Reaction between in-situ generated acid and carbonate rock at ultra-high temperatures: A fusion of kinetic mechanisms and experiments

Abstract

With the advancement of exploration, carbonate resource development has progressed to ultra-deep layers, with reservoir temperatures rising to ultra-high levels (>180 °C). Effective acid fracturing under such conditions requires optimizing acid formulations and understanding acid-rock reaction mechanisms. This study optimized the formulation of in-situ generated acid for ultra-high temperatures and analyzed its acidogenic properties, dissolving capacity. The acid-rock reaction rate was measured using Rotating Disk Apparatus (RDA) and the effect of different factors on the reaction rate was analyzed. Experimental results showed that the optimal acid formulation involved a 1:1.5 molar ratio of ammonium chloride (NH₄Cl) to polyformaldehyde (POM) and a total concentration of 30 %. Acidogenic concentration initially increased rapidly, plateaued, and declined at higher temperatures due to formaldehyde volatilization and decomposition. At 180 °C, higher acid concentrations enhanced reaction rates, intensifying surface etching on limestone and dolomite. Reaction rates decreased with rising temperatures, primarily governed by acidogenic concentration. Increased rotational speed transformed the surface from flat to rough, forming central humps and cavities, with etching pits extending from rock edge to center. Linear velocity significantly influenced reaction rates and etching patterns. Larger cores experienced higher linear velocities at the same rotational speed, resulting to increased reaction rates without area-volume ratio corrections. After correction, differences in reaction rates across core sizes were significantly reduced, with etching morphologies at lower speeds resembling those of smaller cores at higher speeds. This work provided theoretical support for the application of in-situ generated acid in acid fracturing of ultra-high temperature carbonate reservoirs.