Numerical simulation of supercritical CO2 injection into subsurface rock masses

Carbon dioxide (CO2) is considered to be one of the greenhouse gases that may contribute most to global warming on the earth. Disposal of CO2 from stationary sources into subsurface structures has been suggested as a possible means for reducing CO2 emissions into the atmosphere. However, much remains to be done in the issues regarding the safety and reliability of CO2 geological sequestration. In this study, we have developed a simulation code by using the mathematical model of two phase flow in porous media to analyze the flow dynamics in the subsurface. The equation of state for CO2 covering the fluid region from the triple point to the supercritical region is employed to model the states of CO2 gas, liquid and supercritical state. The correct understanding of the CO2 state under the geological formation condition is an important factor to predict the injection pressure and CO2 fluid permeation because the fluid density has a great effect on the injection behavior. The numerical simulation was implemented under several geological conditions including gas, liquid and supercritical states to examine the optimal injection condition. Comparing the numerical results obtained using the equation of state for CO2 with those obtained using the ideal gas equation, it has been shown that the difference in the injection pressure appears to be significant near the condition of the critical point of CO2 and the phase equilibrium curves between the gas and liquid states. The numerical simulation has been implemented to examine the effect of the reservoir condition on the injection behavior. The injection pressure is decreased at the lower reservoir temperature and higher hydrostatic pressure condition. The CO2 permeation is also strongly affected by the reservoir condition, and the spatial CO2 saturation becomes higher with increasing reservoir temperature. It has been demonstrated that the simulation code developed in this study may be useful to provide knowledge required to select the reservoir condition for CO2 geological sequestration.