Hydraulic fracturing is a very complex process that involves the coupling of different competing mechanisms, such as mechanical deformation, fluid flow, and propagation of fractures. Therefore, understanding hydraulic fracturing and its characteristics are both critical and challenging. In the field, seismic monitoring has been used to ascertain the efficiency of the hydraulic fracturing operation by mapping the generated hydraulic fractures and their evolution. Shearing-type mechanisms were found to be dominating, based on the recorded seismic data, despite affiliating tensile fracture opening as the predominant mechanism of hydraulic fracture propagation. This study involved the hydraulic fracturing of true triaxially loaded low permeability crystalline rock cubes using a relatively high viscosity gear oil at different injection rates. Real-time acoustic emission monitoring was employed to not only map and characterize the geometry of the hydraulic fracture network but also to determine the source mechanisms of the individual cracks through moment tensor inversion. Higher breakdown pressures and larger seismic energy were encountered for higher injection rates with tensile events dominating for all tested injection rates.

1. Introduction

Hydraulic fracturing has been used over the past 70 years in diverse applications which include gas and oil production, enhanced geothermal systems, carbon sequestration, rock burst mitigation, coalbed methane development, in-situ stress determination in mining and geotechnical projects, etc. (Hamison and Fairhurst, 1969; Hayashi and Hamison, 1991; Raaen et al., 2001; Kang et al., 2012; Adams and Rowe, 2013; Stoeckhert et al., 2015). In a hydraulic fracturing operation, a high-pressure fluid is injected into a drilled wellbore under a constant injection rate. The downhole pressure is increased to the point where a fracture is initiated, and the pressure-time curve starts to deviate from the linearity. The pressure inside the borehole continues to rise to a maximum value, known as the breakdown pressure, at which point the fracture starts to propagate with the decline in the borehole pressure. Hydraulic fracturing formation and propagation is a complex phenomenon involving different competing mechanisms. These coupled processes involve mechanical deformation of the material (theory of elasticity), fluid flow inside the fracture (lubrication theory), and the propagation of fracture (linear elastic fracture mechanics) (Adachi et al., 2007; O'Keeffe et al., 2018).

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