Injection of fluids in the subsurface can induce seismicity and possibly lead to rock failure. Earthquake nucleation could be triggered by pore fluid diffusion or changes in pore fluid composition. Thus, microcracking processes before the formation of macrofracture in fluid-saturated rock are of major importance. Plane strain compression experiments were performed on dry, oil- and water-saturated Berea sandstone under different boundary conditions. The deformation of the material was measured and the acoustic emission (AE) activity was recorded. Onset of inelastic response coincided with an increase in AE rate. However, released energy and onset of inelastic behavior were influenced by the pore fluid. Presence of oil in the pores did not affect the AE behavior. Earlier onset of recorded AE activity in water-saturated compared to dry and oil-filled specimens is explained by stress corrosion cracking, which resulted in microcracking at relatively low deviatoric stresses. In contrast to the yield envelope, the failure envelope was not affected by type of pore fluid. We suggest performing laboratory experiments that closely replicate the in-situ conditions in terms of applied external stresses, pore pressures, temperatures, and type pore fluids to properly characterize the potential of inducing seismic activity in rock during underground storage.

1. Introduction

Injection of fluids into the subsurface reduces the effective stress and can cause microcracking or slip of existing fractures, which lead to induced seismicity (Ellsworth, 2013). Geothermal energy systems, conventional and unconventional oil and gas recovery, and CO2 storage technologies all involve fluid withdrawal and/or injection, thereby providing the potential to induce seismic events (National Research Council, 2012). An application of new and innovative laboratory methods allows a better understanding of the complex, coupled processes in the subsurface energy extraction applications and corresponding risks of fluid-induced seismicity (Benson et al., 2020). Acoustic emission (AE) monitoring techniques used in the laboratory are similar to microseismicity monitoring applied at field-scale (Lei and Ma, 2014). Generally, AE is a process of elastic wave emission associated with microcrack growth during inelastic deformation; features of the AE represent the process of fault nucleation and allows studying seismicity at laboratory scale (Lockner, 1993).

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