Because of differences in density and viscosity between the injected fluid and the reservoir fluid(s), the miscible process often suffers from poor mobility. Viscous fingering and gravity override frequently occur. The simultaneous injection of a miscible agent and brine may take advantage of the high microscopic displacement efficiency of the miscible process and the high macroscopic displacement efficiency of a waterflood. However, the improvement may not be as good as hoped for since the miscible agent and brine may separate due to density differences, with the miscible agent flowing along the top of the porous medium and the brine along the bottom. Several other variations of the simultaneous injection scheme may be attempted. These typically involve the injection of a miscible agent followed by brine or the altering of miscible agent-brine injection. The latter variation has been named the WAG process (discussed earlier) and has become the most popular. A balance between amounts of injected water and gas has to be achieved. Too much gas will lead to viscous fingering and gravity override of the gas, whereas too much water could lead to the trapping of reservoir oil by the water. The addition of foam generating substances to the brine phase may aid in reducing the mobility of the gas phase.
Operational problems involving miscible processes include transportation of the miscible flooding agent, corrosion of equipment and tubing, and separation and recycling of the miscible flooding agent.
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