Microvisual Analysis of Matrix-Fracture Interaction

摘要

A micromodel employing a two-dimensional representation ofrnthe pore space of a fractured sandstone was used to investigaternmatrix-fracture interactions and water imbibition into thernmatrix. Pore-level mechanisms of wetting and nonwettingrnfluid flow are observed directly with a microscope.rnThe rate of water flow through fractures is shown to bernrelevant to displacement at the pore level. A “filling-fracture”rnregime is marked by relatively slow flow of water throughrnfractures, rapid fluid transfer from the fracture to the matrix,rnimbibition that is microscopically cocurrent, and recovery ofrnthe nonwetting phase that scales linearly with time. On thernother hand, a “filled fracture” regime is noted when fracturesrnfill relatively quickly with water and recovery scales with thernsquare root of time. Additionally, imbibition is found to bernmicroscopically countercurrent. Within a single field of view,rnsome pores are responsible for the uptake of water whilernimmediately adjacent pores expel nonwetting phase into thernfracture. The cocurrent flow mode in the filling fracturernregime was more efficient with respect to recovery of thernnonwetting phase versus the volume of water injected.rnWetting and nonwetting fluids flowed simultaneously inrnboth the fractures and the matrix. These observations providerninsight into the discrepancies among fracture relativernpermeability curves presented in the literature. It is suggestedrnthat fracture relative permeability is sensitive to the total fluidrnvelocity within fractures and the rate of uptake of water by thernmatrix.