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Matrix stimulation treatments (most commonly using acid as a solvent) are aimed at overcoming the effect of near-wellbore damage by dissolving solids and, hence, increasing permeability. The skin factor, a parameter to measure altered flow conditions in the near-wellbore vicinity, is often used as a quantitative indicator of the level of damage. In the absence of completion effects (such as partial completion or poor perforations), a high positive skin factor indicates severe damage; and a negative skin factor is because of a higher effective permeability in the near-wellbore region after a successful stimulation. Selecting an optimal injection rate and appropriate acid system is a difficult process because of uncertainties about the damaged zone and the competing effects of mineral dissolution and reaction-product precipitation. Determining the evolution of skin factor during a matrix stimulation treatment is a means of measuring the effectiveness of acid or other solvent in overcoming formation damage. Treatment-evaluation procedure uses pressure and injection-rate data during a treatment to estimate skin-factor change as a function of pumping time. The procedure can be applied during a treatment as real-time evaluation, or after treatment, as post-treatment evaluation. It is a simple but valuable tool that helps engineers to optimize acid treatment design effectively. In this chapter, the theory of real-time monitoring of skin evolution and the procedure of stimulation evaluation are presented. Special conditions such as deviation from constant injection rate, gas-well treatment, and bottomhole-pressure convergence are also discussed. Field cases in this chapter illustrate the application of the method.

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