A fundamental design criterion for liquid CO₂ injection into shallow sub-seabed aquifer

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 7.3 MPa), 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.