The mechanism of fracture initiation is the basic issue for hydraulic fracture technology.Because of the huge differences in fracture initiation mechanisms for different reservoirs,some successful fracturing techniques applied to porosity reservoirs are ineffectual for fractured reservoirs.Laboratory tests using a process simulation device were performed to confirm the characteristics of fracture initiation and propagation in different reservoirs.The influences of crustal stress field,confining pressure,and natural fractures on the fracture initiation and propagation are discussed.Experimental results demonstrate that stress concentration around the hole would significantly increase the fracture pressure of the rock.At the same time,natural fractures in the borehole wall would eliminate the stress concentration,which leads to a decrease in the fracture initiation pressure.