. RESULTS. 4.1. Pore Morphology Characterization through. FESEM Images. FESEM image...
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Characterization of Whole-Aperture Pore Structure and Its Effect on Methane Adsorption Capacity for Transitional Shales Yinsen Sun†,‡,§ and Shaobin Guo*,†,§ †

School of Energy Resources, China University of Geosciences, Beijing 100083, China Geoscience Center of CNPC Great Wall Drilling Company, Beijing 100083, China § Key Laboratory of Shale Gas Resources Evaluation and Strategy Selection, Ministry of Land and Resources, Beijing, China ‡

ABSTRACT: The structure of shale pores controls gas storage and gas transport. However, because the distribution of shale pore sizes is wide, it is difficult to use only one method to completely characterize the structural characteristics of shale pores. We employed various techniques to characterize the whole-aperture pore structure of 15 shale samples of different levels of maturity collected from a transitional shale formation of the Upper Paleozoic, Ordos Basin, China. Field-emission scanning electron microscopy (FESEM) image analyses indicated that interparticle pores between or within clay minerals and microfractures are commonly developed, while organic matter pores (OMPs) develop poorly, compared with those observed in marine shale. Highpressure mercury intrusion porosimetry (MIP) and low-pressure N2/CO2 adsorption experiments showed that the pore size distribution (PSD) ranges from the nanometer scale to the micrometer scale and is multimodal. The dominant PSD ranges are 0.45−0.65 nm, 0.75−0.95 nm, 3−5 nm, 10−50 nm, 10−40 μm, 2−50 nm (mesopores), and >50 nm (macropores). Mesopores and macropores provide most of the shale pore volume (PV), accounting for 42.9% and 52.7%, respectively. However, micropores only account for 4% of the PV, although micropores (1.0 μm, indicating the presence of a large number of micrometer pores in the shale sample. Between 0.1 MPa and 30 MPa, the change in the mercury injection content was very small, indicating that, within this pressure range, pores of the corresponding size did not develop. Above 30 MPa, the extent of mercury intrusion began to increase significantly, and even at the maximum pressure, mercury intrusion continued to increase rapidly, indicating the presence of many pores smaller than 40 nm in the shale samples. Overall, the hysteresis loop shows the difference between the mercury injection volume and withdrawal volume, which indicates that pores with thin bottlenecks existed in the shale samples. Furthermore, the micropores, mesopores, and macropores were arranged in a tandem configuration with a small pore throat and poor connectivity. Figures 4a−c show the correlation between the pore volume (PV) obtained from the MIP experiment and the pore size distribution (PSD), which indicates that pores with aperture size ranges of 3−30 nm and 10−40 μm contributed most to the PV. Because the MIP experiment mainly measured macropores (>50 nm), the test results pertaining to pore sizes smaller than 50 nm are not accurate; therefore, the pore size range contributing most to the pore volume was 10−40 μm. The contribution to the PV of the other pore size ranges can be ignored, which indicates that the micrometer macropore range of 10−40 μm contributes most to the PV of the Upper Paleozoic transitional shale of the Ordos Basin. Figures 4a−c show that the maturity of the shale samples is increasing. The

distribution characteristics of shale macropores were analyzed by MIP. Figure 3 shows the mercury injection curves and the

Figure 3. Mercury injection and withdrawal curves of Upper Paleozoic shale in Ordos Basin.

mercury withdrawal curves of different shale samples. The highpressure mercury curves show similar morphological characteristics overall. In the low-pressure section (P < 0.1 MPa), with the increase in pressure, the mercury injection content increased appreciably. When the pressure reached ∼0.1 MPa, the mercury intrusion rate decreased. The low-pressure section

Figure 4. Shale macropore volume and SSA distribution determined by MIP experiment. E

DOI: 10.1021/acs.energyfuels.7b03807 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 5. Adsorption−desorption curve and PSD of Upper Paleozoic shale of Ordos Basin determined by N2 adsorption.

shale samples in Figure 4a range from low maturity to mature. With the increase in maturity, the macropore volume increases appreciably. The samples in Figures 4b and 4c are overmature (Ro = 2.6%) and highly overmature (Ro = 3.3%). The samples in Figures 4b and 4c show much higher PVs than the samples in Figure 4a, which indicates that the macropore volume increases with maturity. The maturity levels of the shale samples in Figure 4b are nearly the same. The maturity levels of the samples in Figure 4c are also very similar, while the TOC content shows some differences. Both Figures 4b and 4c show that, with the increase in TOC content, the macropore volume increases. The reasons may be that the thermal evolution of organic matter produces a large number of organic pores and can also produce a large amount of acidic fluid dissolution to expand the pore space and thus lead to an increase in the PV of shale.26,32−35

