Experimental investigation of acid-rock reaction kinetics and fracture conductivity under ultra-high temperature carbonate reservoir conditions

• A novel ultra-high temperature acid-rock reaction device was developed for 200 °C tests. • Weight-loss method is more accurate than titration at 200 °C for reaction rate evaluation. • Gelled acid shows strong etching and excellent conductivity in acid-fractured rock plates. • Self-generating and DTPA acids exhibit superior retarded reaction and uneven etching. Ultra-high-temperature carbonate reservoirs (∼200°C) exhibit extremely rapid acid–rock reactions, leading to shallow penetration and limited effectiveness of conventional acidizing treatments. In this study, a high-temperature rotating disk reactor and rock-plate etching device were used to systematically evaluate the reaction kinetics, etching morphology, and fracture conductivity of five acid systems—HCl, gelled acid, acetic acid, DTPA, and self-generating acid—at 200°C. Results show that the reaction rate follows the order HCl > gelled acid > HAc > DTPA > self-generating acid, while the degree of non-uniform etching exhibits the opposite trend. HCl dissolves rock rapidly but generates nearly uniform surfaces with poor conductivity. Gelled acid produces the strongest non-uniform etching and the highest initial fracture conductivity, though its retardation weakens at higher concentrations due to polymer degradation. Weak acids (HAc, DTPA, and self-generating acid) remain reaction-controlled and exhibit stable retardation at 200°C, maintaining deeper penetration than strong acids. Quantitative morphology analysis and conductivity testing confirm that fracture conductivity is governed jointly by etching depth and heterogeneity, rather than dissolution volume alone. Engineering results suggest that, for reservoirs exceeding 180°C, combining gelled acid with a weak-acid system can effectively control reaction rate, enhance non-uniform etching, and create high-conductivity acid-etched fractures. This study provides the first systematic kinetic and etching-conductivity framework at 200°C, offering practical guidance for acid selection and treatment design in ultra-deep carbonate formations.