The efficiency of an acidizing operation is usually assessed through ordinary well testing methods. These methods are performed only after the acidizing operation, and it is difficult to monitor and evaluate real-time efficiency of acidizing operation. Understanding the performance of the acidizing during the service, by analysis of the acidizing data is a viable approach to this costly and challenging process. The estimation of the skin changes needs an appropriate mathematical method that can handle the main aspects of an acidizing operation. It is tedious to model a real acidizing operation due to the heterogeneity of the reservoir, an unknown reaction rate of acid in reservoir condition, unwanted fluid loss, opening and closing of the wing valve, etc. In this paper, we have presented a new well testing method for calculating the skin variations of open hole vertically drilled carbonate reservoir during an acidizing operation, by using history matching of the surface pressure data. The proposed method is based on continuity equations of acid in the reservoir coupled with analytical equations of ORPM (Output Rate Prediction Method) in the wellbore. The novelty of the proposed method is using a new mass transfer coefficient in continuity equations of acid, and a mathematical approach (ORPM) for the appropriate estimation of bottomhole acid injection rate and fluid loss, that lead to a proper evaluation of the skin variation during a field acidizing operation. (C) 2018 Elsevier Ltd. All rights reserved.