Acoustic emission induced by pore-pressure changes in sandstone samples

An understanding of microseismicity induced by pore-pressure changes in stressed rocks is important for applications in geothermal and hydrocarbon reservoirs as well as for CO2 sequestrations. We have studied the triggering mechanisms of microseismicity (or acoustic emission in the laboratory) as a function of triaxial stress conditions and pore-pressure changes. In investigating the spatiotemporal distribution of acoustic emission activity in water-saturated triaxially stressed Flechtingen Sandstone samples subjected to changes in pore pressure, we assumed that acoustic events were triggered by pore-pressure increase. To estimate pore-pressure changes in the sample, we used an analytical solution of the 1D diffusion equation. A theoretical analysis of the spatiotemporal distribution suggested that for initially insignificantly stressed samples, acoustic events were triggered by the migration of a critical pore-pressure level through the sample. The critical level was controlled by the applied pore pressure of the previous cycle according to an apparent Kaiser effect in terms of pore pressure. This memory effect of the rock vanished if additional axial stress was applied to the sample before the next injection cycle. The behavior of a highly fractured rock in the final stage of the failure experiments was different. During the formation of a final sample-scale fracture, the spatiotemporal distribution of acoustic emission was more likely controlled by propagation of the fracture than by diffusion of a critical pore pressure, showing that the final macroscopic fracture was triggered by low pore pressure. Our work contributes to the characterization of reservoirs using fluid-induced seismicity.