Assessment of large-scale offshore CO2 geological storage in Western Taiwan Basin

A million-ton-scale CO2 geological storage project was planned by the Taiwan Power Company in the Taiwan Basin. The site, half on-shore and half off-shore, has a regional shale formation underlain by two sandy saline aquifers. Numerical models were used to comprehensively assess CO2 geological storage including the CO2 plume evolution, pressure buildup, impacts on the shallow fresh groundwater, phase transition of CO2 potentially moving in a vertical fault, and uplift of the land surface over 500 years for a 50-year CO2 injection at a rate of 5 million tons per year. The results suggested that the reservoir could reliably store and contain the injected CO2 for 500 years. All formations could stand the pressure buildup related to the large injection volume. The modeled CO2 plumes did not either penetrate through the cap rock or reach the fault north to the injection wells. The assumed monitor point for shallow groundwater on the shore showed no changes in water head and water quality during 500 years. Phase transition simulations showed that escape of CO2 from a deep storage reservoir to the land surface through a vertical fault is a complex interaction of multiphase fluid flow and heat transfer. CO2 rising toward the land surface evolves into two-phase mixtures of liquid and gaseous CO2 as temperatures and pressures decrease, inducing three-phase flow of CO2-brine mixtures. The presence of a three-phase zone (aqueous-liquid CO2-gaseous CO2) leads to a small relative permeability for all phases, and in turn makes all phases less mobile and reduces CO2 discharge rates. The proposed injection would induce a couple of centimeters uplift in the land surface during the 50-year injection period, but it would recover to the initial elevation at a slower rate than that of the uplift.