Oil recovery in a fracture of variable aperture with countercurrent imbibition: experimental analysis

摘要

A parallel plate model is widely used to simulate flow in a fracture due to simplicity of idealizing the fracture. However, a real fracture has characteristics of variable aperture in nature. To assess the effect of variable aperture in water flooding, we performed water injection tests through fractured samples initially saturated with oil. A casting method is employed to obtain aperture distribution of real fractures. From these results, we represented the morphology of the fractures using statistical parameters such as mean, coefficient of variation (Cv), skewness (S0, and an anisotropy ratio. Water is injected into unfractured sandstone samples with constant rate and into artificially fractured sandstones with an increasing flow rate. We interpreted cumulative oil recovery curves from the fractured rocks using corresponding statistical parameters in the results. The results show that morphology of a fracture has a significant effect on oil recovery as well as water injection rate. As Cv and S increase, overall efficiency of oil recovery decreases. Anisotropy ratio affects the breakthrough time of injected water and overall oil recovery as significant as Cv. At lower anisotropy ratio such as less than one, early breakthrough and poor oil recovery have occureed. The effect of the statistical parameters is pronounced as injection rate increases. We observed a point where imbibition rate shows identity regardless of fracture morphology, if both flow rate of injected water and matrix characteristics are the same.