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A case study has been conducted on over 90 gas wells in West Texas to optimize fracture conductivity and well profitability. This study provides direct comparisons evaluating the effectiveness of treatments incorporating various designs and breaker systems. The wells stimulated with new designs focusing on methods to improve fracture conductivity have shown significant increases in production rates. These same wells have also shown improved clean-up properties, as compared to surrounding wells, by eliminating swabbing time and returning higher volumes of fracturing fluid. Recent laboratory studies have demonstrated that realistic in-situ fracture conductivity values are significantly lower than previously thought. New testing procedures have concluded that proppant permeability values are greatly reduced when exposed to long term tests. Additionally, fluid damage and polymer concentration further impair these permeability numbers. When breaker concentrations are increased to a level to effectively degrade the concentrated polymer the rheology of the fluid will be adversely affected, thus sacrificing the execution of the treatment. New designs have evolved placing emphasis on proper fluid selection, increasing proppant volumes and concentrations, and improved breaker scheduling. Encapsulation techniques have been developed allowing a controlled delay of breaker activation until after the proppant is placed. Extremely high breaker concentrations are now obtainable without adversely effecting the rheological properties of the fracturing fluids.

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.

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