6.1 Conventional oil

Conventional oil is a loosely defined term covering any crude oil that can be extracted from the ground in the 'conventional' way; that is, it is extracted using standard oil well technology.

Conventional oil is generally found in underground rocks known as reservoirs, where it can sometimes be associated with natural gas. The oil is generally under great pressure and initially when an oil well is drilled, the oil will spontaneously flow to the surface. Indeed the problem at this stage is how to control the flow and avoid a 'blowout', an occurrence that plagued the early oil wells and cost many lives (Figure 8).

Figure 8 The blowout (uncontrolled release of oil and gas under great pressure) of an oil well in Texas in 1901.

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A black and white picture of a drilling rig with a plume of black oil shooting vertically upwards out of the picture, with some of it beginning to fall back down.

Figure 8 The blowout (uncontrolled release of oil and gas under great pressure) of an oil well in Texas in 1901.

Modern oil wells are designed to avoid disastrous blowouts and have sophisticated pressure release systems which allow the controlled release of oil at the surface. However, blowouts still occur occasionally, as you will see in Section 6.2.

The stage at which oil flows spontaneously to the surface is known as the primary recovery phase (Figure 9a) and only a small proportion of the oil in the reservoir can be extracted in this way. Eventually, the pressure in the oil reservoir will drop, and to prolong extraction it becomes necessary to maintain the pressure underground by injecting pressurised water or gas, or both, into the reservoir (Figure 9b). This phase of production is known as the secondary recovery phase.

In order to improve recovery still further, chemical or biological agents may be added to the injected water, or steam may be pumped into the reservoir, in order to reduce the viscosity (thickness) of the crude oil. The term 'viscosity' may be new to you; in scientific terms, viscosity is a measure of how easily a liquid flows - a liquid with low viscosity such as water flows easily, whilst a liquid with high viscosity (e.g. treacle or honey) does not flow easily.

Maximise

Figure 9 The primary and secondary recovery phases of conventional oil extraction. (a) Primary recovery: the oil flows to the surface under its own pressure. (b) Secondary recovery: in this example, water is pumped into the well to force the oil out.

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These are two similar sketches of vertical sections through the Earth's surface. They show a gentle upwards fold of three rock strata below the surface. There are narrow bands of impermeable rock top and bottom and a central larger section of permeable rock. The permeable rock has oil filling the upper part, floating on water below, with a horizontal line separating them. In (a), there is an drilling rig with a pipe down into the oil layer and arrows showing that the oil flows under its own pressure into the pipe and upwards to the surface. In (b), in addition to the oil production well seen in (a), there is also a water tank at the surface with a pipe down into the water layer, labelled water injection well. There are arrows to show that the water is pumped down the pipe and out into the water layer below the oil. This results in the water level below the oil being higher in (b) than in (a). Oil is then forced up into the pipe of the oil production well and up to the surface in the same way as in (a).

Figure 9 The primary and secondary recovery phases of conventional oil extraction. (a) Primary recovery: the oil flows to the surface ...

Under ideal conditions, secondary recovery methods can result in up to 70% of the initial oil present in the reservoir being recovered, but the processes are expensive and for many smaller fields the amount of extra oil recovered may not be worth the investment.