What Is an Oilfield Test Separator: Well Testing, Sizing and Instrumentation Answering the core question: What is an oilfield test separator? An oilfield test separator is a portable, skid-mounted pressure vessel used to measure the individual oil, gas and water production rates of a single well during a production test. It is functionally a small production separator, typically 0.3 to 2 m in diameter, but it is engineered for mobility and measurement rather than for