1. n. [Well Testing]
A multirate test designed as a series of drawdown and buildup sequences at different drawdown flow rates, with each drawdown of the same duration and each buildup reaching stabilization at the same pressure as at the start of the test. The purpose of the test is to determine well deliverability. This type of test is most commonly done in gas wells.
2. n. [Production Testing]
A type of deliverability test conducted in gas wells. This test is used to generate a stabilized gas deliverability curve (IPR) without actually flowing the well for the time required to achieve stabilized conditions (pseudosteady state). This type of test is especially useful for low-permeability reservoirs. In an isochronal test, the well flows at a constant rate and then is shut in, allowing the pressure to build up to the average reservoir pressure. The same procedure typically is repeated four times. It is called isochronal because the flow periods are of the same length. A stabilized point (pseudosteady state) is usually obtained at the end of the test.