# Enhanced oil recovery

Mainly the following process are done for Enhanced oil recovery
→Water injection
→Gas injection
→Reducing residual oil saturation, SOR (alcohol, polymers, surfactants injection)
→Thermal: steam injection (to heating of the reservoir to lower the viscosity)
However, all the above techniques are dependent on reservoir conditions and crude properties.
Water injection/water flooding
Conventionally, based on the type of production and nature of reservoir the following patters are followed an injection well. Once the primary energy of the reservoir tends to deplete it becomes necessary to maintain the pressure inside the reservoir to achieve optimum production and maximise ultimate recovery. In such condition the pressure maintenance can be done by injecting water into the reservoir which is compatible to the formation water present in the reservoir through several water injection wells. Such kind of operation is known as water flooding/Water injection.
In this process, the primary objective is to fill the voidage created by the produced oil fractions thus avoiding the reservoir pressure to decrease with the increased production. When the water is injected in the reservoir, it tends to push the oil upwards, thereby increasing the life and the ultimate recovery of the reservoir. Water injection and water flooding are quite similar terms the only difference being the level at which injection water is being discharged and the displacement phenomena
Water injection:
For water injection operations, the injected water is discharged in the aquifer through several injection wells surrounding the production well. The injected water creates a bottom water drive on the oil zone which pushes the oil upwards. In earlier practices, water injection was done in the later phase of the reservoir life, but now it is often carried out in an earlier phase as to not create voidage in the secondary gas cap. Using water injection in earlier phases helps in improving production because once the secondary gas cap is formed the injected water initially tends to compress the free gas cap and later on pushes the oil, therefore the amount of injection water required is much greater. The water injection is generally carried out when the solution gas drive is present or the water drive is weak. Thus, to reduce costs the water injection is carried out when the reservoir pressure is greater than the saturation pressure.
The selection of injection water {displacing fluid} depends upon the mobility rate between the displacing fluid {injection water} and the displaced fluid {oil}
DISADVANTAGE OF WATER INJECTION
* Reaction of injected water with the formation water can cause formation damage.
* Corrosion of surface and sub-surface equipment.
Water flooding:
In this operation, displacing fluid is injected in the oil zone through the surrounding water injection wells creating an edge water drive flooding oil towards the production well. For better efficiency, the pressure of the reservoir should be such that no secondary gas cap is formed. Water flooding is generally more effective than water injection when no voidage is being created. Water flooding in other regards is similar to water injection including selection parameters of the displacing fluid, the only difference being the displacing phenomenon. However, in practice the above patterns are not necessarily followed It is to be noted that water injection and water flooding are essentially the same process, however, while in the case of former we inject the water in the water zone of the reservoir, in the latter we inject it directly into the hydrocarbon zone