A micromodel investigation of two-phase matrix-fracture transfer mechanisms

Micromodels employing a two-dimensional representation of pore space were used to observe directly (via microscope) water imbibition into a matrix and matrixfracture interactions between wetting and nonwetting fluids. Within a single field of view, some pores are responsible for the uptake of water, whereas immediately adjacent pores expel nonwetting phase into the fracture. When water flow through fractures is relatively slow and fluid transfer from the fracture is relatively rapid, imbibition is microscopically cocurrent and micromodel observations teach that uptake of the wetting phase by the matrix correlates directly with the volume of water injected. This mode of transfer is coined a filling fracture. On the other hand, when fractures fill with water quickly relative to the rate of matrix-fracture transfer, the mass of water imbibed scales with the square root of time. Here imbibition is found to be countercurrent at the pore level. In the countercurrent mode, significant channeling of the nonwetting phase through the continuous wetting phase is observed that reduces the efficiency of water infiltration. Overall, it is found that the rate of water uptake from a fracture into an unsaturated matrix and the pore-level pattern of water infiltration depend critically on the rate of water infiltration through fractures.