Physical simulation of fracture propagation patterns in algal limestone fracturing

In Fengxi block of the Qaidam Basin, the weak surfaces are developed in the algal limestone reservoirs of the Lower Youshashan formation, exhibiting complex lithologies, significant variations in physical properties and strong heterogeneity. The mechanism of fracture propagation during hydraulic fracturing in this area is not well understood. Mineral composition characteristics of the algal limestone were analyzed using rock cuttings electron microscopy scanning experiments, and rock brittleness evaluation based on energy evolution was conducted through triaxial compression experiments. To explore the fracture propagation patterns of algal limestone under different geological and engineering conditions, small-scale true triaxial hydraulic fracturing tests were conducted on algal limestone containing structurally weak surfaces under reservoir temperature and pressure conditions. The pressure curves and fracture morphologies were analyzed. The research results indicate that the algal limestone has a high content of carbonate minerals and shows clear brittle characteristics, with a calculated brittleness index of 0.56 using the energy evolution method. Under experiment conditions of low horizontal stress difference, high injection rate and low fracturing fluid viscosity, the stress-controlled effect on artificial fractures is weakened, the weak structural surfaces are easily activated, and the local curvature of fractures increases, leading to the formation of complex fractures. This study provides a guidance for hydraulic fracturing operations in algal limestone reservoirs.