Analog outcrop

core and well logging data are integrated in this paper to calculate opening pressure of fractures and formation fracture pressure in different reservoir intervals and analyzed various factors affecting opening pressure of fractures based on evaluation of natural fracture characteristics.A sensitivity analysis was performed to determine proper injection pressure for low-permeability reservoirs.High dip-angle fractures are predominant in Chang-6 reservoir in Wangyao block in Ansai oilfield

and their main orientations are NEE-SWW

E-W

S-N and NW-SE

respectively.The opening pressures vary significantly in different reservoir intervals due to the control of buried depth

fracture orientation

pore fluid pressure and current geo-stress.The reasonable injection pressure should be determined by the maturity of local natural fractures accordingly.In areas with few natural fractures

water injection pressure should not be larger than formation fracture pressure

so as to avoid large-scale fracturing that leads to water breakthough.In areas with dense fractures

if the fracture opening pressure is less than the formation fracture pressure

the rational injection pressure should be set less than the formation fracture pressure to avoid large-scale opening and connection of natural fractures; otherwise the injection pressure should be properly set according to the formation fracture pressure to avoid new large-scale fracturing.