Dynamic evolution mechanism of shale structures under fluid-rock interaction based on fractal theory

To investigate the shale damage mechanism caused by long-term contact between water-based drilling fluid and the borehole wall, we systematically studied the dynamic evolution of shale pore structures at different hydration times through fixed-point observation experiments. Results show that after 48 h of shale hydration, the maximum pore diameter increases from 6.52 to 9.66 nm, while the number of pores increases from 136 to 432. Hydration primarily promotes mineral dissolution on the shale surface, exerting a strong influence on pore number and a comparatively smaller effect on pore diameter. The porosity of the core before hydration is 2.1%–5.5% and increases to 2.4%–13.7% after hydration. The porosity increases to varying degrees, and the increase multiple is between 0.14 and 1.50. The effect of hydration is more pronounced in rocks with higher initial porosity. Shale hydration also exhibits clear fractal characteristics, with the fractal dimension increasing from 2.663 to 2.8735 within 48 h. The fractal characteristics of pores before and after hydration and the hydration time conform to the logarithmic function relationship. This study elucidates the evolutionary patterns of fractal characteristics in shale pores and develops a model to describe the relationship between fractal dimension and hydration time. The model provides a scientific foundation for investigating the dynamic evolution of pore structures and the causes of wellbore instability during shale hydration.