Comparative experimental study on porosity, mechanical and CO₂ adsorption characteristics of coal and shale

Abstract

To compare the pore structure, mechanical and CO₂ adsorption properties of coal and shale, a series of experiments were carried out using nuclear magnetic resonance (NMR), uniaxial compression, Brazilian splitting, and high-pressure CO₂ adsorption. The results show that the total porosity of coal is 7.51 times that of shale, and shale is dominated by adsorption pores, while adsorption pores and seepage pores in coal are equally important. Moreover, it is found that the micropores in shale are more advantageous, while meso-macropore in coal are more significant. The adsorption pore surface of coal is rougher than that of shale, and the seepage pore structure of shale is more complex. The uniaxial compressive strength, elastic modulus and absorption energy of shale are 2.01 times, 2.85 times, and 1.27 times that of coal, respectively, indicating that shale has higher compressive capacity and resistance to elastic deformation than coal. The average tensile strength, Brazilian splitting modulus, absorbed energy and brittleness index of shale are 7.92 times, 6.68 times, 10.78 times, and 4.37 times that of coal, respectively, indicating that shale has higher tensile strength and brittleness, but lower ductility, compared with coal. The performed analyses show that under the same conditions, the CO₂ adsorption capacity of coal is greater than that of shale. The present article can provide a theoretical basis to implement CO₂-enhanced coalbed methane (CBM)/shale gas extraction.

Graphical abstract

Keywords

coal / shale / NMR / mechanical properties / adsorption characteristic

References

[1] Aybar U, Eshkalak M O, Sepehrnoori K, Patzek T W (2014). The effect of natural fracture’s closure on long-term gas production from unconventional resources.J Nat Gas Sci Eng, 21: 1205–1213

[2] Cao T T, Song Z G, Luo H Y, Liu G X (2015). The differences of microscopic pore structure characteristics of coal, oil shale and shales and their storage mechanisms.Nat Gas Geosci, 26(11): 2208–2218

[3] Chandra D, Vishal V, Bahadur J, Sen D (2020). A novel approach to identify accessible and inaccessible pores in gas shales using combined low-pressure sorption and SAXS/SANS analysis.Int J Coal Geol, 228: 103556

[4] Chen S, Zhu Y, Wang H, Liu H, Wei W, Luo Y, Li W, Fang J (2010). Research status and trends of shale gas in China.Acta Petrol Sin, 31(4): 689–694

[5] Chen S, Tang D, Tao S, Ji X, Xu H. (2019). Fractal analysis of the dynamic variation in pore-fracture systems under the action of stress using a low-field NMR relaxation method: an experimental study of coals from western Guizhou in China.J Petrol Sci Eng, 173: 617–629

[6] Feng G, Kang Y, Sun Z, Wang X, Hu Y (2019). Effects of supercritical CO₂ adsorption on the mechanical characteristics and failure mechanisms of shale.Energy, 173: 870–882

[7] Gao S, Jia L, Zhou Q, Cheng H, Wang Y (2022). Microscopic pore structure changes in coal induced by a CO₂–H₂O reaction system.J Petrol Sci Eng, 208: 109361

[8] Geng W, Huang G, Guo S, Jiang C, Dong Z, Wang W (2022). Influence of long-term CH₄ and CO₂ treatment on the pore structure and mechanical strength characteristics of Baijiao Coal.Energy, 242: 122986

[9] Gholami R, Rasouli V, Sarmadivaleh M, Minaeian V, Fakhari N (2016). Brittleness of gas shale reservoirs: a case study from the north Perth Basin, Australia.J Nat Gas Sci Eng, 33: 1244–1259

[10] Hou P, Xue Y, Gao F, Dou F, Su S, Cai C, Zhu C (2022). Effect of liquid nitrogen cooling on mechanical characteristics and fracture morphology of layer coal under Brazilian splitting test.Int J Rock Mech Min Sci, 151: 105026

[11] Jin K, Cheng Y, Liu Q, Zhao W, Wang L, Wang F, Wu D (2016). Experimental investigation of pore structure damage in pulverized coal: implications for methane adsorption and diffusion characteristics.Energy Fuels, 30(12): 10383–10395

[12] Joubert J I, Grein C T, Bienstock D (1973). Sorption of methane in moist coal.Fuel, 52(3): 181–185

[13] Langmuir I (1918). The adsorption of gases on plane surfaces of glass, mica and platinum.J Am Chem Soc, 40(9): 1361–1403

[14] Liu X, Zhang C, Nie B, Zhang C, Song D, Yang T, Ma Z (2022a). Mechanical response and mineral dissolution of anthracite induced by supercritical CO₂ saturation: influence of saturation time.Fuel, 319: 123759

[15] Liu Z, Lin X, Cheng Y, Chen R, Zhao L, Wang L, Li W, Wang Z (2022b). Experimental investigation on the diffusion property of different form coal: implication for the selection of CO₂ storage reservoir.Fuel, 318: 123691

[16] Liu Z, Lin X, Wang Z, Zhang Z, Chen R, Wang L, Li W (2022c). Modeling and experimental study on methane diffusivity in coal mass under in-situ high stress conditions: a better understanding of gas extraction.Fuel, 321: 124078

[17] Satya , Harpalani , Basanta , K. , Prusty , Pratik , (2006). Methane/CO₂ sorption modeling for coalbed methane production and CO₂ sequestration.Energy Fuels, 20(4): 1591–1599

[18] Shao X, Pang X, Li H, Zhang X (2017). Fractal analysis of pore network in tight gas sandstones using NMR Method: a case study from the Ordos Basin, China.Energy Fuels, 31(10): 10358–10368

