Tracers are used in the oil industry in order to qualitatively or quantitatively gauge how fluid flows through the reservoir,^[1] as well as being a useful tool for estimating residual oil saturation. Tracers can be used in either interwell tests or single well tests.^[2] In interwell tests, the tracer is injected at one well along with the carrier fluid (water in a waterflood or gas in a gasflood) and detected at a producing well after some period of time, which can be anything from days to years. In single well tests, the tracer is injected into the formation from a well and then produced out the same well. The delay between a tracer that does not react with the formation (a conservative tracer) and one that does (a partitioning tracer) will give an indication of residual oil saturation, a piece of information that is difficult to acquire by other means.

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Tracers can be radioactive or chemical, gas or liquid and have been used extensively in the oil industry and hydrology for decades.^[3]