CO2 permeability of Indian bituminous coals: Implications for carbon sequestration

Abstract

The permeability of coal is one of the most critical parameters in estimating the performance of coal seams as methane reservoirs or carbon dioxide sinks. The reservoir behaviour of coal offers many challenges for the long-term planning of gas production performance and the drainage of water from reservoirs. Moreover, heterogeneity in coal from basin to basin, from seam to seam and within parts of the same seam makes the process more complex. Reservoir-specific understanding is needed for the best possible estimation of its behaviour. Factors such as shrinkage/swelling of coal matrix, gas flow and slippage, geomechanical changes, cleat anisotropy and effective stresses control the flow behaviour of gases in coal. Studies of these aspects have gathered momentum in recent decades for different basins across the world. However, no research is available on swelling-induced permeability characteristics or the role of effective stresses for Gondwana coals in India. Considering the fairly high CO2 storage potential of Indian coals, bituminous coals of the Barakar Formation in Jharia basin, India were tested using gas phase CO2 under triaxial conditions. This paper presents the results of changing effective stresses, in terms of confining stresses and pore pressure, incorporating the role of coal swelling with time on the flow pattern of gaseous CO2. Corresponding to different depths of target coal seams, coal was subjected to different confining stresses in a newly-developed triaxial cell. For each confinement, CO2 was injected at different injection pressures. To minimize the effects of temperature on the characteristics of the gas phase, it was kept constant at 26°C throughout, while use of the drained method of testing maintained the downstream pressure equal to one atmosphere. The results showed that at low confinement, increase in gas injection pressures leads to an initial decrease in permeability of coal, which then becomes almost constant. However, at higher confinements, the permeability increases with increasing injection pressure. Swelling in coal contributes to reduction of permeability. The volumetric expansion in coal reduces the permeability with changing gas injection pressure during the initial confinement. As the confining pressure is increased, coal permeability reduces by one to two orders of magnitude. Finally, an empirical relation accounting for the role of effective stresses and swelling on coal permeability is proposed for Indian coals. At lower effective stresses, the logarithmic decline model is better suited, while the exponential decline relationship fits the data better at effective stresses >12MPa.