Effects of Temperature and Pressure on CO2 Dissolution, Heavy Oil Properties, and Pore-Scale Oil Mobilization

Abstract

Shallow heavy oil reservoirs are prone to steam channeling and poor heat utilization during the middle and late stages of steam thermal recovery. CO2-assisted steam flooding is a promising follow-up recovery method because heating and CO2 dissolution can jointly reduce heavy oil viscosity, induce oil swelling, and promote oil mobilization. However, the limited pressure tolerance of shallow formations restricts CO2 injection pressure. Under these constraints, it remains unclear how effectively heating and CO2 dissolution can improve heavy oil properties and mobilize oil in pores of different sizes. Heavy oil from the Xinjiang Oilfield was studied at 40–120 °C and 3.5–7.0 MPa using PVT phase behavior experiments, viscosity measurements at elevated pressure, and microfluidic visualization. We quantified the relative contributions of heating and CO2 dissolution to oil swelling and viscosity reduction and characterized the resulting oil mobilization at the pore scale. Within the investigated temperature and pressure ranges, CO2 dissolution promoted oil swelling and produced an additional viscosity reduction of 50.31%–97.32% relative to degassed oil at the same temperature. The relative contributions of the two mechanisms to viscosity reduction varied with temperature and pressure. The temperature at which their contributions were equal increased from 62.5 °C at 3.5 MPa to 74.8 °C at 7.0 MPa, indicating that higher pressure partly offset the declining contribution of CO2 dissolution as temperature increased. Microfluidic experiments showed that 92.8%–96.9% of the recovery gain from increasing temperature occurred before CO2 breakthrough. Additional recovery from increasing CO2 pressure declined as temperature rose. This gain was greatest in larger pores at 40 °C and smaller pores at 60 °C, while differences among pore sizes diminished at 90 °C. These results inform the selection of temperature and CO2 injection pressure for steam recovery from shallow heavy oil reservoirs.