Induced stress evolution of hydraulic fracturing in an inclined soft coal seam gas reservoir near a fault

Abstract

Hydraulic fracturing is an efficient technique for enhancing coal seam gas production. Induced stress (or stress shadow) of hydraulic fracturing has been popularly studied recent years. In this study, induced stress was measured in a field during the process of hydraulic fracturing. It was found that the induced stress evolution was closely related to the increase of injection volume. The relation between the average induced stress value and the injection volume was fit by the Michaelis-Menten equation well. When carried out the injection operation, we found that the induced stress increased significantly within the radius of 70 m around the hydraulic fracturing borehole. The induced stress also dropped occasionally during the injection process, which indicated some large fractures’ propagation. When the injection operation was finished, the induced stress began to decrease slowly. The subsequent drilling of gas extraction could release the induced stress. In addition, the induced stresses caused by adjacent fracturing boreholes (spacing was 50 m) substantially influenced each other. The induced stress acted earlier in the upper part of the coal seam than the lower part influenced by the steep incidence of the coal seam. Furthermore, the orientation of the induced stress distribution was eventually parallel to the maximum horizontal principal stress direction controlled by a nearby large fault, although the initial orientation was arbitrary. The results are helpful for hydraulic fracturing evaluation and optimization and numerical simulation research.