Figure 4d shows the contribution of pores of different size scales to the PSSA, which indicates that the 3−30 nm pores contribute most to the PSSA, while the contribution of pores of the other diameters to the PSSA is almost 0. Because the MIP experiment mainly measured macropores, the test results pertaining to pores smaller than 50 nm are not accurate. Therefore, the contribution of macropores to the PSSA of the Upper Paleozoic shale in the Ordos Basin is almost negligible. 4.2.2. Pore Characterization with N2 Adsorption Isotherms. Because of its limited measurement range, MIP cannot accurately characterize the structure of mesopores.36 Therefore, the characteristics of shale mesopores were characterized by N2 adsorption. The adsorption curves are similar to Brunauer− Deming−Deming−Teller (BDDT) type II and type III isotherms (Figure 5). The adsorption curves show no distinct inflection point in the low-relative-pressure section; adsorption rises slowly in this section. In the high-relative-pressure section, F

DOI: 10.1021/acs.energyfuels.7b03807 Energy Fuels XXXX, XXX, XXX−XXX

69.34 56.58 75.33 88.99 88.99 55.14 92.18 99.59 46.48 22.82 56.06 58.38 81.22 85.77 68.66

micropore mesopore marcropore

Proportion (%) G

micropore

2.9453 2.5568 2.1499 0.5964 0.5397 4.3770 0.3882 0.0135 5.7400 9.8630 3.2235 3.1724 5.2010 0.8350 3.3353 6.6659 3.3336 6.5783 4.8375 4.4256 5.4095 4.6381 3.3643 4.9898 2.9226 4.1597 4.4526 22.5235 5.0714 6.1957

mesopore marcropore

0.0026 0.0019 0.0048 0.0020 0.0079 0.0240 0.0052 0.0003 0.0047 0.0224 0.0375 0.0017 0.0060 0.0063 0.0116 6.70 11.65 3.92 1.46 0.94 8.83 0.39 0.03 7.47 9.23 1.51 5.95 6.51 2.85 4.37

micropore mesopore

66.51 47.56 53.90 51.62 38.89 58.13 28.63 21.64 78.54 29.68 7.39 24.62 75.11 66.55 42.92 26.79 40.79 42.18 46.91 60.17 33.04 70.97 78.33 13.99 61.09 91.10 69.43 18.38 30.60 52.71

marcropore TPV

0.0131 0.0066 0.0164 0.0119 0.0163 0.0148 0.0254 0.0177 0.0250 0.0352 0.0734 0.0183 0.0250 0.0095 0.0261 0.0009 0.0008 0.0006 0.0002 0.0002 0.0013 0.0001 0.0000 0.0019 0.0033 0.0011 0.0011 0.0016 0.0003 0.0011

micropore

0.0035 0.0027 0.0069 0.0056 0.0098 0.0049 0.0180 0.0139 0.0035 0.0215 0.0669 0.0127 0.0046 0.0029 0.0159 L1 L2 M3 M4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14 AVE

mesopore marcropore sample ID

0.0087 0.0031 0.0088 0.0062 0.0063 0.0086 0.0073 0.0038 0.0197 0.0104 0.0054 0.0045 0.0188 0.0063 0.0091

Proportion (%) PV (mL/g)

Table 2. Shale Pore Volume and Specific Surface Area Statistics, Upper Paleozoic Shale, Ordos Basin

PSSA (m2/g)

9.6137 5.8923 8.7330 5.4359 4.9732 9.8105 5.0315 3.3782 10.7345 12.8080 7.4206 7.6266 27.7306 5.9127 9.5426

