Experiment and numerical simulation of water injection induced dynamic fractures in ultra-low permeability reservoirs

In order to further clarify the mechanism of water injection induced dynamic fractures in ultra-low permeability reservoirs and its influence on water injection development in ultra-low permeability reservoirs, the genetic mechanism and propagation process of dynamic fractures induced by water injection were expounded based on the laboratory experiments of water injection induced dynamic fractures. A numerical characterization method of water injection induced dynamic fractures was established, and the corresponding reservoir numerical simulation was researched. The results indicate that the water injection induced dynamic fractures include natural closure type, artificial fracturing induced type and super reservoir breakdown pressure type. The main growth mechanism of water injection induced dynamic fractures was that the continuous extension of the fractures developed in rocks or the existing fractures through repeated action of injection pressure and rock breakdown pressure or fracture propagation pressure. The improved water injection induced dynamic fracture experiment shows that the injection pressure presents the characteristics of repeated "up and down", and the injection pressure is the main controlling parameter for the rock to generate water injection induced dynamic fractures. The numerical simulation results verify that the bottom hole pressure of the injection well shows a periodic trend of "pressure-build rise and crack initiation drop". After the dynamic fracture was induced, the pressure and saturation fields in the fracture system change dynamically with the opening and extension of the dynamic fracture and the changes are mainly obvious along the fracture system, and the influence range on both sides of the fracture system is small.