Temporal and spatial distribution characteristics of hydraulic crack propagation under true triaxial loading using the direct current method

Hydraulic fracturing (HF) has been widely used in coal and rock dynamic disasters prevention and control and coalbed methane production in underground coal mines. However, how to effectively and accurately monitor and evaluate the hydraulic crack propagation process and impact range has not been resolved. We developed a novel experimental system for HF-direct current (DC) monitoring under true triaxial loading conditions. With the use of this system, we conducted HF-DC monitoring experiments under true triaxial loading conditions. The response characteristics of the emission current, apparent resistivity, and apparent resistivity difference contour map throughout the entire experimental process were studied. The results showed that the DC monitoring data contain a wealth of effective information on hydraulic cracks propagation. The emission current and apparent resistivity reflected the overall electrical conductivity of the rock. The contour map of the apparent resistivity difference between two measuring lines could accurately characterize the propagation process and spatial distribution of hydraulic cracks. The evolution process of the difference contour map showed that hydraulic cracks were more likely to spread into areas with low homogeneity. The change range and degree of the apparent resistivity were mainly affected by the spatial distribution, water-bearing state of the hydraulic cracks and wet area surrounding the hydraulic cracks. As the hydraulic crack network expanded, a highly conductive channels were formed inside the rock, which could affect the rock conductive properties and patterns. By comprehensively considering the temporal and spatial evolution characteristics of the apparent resistivity difference contour map and the regional apparent resistivity change curve, the dynamic expansion and spatial shape of hydraulic cracks in the rock could be accurately characterized via the DC monitoring method.