Connecting Microseismicity to Lithology via a Model of Slip Avalanches

Abstract

Fluid injection into the earth's crust can induce small and frequent earthquakes in the subsurface. Predicting their sizes and temporal occurrences via statistical analysis is crucial for safe operations in unconventional oil and gas recovery, enhanced geothermal systems, and geologic carbon storage. Here we show that a simple micromechanical model of slip avalanches in slowly deforming solids predicts the slip statistics observed over drastically different spatial scales, namely meter-scale microseismic observations and nanometer- to micrometer-scale nanoindentation experiments can be described with this model. Microseismic catalogs extracted from high-pressure fluid injection operations into geological basins with various lithologies and nanoindentation experiments on shale across a wide range of temperatures and mineral compositions yield statistics consistent with model predictions. This universality across materials, temperatures, and scales is consistent with the prediction that the slip statistics result from only a few basic properties. Previously debated deviations of the statistics in layered sedimentary formations are explained by finite-size and stress-integrative effects resulting from mechanically weak bedding planes. The slip statistics therefore provide important information about the structure and scales of the bedding planes. Conversely, the basin structure can also be used to predict the probability distribution for the sizes of triggered microseismic events.