Hydraulic fracturing in unconsolidated or poorly consolidated formations has been used as a technique for well stimulation and for sand control. Although a large number of hydraulic fracturing operations have been performed in soft formations, the exact mechanisms of failure and fracture propagation remain an unresolved issue. Conventional hydraulic fracturing models based on the theory of linear elastic fracture mechanics (LEFM) often lead to inaccurate results because large inelastic deformations and strong fluid-solid coupling are neglected in such models.

A fully three-dimensional hydraulic fracture growth model for soft sands is developed. Our model can simulate non-planar fracture growth in poro-elasto-plastic materials, fluid flow inside the fracture with proppant transport, and fluid leak-off from the fracture to the porous reservoir. This paper presents the formulation and implementation of a new model. The model is verified by comparisons with analytical solutions. The model predicts considerably higher net fracturing pressure due to plasticity, which is consistent with observations from the field and laboratory experiments. This is because the stress concentrations at the crack tip in a plastic material are lower than in an elastic material and the plastic yielding shields the tip from the fracturing pressure. This induces shorter and wider fractures than without plasticity. Also, fluid leak-off and the pore pressure diffusion in the reservoir are computed numerically in this model. Higher leak-off leads to a poroelastic backstress that builds up around the fracture and results in shorter fractures.