Experimental study on hydraulic fracturing and acoustic emission characteristics of shale

The hydraulic fracturing experiments were conducted in the laboratory under true triaxial condition. Based on acoustic emission localization and acoustic emission characteristic parameters, the process of crack propagation in hydraulic fracturing was studied. The results showed that: (1) According to acoustic emission (AE) characteristics, shale hydraulic fracturing process can be divided into three stages: the initial stage, main fracture stage, and volumetric fracture stage. (2) In the initial stage, the pressure is in an upward phase, with a relatively small number of acoustic emissions and large b-values, mainly consisting of compressive and tensile cracks with low frequency, forming dispersed small cracks inside the rock. Before reaching the peak pressure, the cracks will gradually form a main fracture. (3) In the main fracture stage, as the pressure increases until the reservoir fracture pressure is reached, the acoustic emission rate sharply increases, resulting in a large number of high-energy acoustic emission events. The b-value decreases, and the acoustic emission frequency is higher, forming a tensile type main fracture closed to the wellbore hole. (4) In the stage of volumetric fracture, although the pressure significantly decreases, a large number of micro cracks still occur inside the samples. The fractures mainly propagate in a dispersed manner along the main fracture and form a complex network of volumetric fractures. During this stage, the acoustic emission b value is relatively high, with a large number of acoustic emissions of shear type fractures. (5) The AE characteristics of hydraulic fracturing in the laboratory are consistent with the microseismic monitoring results of on-site hydraulic fracturing. After on-site hydraulic fracturing, a large number of microseismic events will also occur during the well closed stage. The results of this study have important guiding significance for on-site hydraulic fracturing.