Real-time monitoring (on-site evaluation) of acid treatments for oil reservoirs has been developed and applied in the field successfully.1,2,3The technique uses the transient flow equation to estimate the skin factor evolution during the treatment, in order to evaluate and enhance the effectiveness of stimulation and diversion. The equation used in the real- time monitoring model assumes that the reservoir fluid is single-phase and slightly compressible. This approach has worked well in monitoring skin factor change during acidizing of oil wells; however, when acidizing a gas well, injection of liquid-phase acid strongly affects the apparent skin factor because the estimated skin factor depends on the viscosity difference between the reservoir fluid and the injected fluid. The skin factor calculated by the single-phase oil model is wrong if the viscosity contrast is significant. This paper presents a new model to estimate the apparent skin factor for gas reservoirs during acidizing treatments. The model introduces a viscous skin factor to handle the effect of the viscosity difference between the injected fluid and the reservoir fluid on the pressure response. By subtracting the viscous skin factor, which can be calculated as a function of injected acid volume, from the total skin factor obtained from the rate and pressure recorded during the treatment, the true damage skin during an acid treatment is obtained.

Field examples from Brazil show that the real-time monitoring model for acidizing treatments for gas reservoirs has been used to predict the evolving damage skin factor successfully. The examples presented in this paper show that without accounting for the viscous skin effect, the apparent skin factor for the wells rose during the treatment, which could lead to an incorrect interpretation that the treatments were ineffective. However, when the viscous skin was subtracted from the total apparent skin factor, it was seen that the damage skin was decreasing throughout the treatments, as desired. The production data for the wells before and after acidizing confirmed the predicted skin factor behavior by the new model.

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