Fracture Process Zone Evolution of Simulated Natural Gas Hydrate-Bearing Sediment Cores Based on Digital Image Correlation Technique

Natural gas hydrate (NGH)-bearing marine sediments typically possess low hydrate saturation and ultra-low permeability, limiting the efficiency of depressurization-based recovery methods. Hydraulic fracturing has been recognized as a promising stimulation technique for enhancing reservoir permeability. However, accurate prediction of fracture propagation in such complex materials requires a thorough understanding of not only global Mode-I fracture toughness but also local crack-tip processes, particularly the characteristics of the fracture process zone (FPZ). In this study, the digital image correlation (DIC) technique was applied to systematically investigate FPZ evolution in synthetic NGH-bearing sediment analogs subjected to Mode-I loading. Notched semi-circular bend (NSCB) specimens were prepared at ice saturations (S) of 0%, 30%, 50%, and 70%. The FPZ initiation loads increased with saturation, from approximately 65–70% of the peak load for S = 0–30% to around 88–90% for S = 50–70%. FPZ width reached about 4.0 mm in the dry condition and approximately 4.8 mm for S ≥ 30%, remaining almost constant thereafter. In contrast, FPZ length exhibited a stronger dependence on saturation, increasing from 4.4 mm at S = 0% to 9.1 mm at S = 70%. The crack opening displacement (COD) profiles at peak load decreased progressively with increasing y-coordinate, and the COD at the notch tip for the highest saturation was up to ten times greater than that of the dry specimen. These findings provide detailed and quantitative insight into the geometry of the FPZ and the associated COD behavior in NGH-bearing sediment analogs.