Effect of buoyancy force on oil production rate by co-current spontaneous imbibition in tight oil reservior

Surfactant has been widely used to improving oil recovery by spontaneous imbibition in tight oil reservoirs. Surfactant can alter wettability but also reduce IFT. When IFT is ultra-low, the buoyancy force effect in tight matrix cannot be neglected. To quantitatively evaluate buoyancy force effect on oil flow rate and recovery factor by spontaneous imbibition, this paper proposes a new mathematical model for describing core scale spontaneous imbibition process coupling pore scale flow mechanism based on capillary tube model and fractal theory. Firstly, the equation for describing oil-water interface movement due to spontaneous imbibition in a single capillary tube model is established. Then, based on the fractal characteristics of pore size distribution in tight matrix, the core scale co-current spontaneous imbibition model considering buoyancy force effect is developed. Finally, the mathematical model is validated with the experiment data, and the effects of IFT and buoyancy force on imbibition flow rate and oil recovery are investigated. The study demonstrates that when the IFT is ultra-low, buoyancy force has significant influence on the SI flow rate and recovery factor of tight oil reservoir and cannot be neglected.