Oil fields are found in layers of sedimentary rock. The crude oil in these rock layers was made when organic material, such as plankton and algae, deposited millions of years ago decomposed. Crude oil is a thick fluid that, like carbon dioxide, tends to migrate towards the surface. Oil fields are formed when the migrating oil is trapped by nonporous and nonpermeable layers of rock. Many oil fields have existed for millions of years. Today, people have developed sophisticated techniques for finding oil fields and producing the oil from them. When an oil field is first produced, the background pressure in the formation helps to push the oil to the surface. In the early days of oil industry, this sometimes led to the famous “gushers” seen in history books. Typically, the background pressure is great enough to help produce roughly 10% of the oil in an oil field. Once the pressure is released, new means are employed to restore pressure in the oil field and reduce the thickness or viscosity of the oil so that it can move more easily. These methods are generally referred to as Enhanced Oil Recovery or EOR.
There are 3 major approaches used in EOR:
steam is injected to thin the oil and help it flow
polymers or surfactants that behave like soap are injected to break up the oil and help release it from the rock surfaces in the pore space; and,
natural gas (a by-product), nitrogen or carbon dioxide is injected to increase reservoir pressure and to help thin the oil so it can flow to the surface (Figure 1).
The figure above illustrates how gas injection – in this case carbon dioxide – is used for EOR. Carbon dioxide and water are injected through the injection well on the left. The carbon dioxide mixes with the water forming a weak acid that helps to break down the viscosity of the oil. At the same time, the carbon dioxide-water mix restores the background pressure in the oil field, helping to push the oil towards the production well on the right. The resulting oil, carbon dioxide, and water mix is brought to the surface in a closed loop system that allows the operator to separate the carbon dioxide and water for reuse while the oil and related products are sold into the market. In this process, carbon dioxide displaces oil and some of it remains behind permanently. There is not a lot of surplus carbon dioxide available in a form that can be used for EOR so operators try to recover as much of the carbon dioxide as they can. Despite these efforts, at least about 20-30% of the injected carbon dioxide remains in the ground.