Permeability of sub-critical carbon dioxide in naturally fractured Indian bituminous coal at a range of down-hole stress conditions

• Experimental investigation of gaseous CO2 permeability in naturally fractured coal
• Role of confining stresses, effective stresses, and pore pressure on gas permeability
• 5–6 h of flow-related-sorption for maximum closure of the natural fractures
• Role of coal matrix deformation dominates over effective stresses in ‘fractured’ coal.
• An overall exponential decline of permeability with effective stresses was observed.

Understanding reservoir dynamics from a geotechnical and petrophysical perspective is essential for safe and steady fluid extraction, transport or injection in the reservoir. The recent approach on storage of anthropogenic greenhouse gases (GHG) for mitigation of climate change in the deep geologic reservoirs has led to fast-paced research on resolving storage related issues. A coal seam is one such target reservoir where CO2 may be injected for storage; this may be coupled with the enhanced extraction of coalbed methane (CBM). It is known that the interaction of CO2 with coal induces hydro-mechanical changes; however, detailed investigation is mandatory for site-specific operation; the lack of such studies on Indian coal is the motivation for this research. A naturally fractured coal specimen was chosen and the subcritical CO2 flow was maintained by monitoring the injection pressures and ambient temperature; the variations in permeability under various scenarios were also investigated. The results indicate that initial CO2 injection caused coal matrix swelling which took nearly 5–6 h of flow-related-sorption for maximum closure of the natural fractures. Linear variation in flow rate with the pressure gradient across the sample indicated laminar or Darcian flow of CO2 through the coal fractures. Deeper coal seams have high confinements; the CO2 permeability results for such in-situ conditions indicated an exponential decline in naturally fractured coal, due to increasing confining pressures. The observed reduction of CO2 permeability with increase in gas inlet pressure is debated and may be attributed fully to matrix swelling or partly to the Klinkenberg effect. This study is vital for successful GHG sequestration in coal seams and for better understanding of the gas flow behaviour in the seams.