Greenhouse gases: geological CO2 storage, and methane in aquifers overlying gas reservoirs

Abstract

Geological storage of CO₂ involves injecting CO₂ streams underground into suitable reservoirs and has been proposed as part of the energy transition to lower emissions.  CO₂ geological storage is currently undergoing feasibility in the Surat Basin, QLD.  The Precipice Sandstone is the targeted reservoir, with the Evergreen Formation the sealing cap-rock.  The Precipice Sandstone is quartz-rich while the Evergreen Formation is more mineralogically diverse with higher feldspar, clay and carbonate cement content.  During CO₂ storage, injected CO₂ will dissolve into formation water and the resulting acidification can induce rock reactions. These reactions have the potential for dissolution of minerals, alteration of porosity, or mobilisation of metals associated with the mineral phases.  However, subsequent mineral trapping of CO₂ and reactions have the potential to re-sequester metals. Rock drill core was characterised including mineralogy, rock properties, total metals, and trace metal content in specific minerals determined by synchrotron X-ray fluorescence microscopy.  This presentation will discuss experiments performed at reservoir pressure and temperature conditions to react pure and impure CO₂ with formation water and the rock cores, and modelling to understand potential impacts of CO₂ storage. 

The potential for connectivity between water supply aquifers and gas reservoirs raises community, government, and scientific concerns.  Methane can occur naturally, making it difficult to determine whether water bore methane levels are being influenced by nearby gas operations.  This poses a challenge in QLD, where coal seam gas production operates alongside groundwater using industries (feedlots, agriculture, mines).  Water and gas samples were taken from aquifer and Condamine Alluvium water bores, and coal seam gas wells, for stable isotopes of gases, groundwater, and dissolved inorganic carbon, as well as strontium isotopes. Most of the sampled water bores had isotopic signatures distinct from coal seam gas wells, though a minority could not be distinguished from CSG wells.  In those few cases, neither connectivity or dis-connectivity could be confirmed.  Cosmogenic isotopes characterised at ANSTO, including R³⁶Cl and ¹⁴C assisted interpretation. The use of multiple tracers has shown that gas and groundwater in some aquifers can be differentiated from the coal seam gas reservoir and hence are useful tools in identifying where groundwater connectivity occurs.