Full Field Scale Hydrocarbon Gas Enhanced Oil Recovery Project in Offshore Vietnam -Response Analysis and Optimization Practice in Early Stage

Rang Dong field in offshore Vietnam has been producing oil since 1998 from a Lower Miocene sandstone reservoir. Upon the achievement of peak oil production in 2002, gaslift and water injection have been applied to maintain oil production. With primary and secondary oil recovery applications underway, a tertiary recovery technique by immiscible hydrocarbon gas enhanced oil recovery ("HCG-EOR") had been studied since 2009. Subsequent to a successful Pilot Test in 2011, a full field scale HCG-EOR with water-alternating-gas ("WAG") scheme has been implemented since 2014. This HCG-EOR application was the first commercial EOR project in offshore Vietnam and attracted attentions in the region. The project has been successfully executed to effectively minimize the decline and contribute approximately 30 percent of oil production rate as of the date of this paper. Numerous efforts for WAG pattern optimization and accurate incremental oil evaluation were attempted. This paper introduces coupled approach of well performance analysis and reservoir simulation study applied to the WAG optimization and incremental oil evaluation. In line with the theoretical reservoir responses of immiscible gas flooding, all the producers were classified into 3 groups by different types of performances following the WAG injection. By coloring the different groups on the well location map, reservoir engineers were able to understand the regional characteristics of response and managed to optimize WAG by allocating more gas to the regions where incremental production were dominated by tertiary recovery mechanism. For incremental oil evaluation, either decline curve analysis ("DCA") or compositional reservoir simulation is common practice, however, both techniques have its strengths and weaknesses. In this study, reservoir simulations for "past" incremental oil evaluation are always double-checked by DCA, and then prediction runs are performed. Such exercise could deliver the incremental oil evaluation of high confidence. In addition to discussion related to such subsurface evaluations, this paper also introduces lessons leant and mitigations on actual field operation with focus on intensified sand production, injectivity deterioration and back pressure surge after gas breakthrough.