Assessing a potential site for offshore CO2 storage in the Weixinan Sag in the northwestern Beibu Gulf Basin, northern South China Sea

Geologic carbon storage (GCS) activities, especially offshore, are still far insufficient worldwide. Sedimentary basins in the northern South China Sea (nSCS) are identified to be favorable to offshore GCS deployment. This study investigates by numerical reservoir simulations the performance of the Weixinan Sag in the northern depression of the Beibu Gulf Basin (BGB), which is considered the most promising area for CO₂ offshore GCS in saline formations. Simulations of CO₂ injection at a potential site with a normal fault indicate that the pressure buildup induced by CO₂ horizontal injection could penetrate extremely low-permeability faults or caprock formations. The CO₂ plumes under higher injection pressures are more constrained in the horizontal direction and hence appear thicker while CO₂ tends to spread out horizontally in conditions of low pressure. The impermeable fault renders the CO₂ plume about 200 m smaller in size horizontally. Assuming the presence of a fault with a highly permeable core causes leakage to occur after about 30 years of injection, which accounts for only 0.04% the injected amount. For the vertical-well injection case, the potential CO₂ leakage only accounts for around 0.28% of the injection amount, which is far less than the criterion (i.e., 2%) required to make GCS worthwhile. The storage capacities of the formations are mainly controlled by the depths and thicknesses since both their porosities and permeabilities are comparable. The formation Jiaowei-2 and Xiayang have the largest and second largest storage capacities, respectively, when using a fully perforated well for vertical injection. The average storage capacity of the studied site is 13.04 kg m⁻³, which is comparable to that of the formation Xiayang. Average injectivities of formations Jiaowei-2, Xiayang, Weizhou-1, and Weizhou-3 are 8.29 × 10⁻⁵, 1.75 × 10⁻⁴, 6.58 × 10⁻⁵, and 2.99 × 10⁻³ kg s⁻¹ Pa⁻¹, respectively. © 2022 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd.