Influence of Heterogeneity on Nitrogen Foam Flooding in Low-Permeability Light Oil Reservoirs

The strong heterogeneity of (ultra)low-permeability light reservoirs such as the Tuha Oilfield has a serious impact on oil production. Nitrogen foam flooding is an effective means to improve oil recovery in this type of reservoir. However, a quantitative description of the effect of heterogeneity on nitrogen foam flooding has not been comprehensively studied from micro- and macroperspectives. Therefore, microscopic visualization flooding experiments and nuclear magnetic resonance (NMR) monitoring double-core flooding experiments were conducted. The former experiment has the advantage of intuitively showing the dynamic process of nitrogen foam flooding, while the latter experiment can quantitatively study the displacement effect of foam-flooding on pores of different radii and its relationship with permeability contrast. The results show that (1) nitrogen foam flooding can block the main flow channels caused by heterogeneity, thereby displacing large flaky residual oil formed after water-flooding in strongly heterogeneous reservoirs; (2) there is a positive linear relationship between the foam flooding recovery contrast (the ratio of the enhanced recovery rate of foam flooding between the relatively low-permeability core and the relatively high-permeability core) and the permeability contrast in the double-core experiments. When the permeability contrast reached 5, the water-flooding recovery contrast was as high as 2.84; and (3) foam flooding reduces the lower limit of displacement, and the effect of foam flooding on oil in small pores that had not been displaced by water-flooding increases with increasing permeability contrast. When the permeability contrast reaches 5, the enhanced recovery rate of these small pores after foam flooding can be as high as 28.43%.