Experimental Study on the Effect of Microheterogeneity on Multiphase Flow in a Microscopic Visualization Model

Abstract

Nitrogen foam flooding has been widely used in fossil fuel reservoirs for enhanced oil recovery (EOR). However, from a microscopic perspective, the effects of microheterogeneity in various pore structures on both multiphase flow characteristics and EOR during foam flooding after water flooding have not yet been systematically investigated. Therefore, combining computed tomography scanning, casting thin section observations, and microelectronic photolithography, a visual micromodel with three permeability zones was developed. The microheterogeneity in different permeability zones was quantitatively characterized by integrating constant-rate mercury injection data with the fractal theory; then, some experiments were carried out to simulate water and foam flooding injection. Afterward, the multiphase flow characteristics and EOR mechanisms in the zones with different pore structures that exhibited microheterogeneity were analyzed. Finally, the remaining oil distribution and the improvement effect of nitrogen foam flooding in different permeability zones were quantified. The experimental results reveal the following findings: (1) during both water and foam flooding, the flow characteristics vary among the three permeability zones, and the effect of microheterogeneity is greater than that of permeability. The water flooding EOR (59.03%) in the high-permeability zone with the strongest heterogeneity is lower than that in the homogeneous medium-permeability zone (68.48%). (2) For different permeability zones, there are various methods to regenerate foam, which may significantly improve the heterogeneity in the intralayer (microheterogeneity of the pore structure) and interlayer (different permeability zones), and a stronger microheterogeneity results in a greater EOR. Therefore, the EORs in the high-, medium-, and low-permeability zones are 16.39, 8.43, and 14.64%, respectively. (3) Six types of remaining oil may be trapped and occur at various locations. Foam flooding may greatly reduce the remaining oil in the form of flakes and clusters, while the proportion of remaining oil in the form of thin films and columns may increase by 4.36 and 1.44%, respectively. This quantitatively characterizes the EOR micromechanism of foam flooding that transforms contiguous remaining oil into dispersed remaining oil. (4) The gas fraction in foam may gradually decrease from high-to low-permeability zones, indicating that foam is mostly in some larger pores and results in the flow of the liquid toward smaller pores or lower-permeability zones to further expand the sweep area. At the microscopic pore scale, the EOR mechanism of foam flooding that large pores, not small pores, are blocked and the sweep volume is expanded is demonstrated.