Energy Budgeting of Laboratory Hydraulic Fracturing in Granite with Different Viscosity Injection Fluids

Abstract

ABSTRACT In this study, Barre granite cubes were hydraulically fractured in laboratory, using different viscosity injection fluids. Hydraulic Fracturing (HF) experiments were monitored with real-time acoustic emission (AE) setup which consisted of 16 calibrated Nano-30 sensors. The spectral parameters (corner frequency and low-frequency spectral plateau) were determined for each AE event by fitting the Brune Omega-squared model to the detected AE signals. Seismic parameters such as seismic moment, source radius stress drop, and seismic energy were determined after incorporating the focal mechanisms information determined through moment-tensor inversion. Higher breakdown pressures and fracture propagation times were observed for experiment conducted with higher-viscosity fluid. An inverse relationship was observed between corner frequency and seismic moment, similar to those observed for natural and induced seismicity emitted during large-scale events. The stress drop was found to be fairly constant across a wide range of seismic moment. The seismic moment, stress drop, and seismic energy were significantly higher for the higher viscosity injection fluid. However, the corner frequency and source radius were slightly larger for the lower-viscosity fluid. The seismic efficiency was <<1% (10−6-10−4 %) for both experiments but it was much lower for the experiment conducted with lower-viscosity injection fluid, implying a more aseismic response. INTRODUCTION Hydraulic fracturing (HF) is a technique used over the past decades, mainly to increase the reservoir rock permeability for geothermal resource extraction from kms deep crystalline granitic rock. However, HF is a complex phenomenon where it is often difficult to predict the HF growth and the underlying fracture mechanisms. In addition, the HF behavior can be significantly influenced by some key factors including hydro-mechanical properties of the host rock, in-situ stress, and injection parameters (injection fluid rate and viscosity) (Haimson and Fairhurst, 1967). Different viscosity injection fluids have been utilized for the stimulation process mainly to control the HF growth (Ishida et al. 2004). Many previous studies have shown that the attributes of produced fracture network can be drastically different depending on the viscosity of the injection fluid (Tanaka et al. 2020). For example, lower-viscosity injection fluids have been shown to produce more tortuous cracks with many secondary branches (Ishida et al. 2016). Increased stimulated reservoir volume has been shown to achieve with relatively lower-viscosity injection fluids (Warpinski et al. 2005). Understanding the characteristics of the generated HF for different viscosity injection fluids can be crucial to derive appropriate estimations for an efficient HF design.