0.03 0.03 0.05 0.04 0.16 0.24 0.10 0.01 0.04 0.18 0.50 0.02 0.02 0.11 0.14

the adsorption curve increases sharply, and no saturated adsorption plateau occurs. This finding indicates the presence of many mesopores and partial macropores in the test shale samples. According to the classification standard for N2 adsorption hysteresis loops established by the International Union of Pure and Applied Chemistry (IUPAC),37 type H3 and H4 pore structures were determined to occur in the Upper Paleozoic transitional shale of the Ordos Basin. The pore structure of the samples in Figure 4a1 is type H4, which represents slit pores and ink bottle pores. Furthermore, the pore adsorption hysteresis loop is large, indicating that the fine pores of the shale developed very well, and the main pore size distribution (PSD) range is 3−5 nm (Figure 5b1). This pore structure type is the most developed in the Upper Paleozoic transitional shale of the Ordos Basin. Shale samples O9 and O10 shown in Figure 4a2 are type H3 pore structures, representing parallel platelike pores, fractures and wedgeshaped pore structures. The adsorption hysteresis loop is small, indicating that the connectivity of the shale pores is good and the pore size is relatively large, corresponding to the main PSD interval between 10 and 50 nm (Figure 5b2), which represents mostly clay mineral interlayer pores. The pores of shale samples O13 and O15 in Figure 5a3 are composites of plate pores, conical tube pores, and ink bottle pores. The adsorption volume of these composite pores is high, and the adsorption hysteresis loop is irregular, which indicates that the pore structure is complex. The corresponding PSD interval is between 3 nm and 20 nm (Figure 5b3), indicating a multimodal state. Overall, the structure of the mesopores in Upper Paleozoic transitional shale in the Ordos Basin is irregular and mainly contains parallel slit pores, conical plate pores, ink bottle pores, and composite pores. A large number of shale micropores, mesopores, and macropores exist in series with each other to form a complex pore throat system. The BET SSA of the shale samples ranged from 2.92 m2/g to 6.67 m2/g, with an average value of 4.65 m2/g, and the total PV ranged from 3.1 mL/kg to 19.7 mL/kg, with an average value of 8.5 mL/kg (Table 2). Figure 5b shows the contribution of different pore sizes to the PV. It can be concluded that the PV of the transitional shale is mainly determined by pores measuring 3−5 nm, and some samples are dominated by pores measuring 10−50 nm. The SSA shows the same characteristics as the PV. 4.2.3. Pore Characterization with CO 2 Adsorption Isotherms. The two methods described above cannot fully characterize the characteristics of shale micropores, but through the CO2 adsorption method, the distribution of shale micropores can be determined.24,36 Figure 6 shows the contribution of different pore sizes to the PV and PSSA. The distribution of the PSSA of micropores is similar to the distribution of the micropore volume. The main micropore size shows a bimodal distribution: the left pore size peak is between 0.45 nm and 0.65 nm, and the right peak is mainly distributed in the range of 0.75−0.95 nm. Few samples show a narrow peak in the range of 1.0−1.1 nm. Therefore, it can be concluded that the PV and PSSA of the transitional shale are mainly determined by pores measuring 0.45−0.65 nm and 0.75−0.95 nm; few samples with pore sizes ranging from 1.0 to 1.1 nm also contribute partially to the PV and PSSA. The maturity levels of the shale samples in Figures 6a1 and 6b1, Figures 6a2 and 6b2, and Figures 6a3 and 6b3 are increasing. The shale samples in Figures 6a1 and 6b1 range from low maturity to high maturity, while the samples in

30.64 43.39 24.62 10.97 10.85 44.62 7.71 0.40 53.47 77.01 43.44 41.60 18.76 14.12 31.20

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DOI: 10.1021/acs.energyfuels.7b03807 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 6. Shale micropore volume and specific surface area distribution determined by CO2 adsorption.

single method to comprehensively characterize the entire shale PSD. In this study, based on CO2 and N2 adsorption experiments, the distribution characteristics of shale micropores and mesopores were determined. Moreover, MIP was used to determine the distribution of shale macropores. Combining the results of these three experiments, the whole-aperture pore structure characteristics of the Upper Paleozoic transitional shale in the Ordos Basin was characterized (see Figures 7 and 8). Figure 7 shows that the entire PSD of the shale is multimodal; the main PSD ranges are 0.45−0.65 nm, 0.75− 0.95 nm, 3−5 nm, 10−50 nm, and 10−30 μm. Micropores, mesopores, and macropores all contribute to the pore volume of the Upper Paleozoic transitional shale in the Ordos Basin. Mesopores and macropores contribute most to the PV, accounting for ∼95% of the total PV. The average PV of the mesopores was determined to be 0.0091 mL/g, accounting for 42.92% of the total PV. The average PV of the macropores was determined to be 0.0159 mL/g, accounting for 52.71% of the total PV. The contribution of micropores is small. The average PV of the micropores was determined to be very low, ∼0.0011

Figures 6a2 and 6b2 and Figures 6a3 and 6b3 are overmature (Ro = 2.6%) and highly overmature (Ro = 3.3%). The figures show that the shale micropore volume and SSA remain almost constant with the increase in maturity; only for one sample (sample O10) with a high TOC content do the micropore volume and SSA increase appreciably, indicating that TOC content has a greater effect on the development of micropores than does maturity. The maturity levels of the samples in Figures 6a2 and 6b2 are nearly the same; the maturity levels of the samples in Figures 6a3 and 6b3 are also very similar, but the TOC content varies widely. The figures show that, with the increase in TOC content, the micropore volume and specific surface area increase appreciably, indicating that maturity has little effect on the development of micropore volume and specific surface area, while the TOC content has a great effect on the shale micropore volume and SSA.

