Investigating the pore evolution and fractal characteristics of carbonate rocks under dissolution

Abstract The dissolution of carbonate rocks under deep burial conditions significantly affects reservoir quality, garnering recent research attention. The study focuses on a carbonate rock formation in southwest China, where dissolution experiments are conducted using a self-constructed experimental platform. Pore mechanical characteristics, fractal properties, and pore structure of carbonate rocks were analyzed using fractal theory, mercury intrusion method, and rock triaxial compression experiments. Core characteristics and reservoir space types were determined through thin section observation, while the pre- and post-dissolution morphology of rock samples was examined under a microscope. The results suggest that dissolution alters the pore structure of carbonate rock, widening primary fractures, facilitating interconnection, and generating secondary fractures, thereby enhancing pore connectivity. Mean values of macropore fractal dimension (D1), mesopore fractal dimension (D2), and micropore fractal dimension (D3) range from 2 to 3. The fractal dimension (D1) of macropores decreases gradually with increased dissolution time, while that of mesopores (D2) and micropores (D3) increases. There is a robust correlation between fractal dimensions and reservoir physical properties, with higher values indicating a more complex micropore structure. Under constant pore pressure and confining pressure, the peak strength of carbonate rocks diminishes with prolonged dissolution time, while strain increases. Dissolution selectively dissolves minerals based on their composition and rock microstructure, with calcite exhibiting higher solubility than dolomite. Moreover, the specific surface area and porosity of rock sample pores positively correlate with dissolution time. These findings offer a theoretical framework for understanding the origin, distribution, and fractal characteristics of carbonate dissolution pores.