Modeling of Acid Etching Fracture in Carbonate Rocks by Use of Gelled Acid

Summary During acid fracturing of carbonate rocks with gelled acid, changes in acid viscosity strongly affect the morphology of acid-etched fractures. However, most studies ignore that gelled acid, as a non-Newtonian fluid, experiences dynamic viscosity changes in the wellbore and fractures. In this study, we examine how temperature, injection rate, shear duration, and fracture geometry affect gelled acid viscosity during acid fracturing and develop a viscosity model for both wellbore and fracture conditions. By combining a wellbore heat transfer model, acid flow reaction model, and fracture temperature model, we create a comprehensive gelled acid etching model. Using this model, we compare the viscosity behavior and etching patterns of gelled acid and Newtonian regular acid. We also analyze how injection parameters affect the effective acid penetration distance (EAPD) in 220°C reservoirs. Results show that gelled acid viscosity follows an approximate “two concave and one convex” camelback pattern, leading to nonuniform etching that forms short, wide fractures, unlike the uniform, long, narrow fractures formed by Newtonian acid. Fracture morphology and size mainly depend on the gelled acid’s rheological index, reservoir temperature, injection rate, and initial fracture dimensions. To improve EAPD in high-temperature reservoirs, increasing acid concentration and injection rate is effective. For ultradeep well acid systems, attention should shift beyond high-temperature thickeners to include acid corrosion inhibitors and drag reducers. These findings support better design of gelled acid fracturing and acid system development in ultrahigh-temperature carbonate reservoirs.