Pore structure complexity and its significance to the petrophysical properties of coal measure gas reservoirs in Qinshui Basin, China

Abstract

The pore structure of continuous unconventional reservoirs (CURs) in coal measures was investigated using different technologies for 29 samples (9 coal samples, 9 shale samples, and 11 sandstone samples) from Qinshui Basin, China. Results show that coals have relatively high porosities and permeabilities ranging from 4.02% to 5.19% and 0.001 to 0.042 mD, respectively. Micropores (<2 nm) are well-developed in coals and contribute to the majority of pore volume (PV) and specific surface area (SSA). The porosities and permeabilities are between 1.19%–4.11%, and 0.0001–0.004 mD of sandstones with a predominance of macropores (>50 nm). However, shales are characterized by poorly petrophysical properties with low porosity and permeability. Macropores and mesopores (2–50 nm) are well-developed in shales compared with micropores. For coals, abundant organic matters are expected to promote the development of micropores, and clay minerals significantly control the performance of mesopores. For shales and sandstones, micropores are mainly observed in organic matters, whereas clay minerals are the important contributor to mesopores. Moreover, micropore SSA significantly determines the adsorption capacity of CURs and sandstones have the best pore connectivity. The permeability of CURs is positively associated with the macropore PV since macropores serve as the main flow paths for gas seepage. Additionally, we also proposed that effective porosity has a significant effect on the permeability of CURs. The findings of this study could enhance the understanding of the multiscale pore structure of CURs and provide insights into the mechanisms that control gas storage, transport, and subsequent co-production for continuous unconventional natural gas (CUNG) in the Qinshui Basin and other coal-bearing basins worldwide.

Graphical abstract

Keywords

continuous unconventional reservoirs (CURs) / pore structure / adsorption capacity / permeability / effective porosity

References

[1] Ayers W B (2002). Coalbed gas systems, resources, and production and a review of contrasting cases from the San Juan and Powder River basins. AAPG Bull, 86: 1853–1890

[2] Bernard S, Horsfield B, Schulz H, Wirth R, Schreiber A, Sherwood N (2012). Geochemical evolution of organic-rich shales with increasing maturity: a STXM and TEM study of the Posidonia Shale (Lower Toarcian, northern Germany). Mar Pet Geol, 31(1): 70–89

[3] Chen S, Zhu Y, Li W, Wang H (2012). Influence of magma intrusion on gas outburst in a low rank coal mine. Int J Min Sci Technol, 22(2): 259–266

[4] Daigle H, Thomas B, Rowe H, Nieto M (2014). Nuclear magnetic resonance characterization of shallow marine sediments from the Nankai Trough, Integrated Ocean Drilling Program Expedition 333. J Geophys Res Solid Earth, 119(4): 2631–2650

[5] Gao H, Li H A (2016). Pore structure characterization, permeability evaluation and enhanced gas recovery techniques of tight gas sandstones. J Nat Gas Sci Eng, 28: 536–547

[6] Ghomeshi S, Kryuchkov S, Kantzas A (2018). An investigation into the effects of pore connectivity on T₂ NMR relaxation. J Magn Reson, 289: 79–91

[7] Hou X, Liu S, Zhu Y, Yang Y (2020a). Evaluation of gas contents for a multi-seam deep coalbed methane reservoir and their geological controls: in situ direct method versus indirect method. Fuel, 265: 116917

[8] Hou X, Liu S, Zhu Y, Yang Y (2020c). Experimental and theoretical investigation on sorption kinetics and hysteresis of nitrogen, methane, and carbon dioxide in coals. Fuel, 268: 117349

[9] Hou X, Zhu Y, Chen S, Wang Y (2017). Gas flow mechanisms under the effects of pore structures and permeability characteristics in source rocks of coal measures in Qinshui Basin, China. Energ Explor Exploit, 35(3): 338–355

[10] Hou X, Zhu Y, Chen S, Wang Y, Liu Y (2020c). Investigation on pore structure and multifractal of tight sandstone reservoirs in coal bearing strata using LF-NMR measurements. J Petrol Sci Eng, 187: 106757

