Experimental study on fluid flow in micro-fractures contained within matrix

Abstract

Two types of artificially fractured cores were prepared in the lab to investigate the effects of micro-fractures on fluid flow in matrix. Using naturally fractured cores and cores without fractures as control group, we analyzed fluid flow behavior in micro-fractures and their effects on fluid flow in matrix itself from the perspectives of threshold pressure gradient and oil displacement efficiency. The experimental results show that, for cores with similar physical properties, the presence of micro-fractures can significantly reduce the real threshold pressure gradient but has negligible effect on pseudo threshold pressure gradient. The matrix system should be classified as parallel or series system based on its relative location to macro-fractures. Pressure gradient in parallel matrix system is usually small and its real threshold pressure gradient is the key factor deciding whether the oil inside can be recovered. On the other hand, pressure gradient in series matrix system is generally large and its pseudo threshold pressure gradient is the key factor deciding whether effective injection-production communication can be established between two wells. Oil displacement efficiency in micro-fractures increases with the increase of micro-fracture aperture and their relationship can be approximated by a quadratic function within a certain range. However, the relationship between oil displacement efficiency in matrix and micro-fracture aperture is not monotone. Pore volumes of micro-fractures, oil displacement efficiency in micro-fractures, and water channeling within micro-fractures are the three main reasons accounting for this phenomenon.