Fracturing in unconsolidated sands differs from fracture growth in hard brittle rocks because of strong poroelastic effects and elastoplastic material behavior. The key observations from past experimental work are that the fracture propagation in unconsolidated sands is a strong function of fluid rheology and leak off and is accompanied by large inelastic deformations and shear failure leading to higher net fracturing pressures. A new model for fracture propagation in poorly consolidated rocks is presented in this paper in which the fracture propagation criterion is not based on the conventional stress intensity factor approach. Both shear and tensile failure are modeled, with tensile failure at the tip controlling fracture propagation. The model also accounts for reduction in porosity and permeability due to plugging by particles in the injected fluids. The model predicts considerably higher net fracturing pressures due to high leakoff and plastic yielding in shear around the fracture. The results of the sensitivity study conducted on key parameters such as the injection rate, fracturing fluid properties and mechanical properties of the sand are consistent with the observations reported in experiments.
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