Pore-scale study of the effects of grain size on the capillary-associated interfacial area during primary drainage

•The IFA production rate increases with decreasing grain size.•The IFA production rate for fingering is larger than that for stable displacement.•The disconnected IFA depends on the average volume of the clusters.•For stable displacement, the connected IFA is larger for larger grain sizes.•For fingering, the connected IFA is smaller for larger grain sizes. This study investigates the capillary-associated interfacial area during primary drainage in unconsolidated porous media for different mean grain diameters, employing pore-scale numerical simulation with the phase-field method. Five, two-dimensional porous media are generated with the same grain/pore size uniformity, but different mean grain diameters, ranging between 0.24 mm and 0.94 mm. Three cases are considered, representing stable displacement, crossover zone and viscous-fingering regimes. The findings reveal that the capillary-associated interfacial production rate increases with decreasing grain size. Specifically, the capillary-associated interfacial production rate is higher for the viscous-fingering regime compared to the stable displacement. The maximum capillary-associated interfacial area obtained for different grain sizes agrees with the correlation established in our previous study using the kinetic interface-sensitive tracer. Furthermore, the capillary-associated interfacial area is divided into two parts: one related to the disconnected water clusters and the other related to the connected water. The disconnected capillary-associated interfacial area correlates with the average volume of the clusters, following an exponential function. Furthermore, the average volume of the clusters depends on both, the grain size and displacement regime. Moreover, it is found that the displacement regime has a great impact on the connected capillary-associated interfacial area. For stable displacement, the connected capillary-associated interfacial area is larger for larger mean grain diameter. In contrast, for fingering, the connected capillary-associated interfacial area is smaller for increasing mean grain diameter.