Since the advent of hydraulic fracturing as a proven technique for reservoir stimulation, efforts have been made, using empirical and analytical tools. To understand hydraulic fracturing. These efforts have not been entirely successful, and inaccurate hydraulic fracture evaluation remains a problem in the industry. In this study the results of hydraulic fracturing of unconfined 4-inch diameter cylindrical carbonate core samples using water or mineral oil are presented. With a simple laboratory procedure samples were pressurized to failure and associate acoustic emissions analyzed. The breakdown pressures could be associated with the level of AE activity and are functions of treatment fluid viscosity and sample permeability. The hydraulic fractures are not caused by simple tensional failure, and so result in the hydraulic fractures being non-planar at some scale. The microfracturing processes possess overlapping frequency bands between 39 and 391 KHz. Source parameters like stress drops and average displacements of individual shear grain size. The observations are consistent with the reactivation of pre-existing flaws.