Experimental and Modeling Investigation of Fracture Initiation from Open-Hole Horizontal Wells in Permeable Formations

To investigate the evolution of hydraulic fractures in open-hole horizontal wells, a comprehensive experimental and analytical modeling investigation of fluid-driven fracture initiation in open-hole horizontal wells is presented in this paper. A large triaxial experimental system and a three-dimensional (3D) micro-computed tomography (micro-CT) imaging setup were used to simulate and investigate hydraulic fractures similar to those produced in field tests. We used an 8-mm-diameter and 100-mm-long hollow cylinder as a borehole within a 115 mm x 115 mm x 93 mm cement sample. The samples were first loaded in three orthogonal directions at different confining pressures to reproduce in situ reservoir conditions. Subsequently, a viscous fluid used in field operations with a viscosity of 60 mPa.s was injected into the borehole at 9 cc min(-1). During the tests, the injection pressure and rate were monitored. Then, the fracture morphologies were detected by 3D micro-CT scanning. The fracture initiation pressure increases as the wellbore orientation rotates toward the minimum principal stress direction. The 3D images demonstrated that an induced hydraulic fracture first extends along the horizontal wellbore and then turns to align with the preferred fracture plane. An analytical model considering the fluid penetration effect, which is represented by the injection rate, rock permeability and fluid viscosity, was also developed to predict the fracture initiation pressure in open holes in permeable formations. The modeling results of the initiation pressure and the fracture position and orientation fit well with the experimental results. During the stimulation of a permeable formation, some of the fracturing fluid being injected into the wellbore flows from the wellbore into the surrounding formation. The infiltrating fracturing fluid increases the formation pore pressure, causing a compressive circumferential stress around the borehole. This mechanism reduces the fracture initiation pressure. The initiation pressure decreases with the rock permeability and injection rate but increases with the fracturing fluid viscosity. This result is important for petroleum engineering applications, because nearly all reservoirs are permeable to fracturing fluids, and the limitations of previous models preclude them from drawing the same conclusions.