Imbibition plays a key role in the performance of injected fluids in enhanced oil recovery from low-permeability, tight reservoirs. Huff-n-puff with well-soaking and improving reservoir wettability can promote imbibition, resulting in forced imbibition. However, our understanding of its concept, influencing factors and the underlying mechanisms remains limited. Therefore, first, this work systematically reviews the relevant research and clarifies the concept of forced imbibition. Next, the mechanisms and enhanced oil recovery contributions of huff-n-puff and wettability improvement during the forced imbibition process are highlighted and summarized. Huff-n-puff and low-salinity water flooding are two key forced imbibition methods. Regarding huff-n-puff development, this work compares and analyzes the key controlling mechanisms and enhanced oil recovery effects of imbibition enhancement using three fluids: Water, gas and activated water. Gas is currently the most widely used huff-n-puff medium in oilfields because of its stronger mass transfer and diffusion capabilities, making it can enter smaller pore throats. Besides, water is also an irreplaceable huff-n-puff medium because of its fast energy replenishment, low cost, and environmentally friendly. Active water with the addition of surfactants, nanofluids and displacement systems can enhance the effect of water huff-n-puff, achieving a broader application potential. Subsequently, the enhanced oil recovery contribution rate of forced imbibition during the huff-n-puff process is discussed. Regarding low-salinity water flooding, this paper focuses on the mechanism of improving wettability and its effect on enhancing CO₂ imbibition. It not only offers a comprehensive understanding of the concepts mechanisms and enhanced oil recovery effects of the forced imbibition process but also provides valuable insights for theoretical research and field applications of forced imbibition-based enhanced oil recovery technologies.

Huff-and-puff is a key technology for the efficient recovery of oil and gas from tight reservoirs. Active water and CO₂ are two huff-and-puff media with great development potential; however, their effects on enhanced oil recovery and the contribution of imbibition displacement to enhanced oil recovery need further investigation. In this paper, short cores were spliced into long cores for huff-and-puff experiments, and then nuclear magnetic resonance testing was performed to test the transverse relaxation time spectrum of different core sections at different huff-and-puff cycles. Subsequently, the enhanced oil recovery effects, limited effective distances, and influencing factors of active water and CO₂ huff-and-puff were evaluated. Meanwhile, a comparative experiment without well soaking in some specific huff-and-puff cycles was designed to quantitatively split the contribution rate of elastic displacement and imbibition displacement. The results show that active water huff-and-puff mainly mobilizes crude oil in large pores, while CO₂ huff-and-puff can also mobilize crude oil in small pores. The cumulative oil recovery of active water and CO₂ after 4 cycles of huff-and-puff was 24.78% and 40.89%, respectively, and the limited effective distances were 6-8 cm and 8-10 cm, respectively. Elastic displacement is considered the main enhanced oil recovery mechanism of active water and CO₂ huff-and-puff, while imbibition displacement accounts for 20.86% and 31.52%, respectively. Due to its good diffusion and mass transfer ability, CO₂ can more fully participate in the mechanism of imbibition displacement and further improve oil recovery. The findings of this paper can provide valuable theoretical and field data support for the application of huff-and-puff technology in tight reservoirs.

The polymeric surfactant can be used as an efficient agent for enhanced oil recovery (EOR) because of its large bulk viscosity and good interfacial activity. However, there is a sparse understanding of its matching relationship with reservoirs and emulsification occurrence conditions, which may affect its migration and EOR efficiency. One intermolecular association molecule polymeric surfactant (IAM) was synthesized by micellar polymerization and characterized with ¹H NMR, FTIR, and TGA. The matching relationship between IAM and reservoirs was evaluated by comparing the viscosity retention rate of effluent in the core flow experiments. Moreover, the effect of the matching relationship on EOR in the heterogeneous reservoir was clarified with parallel core displacement experiments by considering different flow abilities of IAM in the high-permeability layer. The occurrence conditions of in-situ emulsification of IAM were evaluated via oil-water co-injection experiments under the different injection rates and oil-water ratios. Microscopic visualization displacement was carried out to compare the micro EOR mechanisms of different chemical systems. The results show that IAM features thickening, shearing resistance, viscoelasticity, thermal stability, and interfacial activity. The matching relationship between cores and IAM could be divided as hardly injected, flow limited, and flow smoothly, corresponding to the viscosity retention ratio of < 20%, 20%–80%, and > 80%, respectively. IAM could gain better EOR efficiency (17.69%) when its matching relationship to the high permeability layer was “flow limited”. The defined mixture capillary number shows that only when it is greater than 1 × 10⁻³, the in-situ emulsions can be generated. Compared to HPAM, IAM could reduce IFT and form vortices to more effectively displace film and corner remaining oils by stripping and peeling off crude oil. The formed emulsion accumulated at the pore throat could further increase flow resistance, which benefits swept area enlargement. This work could provide theoretical and data support for the parameters design in the polymeric surfactant practical application.