Carbonate reservoirs are generally highly heterogeneous and fractured. Acid leak-off (infiltration) in acid hydrofracking is particularly serious because it reduces the acid efficiency and limits the fracture length from acid etching. As a result, a favorable reservoir away from the wellbore may fail to be connected, leading to poor results after acid hydrofracking. In this work, a new three-phase (oil-gas-acid) model of acid leak-off (infiltration) in acid hydrofracking of a reservoir that considers dissolution of rock by acid, changes in the width of natural fractures, and the effect of CO (2) was developed and a numerical calculation method was also given. The model allows precise description of the characteristics of acid leak-off in acid-fractured carbonate reservoirs. The results show that the mechanism of acid infiltration into fractured and porous (permeable) reservoirs differs widely. The acid penetrates deep into the reservoir mainly via wide natural fractures and only thereafter through microfissures and the matrix. The acid reacts most vigorously with the rock in wide fractures, leading to further widening of the natural fractures and increased infiltration of the acid into the fractures, which causes deeper penetration of the acid into the fractures. CO (2) reduces the acid leak-off rate, which is less at higher CO (2) concentrations.