A Fundamental Design Criterion for Liquid CO2 Injection into Shallow Sub-Seabed Aquifer.

Abstract

The possibility of CO2 liquid disposal into an aquifer beneath close to the sea-floor was studied using a simple calculation flow model. This study focused on the possibility of CO2 disposal in liquid phase below the critical temperature, because CO2 density can be larger in the low-pressure range than that over the critical temperature.Based on the equilibrium lines in the pressure and temperature map, an aquifer located at about 200 m under the sea-floor with sea depth around 500 m is capable of serving as a targeted zone to carry out CO2 liquid disposal. That means that the sea level of the aquifer is around -700m from the sea surface (absolute pressure is approximately 7.3MPa), and it is expected that the sea floor temperature is in the range of 4-6°C and the aquifer temperature is 15-20°C. For this case, it can be assumed not only that the CO2 goes in solution form with the aquifer water but also that the CO2 liquid flows with replacing the water. This shows that a greater CO2 quantity can be injected compared with that of the supercritical condition. Furthermore, the sediment between the sea-floor and the aquifer functions as an impermeable sealing cap for CO2 leakage to the sea by forming CO2 hydrates with water or sea water. Even if the cap is not formed, since the conditions at the sea-floor satisfy the CO2 liquid condition, there will be no large environmental impact. This study provided a design scheme to decide transmissivity of aquifer, CO2 injection rate, inner diameter of injection tubing and number of wells for CO2 disposal system into the aquifer below the sea-floor in liquid phase below the critical temperature.