Enhanced Oil Recovery - Program Facts [118 KB PDF]

Most oil is produced in three distinct phases: primary, secondary, and tertiary, or enhanced oil recovery (EOR). The definition of tertiary or EOR is that something is added to the reservoir after secondary recovery in order to increase production. This can be gases, chemicals, microbes, heat, or even the addition of energy, such as the stimulation of the oil through vibration energy. The purpose of EOR is to increase oil production, primarily through an increase in temperature, pressure, or an enhancement of the oil’s ability to flow through the reservoir. The challenge of EOR is that the remaining oil often is located in regions of the reservoir that are difficult to access, and the oil is held in the pores by capillary pressure. The goal of DOE’s EOR program is to develop technologies that enable recovery of this remaining oil. During primary recovery, the natural pressure of the reservoir drives oil into the wellbore, and artificial lift techniques (such as pumps) bring the oil to the surface. Only about 10 percent of a reservoir's original-oil-in-place (OOIP) is typically produced during primary recovery.

A pumpjack in Hall-Gurney oilfield near Russell, KS, with an ethanol plant in the background. By-product CO2 from the ethanol plant is being injected into the Lansing-Kansas City formation of Hall-Gurney field as part of the first-ever CO2 flood in Kansas. This NETL project provides a source of CO2 for enhanced oil recovery that is otherwise unavailable in Kansas while creating an opportunity for sequestering the CO2, a chief greenhouse gas.

Since shortly after World War II, producers have employed secondary recovery techniques to extend the productive life of oilfields, usually increasing the recovery rate to 15-40 percent of OOIP. For the most part, those techniques involve injecting water to displace oil, driving it to the wellbore. In some cases, natural gas—often produced simultaneously with the oil—is reinjected to maintain reservoir pressure, thus driving oil into the wellbore.

This broad range in recovery rates stems from the unique reservoir parameters of each oilfield. The main parameters that determine reservoir productivity are the rock properties, fluid saturations, reservoir temperature, and pressure.

Conventional primary and secondary recovery operations typically leave behind two thirds of OOIP. This is reflected in the recovery to date of the U.S. oil endowment. In all, more than 600 billion barrels of oil has been discovered in the United States. Of that total, about 400 billion barrels is unrecoverable by conventional primary and secondary means; of this figure, about 200 billion barrels lies at <5,000 feet subsurface. That shallow remaining volume is the main target for EOR. Most EOR involves the injection of gases or chemicals or thermal enhancement. The injection processes can occur as flooding or as slugs (i.e., batches of fluids injected in phases) or as a combination of both. The combination processes typically include water as a flooding agent or as a slug for one of the phases in an effort to control costs.

Gas injection , particularly in the form of CO2 flooding [brochure- PDF], is the fastest-growing form of EOR in the United States. CO2 floods now account for about 4 percent of the Nation's oil production. The injected gas typically acts as a sort of solvent to reduce oil's viscosity, rendering it more mobile, while helping to sustain reservoir pressure.

Chemical EOR entails injecting chemicals either to reduce interfacial tension between the in-place crude oil and injected water, allowing the oil to be produced, or injecting other chemicals that can shut off excess water production, thus improving the “sweep” of a reservoir.

Thermal EOR entails introducing heat into the reservoir in a controlled manner to reduce oil viscosity. This method typically targets highly viscous, or heavy, crude oils. Because heavy oil is such an important component of the U.S. oil resource base, yet has been underutilized, DOE has devoted a special focus area to it—including “cold” recovery methods for heavy oil outside of thermal EOR.

DOE's EOR program also focuses on “novel” methods outside the industry mainstream. The novel category is a catch-all for any new ideas that don't fit into the traditional EOR world. Such alternatives could include microbial processes or the use of acoustic energy, seismic vibration, or microwave energy to render the oil less viscous and/or more mobile.

Subsurface imaging of EOR projects via computer simulation is an important component of tertiary recovery. Without such tools, an EOR operator cannot successfully optimize injection profiles, well patterns, sweep efficiency, and the like.

As part of its mission to educate the public on oil and gas technology, NETL also offers free EOR process drawings that illustrate the main EOR processes.

More on Improved Recovery Technologies

More on Exploration and Production Technologies