Geomechanical Evolution of the South-West Hub Carbon Capture and Storage Reservoir in Western Australia

Abstract

Characterising the mechanical behaviour of porous subsurface reservoirs is essential for avoiding hazards such as wellbore failure, fault reactivation, seal leakage and gas migration during CO2 storage. Reactivation of faults is a particular concern when CO2 is injected into reservoirs that contain them. To address this, a geomechanical model (GM) was built for the South-West Hub (SWH) Carbon Capture and Storage (CCS) reservoir, drawing on data from the Harvey field wellbores in Western Australia. The 3D GM combined a geological model with seismic data, dynamic-simulation output, and one-dimensional mechanical-earth models. A 4D finite-element model was then used to capture how CO2 injection alters stress and deformation within the storage formation, and to characterise the failure behaviour and performance of the reservoir as injection proceeds. Effective-stress azimuth and magnitude were tracked throughout the injection period. Estimated magnitudes for Sv, SH and Sh across the study area show SH ranging from 22 to 75 MPa and Sh from 21 to 72 MPa. Fault strain responses are compared across a set of injection scenarios. Five separate reservoir-pressure and saturation profiles were used to calibrate the 3D model, which was then re-run repeatedly to refine the stress model against available leak-off test (LOT) data. The calibrated model was subsequently used to run fault-stability analyses and identify which faults are most prone to reactivation under the pore-pressure changes generated by ongoing CO2 injection. Over 30 faults intersecting the reservoir were mapped, mostly trending N-S in keeping with the dominant regional fault orientation. Combining the dynamic and geomechanical results indicates that CO2 injection does not pose a substantial fault-failure risk for the SWH reservoir. This assessment describes the isothermal, pressure-driven response: thermal stresses arising from injecting CO2 colder than the formation are not represented and may be significant near the injection wells.