The determination of phase behavior properties of high-temperature high-pressure and rich condensate gases

Abstract

The recovery process of a rich condensate gas reservoir depends strongly on the phase behavior properties of the reservoir fluid. In this study, three rich condensate gases (referred to as gas 1, gas 2, and gas 3) were sampled from oilfields in northwestern China, with the smallest gas-oil volume ratio was only 983 m3/m3. It was found that when the fluid pressure decreased gradually from its reservoir pressure to the dew point pressure (DPP), a complicated change in the fluid color occurred, which first changed from reddish brown to a pale yellow, then darkened, and finally became a milky red. Finally, when the pressure equaled the DPP, the fluid color turned to reddish brown again, which is different to that of conventional condensate gases. After the fluid pressure decreased to below the DPP, the retrograded liquid amount increased quickly. The maximum percentage of retrograded liquid in gases 1–3 reached 23.44%, 12.16%, and 24.67%, respectively. By combining a suitable equation of state with a component characterization method, a thermodynamic model was further built to simulate the phase behavior properties of rich condensate gases. The simulation results of the DPP and the percentage of retrograded liquid agreed well with the experimental data. Further modeling results showed that with a decrease in temperature, a decrease occurred in the DPP, whereas the retrograde liquid percent increased quickly. This indicated that the retrograde phenomenon in the wellhole will be stronger than that in the reservoir, and thus an appropriate mining technique should be selected.