Water flooding is a widely used stimulation technique for increasing the flow in oil reservoirs.  The principal behind the technique is to maintain or reverse the declining pressure in a reservoir by injecting water under pressure.

To understand the dynamics of water flooding it is important to understand the basic dynamics of an Oil reservoir. Oil reservoirs are generally “Gas Driven”; as a reservoir starts producing, the pressure is reduced, allowing gas in the reservoir to expand which further drives the flow. Imagine a bottle of fizzy water, when the lid is opened, the pressure is reduced, and you can see the trapped gas expanding as bubbles, if the pressure is great enough the water will flow from the bottle along with some of the gas/fiz. 

Like the bottle of fizzy water, the expanding gas in an oil reservoir is free to flow out of the well, reducing the pressure (and flow) until equilibrium is met
Due to the declining pressure the drive experienced from the gas is usually only enough to recover up to a 5th of the total oil in place. If it was possible to maintain the pressure it would be possible to flow more oil from the reservoir.

Injecting water to maintain pressure works for a number of reasons. Firstly, water and oil do not mix which means that injected water will displace the oil present. Secondly and as evident from our discussion, adding water will maintain the pressure which will maintain the flow.

It is understood that the presence of water in oil reservoirs can cause tremendous difficulties and increase the complexity of producing oil. Issues include increased lifting costs, increased strain on equipment and the environmentally safe disposal of produced contaminated water (although in many cases the produced water is re-injected back into the well for further flooding) . Before carrying out water flooding the operators will consider all aspects of the reservoir to reduce risks and to achieve the best possible flows.

Considerations for water flooding include reservoir permeability, fluid saturations, heterogeneity, oil viscosity and gravity, reservoir depth and lifting costs and the availability of a suitable water source.  

Reference: Old Park Lane Capital