Nonequilibrium override phenomenon between gases in condensate gas development

Abstract

The cyclic gas injection technique is widely used in the development of condensate gas reservoirs. However, the override of injected gas induces severe gravity segregation in formations, thereby substantially limiting the application of this technique. At present, few literatures have been found to illustrate the override phenomenon taking place between gases. In this study, experiments were performed, and nonequilibrium thermodynamic theory was used, to examine the phase behavior of injected dry gas and condensate gas in place during the gas injection process. It is observed that in the equilibrium state, the injected gas can potentially reduce the condensate content by about 80%, yet the equilibrium is hard to achieve and the accompanied gas override weakens the effect of injected gas. After gas injection, the visual cell in the pressure-volume-temperature test exhibits a combination of dry gas-condensate gas-condensate oil for the case in which the testing pressure fluctuates around the dew point pressure and a combination of dry gas-condensate gas for the case in which the testing pressure exceeds the dew point pressure. Displacement experiments that were performed on a two-dimensional sandpack helped to determine the dynamic evolution of gas override, and until the end of the tests, the overburden degree increases 13% and 28% for the symmetrical injection mode and the top injection mode, respectively. Further theoretical analysis indicates that after gas injection, the system is prone to becoming trapped in a nonequilibrium steady state in which the fully mixed scenario is unlikely to occur. The results of this study enrich the understanding of the nonequilibrium override phenomenon, especially for that occurs between gas phases, and are crucial to early prediction and intervention of gas override.