Subsurface microorganisms may respond to increased CO2 levels in ways that significantly affect pore fluid chemistry. Changes in CO2 concentration or speciation may result from the injection of supercritical CO2 (scCO2) into deep aquifers. Therefore, understanding subsurface microbial responses to scCO2, or unnaturally high levels of dissolved CO2, will help to evaluate the use of geosequestration to reduce atmospheric CO2 emissions. This study characterized microbial community changes at the 16S rRNA gene level during a scCO2 geosequestration experiment in the 1.4 km-deep Paaratte Formation of the Otway Basin, Australia. One hundred and fifty tons of mixed scCO2 and groundwater was pumped into the sandstone Paaratte aquifer over four days. A novel U-tube sampling system was used to obtain groundwater samples under in situ temperature and pressure conditions for geochemical analyses and DNA extraction. Decreases in pH and temperature of 2.6 log units and 5.8oC, respectively, were observed. Polyethylene glycols (PEGs) were detected in the groundwater prior to scCO2 injection and were interpreted as residual from drilling fluid used during the emplacement of the CO2 injection well. Changes in microbial community structure prior to scCO2 injection revealed a general shift from Firmicutes to Proteobacteria concurrent with the disappearance of PEGs. However, the scCO2 injection event, including changes in response to the associated variables (e.g., pH, temperature and salinity), resulted in increases in the relative abundances of Comamonadaceae and Sphingomonadaceae suggesting the potential for enhanced scCO2 tolerance of these groups. This study demonstrates a successful new in situ sampling approach for detecting microbial community changes associated with an scCO2 geosequestration event.

Coastal to shallow-marine deposits are inherently highly heterogeneous in sediment composition due to variable depositional conditions. Representation of heterogeneity at sub-seismic scales is required for understanding flow and geochemical processes and poses two challenges: Firstly, the representation of different rock types in geological models requires high resolution sampling vertically and laterally. Secondly, the heterogeneity in petrophysical, flow and mineralogical properties within each rock type needs to be sufficiently characterized in order to support (multiphase) flow and reactive transport simulations. This study addresses these challenges for the Paaratte Formation, Otway Basin (Australia), at the CO2CRC’s Otway Research Facility to enable detailed simulations of CO₂ flow and reactions. Based on the analysis of wireline logs, hyperspectral core logs and discrete samples from wells CRC-2 and CRC-3, five rock types are defined and characterized for two coastal to shallow-marine parasequences comprising four depositional facies. A combination of wireline logs is used to derive rock type logs for the wells CRC-2 and CRC-3 at 10 cm vertical resolution and allows high resolution cross-well correlation providing insights into the lateral extent of deposits. Findings of this study will inform future desktop and field studies at the CO2CRC’s Otway Research Facility requiring information on sub-seismic lithological heterogeneity.

Micro-computed tomography (micro-CT) is increasingly utilized to image the pore network and to derive petrophysical properties in combination with modelling software. The effect of micro-CT image resolution and size on the accuracy of the derived petrophysical properties is addressed in this study using a relatively homogenous sandstone and a heterogenous, highly porous bioclastic limestone. Standard laboratory procedures including NMR (nuclear magnetic resonance) analysis, micro-CT analysis at different image resolutions and sizes and pore-scale flow simulations were used to determine and compare petrophysical properties. NMR-derived pore-size distribution (PSD) was comparable to the micro-CT-derived PSD at a resolution of 7 µm for both the rock types. Porosity was higher using the water saturation method as compared to the NMR method in both rocks. The resolution did not show a significant effect on the porosity of the homogeneous sandstone, but porosity in the heterogeneous limestone varies depending on the location of the sub-sample. The transport regime in the sandstone was derived by simulations and changed with the resolution of the micro-CT image. The transport regime in the sandstone was advection-dominated at higher image resolution and diffusion-dominated when using a lower image resolution. In contrast, advection was the dominant transport regime for the limestone based on simulations using higher and lower image resolutions. Simulation-derived permeability for a 400 Voxel³ image at 7 µm resolution in the Berea sandstone matched laboratory results, although local heterogeneity within the rock plays an integral role in the permeability estimation within the sub-sampled images. The simulation-derived permeability was highly variable in the Mount Gambier limestone depending on the image size and resolution with the closest value to a laboratory result simulated with an image resolution of 2.5 µm and a size of 300 Voxel³. Overall, the study demonstrates the need to decide on micro-CT parameters depending on the type of petrophysical property of interest and the degree of heterogeneity within the rock types.