Enhanced Oil Recovery Methods
an abstract by Shunhua Liu

For the near future, there is no economical, abundant substitute for crude oil in the economies of the world. Maintaining the oil supply to propel these economies requires both developing additional crude reserves and improving oil recovery from present reservoirs. The oil recovery methods that are commonly used include pressure depletion and water flooding. Oil production by means of pure pressure depletion may result in an oil recovery of less than 20% of the original oil in place, depending on the initial pressure and the compressibility of the fluids. On the average, water flooding, whose purpose, in part, is to maintain reservoir pressure in order to recover more oil, leaves approximately two thirds of the OOIP as unswept and residual oil in the reservoir for further recovery. In fractured oil-wet reservoirs, this number might be even higher.

Enhanced oil recovery is oil recovery by the injection of materials not normally present in the reservoir. With a few minor exceptions, all EOR methods fall distinctly into one of three categories: thermal, solvent, or chemical. Each method can be divided further into individual processes. And foam flooding could fit into all three.

Thermal methods, particularly steam drive and steam soak, have been commercially successful for over 30 years. Thermal methods rely on several displacement mechanisms to recover oil. The most important mechanism is the reduction of crude viscosity by increasing temperature. Thermal recovery continues to be an attractive means of maximizing the value and reserves from heavy oil assets. However, the viscosity reduction is lower for lighter crude oil. Therefore, thermal methods are not nearly as advantageous for light crudes.

Solvent flooding refers to those EOR techniques whose main oil recovering mechanism is extraction, dissolution, vaporization, solubilization, condensation, or some other phase behavior change involving the crude oil. It includes CO2 flooding, nitrogen injection, natural gas injection, etc. These methods sometimes have additional recovery mechanisms such as viscosity reduction and oil swelling, but their primary mechanism must be extraction.

Chemical methods include polymer methods, surfactant flooding, foam flooding, alkaline flooding, etc. The mechanisms of chemical methods vary depending on the chemical material added to the reservoir; chemical methods may achieve one or more effects: interfacial tension reduction, wettability alteration, emulsification, or mobility control. The technical limitations of chemical flooding methods are insufficient understanding of the mechanisms involved and a lack of scale-up criteria. Furthermore, the process should be cost-effective.