TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractAcidizing is often applied in horizontal wells to improve well performance. Acidizing in horizontal wells differs significantly from the acidizing of vertical wells. The length of acid contact is much longer, the formation properties over the contact length likely vary in a larger range, and the cost is generally higher. To enhance the effectiveness of matrix stimulation, methods to evaluate and optimize acid treatments similar to those used for vertical wells can be adopted for horizontal wells. This paper introduces a simple approach to evaluate horizontal well response to acid treatments. Skin factor was calculated explicitly in the method from pressure and flow rate, which can be directly measured during a treatment. Skin factor is then used to identify the well response to acid injection, and therefore to understand and control the stimulation process. There have been several mathematical models published to simulate the pressure/flow rate behavior of horizontal wells. Most of them have complicated solutions and have to be solved numerically. We selected and modified analytical solutions for horizontal well performance that can be implemented for field evaluation of acid treatments.Field examples presented in the paper show how to apply the evaluation method for horizontal wells. The method can assess the effectiveness of acid treatments and can be used to optimize subsequent treatments.