Use of Acoustic Emissions to Validate Multistage Triaxial Tests—A Key to Characterizing the Subsurface

Acoustic emission (AE) measurements have many uses to evaluate the integrity of materials. AE is often used to detect leakage in pipelines. It has also been used to monitor changes in strength properties of fiber-reinforced concrete. In the oil and gas industry, AE is predominantly used to study fracture initiation and propagation. In particular, characterization of samples is key for evaluating subsurface formations for successful underground storage. Research has been performed to understand the behavior of AE in uniaxial compression and single-stage triaxial compression tests. However, the validity of this method has not been documented in a multistage triaxial test. This characterization is required to understand the stability of the host rock under the related stress changes and potential mineralogical changes that may occur. Typically, there is a shortage of geologic samples. A single multistage triaxial test eliminates the need for twin samples and provides an economic and time-saving protocol compared to conventional methods. A single multistage triaxial (MST) test allows a constitutive model to be developed for a host rock. This work establishes a protocol for performing these tests with minimal corrections to the measurements. Acoustic emissions were measured on five different samples undergoing multistage triaxial tests. Two different behaviors were observed. For the coarse grained samples, designated Group 1 (Miocene sandstone, Wilcox Formation, and Cambrian sandstone), the number of AE events did not show a strong dependence on confining stress. They did show an exponential increase in AE events with increasing deviatoric stress during each stage. In contrast, the Group 2 samples (Niobrara Marl and Niobrara Chalk) exhibited significantly different stress-dependent AE behaviors. The amplitude of the AE events is significantly smaller than in the quartz-dominated samples, indicating a more ductile and diffuse failure mechanism. The correlation between maximum compressive strength and the point of positive dilatancy is 1.2 for both groups of samples, even though a different pattern of AE events is observed.