Numerical simulation of sequential, alternate and modified zipper hydraulic fracturing in horizontal wells using XFEM

With the large-scale commercial exploitation of shale gas around the world, the multi-interval fracturing technique in horizontal well has played an important role to stimulate shale gas reservoirs. The commonly used fracturing methods in reservoir stimulation are the sequential fracturing, the alternate fracturing, and the latest proposed modified zipper fracturing (MZF), which has improved the shale gas production significantly. However, the mechanism of stimulation has not been well understood yet. This paper presents some numerical simulation results for the three different fracturing patterns by use of extended finite element method (XFEM). The numerical solution mainly considers the influences of the in-situ stress difference and the fracturing spacing on fracture propagation. The analytical parameters include the maximum principal stress, the principal stress direction and the fracture width distribution. The numerical results indicate that the induced stress from adjacent fractures is the key factor affecting the fracture configuration. And the stress interference becomes significantly serious when fracture spacing decreases or fracture number increases. Moreover, the in-situ stress difference can counteract the effect of stress interference on the fracture deviation and reduce the extent of deviation. Compared with the other two fracturing techniques, MZF generates larger maximum induced stress, but less change of the principal stress direction, which ensures a desired propagation path. Therefore, the optimal fracture spacing of MZF is smaller than that of sequential fracturing and alternate fracturing. Under the same stress difference and fracture spacing, MZF generally achieves better formation fracturing effects. The results obtained in this paper are of benefit to guide the high-efficient practices of hydraulic fracturing in horizontal wells.