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 mate-rials. AE is often used to detecting 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, charac-terization of samples is key for evaluating subsurface formations for successful under-ground storage. Research has been done 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 mineralogic changes which may occur. Typically, there is a shortage of geolog-ic 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 multi-stage 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 correc-tions to the measurements. Acoustic Emissions were measured on five different samples undergoing Multi-stage Triaxial Tests. Two different behaviors were observed. For the “coarse grained” samples, designated Group 1 (Miocene sandstone, Wilcox and Cambri-an sandstone), the number of AE events did not show a strong dependence on confining stress. They did show an exponential increase of AE events with increasing deviatoric stress during each stage. In contrast, the Group 2 samples (Niobrara Marl and Niobrara Chalk) exhibited a significantly different stress dependent AE behavior. The amplitude of the AE events is significantly smaller than the quartz dominated samples indicating a more ductile and diffuse failure mechanism. The correction between maximum compres-sive strength and the point of positive dilatancy is still 1.2 for these samples, even though a different pattern of AE events is observed.