Transient Analysis of Variable Rate Tests for Solution Gas-Drive Reservoirs

Abstract

Abstract Most of the work on variable rate-variable pressure tests that is available today is related to single-phase systems. The purpose of this study is to examine variable rate-variable pressure tests (VRT), during the transient period, considering multiphase flow effects in the reservoir for solution gas-drive systems. The only procedures present today to evaluate VRT run under multiphase flow conditions use either pressure drop, pressure squared, or pseudopressure function. The first two approaches are empirical in nature and their use is restricted. The direct use of pseudopressure approach is more general but it requires the availability of representative relative permeability data. This paper presents methods that permit the determination of the mechanical skin and sandface effective permeabilities. These methods are derived from the pseudopressure function but they require only traditional information, i.e. rate and wellbore pressure versus time. The expressions developed in this study are useful for both single-phase and solution gas-drive systems. Synthetic and field data are analyzed to illustrate the use of the techniques developed in this work. Rate normalization, using pressure and pressure squared drop, and the use of superposition principle are analyzed, and their advantages and limitations under multiphase flow conditions are compared to the methodology presented in this study.