Investigation on Microscopic Oil Recovery Characteristics of Depletion and Liquid Huff-n-Puff in the Mahu Conglomerate Reservoirs

This study investigates the microscopic oil recovery characteristics of natural depletion and liquid huff-n-puff in ultralow-permeability conglomerate reservoirs of the Mahu Oilfield. A series of experiments were conducted on conglomerate cores to simulate natural depletion, followed by three rounds of liquid huff-n-puff, utilizing nuclear magnetic resonance (NMR) to quantitatively characterize oil recovery and mobile pore-throat radius threshold. The results demonstrate that natural depletion yields limited recovery, with average recoveries of 9.9%, 8.9%, and 4.8% for cores of high, medium, and low permeability, respectively. In contrast, liquid huff-n-puff significantly enhances recovery, with surfactant A solution achieving the highest incremental recovery (24%) due to its superior interfacial tension reduction capability (0.03 mN/m). Surfactant B solution and simulated formation water increased the recovery by 17% and 14%, respectively. During natural depletion, the oil distinctly mobile pore-throat radii ranged from 0.27 to 1.7 μm for dual-peak pore structures (Types I-II) and from 0.04 to 0.09 μm for single-peak cores (Type III). Posthuff-n-puff, these thresholds decreased, indicating enhanced oil drainage from smaller pores. Notably, surfactant injection does not impede water imbibition in nanosubmicron pores, highlighting synergistic effects between interfacial tension reduction and capillary imbibition. Surfactant huff-n-puff is most effective for dual-peak cores with micrometer-scale pores, whereas imbibition dominates recovery in nanopore-dominated systems. Additionally, the first huff-n-puff round contributed the highest recovery increment, with diminishing returns in subsequent rounds. This work provides the first quantitative analysis of surfactant huff-n-puff efficacy postdepletion in conglomerate reservoirs, offering practical guidance for enhancing recovery in similar unconventional systems.