Phase Behaviour of a Near-Critical Gas Condensate Reservoir Fluid from a Conventional Field in the Niger Delta

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Abstract Near-critical gas condensate reservoir fluid like near-critical oil has a very complex hydrocarbon phase behavior and is likely to be encountered as deeper formations are been drilled in an extensive search for hydrocarbon deposits. It is a highly unconventional fluid and lacks adequate characterization approach in the open literature due to its complexity as it is found in the near critical region of a hydrocarbon phase envelope of a multi-component system. All known Equations of State models such as Peng Robinson (PR), Soave-Redlich-Kwong (SRK) etc, have been found to be inadequate in characterizing this fluid. This paper presents an experimental methodology and results of a Pressure-Volume-Temperature (PVT) analysis done on two near-critical gas condensate reservoir fluids from a gas field in the Niger Delta region. Constant Composition Expansion (CCE), single stage flash and reservoir fluid composition analysis were carried out using state-of-the-art visual PVT cell and gas chromatograph. The CCE test performed clearly showed the unusual but unique formation of two liquid phases rather than one liquid phase during the isothermal depressurization below the dew point pressure and the subsequent disappearance of the lower liquid phase. This critical opalescence phase behavior is unique to a near-critical gas condensate fluid. The CCE experiments also showed the maximum liquid drop out (LDO) behavior (about 45% condensate dropout) at a small pressure drop of 100-150 psig below the dew point pressure. The C7+ mole percent (<12.5%) and the produced gas-oil ratio (>5,000 SCF/STB) were other parameters used to further provide a reasonable insight that the fluid under study is a near-critical gas condensate. In conclusion, proper characterization of a near-critical gas condensate reservoir fluid will greatly aid the reservoir and production engineers to have a better and more robust field development strategy to produce and manage near-critical gas condensate reservoir.