A Mechanistic Approach to Understanding Wellbore Phase Redistribution
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AbstractWellbore phase redistribution (WPR) frequently occurs during a shut-in test in wellbores having very compressible fluids, such as low-pressure, single-phase gas and high gas/liquid ratio Multiphase fluid mixture, including steam. The consequence of WPR is a pressure buildup signature that deviates greatly from the commonly used constant-storage model, thereby precipitating test interpretation problems. A lack of physical understanding of the underlying process has led us to rely solely on empirical approaches - exponential and error function models - for analyzing field data. The existing methods have some drawbacks. For example, one may obtain nonunique results. Further, we cannot predict whether a particular well will exhibit WPR a priori. Thus, the available methods are restricted to the test analysis mode only. In this work, we present a mechanistic approach to understanding the principal causes for WPR. A simple physical model, consisting of a liquid column and a small pocket of segregated gas at the top, is assumed to mimic a wellbore. The model well cannot receive any fluid from the reservoir upon shut-in; however, backflow from the wellbore into the reservoir is permitted to relieve excess pressure, generated by a rising bubble. A mathematical model is developed for the idealized well by studying the rise velocity of a single bubble in the liquid column. Interestingly enough, a simplified analytical solution leads to an exponential form to describe the excess pressure behavior, caused by a single-bubble rise, as hypothesized by Fair. Thus, this work provides a basis for Fair's intuitive exponential model. The results show that both the wellhead and bottomhole pressures increase with elapsed time as a single gas bubble ascends up the liquid column. The magnitude of pressure rise is a strong function of flow impediment or skin in the wellbore vicinity, well diameter and its orientation, and wellhead pressure. A comparison of the simplified (exponential) and the rigorous solutions shows good agreement between the two at early times. The proposed model provides a framework for understanding the physical mechanisms that underlie an anomalous pressure rise during a shut-in test. The model, although simplistic in nature, provides some justification for the use of existing empirical Methods for interpreting field data. IntroductionOccurrence of phase redistribution and its consequent impact on pressure buildup tests was first reported in the late 1950s. Although the underlying physical mechanism was speculated, no model was offered to justify such postulation. However, lack of a complete physical understanding of the process did not inhibit the development of empirical models to interpret well tests. Because of the increase in number of model parameters (by two), solution uniqueness cannot always be guaranteed, especially in complex reservoirs. Nonetheless, these models remain the only interpretive tools to date. One drawback to this empirical approach is that one cannot predict whether a well would exhibit WPR a priori; consequently, corrective actions, such as downhole shut-in or flowmetering, cannot be planned. We recently showed how a wellbore/reservoir model could predict WPR phenomena using a host of wellbore arid reservoir input parameters. The model also aids test interpretation. A hybrid approach was used to couple the wellbore (numerical) with the reservoir (analytical). Subsequently, this work was extended by Xiao et al. with a robust wellbore/ reservoir coupling. P. 681^
