Abstract: Due to the highly developed natural fractures and horizontal beddings in shale reservoirs, it is possible to generate complex volume fractures during hydraulic fracture treatment. In order to investigate the propagation process of complex fracture network, a three dimensional finite element model for volume fracture propagation of hydraulic fracture in shale reservoirs is built using fluid-solid coupling basic equations in porous medium and basic theory of damage mechanics. The numerical simulation results are in good accordance with laboratory experiment of shale fracture propagation, which prove the reliability of the numerical model. After a series of numerical simulations, some conclusions are drawn as follws: (1) The horizontal beddings can open and form horizontal fractures in hydraulic fracturing. They can intersect with vertical fractures and finally generate complex volume fracture network. (2) As the horizontal stress difference increases, the length of stimulated reservoir volume (SRV) can increase and the width can decrease, which means the ratio of length to width of SRV can increase. The length is the distribution distance of volume fracture along horizontal maximum principal insitu stress. The width is the distribution distance of volume fracture along horizontal minimum principal insitu stress. (3) As the pump rate of fracturing increases, the SRV length can decrease, width can increase, and the ratio of length to width of SRV can decrease accordingly. (4) When the residual tensile strength of the natural fracture becomes stronger, the width of SRV can decrease, the length and the ratio of length to width of SRV can increase. Research results can offer some references for hydraulic fracture design and operation of shale gas in China.

Key words: shale reservoir, natural fracture, horizontal bedding, fracture propagation, volume fracture