With the accelerated process of the exploration and exploitation of natural fractured reservoir

coal bed me-thane

shale gas

tight gas

tight oil and complex lithology reservoir with low and/or ultra-low permeability

simulation of hydraulic fracture propagation is faced with big challenges from volume fracturing and real-time microseismic monitoring of hydraulic fracture.The propagation behaviors of complex fracture network are affected by reservoir lithology

geomechanics and natural fracture characteristics

etc.Research on the influences of natural fractures on the propagation of hydraulic fractures both at home and abroad was reviewed in this paper.The mechanical behaviors before

during and after the intersection of hydraulic fracture with pre-existing natural fracture determine the propagation of hydraulic fractures and the creation of fracture network.Before their intersection

tensile or shear debonding of cemented pre-existing natural fractures may start when induced fractures tip approaching.During their intersection

the natural fractures might be sheared to cause abundant fracturing fluid filtration

while the hydraulic fractures might penetrate natural fractures and propagate along the original path

or be arrested to propagate along the direction of natural fractures and re-initiate at its terminal or weak structure point.After their intersection

a complex fracture network may be formed with the simultaneous propagation of multiple fracture tips.True triaxial fracturing test system

combined with industrial computed tomography(CT)scan

acoustic emission device and X-ray diffraction

is the principal laboratorial means of studying the generation mechanism of complicated fracture network

while unconventional fracture model and extended finite element method(XFEM)are the main numerical methods for simulation of complex fracture network.XFEM is the most effective approach to deal with discontinuous analysis especially crack propagation problem

and has all the advantages of the finite element methods.As fracture fluid pressure is the driving force for hydraulic fracture propagation

XFEM-based seepage flow-stress-fracture propagation coupling is the future trend of complex fracture network propagation simulation.