A Comprehensive EOS Model for Extra-Heavy Oil Blended with Liquid Solvents: Development, Assessment and Validation

Abstract

Abstract Work is in progress to de-risk and mature liquid solvent injection for extra-heavy oil extraction from the Lower Burgan Abdali (ADBG) reservoir in North Kuwait. This is a deep reservoir (i.e. 9,000 ft) that is not believed to be amenable to steam-based EOR methods. This paper presents a workflow to create a holistic equation of state (EOS) describing the thermophysical properties of the selected solvents, extra-heavy oil, and their blend at different conditions. Such a model is essential to capture subsurface-to-surface variations in the mixture properties, thus allowing proper assessment of solvent-assisted oil production to design a fit-for-purpose pilot for further validation. Two multi-component liquid solvents and representative extra-heavy downhole oil samples were investigated using high-pressure laboratory testing. Different conditions were considered to measure fluid and density. Thereafter, a cubic EOS model was built to reproduce the extra-heavy oil lab-based PVT data. On a parallel track, another EOS model was also developed for the two selected solvents, namely: diesel and Solvarex. These two EOS models were merged into a unified model to generate reliable extra-heavy oil-solvent mixture predictions. A validation technique was suggested to verify the impact of lumping and pseudo-component overlapping on oil recovery and injected fluid breakthrough. Significant differences in the molecular weight of C36+ and the measured viscosity were observed through lab evaluation using representative downhole oil samples with negligible dissolved gas. Upon creating a single EOS model using multi-fluid regression, it was realized that the heaviest fraction had to be split into two pseudo-components, coexisting in both samples, but at different molar fractions. The components existing in extra-heavy oil and solvent were lumped mainly to avoid overlapping. The resulting EOS model was found to be robust as it can accurately reproduce the phase and volumetric behaviour of extra-heavy oil, solvent and their blend at the reservoir temperature and different pressures. Furthermore, it was established that, with such a robust EOS model, an arbitrary number of pseudo-components can be defined for the crude oil and solvents PVT models, without a considerable impact on oil recovery or injected fluid breakthrough.