Recovery

1. Primary recovery

After the well has been drilled into an oil-bearing reservoir and the mud depleted, the natural pressure in the reservoir causes the oil to flow into the production wells and then gush to the surface.

If reservoir conditions are difficult, e.g. rapidly declining pressure or significant flow-resistance factors, this so-called gusher soon stops. The simplest way of maintaining production flow rates is to use downhole pumps. These phases in the production of oil from a field are known as primary recovery. On average, around 5-10% of the original oil in place can be exploited by means of primary recovery.

2. Secondary recovery

Secondary recovery methods were developed to improve the recovery factor. The most common method involves flooding the reservoir with water. This involves injecting water into the reservoir to artificially increase the reservoir pressure and press the oil into the production wells. This process is "reversible" in that gas can be forced into the top of the reservoir and thus press the oil down to deeper production wells. The recovery factor can be increased to around 45% by employing the water flooding method.

3. Tertiary recovery (EOR = enhanced oil recovery)

The following methods can be used to improve the crude's flow qualities:

• Pressing steam into a reservoir heats up the crude and reduces its viscosity.
• The same effect can be achieved by pumping in solvents.

What frequently happens with the water flooding method is that the low-viscosity injection water flows faster than the higher-viscosity oil or, in the case of layers with higher permeability, flows through the oil into the production wells. This can be prevented by placing a high-viscosity polymer bed between oil and injection water.

Up to 60% of the original oil in place can be exploited by means of tertiary recovery.

source : http://www.rwe.com