An analytical model for fracture initiation in elasto-plastic soft formations based on stress path analysis

Abstract

Most models developed for hydraulic fracturing initiation of borehole are based on the elasticity mechanics, and tensile fracturing is considered as the mechanism of fracture initiation. However, the lab and field data show that these models cannot properly predict the fracturing mechanism of soft formations. We proposed an elasto-plastic model to analyze the fracturing mode and fracture initiation pressure in the soft formations that considers the effect of overburden pressure and fluid flow. The analytical stress solutions under six different stress state are obtained, thus the effective stress path on the borehole wall is analyzed to determine the fracturing mode. The effective stress path analysis indicate that both tensile and shear fracturing are considered as the fracturing mode. The fracturing mode and fracture initiation pressure are influenced by the burial depth significantly. The tensile fracturing is more likely to occur in the shallow formation, while the shear fracturing usually occurs in the deep formation. In addition, the formation strength and mud cake also affect fracturing mode and fracture initiation pressure. The shear fracturing is more easily to be induced in the formation with low strength and tight mud cake. The preyielding around the borehole due to the low strength will decrease the fracture initiation pressure without affecting the fracturing mode of the borehole wall.