1. n. [Formation Evaluation]

A system that is in a transient state at the time of a measurement. In the case of permeability measurements on core samples, two transient techniques are used. In the pressure falloff method, the sample is at atmospheric pressure, either in a chamber or under a probe. Fluid at a higher pressure is released into one end of the sample. The decay of pressure with time at that end of the sample is recorded and analyzed by techniques similar to those used for transient well tests. In the pulse-decay method, the sample is held in a chamber and connected to two reservoirs, all of which are filled with a fluid at high pressure. The pressure in one of the reservoirs is increased a small amount and then re-connected to the sample. The change in pressure with time of this sample is recorded and analyzed using flow equations.

Antonyms: steady state

See: core plug,  inertial resistance,  permeameter,  retort method,  routine core analysis