In water drive oil reservoirs, more than half of the initial oil in place is trapped in the water-contacted zone after natural water influx or waterflooding. Gas injection into such reservoirs, with the assistance of gravity, interfacial tension and oil film flow, can cause the displacement of excess water and the redistribution of reservoir fluids in the pore space. As the result of such fluid redistribution, most of the residual oil can be recovered. Moreover, a second water flood following the gas injection can recover the oil in a shorter period of time.

Gravity assisted tertiary gas injection processes include the Double Displacement Process (DDP) and the Second Contact Water Displacement Process (SCWD). The DDP consists of injecting gas into waterflooded oil zones. The SCWD process consists of submitting these gas-flooded zones to a new water displacement process. In this work, the double displacement process (DDP) and the second contact water displacement (SCWD) process were conducted in a transparent sand-pack micromodel, and a pore-level observation was performed to investigate the microscopic mechanisms of the two processes.

Observation of the two processes confirmed that the oil films play a very important role in achieving high recovery efficiencies in the DDP. The oil film was seen clearly. Such observation showed also that oil flowing through oil films and layers was driven not only by its own weight, but also by the increasing volume of the gas. In the SCWD process, trapped gas reduces the possibility of the residual oil being trapped in the center of the pores. Consequently, residual oil can be recovered quickly by a second water flood. Therefore, the SCWD process is suitable to apply in situations where the source of gas is not sufficient, and where the formation has a high irreducible gas saturation.

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