Fracture mode of unconsolidated sandstone based on elastic-plastic with linear strain-hardening

Unconsolidated sandstone reservoirs in Bohai oil region are characterized by shallow burial depths, weak natural compaction, showing obvious non-linear plastic characteristics during frac packing. Analysis and study of the fracture mode for the fracturing process of unconsolidated sandstone strata has important practical significance for prediction of fracture pressure and construction design of frac packing. Based on the constitutive mechanical model of rock for elastic-plastic with linear strain-hardening, this study establishes a stress model around wells in unconsolidated sandstone strata in elastic and plastic zones while considering the percolation threshold effect of fracturing fluid and verifies the correctness of the stress model by theoretical analysis. Based on the tensile failure and shear failure criteria of the rock, the prediction model of the fracture pressure of sidewall and 4 kinds of fracture modes in the frac packing process of unconsolidated sandstones were obtained, and the reliability of the model was verified using field data. The results show that the plastic zone was elliptical at the post-yield sidewall, and the long axis is in the direction of the maximum principal stress; the larger the coefficient of plastic modulus, the smaller the plastic radius, and the smaller the fracture pressure; the larger the yield stress, the smaller the plastic radius. The fracture mode changes from plastic tensile failure, plastic shear failure to elastic tensile failure; as the internal friction angle and cohesion gradually increase, the fracture pressure does not change after increasing to a certain value, and the fracture mode changes from plastic shear to plastic tensile. In terms of the physical properties of conventional unconsolidated sandstone in Bohai oil region, fracture mode is mainly shown as plastic shear failure.