Mineralogical and Microstructural Controls on the Mechanical Weakening and Recovery of Longmaxi Formation Shale during Flowback Fluid Soaking

During postfracturing shut-in, retained flowback fluid can alter the mineralogical, microstructural, and mechanical properties of deep shale. However, the links between these changes and fracture fractal characteristics during soaking remain poorly understood. Here, Longmaxi Formation shale was soaked in field-produced flowback fluid for 0–45 days and characterized using X-ray diffraction, scanning electron microscopy, nanoindentation, Brazilian splitting, triaxial compression, micro-CT, and fractal analysis. The results reveal a nonmonotonic weakening and recovery process. The triaxial compressive strength decreased from 340.27 MPa in the unsoaked state to 256.92 MPa at 7 days, then recovered to 326.40 MPa at 30 days. Tensile strength decreased to 7.50 MPa at 15 days and increased to 10.74 MPa by 45 days. XRD and SEM results show time-dependent changes in mineral composition and surface morphology, together with the development of pores and microcracks and localized granular deposits. Nanoindentation further reveals strong spatial variations in elastic modulus and hardness during soaking. The line and surface fractal dimensions of Brazilian splitting fractures varied with fracture path tortuosity and tensile strength, whereas the CT-derived volumetric fractal dimension captured the evolution of triaxial fracture-network complexity with prolonged soaking. Overall, the results suggest early weakening associated with hydration and dissolution, followed by partial mechanical recovery associated with localized deposits and microstructural reorganization. These findings provide new insights into shale alteration, fracture evolution, and mechanical integrity during shut-in in deep shale gas reservoirs.