Acoustic emission measurements during hydraulic fracturing tests in a salt mine using a special borehole probe

Hydraulic fracturing tests were performed in a horizontal, 20 m long borehole in the salt mine Asse. The minimum principal stress and its orientation could be measured using a special hydraulic fracturing borehole probe. This new-developed probe utilizes acoustic emission (AE) measurements aimed at monitoring and localizing hydraulic fractures. AE analysis pointed out that at small borehole depths in the vicinity of the gallery the developing large fracture planes were nearly perpendicular to the borehole axis. This indicates large deviatoric stresses and a horizontal orientation of the minimum principal stress. In larger borehole depths the events show sometimes zones of dispersed microcracks around the borehole. In other cases small transverse fractures are discernible. This may be attributed to a nearly isotropic state of stress. Stress directions and values of the minimum stress were compared to the results of finite-element calculations considering thermally induced stresses.