Quantifying fracture evolution characteristics of gravelly sandstones based on fractal theory

With the advancement of unconventional oil and gas exploration and development, tight gravelly sandstone reservoirs have garnered increasing attention. In this study, triaxial compression experiments are conducted on gravelly sandstone cores, and correlation analysis is employed to establish the relationships between fractal dimensions of the fractures and rock mechanics parameters. For cores of gravelly sandstone, a positive correlation exists between the fractal dimension and the brittleness index. The prediction model reveals that the error between the predicted and actual values for well 3 is relatively large, which can be attributed to the presence of pure sandstone cores in well 3. Under high confining pressure in the deep strata, rocks exhibit a decreasing trend in fractal dimensions, a phenomenon due to the stress-memory effect. In addition, numerical simulation is further employed to study the effect of the factors that could affect the complexity of the fractures, and the results show that the fractal dimension of gravelly sandstone declines with increasing confining pressure, peak compressive strength, and rock elastic modulus as the loading process intensifies.