A well stimulation modeling method and simulation model for modeling a stimulation treatment involving a chemical reaction between a treatment fluid and a porous medium, such as acid treatment of a carbonate formation. In a wormhole initiation stage or mode, the medium of the cells having a solid saturation above a respective critical solid saturation is comprised of matrix material behaving as a single permeability, single porosity system; and in a wormhole growth stage or mode, the cells having a solid saturation equal to or less than the respective critical sold saturation comprise two different interconnected media, the matrix material and a wormhole material, defined to include wormhole-forming material as well as mature wormholes, having fluid mobility as a function of the solid saturation.