Oil property changes during the waterflooding for reservoirs with condensate gas cap

Zanarol Oilfield is a giant carbonate reservoir with condensate cap, weakly volatile oil, and bioclastic limestone of low porosity and permeability. During the waterflooding stage, formation pressure decreased step by step resulting from the former depleting development and the strong heterogeneity of carbonate reservoirs. The gas cap spread outwards with the formation pressure dropping, causing gas breakthrough in production wells near the oil and gas contact, GOR increased and oil density decreased. In the internal oil region far from the oil-gas contact, the dropping reservoir pressure led to dissolved gas separating out from oil, making crude oil change to black oil from weak volatile oil. With the separated solution gas, the oil density, produced gas oil ratio and oil viscosity increased, the oil well productivity decreased, all these make waterflooding more difficult. The main method for improving development response is to keep formation pressure and improve the utilization factor of water injection.