What Petrophysics and Formation Evaluation Are

A well costs a great deal of money, and most of what it tells us comes from a hole a few tens of centimetres wide. Nobody can walk into the reservoir and look. Instead, we measure physical properties of the rock and its fluids from the borehole and from rock samples, and we turn those measurements into answers.

Petrophysics is the study of the physical and chemical properties of rocks and the fluids they contain, and of how those properties are related. Formation evaluation is the practical work of using measurements made in and around a well to decide what the rock is, what fluid is in it, and how much of that fluid could be produced.

The two words are often used together. Petrophysics supplies the relationships; formation evaluation applies them to a particular well.

The decisions that depend on the answers

• Is the well a discovery? The decision to test, suspend or plug and abandon an exploration well rests largely on the log and pressure data.
• How much is there? Volume estimates use the area of the accumulation, the net thickness, the porosity, the hydrocarbon saturation and a fluid volume factor. Most of these inputs come from formation evaluation: logs give net thickness, porosity and saturation, and fluid samples feed the volume factor.
• Where should the well be completed? Perforation intervals are chosen to produce oil or gas and to stay away from water and gas contacts.
• Where should the next well go? Contacts, pressures and reservoir quality guide appraisal and development wells.
• Is the field performing as expected? Logs run years later show how fluid contacts have moved.