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The high heterogeneity of shale is a deterrent in the accurate assessment of reservoir characteristics, making gas exploitation a challenging task. Besides being a nearly impermeable permeable reservoir type, shale has a complex pore network exhibited by various pore types and pore-size distributions that influence gas storage and transport in the shale matrix [1,2]. Gas in shale is stored as (1) free gas in pores and fractures, (2) adsorbed gas inside organic matter and clay pores; and (3) dissolved gas in water [3,4]. Micropores, present in organic matter and clay minerals, primarily adsorb gas owing to their large specific surface area and more significant adsorption potential in small pores [3,4]. Temperature plays a vital role in modifying shale pore characteristics. Thermally mature shales have larger micropore volume and higher gas sorption capacity than immature shales [5]. With increasing temperature, the thermal cracking of kerogen takes place, leading to a restructuring of organic pores [6]. Although studies exploring the pore structure changes in oil shales through inert heating are plenty, such research rarely exists in the field of gas shale. Due to the probable harmful environmental effect of hydraulic fracturing, more research is being done worldwide to develop a greener solution to improve shale gas resource exploitation [7]. New studies suggest combustion as an alternative technique for porosity enhancement as the process eliminates kerogen and organic matter resulting in higher gas recovery from shale [8].
So far, studies on Indian shales have been analyzing overall pore characteristics and their relation to the organic matter properties and gas adsorption capacities [4,5,9,10]. However, very few works detail the influence of increasing temperature on Indian shale pore, when the thermal treatment takes place in oxygen. Hence, the prime focus of this study is to assess the dynamics of organic pore development in shale when it is combusted in the presence of oxygen. The pore attributes were estimated using low-pressure N2 and CO2 adsorption (LPGA) at each temperature interval, further supported by high-resolution microscopy. The temperature of 300°C is taken as a maximum as a further rise in the temperatures might alter clay, and kerogen's fundamental characteristics in shales [11].
The aliphatic hydrocarbon stretching bands of kerogens, identified from FTIR study, at 2920 cm−1 and 2850 cm−1 display a reduction of almost 13% in the aliphaticity from 100℃ to 300℃. SEM images also confirm the growing connectivity of organic micro-pores with gradual temperature rise. LPGA analysis confirms the appearance of newer micropores with rising temperature and formation of larger pores at 300℃. However, when samples were heated in an inert environment, changes in pore characteristics occur differently. Oxic heating causes a noticeable increase in the surface area and pore volume (almost 70-80%). Along with that, the pore size distribution peaks become sharper and of higher magnitude. But when the samples were heated in the absence of oxygen, pore-blocking occurs due to the generation and solidification of bitumen, resulting in a reduction of surface area and pore volume almost up to 30%.
References
[1] J. Bai, Y. Kang, M. Chen, L. Liang, L. You, X. Li, Investigation of multi-gas transport behavior in shales via a pressure pulse method, Chem. Eng. J. (2019). https://doi.org/10.1016/j.cej.2018.10.197.
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[3] T. Bakshi, B.K. Prusty, K. Pathak, S.K. Pal, Pore characteristics of Damodar valley shale and their effect on gas storage potential, J. Pet. Sci. Eng. 162 (2018) 725–735. https://doi.org/https://doi.org/10.1016/j.petrol.2017.10.091.
[4] T. Bakshi, B.K. Prusty, K. Pathak, B.R. Nayak, D. Mani, S.K. Pal, Source rock characteristics and pore characterization of Indian shale, J. Nat. Gas Sci. Eng. 45 (2017) 761–770. https://doi.org/https://doi.org/10.1016/j.jngse.2017.06.010.
[5] Q. Sun, W. Zhang, H. Qian, Effects of high-temperature thermal treatment on the physical properties of clay, Environ. Earth Sci. 75 (2016) 610. https://doi.org/10.1007/s12665-016-5402-2.
[6] T. Zhang, G.S. Ellis, S.C. Ruppel, K. Milliken, R. Yang, Effect of organic-matter type and thermal maturity on methane adsorption shale-gas systems, Org. Geochem. 47 (2012) 120–131. https://doi.org/10.1016/j.orggeochem.2012.03.012.
[7] W. Chen, Y. Lei, Y. Chen, J. Sun, Pyrolysis and Combustion Enhance Recovery of Gas for Two China Shale Rocks, Energy & Fuels. 30 (2016) 10298–10305. https://doi.org/10.1021/acs.energyfuels.6b02274.
[8] G. Chapiro, J. Bruining, Combustion enhance recovery of shale gas, J. Pet. Sci. Eng. 127 (2015) 179–189. https://doi.org/https://doi.org/10.1016/j.petrol.2015.01.036.
[9] D. Chandra, V. Vishal, A comparison of nano-scale pore attributes of Barakar Formation gas shales from Raniganj and Wardha Basin, India using low-pressure sorption and FEG-SEM analysis, J. Nat. Gas Sci. Eng. (2020) 103453. https://doi.org/10.1016/j.jngse.2020.103453.
[10] B. Hazra, D.A. Wood, V. Vishal, A.K. Singh, Pore Characteristics of Distinct Thermally Mature Shales: Influence of Particle Size on Low-Pressure CO2 and N2 Adsorption, Energy and Fuels. 32 (2018) 8175–8186. https://doi.org/10.1021/acs.energyfuels.8b01439.
[11] R. Holmes, E.C. Rupp, V. Vishal, J. Wilcox, Selection of Shale Preparation Protocol and Outgas Procedures for Applications in Low-Pressure Analysis, Energy and Fuels. 31 (2017) 9043–9051. https://doi.org/10.1021/acs.energyfuels.7b01297.
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