In drilling the reservoir sections of a well, one of the most challenging issues is reducing damage to the reservoir by controlling downhole pressure. Many drilling techniques such as underbalanced drilling (UBD) and managed pressure drilling (MPD) are employed to minimize the risks associated with drilling as well as to minimize damage to the porous rock. Even though chemical and physical damages may be short-term and could be repaired by some stimulation techniques, the long-term effects of mechanical damages in porous and fractured reservoirs have received insufficient attention. Accordingly, not only could the above drilling techniques be applied to solve downhole drilling problems, but they also may be used to reduce induced mechanical damages in fractured rocks.

This article presents a new method for modeling changes in fracture permeability caused by drilling in fractured rocks. As part of the approach, the finite element method (FEM) is employed to conduct a thermo-poroelastic analysis of stress distributions around the borehole and the displacement discontinuity method (DDM) is used to model fracture deformations. Based on different fracture spacings and fracture inclination angles, we have considered models of regular fracture networks in the present study. This study focuses on the differences in permeability in underbalanced and overbalanced drilling operations that are compared together in different models.

Effective stress differences (over 40 MPa) were found along and around borehole periphery. Shear stresses in the oblique fracture network also governed aperture change. Short-term mechanical stresses and long-term thermal and fluid pressures determine the fracture aperture. In the long run, fluid pressure and thermal stresses contribute to long term permeability change of fractures while mechanical stresses cause a short-term change. Underbalanced drilling was simulated to reduce fracture permeability, while cooling and pressurizing of rock encouraged fracture permeability without considering solid particle plugging. Fracture aperture adopts a seesaw pattern in a small-spaced fracture network. When the fracture aperture increases in a fracture, the neighboring fractures experience decreased apertures. Despite the drilling method, fractures intersecting boreholes have reduced permeability after drilling for a long time, as they choked in a few locations along the fracture length.

At present, the industry considers managed pressure and underbalanced drilling to be the priority for resolving drilling problems. This paper investigates stress-induced damages in fractured rocks under overbalanced and underbalanced drilling conditions. It is also of significant interest in geothermal reservoirs, where the temperature difference between the rock and the well bore fluid is large. Furthermore, such an analysis would provide the optimal well location from a geomechanical and reservoir engineering standpoint.

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