Sand-conglomerate reservoirs, comprising cemented sand grains and gravels, exhibit strong heterogeneity and complex flow behavior, which often result in poor sweep efficiency and low recovery during waterflooding. To enhance oil recovery, a clear understanding of residual oil distribution and the enhanced oil recovery (EOR) mechanisms is essential. This study employed Nuclear Magnetic Resonance (NMR) and Computed Tomography (CT) scanning to investigate the distribution of residual oil after waterflooding and to evaluate the EOR mechanisms of CO₂ flooding, CO₂-water-alternating-gas (CO₂-WAG) flooding, and CO₂ intermittent injection. Results show that the residual oil is predominantly “bypassed oil” trapped in macropores behind coarse gravels, with macropores accounting for 77.6% of the total residual oil. This is primarily due to gravel-induced shielding and flow bypassing, which significantly reduce sweep efficiency. All three CO₂ injection strategies effectively mobilize “bypassed oil”, and intermittent CO₂ flooding achieves a higher CO₂-oil displacement efficiency than the other two. During intermittent CO₂ flooding, CO₂ not only partially extracts oil from the dominant channels but also dissolves into the “bypassed oil”, causing oil to swell, enter the flow channel and then be produced. Additionally, increasing soaking time and cycle number can effectively improve the performance of this strategy. Extending soaking time enhance the contact between CO₂ and “bypassed oil”, which is a key EOR mechanism. However, the incremental recovery diminishes with longer soaking time, potentially reducing overall operational efficiency. Thus, optimizing the operational parameters of these strategies requires further investigation.

The sandy conglomerate reservoir is tight and exhibits strong heterogeneity, rendering conventional water flooding and gas drive methods inefficient and challenging for the effective development. CO₂ water alternating gas (CO₂-WAG) injection as an effective enhanced oil recovery (EOR) method has been applied in heterogeneous reservoirs. Simultaneously, it facilitates carbon sequestration, contributing to the green and low-carbon transformation of energy. However, the EOR mechanisms and influencing factors are still unclear for the development of heterogeneous sandy conglomerate reservoirs. In this paper, we conducted core flooding experiments combined nuclear magnetic resonance (NMR) technology to investigate EOR mechanisms of the CO₂-WAG injection on the multiscale (reservoir, layer, and pore). The study compared multiscale oil recovery in sandy conglomerate reservoirs under both miscible and immiscible conditions, while also analyzing the effects of water–gas ratio and injection rate. In the immiscible state, the CO₂-WAG displacement achieves an oil recovery of approximately 22.95%, representing a 7.82% increase compared to CO₂ flooding. This method effectively inhibits CO₂ breakthrough in high-permeability layers while enhancing the oil recovery in medium- and low-permeability layers. Furthermore, CO₂-WAG displacement improves the microscopic oil displacement efficiency within mesopores and micropores. As the water–gas ratio increases, the total oil recovery rises, with enhanced oil recovery in low-permeability layers and micropores. Moreover, a gradual increase in injection rate leads to a decrease in total oil recovery, but it leads to an increase in oil recovery from low-permeability sandy conglomerate layers and micropores. In the miscible state, the displacement efficiency of CO₂-WAG is significantly enhanced, the total oil recovery three times higher than that in the immiscible state. In particular, the oil recovery from low permeability layers and micropores has further improved. Additionally, experimental results indicate that parameters such as water–gas ratio and injection rate do not significantly affect the oil recovery of CO₂-WAG miscible displacement. Therefore, maintaining the reservoir pressure above the minimum miscible pressure is the key to maximizing ultimate recovery factor in these reservoirs.