Experimental Study on Solid Acid Systems for Acid Fracturing in Ultrahigh-Temperature Carbonate Reservoirs

Acid fracturing is a key stimulation technique for carbonate reservoirs. However, current retarded acid systems face significant challenges in ultrahigh-temperature environments, such as overly rapid acid-rock reactions, poor postacidizing fracture conductivity, and severe tubing corrosion. To overcome these limitations, this study proposes a novel acidizing fluid system based on solid acids. A systematic evaluation of different solid acid types and combinations was conducted to determine the optimal formulation. A series of laboratory experiments, including dissolution testing, acid-rock reaction kinetics, etching characterization, and conductivity measurements, were performed to identify the optimal solid acid system for ultrahigh-temperature carbonate reservoirs. Twenty candidate solid acid formulations were evaluated at 453 K. Among them, EDTA, DTPA, and DTPA + HCl were identified as the most promising, demonstrating a high dissolution efficiency and minimal chelate formation. Kinetic tests further confirmed their slower reaction rates with carbonate rocks at elevated temperatures, indicating a strong retardation performance. Additionally, the optimized solid acid systems demonstrated effective fracture etching and superior conductivity at 453 K compared with conventional retarded acids. EDTA, DTPA, and DTPA + HCl exhibited high dissolution capacity, reduced reaction rates, and improved fracture conductivity, confirming their suitability for acid fracturing in ultrahigh-temperature carbonate reservoirs. The optimized solid acid system offers substantial potential for field application.