Experimental investigation on hydromechanical coupling-induced failure and permeability evolution for sandstone with multiple-shape prefabricated fractures

In mineral and geothermal resource co-mining, the underground rock is often affected by mining stress, and fractures of different shapes, such as single fractures, T-shaped fractures, and Y-shaped fractures, are generated. To increase the reservoir permeability, the existing fractures need to be reactivated, causing them to expand under force and propagate in shear and tension modes, generating new fractures and finally forming a fracture network to increase permeability. Waterjet cutting and wire cutting equipment are used to prefabricate sandstone samples with different inclinations and single, T-shaped, and Y-shaped fractures on standard samples. This paper conducts hydromechanical coupling experiments to investigate the possibility of increasing permeability by expanding and merging fractures in prefabricated fractured sandstone samples under triaxial conditions. In addition, the focus is on mechanical properties, such as critical thresholds (crack closure stress, crack initiation stress, damage stress, and peak strength), elastic moduli, and Poisson's ratio, and the failure modes of multiple-shape prefabricated fracture sandstone samples are mainly studied. Simultaneously, the evolution law of acoustic emission and permeability during the progressive failure of fractured rock is studied, and the mechanism of permeability enhancement of fractured rocks under the action of hydraulic coupling is analyzed. The results show that under the action of hydromechanical coupling, all multi-shape prefabricated fracture specimens form secondary cracks that expand in tensile, shearing, or mixed modes through the existing fractures and generate new fractures or fracture networks, which can effectively increase the flow rate. All single-fracture specimens are shear failures, and the T-shaped and Y-shaped fracture specimens have two types of shear failure and tension-shear failure. Furthermore, the weakening effect of water has a smaller effect on strength than the effect of multiple-shape prefabricated fractures. With increasing axial pressure, the rock permeability first decreases and then increases in the pre-peak stage, and the jump coefficient increases when reaching the strength failure. When the stress suddenly drops after the peak of the sample, the permeability reaches the maximum value, and the permeability enhancement effect is the best. The change in the prefabricated fracture angles and shapes has a small influence on the jump coefficient. The average value of the jump coefficients of a single fracture is larger than that of a Y-shaped fracture, which is larger than that of a T-shaped fracture, and the jump coefficients are more than doubled. These observational and experimental results will help to understand fracture failure and fluid flow behavior, which will guide the engineering applications of mineral and geothermal resource co-mining.