How do faults respond to rapid stressing rate changes?: A study of induced earthquakes in geo-reservoirs

University Of Memphis, Memphis TN

Investigators

Abstract

Though slowly accumulating stresses in the Earth's crust are known to cause earthquakes, sudden small changes in stress can briefly cause the rate of earthquakes in some regions to skyrocket. One example of a place where this happens is the Blue Mountain geothermal reservoir in Nevada. It experiences some small earthquakes during its usual operations, but during annual maintenance shut-downs stresses suddenly change and a flurry of earthquakes results. In this project, Goebel and his group will investigate how this happens. They will deploy seismometers at the reservoir to collect detailed and precise earthquake data, analyze these data using machine-learning methods, and develop sophisticated computational models of the reservoir and its faults. The models will be used to compute temperatures, fluid pressures, and stress changes over time as water is injected and withdrawn; and the modeled stress change patterns will be compared with their earthquake data. Goebel and his team will then test whether what they have learned from the Blue Mountain study applies to a large petroleum reservoir in Oklahoma. This project will improve the general understanding of how earthquakes are triggered, and will provide guidance for geothermal operators seeking to reduce their earthquake risks. Goebel will also promote earthquake preparedness in areas with induced seismicity and collaborate with colleagues in El Salvador on reducing earthquake risks from the growing geothermal energy industry in their country. Most seismic events are triggered by small natural and induced stress changes. Triggering intensity is thought to be governed by fluid pressure and dynamic stress. Yet, how a fault will react to specific stress perturbations is hard to predict due to the limited understanding of governing processes and stresses at seismogenic depths. Stress and pressure changes are more easily studied in shallow geothermal reservoirs with induced seismicity. This project investigates induced seismicity and earthquake triggering in the Blue Mountain geothermal reservoir in Nevada which experiences repeated seismicity spikes during annual maintenance shut-downs. Previous observations revealed a rapid seismic response to geothermal operations within ∼24 hours and a gradual decay of seismicity within ∼one week. The respective seismic events occur close to reservoir depths and exhibit strong spatial clustering around a high- volume injection well. The link between seismic events and abrupt operational changes has rarely been examined before and provides a particularly interesting target for the study of earthquake triggering. The researchers will test the hypothesis that rapid injection changes lead to increased seismicity, which is primarily driven by poro-elastic effects. Blue Mountain is especially well-suited to test this hypothesis because of its long operational history, and geologic records and dense seismic and pressure instrumentation. The researchers will examine whether insights from Blue Mountain can be applied more generally by examining rapid injection rate changes and seismicity at larger scales across hydrocarbon basins in Oklahoma. This work will connect induced and natural earthquake triggering to produce a more general understanding of underlying mechanisms. A better documentation of how faults react to different driving stresses has profound implications for seismic hazard mitigation and operational aftershock forecasts. The proposed work will provide insight into the coupling between injection rate changes and seismic activity in different geo-reservoirs, as well as stress conditions that promote earthquake triggering. Identifying specific reservoir operations that amplify a seismogenic response is important for safer subsurface energy production and CO2 sequestration. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.