Carbon dioxide storage leads of the eastern Gippsland Basin, Australia - Terminology, seal and structure considerations in trap integrity assessments

Abstract

Geotechnical assessments of carbon dioxide (CO2) storage sites in the Gippsland Basin have mainly focussed on interpreting stratigraphy, reservoir/seal characteristics, and populating geo-models using one dimensional (1D) well data. Less emphasis has been placed on ascertaining the openness of the fluid-flow pathway that could be anticipated for CO2 migrating within a trap, and using 3D seismic data to interpret any associated faults. This study addresses the structural and seal complexities involved in the assessment of CO2 storage leads (CO2SL) that are proximal to producing petroleum fields where, the data control is optimal and the geology well constrained. The study area is located in the eastern Gippsland Basin, southeast Australia, where we illustrate the concepts introduced and also undertake a fault seal analysis. To assist in the assessment of CO 2SLs, we also introduce new terminology including flow-path distance, flow-path height, sweep area and fluid-flow cell. In the eastern Gippsland Basin, across-fault sand-on-shale contacts are unlikely to be maintained for fault planes that extend up to 20 km along strike; reservoir intervals are also exceptionally sandy. The trap integrity of the CO2SLs studied is likely to be acceptable on condition of a continuous shale smear being present. In the case where sealing by shale smear is considered, mercury injection capillary analyses (MICP) have independently confirmed that the clays of intra-formational seals have mineralogies capable of holding back significant injected columns of CO2. The majority of fault azimuths range from 095 to 140°N and are susceptible to fault reactivation as a result of being subparallel to one of the conjugate directions of shear failure; the latter estimated using a maximum shear stress azimuth of 139°N. However, the sinuous fault planes, possibly rugose in part, have azimuths varying up to ± 55°, which could make fault reactivation less likely as a result of increased shear resistance being anticipated. Finally, the low faulting density interpreted within fluid-flow cells provides some assurance for the unimpeded fluid flow of CO2. © 2011 Published by Elsevier Ltd.