Date of Award

5-31-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering - (Ph.D.)

Department

Civil and Environmental Engineering

First Advisor

Bruno M. Goncalves da Silva

Second Advisor

Jay N. Meegoda

Third Advisor

Antonio Bobet

Fourth Advisor

Siva P.V. Nadimpalli

Fifth Advisor

Matthew J. Bandelt

Sixth Advisor

Rajesh Goteti

Abstract

Hydraulic fracturing is a well-stimulation technique that is employed in field applications, such as enhanced geothermal systems (EGS) and shale oil/gas extraction. This research experimentally investigates the effect of the state of stress and injection rate on the hydraulic fracturing processes. In addition, a displacement discontinuity method (DDM) code, FROCK, is used to model the crack initiation and propagation in a granite specimen under hydraulic fracturing conditions. In order to conduct the experimental work, a test setup capable of applying a triaxial state of stress and water pressure inside pre-fabricated flaws cut in prismatic granite specimens is developed. Additionally, the test setup allows one to monitor the fracturing processes visually and through acoustic emissions (AE).

A set of hydraulic fracturing tests is conducted using three triaxial states of stress (the lateral and out-of-plane pressures are always 2 MPa and the vertical pressures are 1 MPa, 2 MPa, and 4 MPa), but the same injection rate of 3 ml/min. The micro-fracture initiation pressures are obtained based on white patch initiation pressures (visually), and also utilizing a graphical method that relates the changes in pressurization rate (dP/dt) to the fracture initiation. The graphical method identifies when the initial damage occurs earlier than the visual method, which indicates that important micro-damage processes may occur without macroscopic evidence. In terms of AE data, it is noted that the onset of AE activity coincides with the time when there is an inflection in the pressurization rate, supporting that the inflection in (dP/dt) is associated with the initiation of micro-cracking. Additionally, the number of AE events increases significantly after visible crack development and upon the shut-in of the syringe pumps, which is not observed in unconfined tests. The energy budget of the system is used to interpret this phenomenon, leading to the conclusion that due to less energy being dissipated during the crack initiation and propagation (significantly smaller cracks than in the unconfined tests), the energy still stored in the system is converted into the seismic and fracturing energy, with the creation of more micro-cracks. Source mechanism analyses for the lower vertical load (1 MPa) show that shear is the predominant mechanism whereas tension is the prevalent mechanism for the high vertical load (4 MPa). The frequency-magnitude distribution of the AE events follows the Gutenberg-Richter law, with the b-value decreasing for increasing confinement. Additionally, injection rate dependent hydraulic fracturing experiments are also carried out on prismatic granite specimens using three injection rates (i.e., 0.3 ml/min, 3 ml/min, and 30 ml/min) and no confinement. It is observed that the breakdown pressures are injection rate-dependent and also coalescence patterns are direct for the three injection rates. The source mechanism analyses show that 42% of the micro-seismic events are predominantly of shear nature, regardless of the injection rate. Regarding the numerical results, the DDM approach FROCK has proven to be appropriate to simulate the hydraulic fracturing propagation in an isotropic rock. The numerical fracture initiation pressures correspond to the experimental white patching (i.e., micro-damage) initiation pressures for most of the pre-existing geometries. This validates the hypothesis that the critical material properties use in FROCK's fracture initiation criteria are the microscopic properties of the material.

This research significantly contributes to the fundamental understanding of the physical mechanisms responsible for the initiation, propagation and coalescence of hydraulic fractures in crystalline rocks.

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