An investigation on phase behaviors and displacement mechanisms of gas injection in gas condensate reservoir

Abstract

Cyclic gas injection into gas condensate reservoir is regarded as an effective development method to enhance condensate oil recovery. The aim of this work is to investigate the phase behavior and displacement mechanism of gas injection in gas condensate reservoir. Based on the equation of state and the phase equilibrium theory, the phase behavior of condensate oil in the formation mixed with various gas (CO2, N2, CH4, C2H6 and re-injected gas) has been analyzed. By calculating the Minimum Miscible Pressure (MMP), the miscibility of various gas was discussed and the effect of intermediate component in the re-injected gas on miscibility was evaluated. Moreover, a series of numerical simulations modeling the five-spot well network were conducted to investigate the displacement mechanism of gas injection using a compositional simulator. The results show that the injection of N2, CH4 and re-injected gas can raise the bubble point pressure of condensate oil and is conductive to the re-evaporation of condensate oil; whereas, injecting C2H6 or CO2 have little effect on it. The miscibility of re-injected gas increases with the intermediate component fraction. It is observed that at the reservoir pressure of 130 bar and reservoir temperature of 366.5 K, N2 and CH4 can evaporate the condensate oil in the formation by the function of gas extraction, with no miscibility; nevertheless, CO2 and C2H6 can be miscible with condensate oil to enhance the condensate recovery. For the cyclic gas injection, the higher the mole fraction of intermediate component, the more likely the miscibility occurs.