INVESTIGATION OF TRIAXIAL COMPRESSION FAILURE AND ACOUSTIC EMISSION CHARACTERISTICS OF DIFFERENT RESERVOIR ROCKS

With the development of engineering activities such as exploitation of shale gas, waste water injection and carbon dioxide geological sequestration, the issue of induced seismicity due to the change of stress state of reservoir rocks has aroused wide attention, the investigation of failure process and acoustic emission characteristics is of great significance for the understanding of induced seismicity. A series of triaxial compression acoustic emission(AE)tests were conducted on shale, tight sandstone and dolomite, the spatial fracture geometry were obtained, and the failure process of different reservoir rocks were characterized by using acoustic emission monitoring method. The results suggested that: (1)the ratio of dilation stress to peak stress is highest for shale, followed by dolomite and tight sandstone, indicating that the breakdown of shale is dominated by brittle failure. (2)The rock fabric significantly affects fracture propagation and strength, shale typically has developed laminar structure, the compressive strength and failure types present strong anisotropy. Shear-tensile compound failure occurs and massive shear microfractures tend to form when bedding plane inclination is 0°. For shale samples with bedding plane inclinations of 30°and 60°, shear failure tends to occur during compression process. When bedding plane inclination increases to 90°, tensile fractures are likely to create. Tight sandstone sample produces a relative simple shear fracture, and dolomite sample develops two main shear fractures and multiple microfractures. (3)The acoustic emission characteristics of different reservoir rocks vary significantly during the failure process. The acoustic emission activities of shale samples begin to increase near the dilatation stress point, and the number and energy of acoustic emission events increase rapidly when the peak stress is reached. The tight sandstone sample only has a small amount of acoustic emission activity at the moment of breakdown. By comparison, the acoustic emission activities of dolomite samples are obvious in unstable fracture growth stage and post-peak stage, the amplitude, energy and number of events are relative large. Therefore, construction measures should be adjusted according to the microseismic monitoring results in practical engineering to avoid the brittle failure of shale when served as the cap rock and alleviate the induced earthquakes in the stimulation of dolomite formation.