Characterisation of Surat Basin Walloon interburden and overlying Springbok Sandstone: a focus on methane adsorption isotherms, permeability and gas content

The Surat Basin in Queensland is one of the world’s premier producers of natural gas from coal seams. We investigate the potential for clay-rich Walloon Coal interburden and the overlying Springbok Sandstone to hold or produce gas. Seventeen core samples were analysed from two wells from intervals within the Walloon Upper Juandah Coal Measures interburden and the Springbok Sandstone. Samples were characterised using high-pressure methane adsorption isotherms, canister gas desorption tests, moisture contents, ash contents, carbon contents, scanning electron microscopy/energy dispersive X-ray spectrometry, X-ray quantitative clay analysis, permeability, helium pycnometry and mercury intrusion porosimetry. Methane adsorption was conducted at 30°C with up to 8 MPa pressure on dried crushed samples. The adsorption capacity of methane at 8 MPa varied from 3 cc/g (calcite-cemented) up to 25 cc/g (standard temperature and pressure equivalent) (coal). Clay-rich interburden samples adsorbed ~5–14 cc/g (dry). The measured isotherms and methane content from canister desorption tests show that appreciable volumes of gas are contained within some portions of interburden and in the overlying Springbok Sandstone. Gas within the interburden likely represents a large volumetric resource, albeit in low permeability rock that restricts direct productivity. The gas adsorption and gas content results for the Springbok Sandstone help to explain field observations of high gas content in some landholder water wells