Gravity assisted tertiary gas injection processes can produce a large amount of incremental tertiary oil from water drive oil reservoirs. These processes include the Double Displacement Process (DDP) and the Second Contact Water Displacement (SCWD) process. A transparent sandpack micromodel was developed to conduct a pore-level observation to investigate the microscopic mechanisms of the DDP and the SCWD processes. Observation of the two processes confirmed that oil films play a very important role in achieving high recovery efficiencies in the DDP. In the SCWD process, trapped gas reduces the possibility of residual oil being trapped in the centre of the pores in the second water flood. Moreover, reservoir simulations at reservoir scale were performed to investigate the macroscopic level mechanisms of the two processes. The results have shown that both processes are efficient methods to recover waterflood residual oil.