Initiation Pressure and Corresponding Initiation Mode of Drilling Induced Fracture in Pressure Depleted Reservoir

With the production of oil and gas from the reservoir for a long period of time, pore pressure will decline from the initial value to a lower level, which narrows the safety mud weight window, and consequently, makes it easier to generate the drilling induced fracture (DIF). In this paper, a new analytical model is proposed for predicting initiation pressure and corresponding initiation mode of DIF in the pressure depleted reservoir. The effect of pore pressure decline on stress field is considered. Formation around the borehole is divided into plastic zone and elastic zone according to the geomechanical parameters, and small deformation theory is adopted in both of the plastic zone and the elastic zone. For the plastic zone, the nonlinear constitutive relationship is captured using equivalent stress and equivalent strain. In addition, excess pore pressure theory is introduced to describe the pore pressure change during the drilling process owing to the formation of mudcake on the borehole wall. Then, the stress and pore pressure distribution in these two zones and the radius of the plastic zone are obtained. Meanwhile, the theoretical formula of initiation pressure and the corresponding initiation mode of DIF are derived. The reliability of the new model is validated by comparing the obtained results with other published models and the field measured data.