A MECHANISTIC APPROACH TO UNDERSTANDING WELLBORE PHASE REDISTRIBUTION

Wellbore phase redistribution (WPR) frequently occurs during a shut-in test in wellbores having very compressible fluids, such as low-pressu re, single-phase gas and high gas/liquid ratio multiphase fluid mixtur es, Including steam: The consequence of WPR is a pressure-buildup sign ature that deviates greatly from the commonly used constant-storage mo del, thereby precipitating test interpretation problems. In this work, we present a mechanistic approach to understanding the principal caus es for WPR. A simple physical moder,consisting Of a liquid column and a small pocket of segregated gas at the ton, is assumed to mimic a wel lbore. The model well cannot receive any fluid from the reservoir upon shut-in; however, backflow from the wellbore into the reservoir is pe rmitted to relieve excess pressure, generated by a rising bubble. A ma thematical model is developed for the Idealized well by,studying the r ise of a single bubble in the liquid column. The results show that bot h the wellhead and bottomhole pressures increase with elapsed time as a single gas bubble ascends up the liquid column. The magnitude of pre ssure rise is a strong function of flow Impediment or skin in the well bore vicinity, well diameter and its orientation, and wellhead pressur e. A comparison of the simplified (exponential). and the rigorous solu tions shows good agreement between the two at early times. The propose d model provides a framework for understanding the physical mechanisms that underlie an anomalous pressure rise during a shut-in test, The m odel, although simplistic in nature, provides some justification for t he use of existing empirical methods for interpreting field data.