Theoretical Basis of Isochronal and Modified Isochronal Back-Pressure Testing of Gas Wells

The theory of gas well testing is briefly reviewed and the theoretical validity of the isochronal and the modified isochronal tests is demonstrated for conditions of laminar flow. Introduction The isochronal and modified isochronal methods of gas well testing are in common use these days. Both of these methods could be called unsteady-state methods, as the reservoir is not required to reach steady state during the course of these tests. The practical utility of these tests is well established, but there seems to be no completely satisfactory discussion of their theoretical basis. The manual of gas well testing published by the Oil and Gas Conservation Board of Alberta gives a theoretical explanation for the isochronal test using the radius-of-drainage concept. For the modified isochronal test, the analysis presented in the Board manual is incomplete. In this short paper, theoretical bases for both tests are developed from the unsteady-state theory.