5. DISCUSSIONS 5.1. Whole-Aperture Pore Structure Characteristics. Because the shale PSD range is so wide, ranging from several nanometers to dozens of micrometers, it is difficult to use a H

DOI: 10.1021/acs.energyfuels.7b03807 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 7. Whole-aperture distribution of Upper Paleozoic shale PV in Ordos Basin.

the OMPs also affected the pore structure; however, the effect depends on the TOC content and types of organic matter. Therefore, the correlation between shale pores of different size scales and the maturity is highly complex. Figure 8 shows that the PSSA of the Upper Paleozoic transitional shale in the Ordos Basin is mainly determined by shale mesopores, followed by shale micropores, while macropores have a negligible contribution. The micropore size range that contributes the most to the PSSA is 0.5−0.9 nm, whereas the mesopore size range that contributes the most is 3−5 nm; certain samples with a pore size range of 20−30 nm also

mL/g, accounting for 4.37% of the total PV (Table 2). These findings indicate that the shale PV is mainly determined by shale macropores and mesopores, while the contribution of micropores is very low. Compared with marine shale, it can be speculated that In-OMPs rather than OMPs play a dominant role in the Upper Paleozoic transitional shale of the Ordos Basin. The figures show that, with the increase in maturity, the micropore volume increases appreciably in the overmature stage. The mesopore volume increases appreciably, and the macropore volume also increases slowly, because thermal maturity mainly affects the development of OMPs. Therefore, I

DOI: 10.1021/acs.energyfuels.7b03807 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 8. Whole-aperture distribution of Upper Paleozoic shale PSSA in Ordos Basin.

This percentage can practically be ignored, which demonstrates that the SSA is mainly provided by mesopores, followed by micropores. Figure 8 shows that, with the increase in maturity, the PSSA of micropores increases significantly in the overmature stage, but in the low maturity and high maturity stage, the PSSA of micropores decreases, perhaps due to the filling of OMPs with plastic asphalt. The PSSA of the shale mesopores shows no consistent changes, and the PSSA of the macropores shows no change with the increase in maturity. 5.2. Effect on Methane Adsorption Capacity. The CH4 isothermal adsorption experiment can determine the maximum

showed a certain contribution. The contribution of macropores is too small, almost negligible. The contributions of shale micropores, mesopores, and macropores to the specific surface area are listed in Table 2. Micropores provide an average specific surface area of 3.3353 m2/g, accounting for ∼31.2% of the total specific surface area, and mesopores provide an average specific surface area of 6.1957 m2/g, accounting for 68.66% of the total specific surface area; overall, the two account for more than 99% of the total specific surface area. Macropores provide an average specific surface area of 0.0116 m2/g, accounting for ∼0.14% of the total specific surface area. J

DOI: 10.1021/acs.energyfuels.7b03807 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 9. CH4 isothermal adsorption curve and relationship between maximum methane adsorption capacity and TOC, PSSA, PV, clay mineral content, brittle mineral content of Upper Paleozoic shale in the Ordos Basin.

large number of nanopores with thermal evolution. Moreover, OMPs have a greater internal PSSA than other types of pores; therefore, the adsorption capacity of shale is affected greatly by the TOC content.37 As is known, shale gas is stored in shale pores mainly in an adsorption state and a free state; only a small amount exists in a dissolved state. Free gas is mainly stored in shale pores or fractures; thus, its content is mainly controlled by the pore volume and gas saturation. Adsorption gas is mainly adsorbed on the surface of clay minerals and organic matter, and its content is closely related to the SSA of shale.10,38 Figure 9c shows that there is a high positive correlation between the shale specific surface area of micropores and the maximum methane adsorption capacity. The correlation coefficient is 0.87, which indicates that the maximum methane adsorption capacity increases with the increase in the specific surface area of micropores. Figure 9d shows that both the micropore volume and macropore volume have a good correlation with the maximum methane adsorption capacity. The results indicate

methane adsorption capacity of shale under specific temperature conditions, which can reflect the maximum adsorption capacity of shale.10 Figure 9a shows the CH4 isothermal adsorption curves of different shale samples from the Upper Paleozoic in the Ordos Basin. The maximum CH4 adsorption capacity of the shale in the area ranges from 0.84 m3/t to 8.92 m3/t at a test pressure of 12 MPa and temperature of 80 °C. The figure shows that the maximum adsorption capacities of shale samples with different organic matter contents are quite different. Overall, with the increase in organic matter content, the CH4 adsorption capacity of the shale increases correspondingly, which reflects an increase in shale adsorption capacity. At present, scholars16−19,33,36,37 generally believe that organic matter is one of the most important factors controlling the adsorption capacity of shale. Figure 9b shows that the shale TOC has a good positive correlation with the maximum methane adsorption capacity; the correlation coefficient is 0.87, consistent with previous studies. The reason may be that organic matter not only is the source rock but also provides a K

