The Analysis of Gas Productivity by the Influence of Condensation Bank near Well

Abstract

While producing gas from the reservoir to the surface, there exists a unavoidable pressure drop near the wellbore. Due to excessive pressure drops, a condensate drops out from gas and flows with gas towards a well because of compositional changes in PVT phase behavior. Since these all make a productivity of reservoir worse and reduce an overall recovery, many studies of condensate bank phenomena have been performed for a long time. In this study, a reservoir simulation has been conducted to investigate the effect of well's productivity due to the behavior of condensate bank near well. Also this study presents the effects of heterogeneity on condensate banking and conformance control methods to minimize condensate bank problems effectively. From simulational results of condensate bank phenomena in a gas-condensate reservoir, the condensate forms rapidly in the vicinity of the well as soon as gas production begins since the bottom-hole pressure is much lower than the dew-point pressure. The accumulated condensate causes a blockage effect on the movement of reservoir fluids and reduce the effective permeability to gas appreciably, and also causes the decline of well's productivity. The trend of producing GOR shows the abrupt changes which represents typical complex behaviors of gas-condensate reservoirs. In a very heterogeneous reservoir, the bank radius expands up to 100 ft in a day and the maximum saturation of condensate reaches 0.61. Therefore, the mobility of gas is dropped and the overall recovery of oil and gas is decreased. While oil saturation has a highest value near the well in general, in case of reservoir with a strong heterogeneity, some regions away from the wellbore show high level of oil saturation according to local distributions of permeability. To increase the productivity, a different producing strategy was conducted by changing the manner in which the well is brought into flowing condition. The maximum gas and oil production can be achieved by lowering the BHP slowly in the first step of operation period instead of dropping the BHP quickly. The large pressure drops from the beginning of operation makes the formation of condensate bank earlier and increases the producing GOR due to the growth of condensate bank. Accordingly, the implementation of optimal production strategy is essential by considering the condensate content, technical factors, economics depending on market price as well as gas and condensate production profiles in long term.