Tertiary recovery, also known as enhanced oil recovery (EOR), is the third stage used to extract oil from an oil reserve. Since it's more costly than the primary recovery and secondary recovery stages, tertiary recovery is only carried out when the price of oil is high enough to justify the investment. It also involves somღe of the most controversial industry practices.

How Tertiary Recovery Works

Although there are many tertiary recovery techniques, all extract oil by changing its composition to make it easier to remove. This generally means lowering the oil's viscosity or thickness. Let's go through the first two stages to understand better why the methods for the third are used.

The primary recovery stage of extracting oil uses "natural" methods, typically taking advantage of the pressure difference between the oil well's surface and its underground reserves. When you see oil pumps and derricks dotting the landscape, called "nodding donkeys" or pumpjacks, these are typically involved in the primary recovery phase of oil extraction. This method works by increasing the pressure inside the oil reserve using steam or natural gas injections. However, this only recovers from about 5% to 15% of the oil in the reserve because pressure depletes over time.

More techniques are needed to get at more of the oil, which brings us to secondary recovery. The goal is to maintain or increase the reservoir's pressure and expel the oil toward the production wells. The most common secondary recovery technique is waterflooding, which involves injecting water into the reservoir to increase the pressure. Waterflooding is effective because water is readily available and relatively inexpensive as an injectant, though this doesn't begin to account for the potential ecological effects. Another common secondary recovery method is gas injection, which can involve injecting natural gas, nitrogen, or carbon dioxide into the reservoir.

Typically, secondary recovery can extract another 10% to 20% of the reservoir's original oil, although this varies widely depending on the composition of the oil in the reserve and area geology.

Since it comes after the first two methods and costs more, firms that extract oil may abandon wells without progressing to tertiary recovery if the oil price is not high enough to justify the expense. While the first two stages change the reservoir pressure to push the oil o✱ut (or to where it can be extracted more easily), the last changes the oil to make it extractable. There are three primary methods of tertiary recovery:

1. Chemical: Chemical injections involve pumping surfactants, solvents, and polymers into the reservoir. Surfactants and solvents lower the surface tension between the oil and water, and the polymers thicken the injection fluids. As with the other methods, this approach enables the oil to flow more toward the surface, while the increased viscosity of the polymer makes oil recovery more effective. Because of its complexity and environmental concerns, chemical methods are used less often than thermal or gas injection methods.
2. Gas injection: This method accounts for about 60% of EOR production in the U.S. Oil is extracted by pumping gas, such as carbon dioxide, nitrogen, or natural gas, into the reservoir. In the secondary method, these gasses expand, increasing the pressure of the reservoir and pushing the oil toward the surface. In the third stage, the gasses mix with the oil to lower its viscosity and increase its volume, causing it to flow to the production wells. Carbon dioxide and hydrocarbon gasses, like ethane, propane, and butane, can be used in these processes. The added benefit of using carbon dioxide is that some are left behind ("sequestered"), thus capturing it away from the atmosphere, where it can join other climate-changing gasses. In the past, the carbon dioxide came from naturally occurring carbon dioxide reserves. However, it is now possible to harvest carbon dioxide from 澳洲幸运5官方开奖结果体彩网:natural gas processors and fertilizer and ethanol production plants, though most oil fields are far closer to the naturally occurring reserves. Pipelines can then transport carbon dioxide to the injection site, making tertiary recovery more available and efficient than in the past. Nevertheless, much of the CO₂ used was already underground, not in the atmosphere, and ends up emitted into the atmosphere, not sequestered, until the techniques improve.
3. Thermal: These methods heat the reservoir heated by injecting water, which quickly converts into steam. The steam then warms the oil, causing it to thin and flow more easily toward the lower-pressure areas of the surface. 
   

