Investigating residual oil distribution and CO₂-EOR mechanisms in sand-conglomerate reservoirs using integrated CT and NMR techniques

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.