Prediction of the Water-Flooding Efficiency of Low-Permeability Cores by Nanoscale Digital Core Techniques

The application of conventional high-and medium-permeability methods in determining water-flooding efficiency leads to a low dynamic coincidence rate in practical production because of the unfavorable pore structure and abnormal percolation characteristics of the low-permeability reservoir. It takes a long time to experimentally determine the water-flooding efficiency of low-permeability cores, thus leading to large errors and failures in meeting demand in time. This paper proposes a method for predicting the water-flooding efficiency of low-permeability cores by using nano-scaled digital core techniques. On the basis of the characteristics of low-permeability cores, including a small pore throat and high microheterogenicity, a multiscale trunk flow network characterization method was established not only to depict the overall core characteristics but also to retain full throat details. Furthermore, a digital core percolation simulation method based on a trunk flow network was established. The effect of the trunk flow network on the percolation characteristics was analyzed by digitally simulating the oil and water flows of low-permeability sandstone and a low-permeability sandy conglomerate. The results showed that there is a strong correlation between the connectivity of the trunk flow network and the water-flooding efficiency for a low-permeability core, i.e., higher displacement efficiency results from higher connectivity. The water-flooding efficiencies of the low-permeability sandstone and low-permeability sandy conglomerate were calculated by quantitative modeling (45.5% and 63.7%, respectively). The results are highly consistent with those obtained by laboratory experiments (47.1% and 62.9% respectively), thus proving that the new method is dependable.