Self-Generated Organic Acid System for Acid Fracturing in an Ultrahigh-Temperature Carbonate Reservoir.

As an important part of acid fracturing technology for the carbonate reservoir, the performance of the acid system directly affects the stimulation effect. In view of the current problems of an acid fluid system in an ultrahigh-temperature carbonate reservoir, such as fast acid-rock reaction, short effective action distance, and difficulty maintaining the conductivity of acid etching fractures, an experimental study on the self-generated organic acid system was carried out. The results showed that acetic anhydride and ethyl acetate, which had a strong acid generating ability, were suitable for the parent acid types of self-generated organic acids. Preferably, the peak temperature of 25% acetic anhydride is 160 °C, while the peak temperature of 30% ethyl acetate is 180 °C. The acid-rock reaction kinetics experiment shows that the order of activation energy is ethyl acetate > acetic anhydride > cross-linked acid. The reaction rate is arranged as follows: ethyl acetate < acetic anhydride ≪ cross-linked acid. The self-generated organic acid of ethyl acetate has the largest activation energy, the smallest reaction rate, and the best retarding effect. Acetic anhydride forms a strong nonuniform pitting morphology on the surface of the rock plate, with an initial conductivity of 225.4 μm·cm. Ethyl acetate forms an uneven pitting morphology, and the initial conductivity is 53.1 μm·cm. However, the ability of acid etching fracture formed by ethyl acetate to maintain fracture conductivity is stronger than that of acetic anhydride. Ethyl acetate is more suitable for a deep ultrahigh-temperature carbonate reservoir.