Exploring the Dynamic Evolution of Shallow and Deep Coal Nanopore Structures Under Acidic Fracturing Fluids Using Synchrotron Radiation Small-Angle X-Ray Scattering

Acid fracturing technology is one of the most effective methods for resolving mineral plugging and for improving the pore structure of coal reservoirs. To investigate the characteristics of shallow and deep coal nanopore structures under the influence of acidic fracturing fluids, experiments using synchrotron radiation small-angle X-ray scattering were conducted on shallow and deep coal samples soaked in acidic fracturing fluids of different concentrations for varying durations. This quantitatively characterized the different nanoscale pore scattering intensity ratios (A_I), fractal dimensions, and nanopore parameters. The research indicates that, under the influence of acidic fracturing fluids, the shallow coal nanopore structure tends to become more complex while that of deep coal becomes simpler. The impact of 20% acidic fracturing fluid is greatest on shallow coal nanopore structure, while deep coal nanopore structure is more susceptible to 12% acidic fracturing fluid, with these effects primarily concentrated in the 2–10 nm pores. Acidic fracturing fluids primarily affect the shallow and deep coal nanopore structures by dissolving, among others, carbonate minerals, pyrite, and clay minerals, resulting in the dynamic evolution of the shallow and deep coal nanopore structures during the soaking process.