The reactivity of acid with rock in an acidizing process depends on the abundance and the geometrical arrangement of various minerals with respect to each other. A model is developed to compute the rate of acid consumption and the change in rock porosity as a function of time. The minerals are located in accordance with thin section images. Under specified reaction kinetics, real time simulation of mineral dissolution is conducted graphically on a computer. Since the rate of reaction of each mineral is known, an erosion process is used to update the image of the rock as a function of time. This procedure can be recorded by a series of images and the porosity, permeability and cumulative acid consumption can be computed. It is shown that this model can provide substantially different results than a simple model that is based only on the mineral abundance in the rock matrix. In some cases this can have a significant impact on acidizing simulators that are currently used. Such simulators do not use petrographic information on the relative location of minerals in modelling acid consumption rates.