[19] Sing K (1982). Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Provisional).Pure Appl Chem, 54(11): 2201–2218

[20] Song Y, Zou Q, Su E, Zhang Y, Sun Y (2020). Changes in the microstructure of low-rank coal after supercritical CO₂ and water treatment.Fuel, 279: 118493

[21] Su E, Liang Y, Chang X, Zou Q, Xu M, Sasmito AP (2020). Effects of cyclic saturation of supercritical CO₂ on the pore structures and mechanical properties of bituminous coal: an experimental study.J CO₂ Util, 40: 101208

[22] Wang G, Qin X, Shen J, Zhang Z, Han D, Jiang C (2019). Quantitative analysis of microscopic structure and gas seepage characteristics of low-rank coal based on CT three-dimensional reconstruction of CT images and fractal theory.Fuel, 256: 115900

[23] Wang H M, Zhu Y M, Li W, Zhang J S, Luo Y (2011). Two major geological control factors of occurrence characteristics of CBM. J China Coal Soc, 36(7): 1129–1134 (in Chinese)

[24] Wang X, Cheng Y, Zhang D, Liu Z, Wang Z, Jiang Z (2021a). Influence of tectonic evolution on pore structure and fractal characteristics of coal by low pressure gas adsorption.J Nat Gas Sci Eng, 87: 103788

[25] Wang X, Cheng Y, Zhang D, Yang H, Zhou X, Jiang Z (2021b). Experimental study on methane adsorption and time-dependent dynamic diffusion coefficient of intact and tectonic coals: implications for CO₂-enhanced coalbed methane projects.Process Saf Environ Prot, 156: 568–580

[26] Wang X, Geng J, Zhang D, Xiao W, Chen Y, Zhang H (2022a). Influence of sub-supercritical CO₂ on pore structure and fractal characteristics of anthracite: an experimental study.Energy, 261: 125115

[27] Wang X, Liu H, Zhang D, Yuan X, Zeng P, Zhang H (2022b). Effects of CO₂ adsorption on molecular structure characteristics of coal: implications for CO₂ geological sequestration.Fuel, 321: 124155

[28] Wang X, Zhang D, Liu H, Jin Z, Yue T, Zhang H (2022c). Investigation on the influences of CO₂ adsorption on the mechanical properties of anthracite by Brazilian splitting test.Energy, 259: 125053

[29] Wang X, Zhang D, Su E, Jiang Z, Wang C, Chu Y, Ye C (2020). Pore structure and diffusion characteristics of intact and tectonic coals: implications for selection of CO₂ geological sequestration site.J Nat Gas Sci Eng, 81: 103388

[30] Wang Z, Fu X, Hao M, Li G, Pan J, Niu Q, Zhou H (2021c). Experimental insights into the adsorption-desorption of CH₄/N₂ and induced strain for medium-rank coals.J Petrol Sci Eng, 204: 108705

[31] Wang Z, Pan J, Hou Q, Yu B, Li M, Niu Q (2018a). Anisotropic characteristics of low-rank coal fractures in the Fukang mining area, China.Fuel, 211: 182–193

[32] Wang Z, Pan J, Hou Q, Niu Q, Tian J, Wang H, Fu X (2018b). Changes in the anisotropic permeability of low-rank coal under varying effective stress in Fukang mining area, China.Fuel, 234: 1481–1497

[33] Wu T, Du X, Li Q (2020). Comparative analysis of pore structure and adsorption characteristics of coal and shale.Safe Coal Mines, 51(11): 6

[34] Yang K, Zhou J, Xian X, Jiang Y, Zhang C, Lu Z, Yin H (2022a). Gas adsorption characteristics changes in shale after supercritical CO₂-water exposure at different pressures and temperatures.Fuel, 310: 122260

[35] Yang K, Zhou J, Xian X, Zhou L, Zhang C, Tian S, Lu Z, Zhang F (2022b). Chemical-mechanical coupling effects on the permeability of shale subjected to supercritical CO₂-water exposure.Energy, 248: 123591

[36] Yang Q, Li W, Jin K (2020). Supercritical CO₂ interaction induced pore morphology alterations of various ranked coals: a comparative analysis using corrected mercury intrusion porosimetry and low-pressure N₂ gas adsorption.ACS Omega, 5(16): 9276–9290

[37] Yu Y, Wang T (2004). Study on relationship between splitting behaviour and elastic modulus of Three Gorges granite.Chinese J Rock Mechan Eng, 23(19): 3258–3261

[38] Zhang K, Hou CH, Zhao DF, Guo YH, Hui XU (2017). Comparison of pore structure characteristics fractal characteristics between coal and shale through nitrogen adsorption experiment with the example of Shanxi Formation 15# Coal and shale in Yangquan Area.Sci Techn Eng, 29: 68–75

[39] Zhang X G, Ranjith P G, Ranathunga A S, Li D Y (2019). Variation of mechanical properties of bituminous coal under CO₂ and H₂O saturation.J Nat Gas Sci Eng, 61: 158–168

[40] Zheng J, Huang G, Cheng Q, Zhen L, Cai Y, Wang W (2022). Degradation of mechanical and microporous properties of coal subjected to long-term sorption.Fuel, 315: 123245

[41] Zhou J, Xie S, Jiang Y, Xian X, Liu Q, Lu Z, Lyu Q (2018). Influence of supercritical CO₂ exposure on CH₄ and CO₂ adsorption behaviors of shale: implications for CO₂ sequestration.Energy Fuels, 32(5): 6073–6089

RIGHTS & PERMISSIONS

Higher Education Press