Geomechanical assessment of the Cenozoic stratigraphy of the Tui area, Taranaki Basin, offshore New Zealand – Implications for geological storage in the Paleocene and Eocene sandstones

This study presents a geomechanical assessment of the 4km thick Paleocene-Pleistocene succession of the Tui field area from Taranaki Basin, offshore New Zealand. Based on the core measurements, suitable rock-mechanical models have been presented for static and dynamic elastic properties and rock strength. The Cenozoic stratigraphy is inferred to be normally compacted and devoid of any notable overpressure. Based on the C-quality stress indicators, we infer a 16.48 MPa/km minimum horizontal stress gradient, while the static elastic property-based model suggests a maximum horizontal stress gradient of around 21.15 MPa/km. The estimated in-situ stress magnitudes of the Paleocene-Miocene interval indicate a normal to strike-slip transitional stress regime (SHMax ∼Sv> Shmin). The petrographic and routine core analysis reported medium to coarse-grained, macro-megaporous sub-arkose arenites within the Paleocene Farewell sandstone and Eocene Kaimiro sandstone, which were considered as suitable candidates for geological storage. We analysed the injection stress paths for these two storage units as a result of pore pressure build-up and consequent stress perturbations. The maximum sustainable injection threshold is determined to ensure storage integrity. The conventional approach exhibits a 5-6 MPa repressurization window, while a much higher build-up threshold has been inferred from the model by utilizing pore pressure-stress coupling effects.