Title: Analysis of Progressive Failure from Hydrofracturing Tests in Boise Sandstone

Program: Civil Engineering MS

Committee Chair: Nick Hudyma

Committee: Nick Hudyma, Bhaskar Chittoori, Yang Lu

Abstract: Sealed borehole laboratory hydrofracturing tests with acoustic emission monitoring were used to investigate the acoustic emission activity, damage thresholds,
progressive failure, and tensile strength as a function of injection rate for twenty Boise Sandstone specimens. Four injection rates 15.7, 31.4, 62.8, and 125.6 mL/minute were used in the testing.
Increasing the injection rate initially increased and then decreased acoustic
emission activity. The highest injection rate resulted in the lowest number of AE hits. The acoustic emission data was collected via two ultrasonic sensors. The highest average AE hits at the 31.4 mL/min injection rate was 613 and the lowest average AE hits at the
125.6 mL/min injection rate was 399.
Damage thresholds were investigated by determining the crack initiation stress, which is the stress where microcracks formation rates increase with increased loading. This stress was determined using two lateral strain methods and two acoustic emission activity methods. Increasing injection rates showed lower crack initiation stresses. Of the four crack initiation methods investigated, the AE Hit Line method estimated the highest crack initiation stresses. The Cumulative Acoustic Emission Hits method was very sensitive to fluctuations in the AE data. The lateral strain method is more subjective than the lateral strain response method. Overall, the crack initiation stress was found to be between 47 to 66 percent of the tensile strength.
Progressive failure was assessed by classifying microcracks using the dominant frequency determined using a Fast Fourier Transform of the AE waveform. Tensile microcracks were most abundant at the beginning of the tests. As the test progressed,
tension dominant mixed-mode microcracks became more abundant, and near the end of the test, shear microcracks were the most abundant. Overall, the average percentages of microcracks were 9.8%, 9.6%, 74.8%, and 5.8% for tension, tension dominated mixed-
mode, shear, and other, respectively.
Tensile strengths of the specimens decreased with increasing injection rates for the first four injection rates but the tensile strength was similar between the second highest and highest injection rate. The average tensile strength at the lowest injection rate was 9.4
MPa and the lowest tensile strength at the two highest injection rates was 7.0 MPa, a 25.5 percent decrease in tensile strength. The hydrofracture tensile strengths were 3.5 to 5.8 times the Brazilian tensile strengths.