Theoretical Development of a Thermodynamically Consistent Equation of State for Nigerian Crude Oil Mixtures Incorporating Heavy End Characterisation and Asphaltene Precipitation Modelling

The accurate representation of Nigerian crude oil fluid behaviour across the full range of production conditions from reservoir to processing plant requires an equation of state that captures both the complex heavy end compositional distribution and the thermodynamic conditions for asphaltene precipitation, neither of which is adequately treated in standard cubic equations of state. This study develops a thermodynamically consistent equation of state framework for Nigerian crude oil mixtures by extending the PC-SAFT equation of state with a modified colloidal instability model for asphaltene precipitation and an original C7 plus characterisation scheme calibrated to Nigerian crude compositional data. The theoretical framework introduces association parameters for asphaltene sub-fractions derived from systematic fractionation and molecular weight characterisation of twenty-two Nigerian crude oil samples spanning the full range of API gravity from 20 to 46 degrees. Thermodynamic consistency requirements including the Gibbs-Duhem stability criterion are rigorously enforced during parameter regression to prevent unphysical phase behaviour predictions. The developed equation of state framework is validated against seventeen independent PVT datasets not used in parameterisation, achieving a mean absolute deviation in saturation pressure prediction of 2.8 percent and asphaltene onset pressure prediction of 4.1 percent, compared to deviations of 6.4 and 12.7 percent respectively for the standard PR equation of state with conventional C7 plus splitting. Keywords: equation of state, PC-SAFT, asphaltene precipitation, Nigerian crude oil, PVT modelling.