Non-linear relations between acoustic emissions and fluid pressure in saturated and critically-stressed sandstone

Abstract

Injection-induced seismicity poses a risk to public safety, infrastructure and perception. The disposal of wastewater deep-underground presents an especially large hazard in the central United States. Relations currently used in forecasting earthquakes are based on injection volume and rate. However, the triggering mechanism for induced seismicity is usually formulated as an increase in uid pressure that reduces frictional strength and destabilises a fault. Laboratory experiments on Bentheim and Flechtingen sandstone were performed to establish quantitative relations between acoustic emissions, the elastic waves that accompany microscopic damage, and uid pressure and test the hypothesis of a linear relation between uid pressure and seismicity. Acoustic emissions are a micro-scale analogue for seismicity and obey empirical relations such as Omori's and Gutenberg-Richter's laws. The acoustic emission experiments consisted of four phases: saturation, con ning, loading to a critically-stressed state, and undrained injection to macroscopic failure. P-wave piezoelectric sensors actively measured wave velocity and passively monitored acoustic emissions throughout the procedure. The injection pressure was increased at the boundary in a series of steps to allow time for uid pressure to equilibrate and the acoustic emission rate to decay. It was concluded that the number of acoustic emissions produced per unit increase of

uid pressure followed a power-law function of the total acoustic emissions produced during the injection, once a critical pressure was exceeded. The observed behaviour could be explained by a damage model where former weakening by microscopic crack formation may have provided defects for subsequent acoustic emissions.