Coupled thermo-poroelastic analysis of drilling induced mechanical damage in fractured rocks

Abstract

The wellbore represents one of the most crucial components in the hydrocarbon and geothermal reservoir system, as it is the sole conduit to the reservoir for fluid production or injection. Therefore, predicting and controlling of the permeability variations close to the wellbore has been one of the most challenging issues in geothermal and petroleum reservoir systems. 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. Four models of fracture networks with different facture spacing and varied inclination angle are considered. Permeability variations in underbalanced and overbalanced drilling operations are compared together in different models. The results indicate the difference in effective stress values along x and y directions exceed over 40MPa around borehole and along fracture surface. It was proved that the mechanical stresses caused by excavation of the rock contribute to short time while fluid pressure and thermal stresses contribute to long term permeability change of fractures. The application of the approach illustrates that the maximum variation of fracture aperture occurs near to the borehole and becomes negligible at large distance away from the borehole. Regardless of the drilling operation method, either overbalanced or underbalanced conditions, the permeability of fractures intersecting borehole decreases for large time after drilling. The maximum variations in fracture permeability occur in fracture network with inclination angle of 45 degrees where maximum variation of effective normal and shear stresses occur. Also, initial poroelastic effect of stresses and fluid pressure on the permeability is maximized by increasing fracture spacing.