Migration and retention of CO₂ and methane in the Otway Basin and south-east Australia: an integrated geochemical and structural analysis
Carbon Capture and Storage (CCS) is the only means currently available to directly reduce the CO₂ emissions produced by the combustion of fossil fuels. CO₂ can be captured from various energy producing sources and can then be injected at supercritical pressures into deep-seated saline aquifers or depleted oil and gas reservoirs. Assurance of the safety and security of geological CO₂ storage can be provided through geochemical tracing techniques, allowing the monitoring of gas migration within the reservoir to verify its retention in the subsurface. Fluid migration and retention is often facilitated by fault zones, which can either act as fluid pathways to decrease the maximum storage capacity of the reservoir, or behave as unwanted barriers to fluid migration along the planned injection pathway, causing pressure increase and limiting the maximum rate of injection. This thesis undertakes an integrated approach to evaluate the fault control on geochemical fluid composition and outline the implications of such an approach to the safe deployment of CCS. This case study of gas migration in the Otway Basin in south-east Australia encompasses many of the process applicable to retention of CO₂ in an engineered storage site. The basin contains natural accumulations of methane, CO₂, and their mixtures, supplied by multiple charge events. The traps are bound by faults, which structurally control fluid migration and the resulting geochemical composition of the gas fields. CO₂-rich spring waters emanate at the ground surface within the extent and north of the basin, located in the vicinity of recently active fault zones and areas of recent volcanism. The results of noble gas, stable isotope and bulk gas composition analysis identify an unambiguous mantle source in the well gases and CO₂ springs. The variability of ³He/⁴He in the well gases is controlled by the gas residence time in the reservoir and associated radiogenic ⁴He accumulation. ³He/⁴He in CO₂ springs is controlled by hydrodynamic dispersion. Elevated CO₂/³He ratios, commonly associated with an input from a crustal source, can be explained solely by near-surface solubility fractionation. Taking these processes into account, the composition of CO₂ in the reservoirs and the springs is traced back to a single end-member of 3.07 - 3.65 R/Rₐ, proving a common mantle source. Geochemical tracing techniques are used to provide evidence multiple gas charge events into the traps and differentiate between chemical and physical processes such as dissolution and mineralisation, occurring during gas transfer through the subsurface and to the surface. This shows that significant CO₂ loss to dissolution and mineralisation is occurring within the Ladbroke Grove field. Solubility fractionation modelling of the atmospheric noble gas component is used to differentiate between fault-bounded traps that are acting as open and closed systems relative to the formation water and discern multiple gas injection events into the system. The geochemical analysis results are integrated with structural and fault seal analysis of the fault zones. Fault seal modelling techniques, commonly used in hydrocarbon exploration, are reviewed and adapted for use in CO₂ sequestration context by defining the uncertainties associated to the fluid properties of CO₂. The findings show that fault seal modelling techniques explain the gas migration and the associated gas compositions observed in the Otway Basin and can be successfully applied to CCS. Overall, the project establishes a comprehensive structural and geochemical model to account for the differences in gas retention and migration in fault-bounded traps and CO₂ springs in the Otway Basin. The presented methodology is discussed in the context of adaptation to CCS, hydrocarbon exploration and environmental monitoring settings.