Fully coupled analysis of interaction between the borehole and pre-existing fractures

The coupling of rock and thermal stresses along with fluid pressure are particularly important in fractured rock masses, since stress-induced changes in permeability can be large and irreversible under perturbations resulting from various natural and induced activities. A new method is presented to model fracture permeability changes during drilling in fractured rocks. The approach includes finite element method (FEM) for fully coupled thermo-poroelastic analysis of stress distribution around borehole and displacement discontinuity method (DDM) to model fracture deformation. Three cases of overbalanced, underbalanced, and balanced drilling fluid pressure conditions are employed. The application of the approach illustrates that the maximum variation of aperture occurs near to the borehole and become negligible at large distances away from the borehole. It was shown that mechanical stresses caused by excavation of the rock contribute to short time while fluid pressure and thermal stresses are responsible for long term permeability variation of fractures. The results show that the fracture permeability decreases with depletion of the fracture and the rock matrix while increases with pressurization and cooling of fracture during overbalanced condition. The fracture permeability reduces during balanced fluid pressure condition within a short time then enhances by cooling of the rock surrounding the borehole.