Quantification of Gas Exsolution Dynamics for CO2/CH4-Heavy Oil Systems with Population Balance Equations

Abstract

Although foamy oil phenomenon has been considered as the key factor that dominates heavy oil recovery, the existing models cannot be used to accurately quantify gas exsolution dynamics in foamy oil under various conditions due to the inherent physics and complex flow behaviour. In this study, experimental and theoretical techniques have been developed to quantify gas exsolution dynamics of CO2/CH4-heavy oil systems while considering gas bubble nucleation mobilization, and binary coalescence. Experimentally, constant composition expansion (CCE) tests were performed with a sealed PVT apparatus for the CO2/CH4-heavy oil systems to induce foamy oil behaviour by gradually depleting pressure at a constant temperature, during which the pressures and volume changes were monitored and recorded continuously. Theoretically, the Fick's law, equation of state, classical nucleation theory, and population balance equation have been integrated to describe the gas exsolution dynamics, during which gas bubbles are discretized with the fixed-pivot technique. The gas bubble number and size distribution in the induced foamy oil can then be determined once the deviations between the measured and calculated parameters, including liquid volume and pseudo-bubble point pressure, have been minimized with the genetic algorithm. For both CO2- and CH4-heavy oil systems, not only can a reducing pressure depletion rate or an increasing temperature result in a higher pseudo-bubblepoint pressure, but also gas bubble growth is strongly dependent on both temperature and diffusion of a gas component in heavy oil, while increasing the solvent concentration in the heavy oil tends to hinder the gas bubble nucleation and mitigation due to the higher pressure set for the experiments. During the generation of foamy oil, a higher temperature reduces heavy oil viscosity to accelerate the diffusion process, positively contributing to the gas bubble nucleation, binary coalescence, and bubble mobilization, respectively. Compared with CO2, CH4 induces a stronger and more stable foamy oil, illustrating that, at a lower temperature, foamy oil is more stable with more dispersed gas bubbles. In this study, the newly developed theoretical techniques are able to reproduce gas exsolution dynamics at the bubble level, allowing us to seamlessly integrate them with any reservoir simulators to not only accurately characterize foamy oil behaviour, but also evaluate the associated recovery performance.