A preliminary study of oxidant stimulation for enhancing coal seam permeability: Effects of sodium hypochlorite oxidation on subbituminous and bituminous Australian coals

Chemical oxidation is proposed as an effective means to react and dissolve small regions of coal in the near wellbore region, thereby raising permeability for gas flow. In this study, we investigated the effect of sodium hypochlorite (NaClO) treatment on the structure of bituminous coal (Coal B) and subbituminous coal (Coal S) separately from the Bowen and Surat basins in Queensland, Australia. Swelling and leaching tests showed that both coals swelled, dissolved and broke in 5%wt. aqueous solutions of NaClO. Coal S reacted more vigorously in 5% NaClO, with 49% mass loss and 3840 mg/L of dissolved organic carbon (DOC) measured in the oxidant filtrate, than Coal B. The Coal B mass loss in 5% NaClO was 4.5% with 430 mg/L DOC measured in the filtrate. After NaClO treatment the total accessible pore volume of Coal S particles increased from 4.6% to 6.1%, and the porosity of Coal B increased from 8.6% to 8.9%. Pore size distributions determined from mercury intrusion porosimetry (MIP) indicated that oxidation enlarged the pores in Coal S more significantly than Coal B. Scanning electron microscopy (SEM) confirmed oxygen generated large pores on the surface of Coal S particles, but there were no significant changes on Coal B. We used a microfluidic cleat flow cell (CFC) to inject NaClO into artificial channels scribed in polished samples of Coal S and Coal B, and measured an increase in the widths of the channels after NaClO treatment. The increase in channel width observed in the CFC indicated that coal solubilisation was a more dominant mechanism than coal swelling. Similarly, the channel aperture of Coal S increased more than Coal B. CFC results also showed that NaClO etched dull coal bands (inertinite-rich) more significantly than bright coal bands (vitrinite-rich), and we proposed this result was due to the greater porosity in semi-fusinite, which allowed greater penetration of NaClO in dull coal bands than in bright coal bands. The low coal rank sample (Coal S) with higher liptinite content and more oxygen content was more susceptible to oxidisation by NaClO than Coal B.