Analysis of influencing factors for breakdown point in horizontal wells under fracture interference

In order to understand the influencing factors for subsequent fracture breakdown point locations under the interference of induced stress by previously created fractures during staged fracturing in horizontal wells and their influencing law, and to guide the selection of perforation locations in staged fractured horizontal wells, the mathematical model for hydraulic fracture induced stress interference was built which had non-equal fracture half-length, non-equidistance and random fracture dip, and a mathematic model for calculation of breakdown point was established based on this. The results show that the previously created fracture parameters, including the number of fractures, their length and net pressure, and original dominant stress status azimuth of horizontal well and completion method can all affect the size of subsequent fracturing pressure, hence affecting the location of breakdown points. The original geostress, the number and length of previously created fractures and completion method pose a significant effect on the location of breakdown points, while the azimuth of horizontal well and net pressure of previously created fractures have little impact on the breakdown points. For sandstone reservoirs with small difference of original horizontal dominant stress, the severe interference between fractures may cause inversion of stress and increase the fracturing pressure of subsequently initiated fractures, causing difficulty in fracturing job. So the inter-fracture interference should be avoided as much as possible. For shale reservoirs, the staged multi-cluster perforation technique is used at present, and the fracturing pressure is even lower when the interval between stages is set at about 60 m, which helps fracture initiation.