The study took Chang 6 reservoir in Ansai oilfield of Ordos Basin as an object to demostrate the essential ch-aracteristics and the formation mechanisms of waterflood induced fractures

based on the analysis of dynamic production features of the water-injecting lowpermeability reservoir.And the main formation mechanisms of waterflood induced fractures in the study area were numerically simulated in the end.Waterflood induced fracture is an injector-centered open fracture or a fast flow channel

which is formed when the water injection pressure exceeds the opening pressure of various fractures or the formation fracture pressure during the long-period waterflooding of low-permeability reservoirs.The fractures are an artificial geological attribute and the main cause of heterogeneity for waterflood development of low-permeability reservoirs

and they are widely occurring and inevitable in these reservoirs.Three formation mechanisms of waterflood induced fractures were summarized as follows:(1) when the water injection pressure exceeds the opening pressure of natural fractures

the natural fractures become open and are extended; (2) When the water injection pressure exceeds the formation fracture pressure

new fractures are developed throughout the injection life; (3) Other types of artificial fractures are generated by production or stimulation measures such as perforating and fracturing around the injector.In Chang 6 reservoir

high angle structural shearing fractures

in en echelon arrangement

are widespread approximately along the direction of major principal stress

and the dominate fractures are the first type of waterflood induced fractures.According to numerical simulation results

the bottom hole pressure of the injeciton well exhibits a continuous irregular periodic change with the continuous expansion of the waterflood induced fractures.