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Alkaline flooding or caustic flooding is a chemical enhanced oil recovery technique consisting of injecting an alkaline solution into a crude oil reservoir, either as a stand-alone process or in combination with surfactant-polymer injection. The commonly used chemicals include sodium hydroxide, sodium carbonate, sodium orthosilicate, sodium tripolyphosphate, sodium metaborate, ammonium hydroxide, and ammonium carbonate. The preferred alkali used in oil fields is sodium hydroxide, as it is cheaper than the other mentioned options. Actually, it is the most inexpensive chemical-EOR technique.
The injected alkali will react with the crude oil's acidic component, forming an in-situ surfactant (soaps). Subsequently, the produced surfactant reduces the oil and water interfacial tension, resulting in a lower residual oil saturation. Moreover, the low interfacial tension would promote the formation of an emulsion which, in turn, would make the effective mobility ratio less unfavorable.
The mechanism of alkaline flooding are as follows:
- The addition of the alkali increases the pH and lowers the surfactant adsorption so that very low surfactant concentrations can be used to reduce the overall cost
- A reduction of interfacial tension resulting from the produced surfactants
- A change in wettability from oil-wet to water-wet or water-wet to oil-wet
- Emulsification and entrainment of crude oil in the flowing water
- Emulsification and entrapment of oil to assist mobility control
- Solubilization of rigid oil films at oil-water interfaces
It is to be noted that not all mechanisms mentioned above are effective in each reservoir.
It is not recommended to implement alkaline flooding in a carbonate reservoir due to high calcium concentration. When an alkaline chemical is mixed with calcium ions, hydroxide precipitation is produced. This precipitation may cause damage to the formation.
Not every reservoir is able or suitable to utilize the alkaline flooding method. Higher molecular weight acidic components of crude oil are favorable for the formation of surfactants. Before implementing this technique, chemical tests are available to assess a given crude oil's sensitivity to alkaline flooding. These tests can help assess the amount of alkali needed to neutralize one gram of oil.
Rule of thumb
The higher the acid number is, the more reactive the oil is. This means that it will take less time to produce surfactants.
The addition of silicates to caustic flooding is necessary to optimize the efficiency of this latter. Those added silicates have two principal roles:
- Safeguard: they maintain a constant high PH level yielding into a minimal interfacial tension
- Catalyzer: they improve surfactants' efficiency by removing hardness ions from reservoir brines, consequently diminishing surfactants' adsorption at the rock's surface.
Alkaline flooding efficiency is greatly improved if the reservoir's oil acid level is comparatively high. Similar effectiveness is achieved for reservoirs with high acid crude oil, which is not necessarily a heavy oil.
- Lithology: this Chemical-EOR technique should rarely be carried out in carbonate formation. This is because anhydrite, which leads to high precipitation and high alkaline consumption, is present in carbonate formations. For this flooding to be carried out, the clay content must be minimal. High clay content leads to high alkaline consumption.
- Oil composition and viscosity: for this flooding, a high acid number is most suitable as an alkali will produce surfactants in situ when in contact with crude oil. It is also advised that this flooding be performed in reservoirs with a viscosity of oil lower than 150cp. The oil gravity is less critical than the viscosity and acid number. For alkaline injection with surfactant and polymer flooding, a light oil reservoir is preferred to a heavy oil reservoir according to conventional screening criteria.
- Oil saturation before flooding: before carrying out the flooding, the oil saturation must be high because a process of caustic flooding depends on the reaction between the crude oil and the alkali. The oil saturation should be at least 0.35. Furthermore, alkaline injection wells should be drilled in the oil zone.
- Formation water salinity and divalent: the hardness components should be at a low-level since their reaction with alkalis will lead to precipitation. For the salinity to be within the required salinity levels of surfactants, the Total Dissolved Salt (TDS) should be low. The recommended maximum TDS and multivalent cations concentrations are 50,000 ppm and 100 ppm, respectively. To avoid precipitation problems, alkalis may have to be replaced by Na4EDTA and Na3NTA, which are high Ph chelating agents. Sweep efficiency is improved as precipitates initially reduce permeability in high permeability channels.
- Reservoir temperature: the reservoir temperature should not be a hindrance to flooding. The formation depth is also not be critical.
- Formation permeability: the formation permeability is recommended to be 10mD or higher.
- Gas cap and aquifer support: pressure support is provided when the alkaline solution is injected. The pressure support from the injected alkaline solution becomes less significant when the reservoir initially has strong gas and or aquifer support.
- Measuring interfacial tension (IFT) between the alkaline solution and the crude oil
- Measuring static alkaline consumption
- Conducting core flood tests
- Acid number test
- Precipitation test
- Scale prediction test
- Filtration test
- Scanning electron microscopy and petrographic image analysis
- Alkaline/reservoir fluid compatibility test
- Alkaline emulsion stability test
The main issues in alkaline flooding are precipitation and scaling. When an alkaline solution is injected into a formation, the concentrations of OH-, CO32- and SiO32- increase. These ions react with the divalent like Ca2+ and Mg2+ to form inorganic scales and precipitates. The scaling and precipitates may cause formation damage.
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