COUPLED WELLBORE EROSION AND STABILITY ANALYSIS

This paper extends earlier work(1) on sand erosion and presents an att empt to couple sand erosion to mechanical damage of rock around a well bore. Porosity which evolves in time and space as surface erosion prog resses, is chosen as the coupling parameter. Both rock elasticity and strength (cohesion) are assumed to depend on porosity in such a way th at the material becomes weaker with increasing porosity. The mathemati cal model, consists of erosion equations, mixture flow equations and s tress equilibrium equations, is solved numerically by Galerkin finite element method. Numerical results suggest that erosion, resulting in s and production, is high close to the free surface. Erosion is accompai ned by changes in porosity and a significant permeability increase. Er osion in the vicinity of the wellbore induces alterations in the mecha nical behaviour of the medium. Weakening of rock stiffness leads to se vere alteration of both effective stresses and pore pressure near the cavity. Since cohesion decreases with increasing porosity, one can als o identify the time instant at which rock mechanical failure starts. ( C) 1998 John Wiley & Sons, Ltd.