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Abstract Hydraulic fracturing is used widely in petroleum industry，which plays an important role in the production of low permeability reservoir. And hydraulic fracturing treatments are greatly affected by the selected perforation parameters. The geometric configuration is a key problem in hydraulic fracturing. The strata are simulated by a triaxial rock mechanics test system on artificial specimens with size of 300 mm×300 mm×300 mm. The fracture propagation mechanism of hydraulic fracturing is studied. Experiments are designed to investigate hydraulic fracture initiation sites，breakdown pressure and geometries，specially the tortuous fracture geometries resulting from fracture initiation and propagation through perforated vertical wellbore. The experimental results show that the tortuous fracture can be generated by using oriented perforation fracturing technology. And a model for prediction of hydraulic fracture initiation pressure from vertical wellbores with perforations has been presented under given in-situ stress condition. To investigate the sensitivity of the results to variable conditions，by changing the perforation angle and the horizontal stress difference，it is found that the breakdown pressure and hydraulic fracture reorientation path are functions of the perforation orientation. To reduce the breakdown pressure and reorientation path，the perforations should be placed with a smaller perforation angle. The effect of micro-annulus near the wellbore on the fracture initiation and its geometry is also discussed；it makes the fracture initiation as from an open-hole and the fracture geometry very simple. The conclusions drawn above provide valuable guidance to the design of hydraulic fracturing treatment and oil engineering practice.
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Received: 09 October 2008
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