DOI: 10.1021/acs.energyfuels.7b03807 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels that micropores are one of the main factors controlling the adsorbed gas content of the Upper Paleozoic transitional shale in the Ordos Basin, which is consistent with the research of scholars at home and abroad on the adsorption capacity of shales of different regions. Chalmers and Bustin39 found that the isothermal adsorption of CH4 in Cretaceous shale increases with micropore volume. Zhong et al.40 studied the adsorption capacity of continental shale in the Yanchang Formation of the Ordos Basin through shale thermal evolution and methane adsorption experiments. A linear equation correlating micropores, other factors, and the adsorption capacity was established. It was found that the coefficient relating micropores to the adsorption capacity was much larger than other factors, which indicates that the effect of micropores on the methane adsorption capacity is the most important. Lin et al.37 conducted many studies on the shale in the Longmaxi Formation, Sichuan Basin, and confirmed that the shale adsorption capacity is mainly controlled by micropores. Many researchers believe that rock mineral composition also has a certain effect on the shale adsorption capacity. Figures 9e and 9f respectively illustrate the correlations of the maximum methane adsorption capacity with the clay mineral composition and brittle mineral composition. It can be observed that, after eliminating the red triangular point, which is an outlier (the organic matter content is very high at that point, which leads to a large amount of methane adsorption), the methane adsorption capacity increases with the clay mineral content (Figure 9e), while it decreases with an increase in the brittle mineral content (Figure 9f). This difference is mainly caused by differences in mineral composition and adsorption capacity. Clay minerals have a large mineral specific surface area and strong adsorption capacity, while quartz and other brittle minerals have a relatively small adsorption surface area; thus, they possess a lower adsorption capacity.41 Zhang et al.42 examined the methane adsorption capacity of shale in the Sichuan Basin and found that organic matter’s capacity for adsorbing gas was 11.3 times that of clay minerals; brittle minerals such as carbonate rocks and quartz have a negative effect on shale gas production.

68.6%, respectively, of the total specific surface area. The contribution of macropores is almost negligible. (3) The pore structure of the transitional shale is irregular, mainly featuring parallel slit pores and some conical plate pores, ink bottle pores, and composite pores. Different pore structures correspond to different ranges within the PSD. Parallel slit pores correspond to the PSD range of 10−50 nm, while ink bottle pores correspond to the PSD range of 3−5 nm. Composite pores correspond to a PSD range between the two. (4) The methane adsorption capacity of the transitional shale is mainly controlled by the specific surface area of micropores, and mesopores also affect the adsorption capacity; the effect of macropores, however, is almost negligible. Organic matter content and clay mineral composition also contribute significantly to the methane adsorption capacity of the shale, while brittle mineral content is negatively correlated with the shale methane adsorption capacity.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shaobin Guo: 0000-0003-3391-8806 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is a part of the research of Study on Formation Mechanism and Enrichment Regularity of Different Types of Shale Gas, which is financially supported by the National Science and Technology Major Project of the Ministry of Science and Technology of China (No. 2016ZX05034-001).

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REFERENCES

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6. CONCLUSIONS The whole-aperture pore structure of transitional shale was characterized by FESEM, MIP, and low-pressure N2/CO2 adsorption techniques, and the effects of this structure on methane adsorption capacity was investigated based on a series of geochemical and petrological measurements. Our main conclusions are summarized as follows: (1) Inorganic mineral pores dominate the transitional shale pore system; clay mineral interlayer pores are the most common type; and the OMPs in the transitional shale are much larger than those in marine shale. The size of OMPs in marine shale ranges from a few nanometers to tens of nanometers, while that of transitional OMPs ranges from tens of nanometers to several hundred nanometers. (2) The entire PSD of transitional shale in the Ordos Basin shows multiple kurtosis values; the main distribution ranges are 0.45−0.65 nm, 0.75−0.95 nm, 10−40 nm, 10−40 μm, 2−50 nm (mesopores), and >50 nm (macropores). The latter two contribute most to the PV of the shale, accounting for 42.9% and 52.7%, respectively; micropores (