Fractal Perspective on the Effects of the Acid-Rock Interaction on the Shale Pore Structure

Adding acid to fracturing fluids is a promising method to improve the stimulated reservoir volume of shale reservoirs. It is of great significance to comprehensively understand the pore structure change during the acid-rock interaction for the purpose of maximizing its effects on long-term productivity enhancement. In this study, the nanopore fractal dimension and surface fractal dimension of two shales with different mineral compositions before and after the acid-rock interaction are determined from N2 adsorption results. Besides, the micropore fractal dimension, lacunarity, and succolarity are calculated using three-dimensional microcomputed tomography images. Through these fractal parameters, the effects of the acid-rock interaction on the shale pore structure are evaluated quantitatively. Moreover, the correlations of fractal parameters with porosity and pore structure parameters are analyzed. The results show that the surface fractal dimension decreases while the micropore fractal dimension increases for both shales after the acid-rock interaction. It indicates that nanopore surfaces become smoother; meanwhile, micropore distributions become more irregular. Carbonate-rich shale with an increased nanopore fractal dimension has a more uneven nanopore distribution, whereas that of carbonate-poor shale becomes more uniform. The lacunarity increases and the pore structure becomes more heterogeneous for both shales. The increased succolarity indicates that the pore structure connectivity is greatly improved after the acid-rock interaction. Furthermore, the differences between the succolarity values corresponding to different directions become smaller, suggesting that the anisotropy of shale is weaken by the acid-rock interacion. The complementary effects of the fractal dimension, lacunarity, and succolarity help to better understand the pore structure changes of shale undergoing complicated physicochemical reactions.

Automated summary

Adding acid to fracturing fluids has potential to improve the stimulated reservoir volume of shale reservoirs. This study evaluates the effects of acid-rock interaction on shale pore structure using fractal parameters. The results show that the surface fractal dimension decreases while the micropore fractal dimension increases for both shales after the acid-rock interaction. The lacunarity increases and the pore structure becomes more heterogeneous. The increased succolarity indicates improved pore structure connectivity. The complementary effects of fractal dimension, lacunarity, and succolarity help understand the changes in shale pore structure during complicated physicochemical reactions.