The matrix reactivity of sandstone formations with mixtures of hydrofluoric (HF) and hydrochloric (HC1) acids has been studied experimentally using natural cores. A systematic approach, which includes laboratory analysis and computer modelling, has been used to design and plan acid treatment for sandstone formations. Matrix reactivity to acid mixtures (reaction rate) and the relationship between the porosity and permeability are established by subjecting the Pacoota Sandstone core samples to different acid concentrations and injection rates at different temperatures. Based on material balance and reaction kinetics a numerical simulator has been developed and verified in the laboratory. This simulator can adequately predict spent-acid concentration and changes in porosity and permeability as a function of acid penetration depth for given acid treatment conditions (acid concentrations, injection rates and treatment temperatures).