Preparation and Optimization of Synthetic Cores for Weakly Cemented Sandstones and their Application in True Triaxial Hydraulic Fracturing Experiments

Weakly cemented unconsolidated sandstones exhibit strong heterogeneity, and obtaining natural core samples is highly challenging. Consequently, most true triaxial hydraulic fracturing experiments currently rely on synthetic cores. Selecting an appropriate cementation method to accurately control core physical properties has become critical to research in this area. In this study, three commonly used solid cementing agents—cement, phenolic resin, and epoxy resin—were selected and systematically mixed with typical formation sands representative of unconsolidated sandstone reservoirs. Through continuous optimization of the mixing ratios of formation sand, water, and cementing agent, an optimal formulation was developed. The permeability, porosity, and uniaxial compressive strength of cores prepared with different formulations were systematically tested, followed by data fitting and analysis to compare the mechanical stability and flow characteristics associated with different cementing agents. The results demonstrate that epoxy resin–cemented cores exhibit the best overall performance in balancing permeability–porosity characteristics and mechanical strength, making them well-suited to simulate the behavior of weakly cemented natural sandstones. Furthermore, using the optimized epoxy resin cores, large-scale true triaxial hydraulic fracturing experiments were conducted. Diverse fracture morphologies—including primary fractures, secondary (branch) fractures, and localized crushed zones—were observed during the fracturing process, providing a solid experimental foundation for further investigations into fracture propagation mechanisms in weakly cemented unconsolidated sandstones.