Dynamic immiscible displacement mechanisms in pore doublets: Theory versus experiment

This paper presents a critical analysis of the immiscible displacement mechanisms at the pore level with the aid of the so-called “pore doublet model” (PDM), a parallel arrangement of small- and large-diameter capillary tubes. The PDM has been used by a number of investigators in past studies as an idealized model of the pore structure for the interpretation of the trapping of one phase by another immiscible phase during immiscible displacements in permeable porous media. The emphasis has been on the entrapment of oil under various conditions of waterflooding and on the distribution of the immiscible phases in the porous media in both drainage type and imbibition type of displacements. The results of this paper are not in agreement with some of the conclusions reached previously by other authors. Theoretical considerations of the motion of fluid-fluid interfaces in pore doublets are presented. These considerations are supported by the results of displacement experiments carried out in transparent capillary micromodels of pore doublets and pore multiplets connected in series. Some of the basic laws of immiscible displacement in pore doublets have been discovered and are described. These establish a hierarchy of displacement from various, different branches of a pore doublet or pore multiplet as the fluid-fluid interfaces advance from one node of the PDM to the next node downstream. The only factor that determines the entrapment of the wetting phase is the pore structure, whereas the entrapment of the nonwetting phase is determined by (i) the pore structure, (ii) the magnitude of capillary forces in relation to the magnitude of viscous forces, and (iii) under strongly wetting conditions, the mobility of the wetting phase in the form of bulk liquid films which, in the case of irregular pore cross sections and neck-bulge-neck type of pore arrangements, as a result of the instability of the liquid films, give rise to the snap-off of the nonwetting phase at pore constrictions ahead of the leading fluid-fluid interfaces.