Tertiary recovery, also known as enhanced oil recovery (EOR), is a method used to extract additional oil from reservoirs after primary and secondary recovery techniques have been exhausted. This process typically involves injecting substances such as water, steam, or chemicals into the reservoir to increase pressure and improve oil flow. The goal of tertiary recovery is to recover a higher percentage of the original oil in place, often exceeding 30% of the total oil. Techniques can include thermal recovery, gas injection, and chemical flooding, each designed to optimize extraction and maximize production efficiency in oil fields.