Hydraulic Fracturing of Tight Shale Monitored by Acoustic Emission and Ultrasonic Transmission

In this work we study the effect of fluid viscosity on hydraulic fracturing initiation and near-wellbore propagation on block samples of tight shales subjected to representative effective in-situ stress conditions. Combined analysis of acoustic emission, ultrasonic transmission and volumetric deformation indicates that the viscosity of the injected fluid had a strong influence on hydraulic fracturing initiation, fracture propagation and fracture geometry. Injection of high viscosity fluid into the stressed tight shale resulted in fracture initiation at a bore pressure higher than the overburden stress and occurred significantly earlier than the borehole breakdown pressure. After initiation, the hydraulic fracture propagated symmetrically from the borehole in the direction parallel to the maximal horizontal stress, causing significant volumetric deformation of the rock. In the case of injecting a low viscosity fluid into the stressed block, fracture initiation occurred at a borehole pressure significantly lower than it was required with the higher viscosity fluid, and occurred almost simultaneously with the bore pressure breakdown. AE measurements during hydraulic fracturing allowed us to estimate that the average speed of hydraulic fracture propagation was approximately thousand times faster for the low viscosity fluid than for the high viscosity fluid.