Enhanced Oil Recovery

The third stage of hydrocarbon production during which sophisticated techniques that alter the original properties of the oil are used. Enhanced oil recovery can begin after a secondary recovery process or at any time during the productive life of an oil reservoir. Its purpose is not only to restore formation pressure, but also to improve oil displacement or fluid flow in the reservoir.

The three major types of enhanced oil recovery operations are chemical flooding (alkaline flooding or micellar-polymer flooding), miscible displacement (carbon dioxide [CO2] injection or hydrocarbon injection), and thermal recovery (steamflood or in-situ combustion). The optimal application of each type depends on reservoir temperature, pressure, depth, net pay, permeability, residual oil and water saturations, porosity and fluid properties such as oil API gravity and viscosity.

Enhanced oil recovery is also called improved oil recovery or tertiary recovery and it is abbreviated EOR.

CO2-EOR is a commercially proven technology. CO2-EOR has been used extensively in the USA, where 74 projects are now operating. CO2-EOR can be either miscible or immiscible depending primarily on the pressure of the injection gas into the reservoir. In miscible CO2-EOR, the CO2 mixes with the crude oil causing it to swell and reduce its viscosity, whilst also increasing or maintaining reservoir pressure (Figure 2). The combination of these processes enables more of the crude oil in the reservoir to flow freely to the production wells from which it can be recovered. Additional oil recovery of 10 to 15% or more of the original oil in place is often quoted as typical, but actual results vary considerably. In immiscible CO2-EOR, the CO2 is used to re-pressure the reservoir and as a sweep gas, to move the oil towards the production well. A side benefit of CO2 injection is that the CO2 preferentially mobilises the lighter fractions of the oil, which slightly improves the quality of the produced oil.

Most CO2-EOR projects undertaken to date have been designed to minimise CO2 losses in the reservoir since such losses represent an additional production cost. Accordingly, most operations have not considered the fate of the stored CO2 or attempted to determine how much CO2 can be stored and the relative merits of storage versus enhanced oil production. The Weyburn CO2 Monitoring project will aim to close this gap in knowledge.

Few of the existing CO2-EOR projects use CO2 from anthropogenic sources. The CO2 injected into the Weyburn field will come from the Great Plains Synfuels plant, owned and operated by the Dakota Gasification Company. The sequestration of CO2 in this field will make a direct contribution to reducing anthropogenic emissions of CO2 and will provide an example for future sequestration projects of the potential available from use of this technique.