Mechanical Skin Damage in Weils

Abstract

This paper describes and quantifies a mechanical skin damage resulting from the redistribution of stresses caused by drilling the well. The damage is localized within a radial ring around the borehole wall. The stresses (and related positive skin), increase with depth, angle of inclination, and well production. They rapidly fade with lateral radial distance. The damage becomes insignificant at approximately three times the borehole radius. Introduction After drilling a well, the in-situ stresses that originally were supported by the drilled material concentrate near the borehole wall compressing and sometimes crushing the rock. As a consequence, there is a localized permeability damage that, in this paper, is termed mechanical skin damage. Examples of this damage are the residual positive skins that are frequently measured by well tests after acid clean-ups and other remedial treatments. For example, Reference 9 shows positive skins varying from 5 to 50 units for gravel packed wells and skins varying from 0 to 12 units for hydraulic fractured wells. The fractured wells remove the stress concentrations by redistributing them into the fracture walls. This paper uses the existing solutions, described below, to evaluate the borehole stresses and mechanical skin in vertical and inclined wells. Kirsch and Timoshenko obtained linear elastic solutions to quantify the distribution of stresses around a cylindrical cavity. Other authors, expanded these solutions to include the effects of pore pressures and internal well pressures. To consider the well inclination and azimuth, Yew, et. al. and Li, presented a directional trigonometric matrix to obtain the stress components with respect to the coordinates of inclined wells. In this paper, the reduction of permeability caused by the stress concentrations is evaluated by hydrostatic laboratory tests and the corresponding skins are quantified with the Hawkins equation. Evaluation of Borehole Stresses and Mechanical Skin Three steps are used to evaluate the mechanical skin damage in vertical or inclined wells:Given the global principal stresses (v, hmax, hmin) aligned with reference to a rectangular coordinate system with axis 1, 2, and 3 (Figure 1), the local stresses for the inclined borehole are evaluated using the rotational matrix given by Equation 1,(1)As shown Figure 2, this matrix rotates the principal stresses through the azimuth angle, (counterclockwise rotation with respect to the 1-axis to the coordinate system x', y', z'). Then, as shown in Figure 3, the matrix rotates the stresses through an inclination angle, (clockwise rotation through the y'axis) to the well coordinate system x, y, and z.