Fracturing fluid–rock interaction and pore structure evolution in shale oil reservoirs

Hydraulic fracturing has significantly advanced shale oil extraction, yet the long-term effects of fracturing fluids on pore structure remain inadequately understood. This study marks the first quantitative evaluation of the pore structure evolution induced by the fracturing fluid–rock interaction. The mineral composition, pore types, morphology, pore size distribution (PSD), pore volume (PV), and specific surface area (SSA) for the pre- or post-treatment shale samples were measured and compared through a series of experiments, including x-ray diffraction, large-field scanning electron microscopy, focused ion beam scanning electron microscopy, field emission scanning electron microscopy, micro-computed tomography (μCT), gas adsorption, and high-pressure mercury injection. The results reveal that fracturing fluids induce mineralogical and structural modifications. After treatment, the carbonate mineral content decreased from 60.19% to 56.00%, while the clay mineral content increased from 11.72% to 15.66%. Pore structure analysis revealed the development of microfractures, inorganic pores, and organic pores, each exhibiting multi-morphological and multi-scale characteristics. Post-treatment observations showed the emergence of dissolution pores and the opening of microfractures. However, these benefits were offset by mineral detachment, migration, and water–rock interactions. According to the full-scale PSD, micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) account for 4%, 44%, and 52% of the total volume, respectively. The measured results in this work suggested that the total PV and SSA of treated samples decreased by 29.6% and 46.8%. These findings provide critical insight into the long-term implications of hydraulic fracturing on reservoir quality and hydrocarbon flow efficiency.