Effect of trapped gas saturation on oil recovery during the application of secondary recovery methods in exploitation of petroleum reservoirs

The paper presents the effect of trapped gas during the application of secondary processes, or more precisely, in the period of application of conventional waterflooding of partially depleted oil reservoirs. These processes can be simulated by analogous tests on reservoir rock samples, on synthetic models under laboratory conditions or by numerical models. An explanation of physical mechanisms that take place during such processes is required to understand better the benefits of gas phase saturation, and additionally to that, favourable and unfavourable factors present during such processes are explained. Besides, here are as well explained procedures for finding optimal trapped gas saturation, by which, for the selected partially depleted reservoir and the selected combination of fluids, and by waterflooding with maintenance of a nearly constant reservoir pressure, maximal ultimate recovery is achieved. At the same time, it is a guideline that waterflooding should start when, during the natural process of recovery by solution gas drive, mentioned optimal gas saturation is accomplished. Finally, the paper gives a short review of the results of different simulation studies of such processes in which, inter alia, three-phase relative permeability curves with expressed effects of hysteresis are used.