Double-porosity poromechanical models for wellbore stability of inclined borehole drilled through the naturally fractured porous rocks

Abstract

Fractured porous rock comprises the matrix and fracture systems with double-porosity and double-permeability. This paper treats the fracture system as a porous media due to the fracture-occupied space filled by the depositional minerals or assumes it to be fully saturated with the formation fluid. Thus, one considers the double-porosity poromechanical models and provides the semi-analytical dual-poroelastic solutions to the field variables around arbitrally inclined boreholes subjected to non-hydrostatic stresses. The plane strain assumption holds for deriving solutions to field variables. The field variables include displacements, stresses, and two pore-pressures in the matrix system and fracture system. This work conducts a quantitative study of the relation between fracture spacing related to fracture density, fracture width, and the safe pressure ratios window (SPRW). One defines that the upper limit of SPRW is related to the ratio of fracture pressure to the original formation pore pressure, and the lower limit is referred to as the ratio of collapse pressure to formation pore pressure. The main results show that the dual-porosity medium displays a higher wellbore instability potential than the single-porosity one. The fracture system that is fully saturated with formation fluid has a narrower SPRW than that of the minerals-filled one referred to as a porous medium. When the coupled hydro-mechanical model is considered, the drilling mud-pressure window narrows with increasing time. It suggests that the drilling engineer considers fracture spacing, and fracture width in the predrilling design of the time-dependent SPRW of naturally fractured porous rocks.