[11] Hou X, Zhu Y, Wang Y, Liu Y (2019). Experimental study of the interplay between pore system and permeability using pore compressibility for high rank coal reservoirs. Fuel, 254: 115712

[12] Hou X, Zhu Y, Yao H (2018). Coupled accumulation characteristics of Carboniferous-Permian coal measure gases in the Northern Ordos Basin, China. Arab J Geosci, 11(7): 156

[13] Huang H, Sun W, Ji W, Zhang R, Du K, Zhang S, Ren D, Wang Y, Chen L, Zhang X (2018). Effects of pore-throat structure on gas permeability in the tight sandstone reservoirs of the Upper Triassic Yanchang formation in the Western Ordos Basin, China. J Petrol Sci Eng, 162: 602–616

[14] Krooss B M, van Bergen F, Gensterblum Y, Siemons N, Pagnier H J M, David P (2002). High-pressure methane and carbon dioxide adsorption on dry and moisture-equilibrated Pennsylvanian coals. Int J Coal Geol, 51(2): 69–92

[15] Lai J, Wang G, Cao J, Xiao C, Wang S, Pang X, Dai Q, He Z, Fan X, Yang L, Qin Z (2018). Investigation of pore structure and petrophysical property in tight sandstones. Mar Pet Geol, 91: 179–189

[16] Law B E, Curtis J B (2002). Introduction to unconventional petroleum systems. AAPG Bull, 86: 1851–1852

[17] Li X, Kang Y, Haghighi M (2018). Investigation of pore size distributions of coals with different structures by nuclear magnetic resonance (NMR) and mercury intrusion porosimetry (MIP). Measurement, 116: 122–128

[18] Li Y, Cheng Z, Tang D, Gan Q, Niu X, Wang K, Shen R (2017). Coal pore size distributions controlled by the coalification process: An experimental study of coals from the Junggar, Ordos and Qinshui basins in China. Fuel, 206: 352–363

[19] Li Y, Song D, Liu S, Pan J (2020). Characterization of ultramicropores and analysis of their evolution in tectonically deformed coals by low-pressure CO₂ adsorption, XRD, and HRTEM techniques. Energy Fuels, 34(8): 9436–9449

[20] Li Y, Tang D, Wu P, Niu X, Wang K, Qiao P, Wang Z (2016). Continuous unconventional natural gas accumulations of Carboniferous-Permian coal-bearing strata in the Linxing area, northeastern Ordos basin, China. J Nat Gas Sci Eng, 36: 314–327

[21] Li Y, Wang Z, Pan Z, Niu X, Yu Y, Meng S (2019b). Pore structure and its fractal dimensions of transitional shale: a cross section from east margin of the Ordos Basin, China. Fuel, 241: 417–431

[22] Li Y, Yang J, Pan Z, Meng S, Wang K, Niu X (2019a). Unconventional natural gas accumulations in stacked deposits: a discussion of upper paleozoic coal-bearing strata in the east margin of the Ordos basin, China. Acta Geol Sin-Engl, 93(1): 111–129

[23] Liu Y, Zhu Y, Li W, Zhang C, Wang Y (2017). Ultra micropores in macromolecular structure of subbituminous coal vitrinite. Fuel, 210: 298–306

[24] Loucks R G, Reed R M, Ruppel S C, Jarvie D M (2009). Morphology, genesis, and distribution of nanometer-scale pores in siliceous mudstones of the Mississippian Barnett Shale. J Sediment Res, 79(12): 848–861

[25] Mishra D K, Samad S K, Varma A K, Mendhe V A (2018). Pore geometrical complexity and fractal facets of Permian shales and coals from Auranga Basin, Jharkhand, India. J Nat Gas Sci Eng, 52: 25–43

[26] Monaghan A A (2017). Unconventional energy resources in a crowded subsurface: reducing uncertainty and developing a separation zone concept for resource estimation and deep 3D subsurface planning using legacy mining data. Sci Total Environ, 601-602: 45–56

[27] Nie B, Liu X, Yang L, Meng J, Li X (2015). Pore structure characterization of different rank coals using gas adsorption and scanning electron microscopy. Fuel, 158: 908–917

