Hydraulic fracturing initiation and near-wellbore nonplanar propagation from horizontal perforated boreholes in tight formation

Tight formations, which are rich in reserves in China, have a high economic development value. Perforated fracturing would need to be used widely to exploit these formations because of reservoir characteristics such as low permeability, low porosity and high density, resulting in high fracture pressure, limited fracture propagation, and complex fracture geometry. To address these issues, the mechanism of fracture propagation under different construction conditions must be understood and described. Therefore, the effect of different perforation parameters (shot density, shot length, shot diameter, shot phase, and shot interval spacing) and different horizontal stress contrasts on fracture geometry, initiation, and propagation from perforated horizontal wells in tight formations is studied based on numerous triaxial hydraulic fracturing simulation experiments. Three types of fracture geometry are observed in these experiments: A single flat fracture can be created by an initiated perforation or more initiated perforations with good connectivity, spiral-shaped fractures are generated by a large perforation diameter or high perforation density, and multiple-parallel fractures are induced by a high perforation phase. Moreover, each fracture type has certain pressure behaviors corresponding to a fracture curve shape. The perforations are likely to initiate when the angle between the perforation and maximum horizontal stress is small, and the initiation position is most likely at the base of the perforation. It is difficult to predict the initiation order of perforations; however, the first initiated perforation can be ensured by the profile of the relief pressure. In addition, a higher horizontal stress contrast value results in more initiated perforations, lower fracture pressure, and shorter breakage time. Ultimately, a large perforation diameter, high perforation density, and perforation phase of 60° should be chosen for fracturing at low fracture pressure and simple fracture geometry in tight formations.