Experimental Study and Mathematical Model of Residual Oil Distribution during Gas Flooding in Unconventional Oil Reservoirs with Low-Field NMR

Gas flooding is a promising way to enhance oil recovery in unconventional oil reservoirs, but accurate identification of the distributions of movable and residual oil during gas flooding is difficult. In this study, the movable and residual oil distributions of tight conglomerate oil reservoirs during gas flooding are monitored with low-field nuclear magnetic resonance (NMR). The NMR T 2 spectra are converted into pore throat size distributions using a nonlinear conversion method in conjunction with high-pressure mercury intrusion, and then the lower limit of the pore throat size of movable oil under different pressure differences is determined. In addition, a mathematical model is proposed to predict the lower limit of the pore throat size of movable oil during gas flooding based on the capillary tube model and the fractal characteristics of pore structures in conglomerates. The research studies show that the model prediction results are very close to the results measured from experiments. The lower limit of the pore throat size of movable oil decreases with the increasing pressure difference of gas injection and decreasing core permeability. This study will be beneficial for characterizing residual oil distribution after gas flooding in unconventional oil reservoirs.