Enhanced oil recovery (EOR) is a process for extracting oil from a well that has already gone through the primary and secondary stages of oil recovery. It involves altering the chemical composition of the oil to make it easier to extract. EOR techniques can be expensive and have environmental impacts, such as releasing harmful chemicals into the groundwater

There are three main types of EOR techniques: gas injection, thermal injection, and chemical injection

• Gas injection: This technique uses gases such as natural gas, nitrogen, or carbon dioxide (CO2) to force the oil to the surface and reduce its viscosity. CO2 is the most commonly used gas because it is cheap and abundant. However, it also contributes to greenhouse gas emissions and climate change
• Thermal injection: This technique uses heat to lower the viscosity and increase the flow of the oil. The heat can be generated by injecting steam, burning fuel, or using electric heaters. Thermal injection is mostly used in heavy oil reservoirs, such as those in California
• Chemical injection: This technique uses chemicals such as polymers, surfactants, or alkaline agents to improve the mobility and recovery of the oil. The chemicals can either reduce the surface tension between oil and water, increase the viscosity of the water, or alter the pH of the reservoir. Chemical injection is often used in combination with other EOR techniques

If you want to learn more about EOR, you can visit the Department of Energy website or read the Wikipedia article on this topic.