We present numerical results of a propagating hydraulic fracture in weak rock formations. The results were obtained from a Finite Element Hydraulic Fracturing model which simulates all the involved non-linear and coupled processes. The fracture is driven by pumping of an incompressible viscous fluid with Newtonian rheology. The fracturing fluid leak-off in the host rock formation is also considered. For propagation criterion a cohesive-softening model is implemented in interface finite elements and the bulk rock deformation is modelled by Mohr-Coulomb yield criterion. The main objective of this research work is to determine the influence of the stress-field, pore pressure field and pumping parameters on rock plastic deformation and its implications on fracturing net-pressure and fracture dimensions. It has been previously shown that in weak formations plastic yielding and the associated rock dilation create shielding of the fracture tip and hence larger pressures are needed to propagate the fractures and the created fractures are shorter and wider. We extend these studies to investigate the influence of the pore pressure which has been previously ignored. We found that high anisotropic stress field and high initial pore-pressure fields increase the plastic yielding in a rock formation demanding higher net-pressures for propagating a fracture and the created fractures are shorter and wider. This non-linear analysis and results may explain the differences observed in net-pressures between field measurements and conventional model predictions. These findings are important in improving the numerical simulators of modelling hydraulic fracturing in particular for short fractures in weak formations for sand control applications. The results are also important to better prediction of vertical fracture growth and containment in shale-natural gas stimulations where there are serious environmental concerns on the risk of ground-water contamination.

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