Investigation of Reservoir Fluid Characterisation Methods for Gas Condensate Reservoirs in the Offshore Niger Delta

Accurate reservoir fluid characterisation is foundational to development planning for gas condensate fields, where complex phase behaviour significantly affects production forecasting and facility design. This study investigates the accuracy and reliability of reservoir fluid characterisation methods applied to gas condensate reservoirs in the offshore Niger Delta, examining recombined wellhead sample analysis, downhole sampling, and equation of state (EOS) modelling approaches. The research employs an analytical comparative methodology, utilising compositional data from seven gas condensate wells across three offshore blocks, supplemented by pressure-volume-temperature (PVT) laboratory reports and EOS simulation using PVTsim software. The study critically evaluates the effects of sampling protocol quality on characterisation accuracy, investigating the impact of retrograde condensation during downhole sampling, recombination ratio errors in wellhead sampling, and the adequacy of common EOS models (Peng-Robinson and Soave-Redlich-Kwong) in representing near-critical gas condensate behaviour. A key gap addressed is the limited published literature on EOS tuning strategies specifically calibrated for high-CO2 Niger Delta gas condensates, which exhibit distinctive fluid behaviour relative to North Sea benchmarks. Findings reveal that EOS models without specific tuning for CO2 content systematically underpredict condensate dropout in the retrograde region by up to 18 percent, with significant implications for facility separator design and liquid handling capacity. The study proposes a standardised EOS characterisation workflow for Niger Delta gas condensate applications. Recommendations include mandatory PVT quality audits for high-value gas condensate development wells and industry adoption of the proposed EOS workflow. Keywords: gas condensate, PVT analysis, EOS modelling, reservoir characterisation, Niger Delta.