Effective Pore-Throat Sweep Limits and Microscopic Oil Mobilization during CO2-WAG Flooding in Low-Permeability Conglomerate Reservoirs
Xiaoguang Wang1,2, Dehua Liu1,2,*, Zhenping Liu3, Zhen Luo3, Guihua Yang1,2, Zongfa Li1,2, Lijuan Huang1,2,*
1 State Key Laboratory of Low Carbon Catalysis and Carbon Dioxide Utilization (Yangtze University), Wuhan, China
2 School of Petroleum Engineering, Yangtze University, Wuhan, China
3 Research Institute of Exploration and Development, Xinjiang Oilfield Company, PetroChina, Karamay, China
* Corresponding Author: Dehua Liu. Email: 
; Lijuan Huang. Email: 
(This article belongs to the Special Issue: Complex Flows in Geological and Surface Processes)
Fluid Dynamics & Materials Processing https://doi.org/10.32604/fdmp.2026.079864
Received 29 January 2026; Accepted 11 June 2026; Published online 07 July 2026
Abstract
Low-permeability conglomerate reservoirs are characterized by complex pore structures and poor sweep efficiency, making the optimization of carbon dioxide-water alternating gas (CO
2-WAG) injection critical for enhanced oil recovery. To evaluate the displacement performance of CO
2-WAG and determine the lower limit of effectively swept pore throats, this study presents long-core flooding experiments conducted on cores from the KS Formation of the B Oilfield in the Junggar Basin under reservoir conditions (20.1 MPa and 58°C). Continuous CO
2 flooding and CO
2-WAG schemes with varying cycle numbers and gas-water ratios (GWRs) were systematically compared.
In situ online nuclear magnetic resonance (NMR) monitoring was employed to quantitatively characterize the mobilization of microscopic remaining oil during flooding. The results demonstrate that CO
2-WAG significantly outperforms continuous CO
2 flooding. Among the tested scenarios, three alternating cycles and a GWR of 1:1 yielded the highest oil displacement efficiencies of 64.8% and 66.13%, respectively, representing improvements of 23.6% and 24.53% over water flooding and 4.75% and 6.08% over continuous CO
2 flooding. NMR analysis revealed that water flooding primarily mobilizes crude oil in larger pore throats, with an effective sweep limit of approximately 18.2 nm. In contrast, CO
2-WAG extends the lower limit of effectively swept pore throats to 9.52 nm, enabling the recovery of oil trapped within mesopores and micro/nano-scale pore systems. Mechanistically, the injected water slugs improve sweep efficiency and suppress gas channeling, while the CO
2 slugs reduce crude oil viscosity and enhance displacement within smaller pores. This synergistic combination of macroscopic sweep improvement and microscopic displacement enhancement substantially increases oil recovery.
Keywords
CO
2-WAG; conglomerate reservoirs; oil displacement efficiency; nuclear magnetic resonance; producing limit
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