Changes in reservoir properties from injection of supercritical CO2 into coal seams — A laboratory study

Two Australian Permian coals of similar rank but different texture, one predominantly dull and the other predominantly bright, were investigated in laboratory experiments to assess changes in reservoir properties following exposure to supercritical CO2 (SCCO2) and water. Both powdered coal (0.180–0.220 mm) and small cubes (15 mm to a side) were tested in a high-pressure (HP) batch reactor for up to 120 h. Two reaction fluids were used, each on separate sister coal samples: de-ionised water (DH2O) only, and a SCCO2 and DH2O mixture. Properties were measured before and after treatment with: high-pressure CO2 adsorption isotherms (storage capacity); helium pycnometry, mercury porosimetry and low-pressure (LP) CO2 adsorption (density, porosity and PSD effects); leachate chemical analysis for dissolved mineral matter; and water and CO2 permeabilities at in situ conditions during core flood experiments on an 80 mm cube.Micro and meso porosities for both coals showed significant increases after reactions with the SCCO2 and DH2O mixture. The macroporosity decreased significantly for the dull coal, but increased marginally for the bright coal. Total accessible porosity for dull coal showed virtually no change (0.5%); the bright coal exhibited 3.4% increase from a pre-treatment total porosity of 11.0%. On powdered samples reacted with the mixture, 80%+ increases in internal surface areas, measured using LP CO2 sorption at 0 °C, were noted for both coals. The HP CO2 excess adsorption isotherms on both coals increased after treatment, varying with pressure level, coal type and coal texture. The core flood tests indicated the permeability to CO2, after a waterflood stage, increased significantly. The second stage waterflood exhibited an over 600% increase on the pre-CO2 first stage waterflood permeability. These combined results indicate that mineral matter in Permian coals is dissolved and mobilized by the carbonic acid formed during CO2 dissolution in water, leading to increased porosity, permeability and HP CO2 excess adsorption.