With the increasing wide use of hydraulic fractures in the petroleum industry it is essential to accurately predict the behavior of fractures based on the understanding of fundamental mechanisms governing the process. The prevailing approach for hydraulic fracture modeling relies on Linear Elastic Fracture Mechanics (LEFM). Generally, LEFM that uses stress intensity factor at the fracture tip, gives reasonable predictions for hard rock hydraulic fracturing processes, but often fails to give accurate predictions of fracture geometry and propagation pressure in soft/unconsolidated formations. The reasons are that the fracture process zone ahead of the crack tip, elasto-plastic material behavior and strong coupling between flow and stress cannot be neglected in these formations. Recent laboratory testing has revealed that in many cases fracture propagation conditions cannot be described by traditional LEFM models. Rather, fractures develop in cohesive zones. In this study, we developed a fully coupled poroelastic and poroplastic hydraulic fracturing model with cohesive zone method which is able to model crack initiation and growth considering process zone effects. The impact of formation plastic properties on fracture process is investigated for both short term and long term injection and the results are compared with elastic formation. In addition, the main factors that affect the poroelastic backstress and the effects of formation plasticity on a hydraulic fracture are also investigated. The results indicate that both poroelastic backstress and formation plasticity lead to higher net pressure, but they have an opposite impact on fracture geometry. Poroelastic backstress tends to reduce fracture width while formation plasticity tends to increase fracture width. It was also observed that plasticity can reduce closure pressure below minimum horizontal stress. Poroelastic backstress is mostly controlled by the leak-off rate and formation permeability and the effects of plasticity are mostly influenced by in-situ stress, plastic properties and pore pressure. We also found that the effective toughness method which is used to quantify fracture process zone effect in soft formation tends to underestimate the fracture width and overestimate fracture length even if it can match the net pressure. Ignoring plasticity in certain formations can lead to inaccuracy of fracture geometry, net pressure prediction and fluid efficiency calculations. The deviations of real fracture geometry from elastic formation model can have great impact on well production, especially in unconventional, low-permeability reservoirs. So for a more accurate modeling of fracturing in plastic formation, the whole affected region around the fracture should be considered, especially when the plastic deformation area is large.

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