Effect of the Triaxial State of Stress in the Hydraulic Fracturing Processes of Granite: Part 1—Visual Observations and Interpretation

Abstract

A set of hydraulic fracturing tests were conducted in prismatic granite specimens to investigate the influence of the triaxial state of stress on the cracking processes. The tests considered three states of stress, in which the horizontal and out-of-plane loads were 2 MPa, while the vertical loads were either 1 MPa, 2 MPa, or 4 MPa. A hydraulic pressure inside pre-fabricated flaws was increased at a constant injection rate until hydraulic fractures initiated and propagated. In order to materialize this state of stress, a test setup was developed to independently apply and control three orthogonal stresses in the prismatic granite specimens while simultaneously applying a hydraulic pressure inside pre-fabricated flaws. More importantly, the setup also allowed one to observe the damage processes visually and through acoustic emission monitoring. This paper (the first of two) looks into the visual (macroscopic) observations, whereas the companion paper focuses on the acoustic emission monitoring. In particular, the present paper looks into the white patching (micro-damage) development, crack initiation, propagation and coalescence, and relates these with fracture initiation and breakdown pressures. From the tests, it was observed that the initiation of visible cracks is, generally, preceded by white patching and occurs at or near the pre-fabricated flaws. The visual analysis showed that the induced fractures were primarily intragranular when the triaxial stresses were same in the three orthogonal directions and intergranular otherwise. Additionally, micro-fracture initiation pressures were obtained based on white patch initiation pressures (visually) and using a graphical method which relates the changes in pressurization rate to fracture initiation. It was observed that the graphical method identifies when the initial damage occurred significantly earlier than the visual method, which indicates that important micro-damage processes may occur without macroscopic evidence. Most importantly, the coalescence patterns and the ratio between the breakdown pressure and the vertical stress are stress-state dependent. In fact, for lower vertical loads (VL = 1 MPa), the ratio between the breakdown pressure and vertical load is approximately 10 and for higher vertical loads (VL = 4 MPa) approaches 1.0, which is in agreement with data observed in the field.