The use of alternating stages of gelled water and acid to obtain deeply penetrating, highly conductive fractures is common practice in carbonate formations.5 By monitoring bottom-hole pressure during this type of treatment, and using a pseudo-threedimensional simulator, a history match of stimulation pressure response can be made to help quantify various effects of the treatment, and to proceed a step closer to optimization. This paper presents bottomhole pressure data from several wells during acid fracturing. The data include treatments with various acids and varying proportions of pad-to-acid. The results of the simulations indicate that once acid is introduced into a hydraulic fracture system, certain parameters (which are assumed constant during conventional fracturing) can vary greatly. Therefore, onsite interpretation of bottom-hole fracturing pressure becomes more difficult. The potential may also exist to apply this information to proppant fracturing of carbonate formations. Acid may be used to etch the fracture face to reduce fracture friction pressure, thus creating an artificial control upon fracture height growth.