Energy evolution and fracture network complexity in deep carbonate reservoirs

Deep carbonate reservoirs are commonly characterized by low porosity, low permeability, and strong heterogeneity, resulting in complex fracture propagation behaviors and great challenges for reservoir stimulation. Based on a 10,000-ton-capacity ultra-large true triaxial hydraulic fracturing physical simulation platform, large-scale carbonate specimens with dimensions of 2 m×2 m×1 m were used to conduct hydraulic fracturing experiments under three typical completion schemes: cased perforation in vertical wells, open-hole vertical wells, and open-hole horizontal wells. By integrating high-frequency pressure monitoring with multi-channel microseismic monitoring, fracture initiation, propagation, and spatial distribution characteristics were systematically characterized. The results indicate that open-hole horizontal wells (energy utilization efficiency 0.70, fractal dimension 1.78) are most effective in activating natural fractures and dissolution cavities, forming highly complex three-dimensional fracture networks, with the stimulated reservoir volume (SRV) approximately 2.3 times that of perforated completions. Cased perforation completions exhibit well-controlled fracture orientations but the lowest fracture complexity, whereas open-hole vertical wells show lower fracture initiation pressure and relatively stable fracture geometries. Energy analysis reveals a significant positive correlation between energy utilization efficiency and fracture complexity, with an increase of approximately 0.1 in the fractal dimension for every 0.05 increase in energy utilization. A quantitative relationship among energy utilization, fracture complexity, and SRV is established, providing a basis for fracturing parameter design and completion selection in deep carbonate reservoirs.