Multi-Factor Integrated Geomechanical Fracability Evaluation of Ultra-Low Permeability Sandstone Reservoirs: A Case Study of the Chang 6 Formation in Block H

Fracability evaluation is essential for hydraulic fracturing interval selection and stimulation optimization in ultra-low permeability sandstone reservoirs. Conventional brittleness-based methods derived from shale reservoirs are insufficient for characterizing fracture initiation difficulty, fracture propagation resistance, natural fracture interaction, and post-fracture conductivity in tight sandstone formations. In this study, the Chang 6 ultra-low permeability sandstone reservoir in Block H was investigated by integrating triaxial rock mechanical testing, Kaiser acoustic emission stress measurement, FMI/MCI image-log interpretation, and logging-based dynamic-to-static mechanical parameter conversion. The results show that the reservoir is characterized by relatively high stiffness and strength, with an average static Young’s modulus, Poisson’s ratio, and compressive strength of 24.05 GPa, 0.21, and 131.97 MPa, respectively. The all-sample average maximum and minimum horizontal principal stresses are 35.70 MPa and 29.91 MPa, respectively. After excluding the anomalous C6-19 stress-memory response, the representative average σH and σh are 37.06 MPa and 30.95 MPa, respectively, with a representative stress difference of 6.12 MPa. A multi-factor integrated fracability index was established by considering brittleness, natural fracture development, compressive strength, equivalent fracture propagation resistance, and effective confining pressure. The average fracability indices of Wells L7 and L26 are 0.624 and 0.596, respectively, indicating relatively favorable fracturing potential. The proposed workflow provides a geomechanically constrained method for relative sweet-spot ranking and preliminary hydraulic fracturing design in ultra-low permeability sandstone reservoirs.