Technology Update Tertiary oil recovery technologies can extend the economic life of maturing waterflooded reservoirs. This article describes the results from a biologically based enhanced oil recovery (EOR) technology that has improved waterflood efficiency by increasing oil production and decreasing the decline rates, thereby significantly increasing the recovery factor. Traditional tertiary recovery processes such as thermal methods, CO2 flooding, and chemical flooding require significant changes to field infrastructure and usually involve relatively high operating expenditures of up to USD 50 per incremental barrel of oil produced. Called Activated Environment for the Recovery of Oil (AERO), the technology used in this project represents a breakthrough in biologically based EOR by using a continuous injection of inorganic nutrients to stimulate indigenous microbes. The use of continuous injection (water and nutrients), which differentiates AERO from most previously attempted microbial EOR methods, prevents production disruptions and makes it easier to accurately measure, assess, and document the production benefits. The key advantages of this method are Decreased decline rates, enabling significant reserve gains Increased oil production Low capital expenditures Low operating costs Rapid response Biological EOR uses inorganic nutrients to activate indigenous microbes, those native to the field. Because no organic carbon is introduced, the microbial growth is restricted to the interface between the injection water and the oil, the carbon source for growing the microbes. The use of indigenous microbes is advantageous because they are perfectly suited to the local conditions and, unlike externally originated organisms, are neither costly to produce nor prone to rapid death in the reservoir and are thus in need of replacement. In addition, the concentrations of the nutrients required are relatively low, a major reason for the feasibility of continuous injection and stimulation. The technology is among the most inexpensive tertiary recovery methods available and requires only minimal changes to waterflood facilities for deployment. It can likewise be used with relatively minor modifications in fields without an operating waterflood, such as fields with a natural waterdrive or very mature fields where waterflooding has ceased. The biological EOR technology is producing a growing body of positive results, such as the following example from Canada.