Quantitative Analysis of Micro-structural Changes in a Bituminous Coal After Exposure to Supercritical CO2 and Water
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High-volatile bituminous coal samples were reacted in deionized water with supercritical CO2 (ScCO2–water) under simulated in situ pressure and temperature conditions (8 MPa and 40 °C) in unconfined stress state for 14 days, in order to characterize potential CO2–water–coal reactions. Micro-structural changes were identified pre- and post-experiment using X-ray powder diffraction (XRD) analysis for powdered coal (mineralogical changes), optical microscopy and scanning electron microscopy (SEM) for polished thin sections (surface feature changes) and micro-CT scanning for a small core (porosity and permeability changes). XRD analysis revealed that carbonic acid leaches out mineral matters in coal, including carbonates (calcite) and silicate minerals (albite, illite and kaolinite). Optical microscopy, SEM and CT images confirmed that the interaction of coal with ScCO2–water causes an abundance of micro-cracks to open or propagate in unconfined coal samples. Most micro-cracks preferably propagated along maceral–mineral and maceral–maceral interfaces, which demonstrates that the micro-cracking was caused by differential swelling of different coal lithotypes. Wormhole formation was observed in coal caused by mineral dissolution and hydrocarbon mobilization, which significantly increases coal porosity compared with swelling-induced cracking. 3-D pore network models extracted from CT images show that ScCO2–water treatment enlarges the pore and throat size, increases the numbers of pores and throats and improves pore network connectivity. Overall, CO2–water–coal interactions under unconfined conditions enhance coal porosity, connectivity and permeability, which can be attributed to the combined effect of micro-cracking, mineral dissolution and hydrocarbon mobilization.
KeywordsSupercritical CO2 Micro-CT Pore network model Connectivity
The CT scanning was undertaken on the Imaging and Medical beamline at the Australian Synchrotron, and we record our great appreciation of Dr. Anton Maksimenko and Dr. Chris Hall for their assistance in recording the CT images.
- Dong, H., Fjeldstad, S., Alberts, L., Roth, S., Bakke, S., & Øren, P.-E. (2008). Pore network modelling on carbonate: A comparative study of different micro-CT Network extraction methods. In International symposium of the society of core analysts, Society of Core Analysts, 2008.Google Scholar
- Grigg, R., & Svec, R. (2003). Co-injected CO2-brine interactions with Indiana Limestone. In SCA2003-19, presented at the Society of Core Analysts Convention SCA, 2003.Google Scholar
- Grigg, R. B., & Svec, R. (2008). Injectivity changes and CO2 retention for EOR and sequestration projects. In SPE symposium on improved oil recovery, 2008. Society of Petroleum Engineers.Google Scholar
- Hiscock, K. M. (2009). Hydrogeology: Principles and practice. Hoboken: Wiley.Google Scholar
- Izgec, O., Demiral, B., Bertin, H. J., & Akin, S. (2006). Experimental and numerical modeling of direct injection of CO2 into carbonate formations. In SPE annual technical conference and exhibition, 2006. Society of Petroleum Engineers.Google Scholar
- Massarotto, P., Golding, S. D., Bae, J. S., Iyer, R., & Rudolph, V. (2010). Changes in reservoir properties from injection of supercritical CO2 into coal seams—A laboratory study. International Journal of Coal Geology, 82(3), 269–279. https://doi.org/10.1016/j.coal.2009.11.002.CrossRefGoogle Scholar
- Ranathunga, A. S., Perera, M. S. A., Ranjith, P. G., & Bui, H. (2016b). Super-critical CO2 saturation-induced mechanical property alterations in low rank coal: An experimental study. The Journal of Supercritical Fluids, 109, 134–140. https://doi.org/10.1016/j.supflu.2015.11.010.CrossRefGoogle Scholar
- Yang, J., Lian, H., Liang, W., Nguyen, V. P., & Chen, Y. (2018). Experimental investigation of the effects of supercritical carbon dioxide on fracture toughness of bituminous coals. International Journal of Rock Mechanics and Mining Sciences, 107, 233–242. https://doi.org/10.1016/j.ijrmms.2018.04.033.CrossRefGoogle Scholar
- Zhang, G., Ranjith, P., Perera, M., Haque, A., Choi, X., & Sampath, K. (2018b). Characterization of coal porosity and permeability evolution by demineralisation using image processing techniques: A micro-computed tomography study. Journal of Natural Gas Science and Engineering, 56, 384–396.CrossRefGoogle Scholar