Original Contribution to the Thermodynamic Modelling of Phase Behaviour in High-CO2 Niger Delta Gas Reservoirs

High carbon dioxide concentrations in certain Niger Delta gas reservoirs create complex phase behaviour that challenges the predictive capabilities of standard cubic equations of state, leading to systematic errors in reservoir depletion forecasting and facility design calculations. This study makes an original theoretical contribution to thermodynamic modelling of phase behaviour in high-CO2 Niger Delta gas reservoirs, developing and validating modified equation of state (EOS) formulations specifically parameterised for the CO2-hydrocarbon systems characteristic of these reservoirs. The research employs a fundamental thermodynamic research methodology, beginning with synthesis and critical evaluation of existing CO2-hydrocarbon binary interaction parameter data in the literature, followed by high-pressure PVT laboratory experiments on CO2-enriched gas samples from three Niger Delta reservoirs with CO2 contents ranging from 8 to 22 percent molar. Experimental measurements include constant composition expansion, constant volume depletion, separator tests, and retrograde condensation volumes at reservoir conditions. The Peng-Robinson and PC-SAFT equations of state are systematically re-parameterised and evaluated against the experimental data, with novel temperature-dependent binary interaction parameter correlations derived for CO2-CH4 and CO2-C3+ pairs at Niger Delta reservoir conditions. The study's original contributions include: a Niger Delta-specific EOS characterisation protocol, a CO2-enriched gas viscosity correlation validated against falling cylinder viscometer data, and a phase envelope shift quantification demonstrating the practical implications of CO2-induced dew point suppression for well deliverability calculations. Validation demonstrates a reduction in dew point prediction error from 18 percent (standard PR-EOS) to under 5 percent (proposed modified EOS). Recommendations include adoption of the modified EOS into Nigeria's standard PVT reporting requirements and integration into commercial reservoir simulation software. Keywords: thermodynamic modelling, CO2, phase behaviour, Niger Delta, equation of state.