Oil Recovery Stages vs. EPA Classes

While primary, secondary, and tertiary recovery refer to techniques for extracting oil from underground reservoirs, they don't correlate directly with the classifications used by the Environmental Protection Agency (EPA) under the Safe Drinking Water Act. Below, we make distinguishinཧg them easier with a table:

The EPA also has further classes, four through six, which generally don't involve oil production. Class IV wells are those that inject hazardous or radioactive waste into the ground that includes part of the water supply for an area. Class V is for injecting nonhazardous fluids into or above underground drinking water supplies. Class VI is for the underground, long-term storage of carbon dioxide.

Pros and Cons of Tertiary Recovery

The tertiary recovery method has several advantages. First, it enables the recovery of oil that couldn’t be retrieved from primary or secondary means, increasing productivity in an oil reservoir. Second, some EOR techniques, like nanobubbles, are free of the disadvantages of approaches and could, therefore, be more efficient. Finally, some EOR technologies could sequester some carbon dioxide, reducing their emission into the atmosphere.

Nevertheless, tertiary recovery has significant disadvantages. The high cost at the tertiary stage over the primary and secondary recovery stages i𓆏s its major drawback within the oil industry. This makes it too expensive unless🍃 oil prices are high. In addition, not all the oil can be recovered by this method. For instance, in the case of oil sands, only 75% of its tar-like bitumen can be recovered, leaving behind a huge amount of waste.

Most notably, besꦇides their cost and complexity, these techniques are among the most controversial of all modern industrial processes. Tertiary techniques can produce environmental, social, and health effects. Here are some methods best known👍 by the public:

• Acid: This is often what's meant by the chemicals discussed in EOR. It thus plays a significant role in tertiary oil recovery. As fields mature and primary and secondary recovery methods become less effective, acidizing can help move the remaining oil. For example, it can remove scale, drilling mud, and other substances that block pore spaces and reduce ways for the oil to move toward wells. Also, in reservoirs where other EOR methods like steam or carbon dioxide injections can't be used, acidizing can dissolve rock and make it more permeable for recovering the oil.
• Arctic drilling: EOR methods can play a significant role in Arctic drilling. Given the harsh conditions and high extraction costs, maximizing the recovery of oil from each well is crucial for the firms involved. EOR techniques such as gas injection can help by mobilizing the oil that remains after primary and secondary recovery methods. The Arctic environment is fragile, and any oil spill or mishap would have devastating effects on the local ecosystem.
• Hydraulic fracturing (fracking): Mostly used in the primary recovery phase but can also be used as a tertiary method. This involves injecting high-pressure fluid into 澳洲幸运5官方开奖结果体彩网:shale formations to create fractures, allowing oil or natural gas to flow more freely to the production wells. The method has drawn widespread concerns over groundwater contamination, induced seismic activity (earthquakes), excessive water use, and ecological degradation.
• Offshore deep-water drilling: Mainly primary recovery, though secondary and tertiary techniques are applied in later stages. These methods risk major oil spills and impact marine ecosystems even when they don't occur.
• Tar sands (oil sands) extraction: This often relies on techniques involved in tertiary recovery because of the need to reduce the viscosity of the tar-like bitumen for extraction. About 10% of all oil reserves are in the so-called 澳洲幸运5官方开奖结果体彩网:tar sands of Alberta, Canada, making it valuable when the price of oil goes up. Methods include surface mining and techniques that require great amounts of energy and water, cause great damage to surrounding land and ecology, cause greenhouse gas emissions, and pollute local groundwater.

As oil recovery requires more and more expense and effort, the lines between these stages can blur, especially as the sophistication of the processes involved increases with each passing decade, and the materials used (water, gasse♏s, etc.) are put to use in different stages.

The Bottom Line

Tertiary recovery is the third oil extraction phase used when the first two stages aren't enough. It uses heat, gas, and chemical injection to extract hard-to-reach oil. CO₂ could, in certain instances, be used to extract the oil and be requested underground where it can't help further heat the planet, but this is still very much in the offing, especially when oil prices go lower. EOR involves costly processes with significant environmental risks, making some of its techniques among the most well-known by the public. Nevertheless, these processes enable the recovery of vast amounts of oil that are used to power many of our modern economies.