When an alkaline solution is mixed with certain crude oils, surfactant molecules are formed. When the formation of surfactant molecules occurs in situ, the interfacial tension between the brine and oil phases could be reduced. The reduction of interfacial tension causes the microscopic displacement efficiency to increase, thereby increasing oil recovery.
Alkaline substances that have been used include sodium hydroxide, sodium orthosilicate, sodium metasilicate, sodium carbonate, ammonia, and ammonium hydroxide. Sodium hydroxide has been the most popular. Sodium orthosilicate has some advantages in brines with a high divalent ion content.
There are optimum concentrations of alkaline and salt and optimum pH, where the interfacial tension values experience a minimum. Finding these requires a screening procedure similar to the one discussed previously for the micellar-polymer process. When the interfacial tension is lowered to a point where the capillary number is greater than 10–5, oil can be mobilized and displaced.
Several mechanisms have been identified that aid oil recovery in the alkaline process. These include the following: lowering of interfacial tension, emulsification of oil, and wettability changes in the rock formation. All three mechanisms can affect the microscopic displacement efficiency, and emulsification can also affect the macroscopic displacement efficiency. If a wettability change is desired, a high (2.0–5.0 wt%) concentration of alkaline should be used. Otherwise, concentrations of the order of 0.5–2.0 wt% of alkaline are used.
The emulsification mechanism has been suggested to work by either of two methods. The first is by forming an emulsion, which becomes mobile and later trapped in downstream pores. The emulsion “blocks” the pores, which thereby diverts flow and increases the sweep efficiency. The second mechanism is by again forming an emulsion, which becomes mobile and carries oil droplets that it has entrained to downstream production sites.
The wettability changes that sometimes occur with the use of alkaline affect relative permeability characteristics, which in turn affect mobility and sweep efficiencies.
Mobility control is an important consideration in the alkaline process, as it is in all tertiary processes. Often, it is necessary to include polymer in the alkaline solution in order to reduce the tendency of viscous fingering to take place.
Not all crude oils are amenable to alkaline flooding. The surfactant molecules are formed with the heavier, acidic components of the crude oil. Tests have been designed to determine the susceptibility of a given crude oil to alkaline flooding. One of these tests involves titrating the oil with potassium hydroxide (KOH). An acid number is found by determining the number of milligrams of KOH required to neutralize 1 g of oil. The higher the acid number, the more reactive the oil will be and the more readily it will form surfactants. An acid number larger than ~0.2 mg KOH suggests potential for alkaline flooding.
In general, alkaline projects have been inexpensive to conduct, but recoveries have not been large in the past field pilots.19–20
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