Experimental investigation on macroscopic fracture and microstructural evolution of granite under high-temperature steam action

Abstract

This study conducted simulated experiments on the thermal fracturing of granite under non-uniform stress conditions using the method of injecting high-temperature and high-pressure steam. Additionally, a comprehensive investigation was carried out on the thermal fracturing characteristics of granite after steam treatment, as well as the distribution patterns of cracks. The results show that: Firstly, the process of fracturing granite using high-temperature and high-pressure steam as a medium can be divided into five stages. The threshold temperature for thermal fracturing of granite is 260 ℃, at which point a large number of cracks develop or form interconnected networks. Secondly, under high-temperature conditions, the tensile strength of granite is greatly reduced. The temperature at which brittle fracture occurs in granite during the experiment was found to be 427 ℃ at a steam pressure of 10.6 MPa. Thirdly, under the influence of high-temperature and high-pressure steam, granite not only generates primary cracks along the direction perpendicular to the minimum principal stress but also produces a certain number of secondary cracks in other directions. The crack propagation direction can be determined from the velocity and permeability distribution maps of fractured granite. Finally, the fracture surface of granite exhibits significant brittle characteristics. The fracture morphology mainly includes transgranular and intergranular fractures. In the experiment, the fracture surface also showed fragmented and fatigue-induced fracture features. The fractal dimension of micro-morphological features can be used to assess the extent of thermal fracturing at different locations on the fracture surface. Through macroscopic fracturing experiments and comprehensive analysis of theory, wave velocity, and micro-morphology, the study obtained relevant crack propagation patterns and the formation of cracks in different directions. This is of practical significance in the formation of interconnected spatial fracture networks, thereby helping to increase the thermal exchange surface area of artificial reservoirs and improve the efficiency of geothermal exploitation.