Mitigation of global warming is the need of today. The anthropogenic emission of CO2 in the atmosphere is the primary reason for global warming. Therefore, it is necessary to restrict CO2 emissions. CO2 sequestration in a deep saline storage reservoir is the most efficient and cost-effective method to store CO2 for more than thousands of years. Existing literature has multiple levels of simplifications that limits the overall understanding of the sequestration process. In this thesis, we have demonstrated and hypothesized new mechanisms that govern the realistic CO2 trapping processes in a deep saline reservoir. We have numerically modeled CO2 sequestration by dissolution and local capillary trapping methods. We have also demonstrated the possibility of a small size CO2-based geothermal energy extraction system in a sedimentary reservoir. We have modeled two-phase fluid flow and solute transport and identified mechanisms like capillary transition zone, CO2 plume evolution, gravitational instability, and solutal fingering. The most striking findings of this work are injection-induced capillary transition zone, gravity trunk formation, large scale convective rolls, reverse fingering, and associated CO2 dissolution. The supercritical CO2 plume tip propagates below a caprock following a power law with time. The fraction of dissolved CO2 increases after the onset of fingering and onset time is inversely proportional to permeability for the same injection pressure. Further, we found that gravity trunk formation, solutal fingering, and large convective rolls are visible for nonisothermal conditions. We estimated the spatial plume moments to show a restricted lateral spread of free phase CO2 and faster finger growth with increasing heterogeneity indicating strong local capillary trapping. A strong negative correlation is found between heterogeneous capillary entry pressure and patchy CO2 saturation distribution. The buoyancy of less dense CO2 plays a significant role in thermal drawdown at the production well during geothermal energy extraction modeling.