Imbibition Characteristics and Pore Oil Mobilization of Chang 8 Tight Sandstone Under Different Imbibition Agents: An Experimental Study

To evaluate how field-applied imbibition agents affect oil recovery and pore-scale oil mobilization in Chang 8 tight sandstone, natural cores from Longdong, Ordos Basin, were tested using CNI-A, CQH-1, and EOS-3. Surface tension, oil–water interfacial tension, and contact angle were measured at different concentrations. Solutions with a mass fraction of 0.3% were selected for 144 h macroscopic imbibition tests and 96 h nuclear magnetic resonance measurements at 18 MPa. All three agents reduced surface and oil–water interfacial tension and altered core wettability. At a mass fraction of 0.3%, EOS-3 achieved the lowest oil–water interfacial tension (0.03 mN/m), whereas CQH-1 produced the highest average recovery after 144 h (32.9%), compared with 21.9% for CNI-A and 20.6% for EOS-3. Nuclear magnetic resonance relaxation-time distributions indicated oil mobilization across different pore populations and supported the stronger sustained performance of CQH-1. This contrast shows that interfacial tension alone cannot determine imbibition performance. In the field trial, Well X treated with CQH-1 showed a 28.4% increase in average daily oil production during the first month and a 23.1% increase in cumulative oil production over 90 days. Imbibition performance reflects the combined effects of wettability alteration, capillary driving force, interfacial resistance, and pore-throat structure. This study provides an experimental basis for selecting imbibition agents for Chang 8 tight sandstone reservoirs in Longdong.