Abstract The Naylor structure in the Port Campbell Embayment, Otway Basin, South Australia is proposed as a demonstration site for the subsurface geological storage of carbon dioxide (CO 2 ). The Naylor structure is a fault-bounded high with normal faults to the north and west to SW. Seismic interpretation shows evidence of recent fault reactivation in the Otway Basin. It is postulated that residual hydrocarbon columns (accumulated and leaked prior to present day) in the Otway Basin leaked due to fault reactivation. Thus, a critical issue in the geological storage of CO 2 in the Port Campbell Embayment is the potential for the reactivation of faults bounding the Naylor structure. The propensity of faults to be reactivated is assessed by determining the in-situ stress field, the mechanical properties of the fault rock and the orientations of the existing faults. The in-situ stress field lies on the boundary of a strike-slip and reverse faulting regime in the Port Campbell Embayment. The vertical, minimum horizontal and maximum horizontal stress gradients are 21 MPa km −1 , 19 MPa km −1 and 38 MPa km −1 respectively and the pore pressure gradient is hydrostatic. The maximum horizontal stress in the Port Campbell Embayment is oriented at 150°N. One planar and two curviplanar faults were identified within the Naylor structure. Two fault segments act to trap accumulations at the crest of the structure. These fault segments have relatively low propensities to reactivate near the crest of the structure. The intended migration pathway of the CO 2 plume does not intersect the identified faults until it reaches the crest of the Naylor structure. However, reservoir heterogeneities such as sub-seismic faults may cause the migrating CO 2 plume to move towards identified fault segments which are not intended to trap the injected CO 2 and have a relatively high propensity to reactivate.

Abstract Tracers can be added to the injected CO 2 stream in geosequestration projects to demonstrate safety and containment to stakeholders by verifying the presence of injected CO 2 (other CO 2 may exist), monitoring the possible leakage of CO 2 (fingerprinting tool), and measuring the behavior of CO 2 under injection (e.g., saturation, fluid mixing, calibrating models, etc.). Monitoring may be required in a variety of settings (e.g., reservoir fluids, shallow groundwater, deep soil, surface emissions, and formation gases). Thus, an appropriate tracer or suite of tracers is required for clear identification if the CO 2 migrates out of the injection zone. Although the injected CO 2 in some study areas has been isotopically distinct from the existing CO 2 present in the aquifer or reservoir (e.g., the Encana Enhanced Oil Recovery Program at Weyburn, Saskatchewan, Canada), this is not always the case. Tracers are required to be chemically inert, environmentally safe, nontoxic, persistent, and stable for purposes of the desired monitoring time scale. Low-volume usage (for cost, availability, and ease of handling) and sensitivity to detection by analytical methods are also of key importance. Application methods (e.g., pulsed or continuous addition) may also affect costs. More commonly than not, the analytical costs are seen to be the most prohibitive cost overall (specialist tracers, very precise measurements by specialized methods or equipment, etc.). Based on the general requirements for carbon storage, several possible tracer compounds have been identified by various authors and summarized here. Their behavior, availability, ease of analysis, and indicative costs are evaluated. This list is by no means exhaustive for all likely tracers. The Cooperative Research Centre for Greenhouse Gas Technologies (CO 2 CRC) is conducting a CO 2 storage pilot program in Victoria, Australia. The Otway Project site is a depleted natural gas reservoir with roughly 20% residual methane saturation and a remaining methane-rich gas cap. In addition, the natural source of CO 2 for injection is mixed with about 20% methane content. Thus, tracers are required that may give information on the fate of the injected CO 2 -methane combination. Some preliminary work on CD 4 (fully deuterated methane) is presented, and further work on the evaluation of this potential tracer is discussed.