Abstract: CO₂ generates in the course of acidizing treatment with the reaction of acid and rock, which will influence the mechanism of acid leaking. An acid filtration model considering CO₂ effect has been set up based on the conventional acid filtration model in order to study the CO₂ effect on acid liquid loss and to determine the volume of free CO₂. Simulation results showed that the volume of free CO₂ increases with increasing acid concentration and decreases with increasing pressure. In acid corrosion fractures, when free CO₂ exists, filtration law is the same as foam fracturing, and the volume of acid fluid loss reduces with increasing CO₂ volume.