Investigation of the Acid Generation Pattern and Flow-Reaction Behavior of Solid Retarded Acid

The carbonate reservoir of the Shunbei oilfield is characterized by deep burial depth and high temperature. During acid fracturing, the reaction rate between conventional acid systems and the rock is relatively fast, leading to a limited effective acid penetration distance. To extend the acid penetration distance, a combination of solid retarded acid and conventional acid was used in field operations. The effectiveness of the solid retarded acid depends on its acid generation pattern, making it necessary to study the acid generation behavior of the solid retarded acid. This paper establishes a frame for evaluating the solid retarded acid, including tests for solid retarded acid solubility, acid concentration, and acid-etched fracture conductivity. Based on the test results, the acid generation pattern of solid retarded acid was analyzed, its slow-generation performance was evaluated, and an acid generation model was established. Finally, by integrating the acid generation model with the existing acid fracturing model, the effective distance of solid retarded acid was predicted. The study shows that the solubility of acid-generating materials is influenced by both temperature and solid retarded acid concentration. When the concentration of solid retarded acid exceeds 25%, it does not completely dissolve at room temperature, but can fully dissolve after 40 min at 120 °C. The acid concentration is significantly affected by temperature, with an acid concentration of about 1.6 mol/L at room temperature and up to 3.1 mol/L at high temperatures, comparable to a 12% hydrochloric acid concentration. Solid retarded acid exhibits good slow-generation performance, with a comprehensive reaction rate approximately one-third of that of cross-linked acid. When the acid-rock contact time is around 3 h, the acid-etched fracture conductivity of solid retarded acid can remain above 5 D·cm under a closure pressure of 60 MPa. The predicted effective acid penetration distance of solid retarded acid can reach over 150 m, under typical conditions of Shunbei oilfield. The findings of this study can serve as a reference for the design and optimization of solid retarded acid fracturing.