Volume 50

Multi-Type of Miscible Characterization Methods during CO2 Flooding in Low-Permeability Reservoirs with Different Water Cut Shiqiang Guo, Keliu Wu, Qingyuan Zhu, Shengting Zhang, Hong Ran, Kaifen Li, Jiahong Jiang

Abstract

Injecting CO2 into low-permeability reservoirs can synergistically enhance oil recovery and achieve carbon sequestration goals. The miscibility between CO2 and crude oil during the displacement process is influenced by pore scale and water saturation. To elucidate the characteristics of multiple types of miscibility in CO2 flooding at different water saturation levels in the low-permeability reservoirs of Block H in the JY Oilfield and to guide the design and parameter optimization of CO2-EOR schemes, we conducted CO2-crude oil phase behavior experiments, established a compositional model for CO2 flooding numerical simulation in low-permeability reservoirs, and proposed standards and characterization methods for the classification of multiple types of CO2-crude oil miscibility states under different water saturation levels. The influence of gas injection factors on miscibility state transitions was also determined. The study results indicate that Block H of the JY Oilfield contains medium-light crude oil with a saturation pressure of 4.762 MPa and a minimum miscibility pressure of 17.26 MPa. Under different water saturations, four types of CO2-crude oil miscibility states exist. As water saturation increases, the miscibility pressure threshold rises, and oil recovery decreases. Pore size and residual water significantly affect the miscibility state, with higher water saturation reducing the effectiveness of CO2 in improving crude oil mobility. Increasing the number of pore volumes injected and using high-purity CO2 favor achieving miscible and fully miscible flooding, resulting in high displacement efficiency and increased ultimate recovery. The study concludes that CO2-EOR scheme design for low-permeability reservoirs should prioritize blocks with medium to low water saturation and optimize injection and production parameters based on reservoir structure characteristics. For blocks with higher water saturation, employing near-miscible to miscible flooding at lower pressures can significantly reduce development costs and maximize economic benefits.

Keywords CCUS-EOR, miscibility state, enhanced oil recovery, low-permeability reservoirs, water cut, oil mobility ratio