Mechanisms And Geological Modeling Of CO₂ Sequestration In Saline Aquifers

Carbon sequestration in saline aquifers has been identified as a promising method of reducing atmospheric CO₂ in response to growing concerns over climate change. Saline aquifers are attractive for such sequestration because of their large capacity and broad distribution. Modeling CO₂ sequestration in saline aquifers is needed to direct injection, site selection, and economic evaluation. With data extracted from publications, deep and shallow saline aquifer models were built using compositional reservoir simulator CMG-GEM to investigate CO₂ transportation, distribution, CO₂ storage capacity, CO₂ injectivity, and the type and extend of sequestration mechanisms. The effect of reservoir properties and injection rate on CO₂ storage and injectivity in saline aquifers were then studied. Sensitivity analysis was carned out using design of "experiments (DOE) to determine the dominant factors. In addition, engineering aspects were discussed to optimize CO₂ storage capacity, including water withdraw and completion methods as partial perforation, well geometry, orientation, location and length.Simulation results show that CO₂ moves upward in saline aquifers and the distribution pattern is affected by reservoir heterogeneity. The storage capacity CO₂ in deep saline aquifer is high than that of shallow aquifer, but CO₂ injectivity is lower. Most CO₂ present as free gas, some are residual gas and dissolved phase, and very few CO₂ are mineralized. Two main trapping mechanisms occur in saline aquifers are residual gas trapping and solubility trapping.Generally, heterogeneity has very little effect on CO₂ storage capacity and injectivity. Relatively, mean permeability affects CO₂ storage capacity and injectivity the most. More CO₂ can be stored in the heterogeneous reservoirs with low mean permeability; however, high injectivity can be achieved in the uniform reservoirs with high mean permeability.Among all the properties discussed in the model, the fracture pressure gradient of reservoir affects CO₂ storage capacity the most, while the maximum injection rate affects CO₂ injectivity the most. Reservoir depth doesn't affect CO₂ sequestration significantly, suggesting the possibility of CO₂ sequestration in shallow aquifers. Increasing reservoir depth, porosity, and the maximum injection rate can improve CO₂ storage capacity and injectivity.Comparing to deviated wells and vertical wells, horizontal wells are better choices for CO₂ injection in that it improves CO₂ storage capacity, especially when set in upper layers of reservoir. The optimal length can be determined according to CO₂ storage capacity and the sequestration efficiency. For the aquifer examined, the value is 280 m. Completing more layers in the upper reservoir gets more CO₂ injected. Water withdraw can improve CO₂ storage capacity greatly and benefit CO₂ dissolution.