Oil reservoirs have several stages in their production life, each successive stage requiring an increasing level of technology.

At first, the natural pressure in a reservoir will often be enough to drive the oil to the surface through wells. As more and more oil is extracted, however, the reservoir pressure drops, and eventually no further oil will flow up the wells. At this point, a pump can be installed, drawing oil into the well and up to the surface.

These first-stage methods of oil recovery are known as primary recovery, and as much as a fifth of the oil originally in the reservoirs can usually be extracted with them. Once the natural pressures have fallen below the point at which primary recovery can be effective, two forms of secondary recovery can then be employed: water flooding and immiscible gas injection. Water flooding involves injecting water into the reservoir from one well, so that the water displaces the oil into other wells that bring it to the surface. In addition to sweeping the oil to the producing wells, the water helps to maintain the reservoir pressure. Immiscible gas is so called because it doesn’t dissolve into the oil. It is injected into a reservoir to maintain pressure, and therefore the flow of oil, to the producing wells.

The production by means of secondary recovery techniques eventually peters out too. In order to get a third ’crop’ of oil from the reservoir, yet more advanced techniques must be employed. These tertiary recovery techniques are grouped under the heading enhanced oil recovery (EOR).

Petroleum Development Oman (PDO) has been investing a great deal of time and money in EOR techniques during recent years, as it looks to boost the overall percentage of oil recovered from its fields.