Effective Parameters on Breakdown Pressure in Hydraulic Fracturing, Modeling with Finite Difference Method

Although hydraulic fracturing has many applications, but breakdown pressure from hydraulic fracturing process is very important, since this pressure is related to the in situ stresses. In hydraulic fracturing fluid over time, injected into a borehole until it reaches to the limit such that tensile fractures occur in the wellbore wall. At the moment of occurrence of fracture, fluid pressure within the wellbore, said the breakdown pressure that is equivalent to the peak point of the pressure-time curve. There are simple and classic relations that related breakdown pressure to the in situ stresses. Estimation of in-situ stresses is a major challenge in Geomechanic. In this paper, the finite difference modeling of hydraulic fracturing will be discussed. Modeling is base on two-dimensional plane strain assumptions. The purpose of the modeling is to study on parameters affecting the breakdown pressure, parameters that do not exist in the classical relations but affect the breakdown pressure. After validation of the model and in accordance with the results of this paper breakdown pressure not only is related to the in-situ stresses and rock tensile strength but also wellbore radius and pre-existing cracks in the wall of the wellbore are parameters that involved in hydraulic fracturing and breakdown pressure. For isotropic in situ stresses variation of wellbore radius dont effect on the breakdown pressure but for non-isotropic in situ stresses with increasing wellbore radius breakdown pressure decreases and with increasing deviatoric stresses (difference between in situ stresses), the rate of breakdown pressure reduction increases.