Hydraulic Fracturing with Variable Injection Rate in Layered Tight Formations Monitored by Acoustic Emission

Laboratory fracturing with variable injection rate was performed on natural shale specimens containing multiple bedding planes (BPs). The influence of injection rate on hydraulic fracture (HF) propagation and stimulated reservoir volume (SRV) was analyzed by monitoring dynamic process of hydraulic fracturing using acoustic emission (AE). Experimental results show that the location of small few AE events along BP near the wellbore and stable injection pressure indicated the fracturing fluid leaking into the dilated BP under low injection rate (= 10 mL/min). In this case the SRV was limited. Increasing the injection rate to 20 mL/min is helpful for HF initiation and propagation. Intense AE activity was simultaneously monitored. Three-dimensionally distributed AE events indicated the generation of complex fracture network. Consequently, the SRV significantly increased from 624 to 3126 cm³. When re-fracturing was conducted after the HF had breakthrough the rock boundary, the initial fracture network was reopened under the injection rate of 20 mL/min. Low AE activity and stable injection pressure were monitored. On the contrary, a higher injection rate of 50 mL/min resulted in new failure in the rock. The SRV calculated from the AE distribution was further increased to 4494 cm³.