A micromodel employing a two-dimensional representation of the pore space of a fractured sandstone was used to investigate matrix-fracture interactions and water imbibition into the matrix. Pore-level mechanisms of wetting and nonwetting fluid flow are observed directly with a microscope.

The rate of water flow through fractures is shown to be relevant to displacement at the pore level. A "filling-fracture" regime is marked by relatively slow flow of water through fractures, rapid fluid transfer from the fracture to the matrix, imbibition that is microscopically cocurrent, and recovery of the nonwetting phase that scales linearly with time. On the other hand, a "filled fracture" regime is noted when fractures fill relatively quickly with water and recovery scales with the square root of time. Additionally, imbibition is found to be microscopically countercurrent. Within a single field of view, some pores are responsible for the uptake of water while immediately adjacent pores expel nonwetting phase into the fracture. The cocurrent flow mode in the filling fracture regime was more efficient with respect to recovery of the nonwetting phase versus the volume of water injected.

Wetting and nonwetting fluids flowed simultaneously in both the fractures and the matrix. These observations provide insight into the discrepancies among fracture relative permeability curves presented in the literature. It is suggested that fracture relative permeability is sensitive to the total fluid velocity within fractures and the rate of uptake of water by the matrix.

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