Experimental investigation of microscale mechanical alterations in shale induced by fracturing fluid contact

Understanding the changes in the mechanical properties of shale exposed to fracturing fluids is crucial for optimizing parameters in hydraulic fracturing. However, the dynamic alterations in mechanical properties still need to be disclosed due to the high heterogeneity of shale, particularly at the microscale. This paper aims to in-situ unravel the microscale changes in the dynamic mechanical properties of shale affected by fracturing fluids. A combination of nanoindentation, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrum (EDS) techniques was employed. The nanoindentation results demonstrate a rapid initial decrease followed by a slower decline after 4 days in both Young's modulus and the hardness of shale exposed to fracturing fluids. Specifically, the average Young's modulus of shale ranges from 62.45 GPa to 56.275 GPa, 52.575 GPa, 40.15 GPa, and 39.5 GPa while the average hardness modulus varies from 3.305 GPa to 2.2125 GPa, 1.8175 GPa, 1.24 GPa and 1.1525 GPa with the treatment time of 0, 1, 2, 4, and 7 days, respectively. An exponential expression well describes the relationship between mechanical properties with the treatment time. XRD analysis shows carbonate and clay mineral content decrease, while quartz content increases. Moreover, in-situ SEM results highlight the emergence of many dissolution pores when shale is exposed to fracturing fluid. Further pH testing and EDS analysis indicates a corrosion effect, with hydrogen ions reacting with carbonate minerals in shale, leading to a significant reduction in calcite and dolomite. Consequently, the corrosion-induced dissolution pores weaken the stability of the shale matrix, leading to a reduction in mechanical properties. This study sheds light on the mechanism behind the dynamic mechanical properties of rock and provides valuable insights for optimizing the soaking time for fracturing fluids.