Study on imbibition mechanisms in tight oil reservoirs based on nuclear magnetic resonance and pore-scale simulation

Imbibition plays a crucial role in waterflood development and the soaking stage after fracturing in tight oil reservoirs, serving as an effective method to enhance oil recovery. To investigate the effects of complex pore structures and rock-fluid interactions on imbibition mechanisms in tight reservoirs, this study combined nuclear magnetic resonance (NMR) technology with pore-scale imbibition numerical simulation techniques, conducting imbibition experiments and pore-scale imbibition numerical simulations on tight cores with different pore-throat characteristics. In the imbibition experiments, NMR T2 spectra (transverse relaxation time) at different times were monitored in real time, which revealed the dynamic influencing patterns of pore structure on imbibition efficiency. In the pore-scale imbibition numerical simulations, realistic pore-scale physical models of tight sandstone were constructed based on thin sections, and the pore-scale imbibition process of tight sandstone was simulated by solving the Navier-Stokes equations combined with the phase field method. Based on the mutual verification of experimental and simulation results, the effects of contact angle, crude oil viscosity, and reservoir physical properties on imbibition efficiency were analyzed in detail. The results showed that the pore-scale imbibition numerical simulation results were in good agreement with the experimental data. The complexity of the pore structures significantly affected the oil displacement characteristics of imbibition, showing a relatively fast imbibition rate that gradually decreased with the extension of imbibition time. The aqueous phase preferentially entered smaller pores and then displaced the oil phase in larger pores. The smaller the contact angle resulting from rock-fluid interaction (i.e., the stronger the hydrophilicity of the rock), the greater the oil-water displacement driving force in the imbibition process and the higher the imbibition efficiency. In addition, a lower oil-water viscosity ratio and lower core permeability both generated stronger imbibition driving force. The research findings deepen the understanding of imbibition mechanisms in tight oil reservoirs at the microscopic level and provide theoretical foundation and experimental support for improving the development efficiency of tight oil reservoirs.