This paper investigates the effect of pore pressure change on the interaction of induced hydraulic fracture and natural fracture and the effect of natural fracture orientation. Warpiniski and Teufel's interaction criteria were adopted to evaluate the fracture propagation that would occur after an induced fracture intersects an existing natural fracture.
A fully coupled finite element based model is used to simulate hydraulic fracture propagation in a poroelastic formation with existing natural fracture. In order to study the effect of pore pressure change on interaction zone, a poro-elastic solution for the stresses in the interaction zone has been used as a basis for fracture interaction criteria. From the results of this study, the following were observed when the induced fracture interacts with the natural fracture with changing pore pressure.At low angles of approach with low to high differential stress, the induced fracture is arrested by the natural fracture, then the natural fracture begins to open, diverting the fracturing fluid and preventing the induced fracture from crossing.At medium (60 deg and above) angles of approach fracture opening and crossing both was observed depending on the differential stress. In low differential stress opening occurred and in high differential stress (more than 600 psi) induced hydraulic fracture cross the natural fracture.At high angles of approach crossing occurred at all differential stresses.Pore pressure change does not have any direct impact on interaction type but it accelerates the reinnitiation pressure needed to cross the natural fracture. The pressure build up rate inside the natural fracture is influenced by the pore pressure change.
This paper depicts that the effect of pore pressure change and the orientation of the natural fracture is of immense significance in hydraulic fracture propagation when it interacts with natural fracture. The study includes small to large natural fracture and low permeable formation.
Change in pore pressure in chemically active rocks such as shale is caused by several mechanisms and numerous studies have been carried out to investigate these mechanisms. However, some important coupling terms or driving forces have been neglected in these studies due to simplifying assumptions. In this study, a hydro-chemo-thermo-electrical model based on finite element method is presented to investigate the change in pore pressure in shale formations resulted from thermal, hydraulic, chemical and electric potential gradients. The change in pore pressure is induced by hydraulic conduction, chemical, electrical and thermal osmotic flow. In order to solve the problem of ion transfer under the influence of an electrical field, the Nernst-Planck equation is used. In addition, ion advection is considered to investigate its possible effect on ion transfer for the range of shale permeability. All equations are derived based on the thermodynamics of irreversible processes in a discontinuous system. The numerical results are compared against existing and derived uncoupled analytical solutions and good agreement is observed. The numerical results showed that the ion transfer and pore pressure are considerably affected by the electric field in the vicinity of the wellbore. It was also found that advection can play a remarkable role in ion transfer in shale formations. It was further shown that the change in pore pressure in shale formation is characterized by the combined effect of hydraulic, chemical, thermal and electro osmotic flow.
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