Oil Reservoir on a Chip: Pore-Scale Study of Multiphase Flow During Near-Miscible CO2 EOR and Storage

Abstract

CO2 injection into oil reservoirs is widely accepted as an effective enhanced oil recovery and CO2 storage technique. While oil recovery and CO2 storage potential of this technique have been studied extensively at the core-scale, complex multiphase flow and fluid–fluid interactions at the pore scale during near-miscible CO2 injection have not, and this area needs more study. To address this, a unique high-pressure microfluidic system was implemented which allows for the optical visualisation of the flow using optical microscopy. The results show that during tertiary near-miscible CO2 injection, when CO2 phase contacts the oil, the oil spreads as a layer between the CO2 phase and water preventing CO2 phase from contacting the water phase. This is attributed to the positive value of the spreading coefficient. Furthermore, due to the presence of pore-scale heterogeneity in the chip, an early breakthrough of CO2 was observed causing a large amount of oil to be bypassed. However, after CO2 breakthrough, CO2 gradually started to diffuse and flow inside the bypassed oil zones in the transverse directions which is a characteristic of capillary crossflow. The driving force for this capillary crossflow was the interfacial tension gradient formed by the diffusion of CO2 into the oil phase and the extraction of light to medium hydrocarbon components from the oil into the CO2 phase. The same mechanism led to the recovery of the bypassed oil trapped in dead-end pores. This unique mechanism produced the majority of the bypassed oil after CO2 breakthrough and significantly increased the oil recovery. In our three-phase flow water-wet system, CO2 flow displaced the water through a multiple displacement mechanism which is unique to three-phase flow. CO2 displaced the oil in oil-filled pores thorough bulk flow, and the spreading oil layers were gradually produced by layer flow.