[28] Ross D J K, Bustin R M (2008). Characterizing the shale gas resource potential of Devonian-Mississippian strata in the Western Canada sedimentary basin: application of an integrated formation evaluation. AAPG Bull, 92(1): 87–125

[29] Rouquerol J, Avnir D, Fairbridge C W, Everett D H, Haynes J M, Pernicone N, Ramsay J D F, Sing K S W, Unger K K, and the IUPAC (International Union of Pure and Applied Chemistry) (1994). Physical chemistry division commission on colloid and surface chemistry, subcommittee on characterization of porous solids: recommendations for the characterization of porous solids (technical report). Pure Appl Chem, 66(8): 1739–1758

[30] Schmitt M, Fernandes C P, Wolf F G, Da Cunha Neto J A, Rahner C P, Santiago Dos Santos V S(2015). Characterization of Brazilian tight gas sandstones relating permeability and Angstrom-to micron-scale pore structures. J Nat Gas Sci Eng, 27: 785–807

[31] Song D, Ji X, Li Y, Zhao H, Song B, He K (2020). Heterogeneous development of micropores in medium-high rank coal and its relationship with adsorption capacity. Int J Coal Geol, 226: 103497

[32] Song Y, Jiang B L J (2017). 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. Energy Fuels, 31(7): 6711–6723

[33] Sun C, Tang S, Zhang S, Wei J, Hou Y, Zhang T (2017). Nanopore characteristics of Late Paleozoic transitional facies coal-bearing shale in Ningwu Basin, China investigated by nuclear magnetic resonance and low-pressure nitrogen adsorption. J Nanosci Nanotechnol, 17(9): 6433–6444

[34] Wang D, Shao L, Li Z, Li M, Lv D, Liu H (2016). Hydrocarbon generation characteristics, reserving performance and preservation conditions of continental coal measure shale gas: a case study of Mid-Jurassic shale gas in the Yan’an Formation, Ordos Basin. J Petrol Sci Eng, 145: 609–628

[35] Wang Y, Zhu Y, Chen S, Li W (2014). Characteristics of the nanoscale pore structure in northwestern Hunan Shale Gas Reservoirs using field emission scanning electron microscopy, high-pressure mercury intrusion, and gas adsorption. Energy Fuels, 28(2): 945–955

[36] Wang Y, Zhu Y, Zhang R, Anovitz L, Bleuel M, Liu S, Chen S (2020). SANS coupled with fluid invasion approaches for characterization of overall nanopore structure and mesopore connectivity of organic-rich marine shales in China. Int J Coal Geol, 217: 103343

[37] Xiao D, Jiang S, Thul D, Lu S, Zhang L, Li B (2018). Impacts of clay on pore structure, storage and percolation of tight sandstones from the Songliao Basin, China: implications for genetic classification of tight sandstone reservoirs. Fuel, 211: 390–404

[38] Xiong F, Jiang Z, Li P, Wang X, Bi H, Li Y, Wang Z, Amooie M A, Soltanian M R, Moortgat J (2017). Pore structure of transitional shales in the Ordos Basin, NW China: effects of composition on gas storage capacity. Fuel, 206: 504–515

[39] Yang Y, Liu S (2020). Laboratory study of cryogenic treatment induced pore-scale structural alterations of Illinois coal and their implications on gas sorption and diffusion behaviors. J Petrol Sci Eng, 194: 107507

[40] Yao Y, Liu D (2012). Comparison of low-field NMR and mercury intrusion porosimetry in characterizing pore size distributions of coals. Fuel, 95: 152–158

[41] Zhang R, Ning Z, Yang F, Zhao H, Wang Q (2016). A laboratory study of the porosity-permeability relationships of shale and sandstone under effective stress. Int J Rock Mech Min, 81: 19–27

[42] Zhao J, Xu H, Tang D, Mathews J P, Li S, Tao S (2016). Coal seam porosity and fracture heterogeneity of macrolithotypes in the Hancheng Block, eastern margin, Ordos Basin, China. Int J Coal Geol, 159: 18–29

RIGHTS & PERMISSIONS

Higher Education Press