On the edge?

Lead Research Organisation: University of Edinburgh


This project addresses the current imperative to move to a net zero carbon economy. Many practical solutions involve engineering of the ground beneath our feet, for example through geothermal energy production, radioactive waste disposal, and subsurface storage of carbon dioxide or hydrogen. The problem is that these activities add new stresses to underground reservoirs or storage sites already suffering from ambient stresses due to plate tectonics. Hence, there is a risk of even small additional stress triggering earthquakes, potentially leading to damage or nuisance from ground motion and/or allowing harmful fluids to escape to the surface, and hence losing public confidence in such solutions. For example, the onshore fracking industry in the UK triggered earthquakes as large as magnitude 2.9 in Lancashire, despite the introduction of a 'traffic light system' to manage the risk. The UK traffic light system operated to modify operations at a threshold magnitudes of 0.5 (amber) or to stop them for the day at magnitude 1.0 (red). The failure to prevent the magnitude 2.9 earthquake resulted in a moratorium on the fracking industry to the present day. It would be tragic if a similar fate awaited the net zero solutions involving engineering of the sub-surface. Here we will address the problem: can we do better?
One of the main barriers to developing an effective risk management strategy is that we often do not know how close the Earth is to failure on the scale of the engineered system - are we 'on the edge' of failure (or not)? The susceptibility to small stress perturbations in the Earth is highly variable, and one of the biggest 'known unknowns' in this field. Here we will carry out a series of experiments to understand the processes involved in the triggering of fracture and earthquakes at different starting stresses, to see if we can characterise this sensitivity before we start operations, and to control the risk of extreme events during operations better than the current traffic light system.
We will use new methods of accurately measuring seismic wave velocity changes that are sensitive to stress changes, and use these, and other attributes of the induced seismicity such as event rate, fault or fracture type, and the amount of associated deformation, to see if we can do better in a controlled environment. We will construct a scale model system in the laboratory, where we can mimic field conditions in terms of stress and fluid pressure. While we are deforming the rock by changing stress or pore pressure, we will record tiny micro-earthquakes caused by damage in the form of micro-cracking, and monitor changes in seismic velocity and fluid permeability. In particular, we are interested in tiny but detectable velocity transients (step changes followed by a gradual decay) associated with very small stress perturbations. Transients are thought to be caused by the sudden induced damage and subsequent slower healing of the material when stresses change. They have been observed in a variety of settings in the Earth, but their causes remain enigmatic. Here we will conduct the live experiments in a synchrotron, so we can 'see' the actual processes of deformation at the pore scale. We will build a unique, purpose-built portable deformation rig to maintain the UK global lead in this type of work. This will allow us to combine seismic 'sound' with x-ray 'vision', and hence help us understand the meaning of seismic data on the operational scale, where we cannot see the processes.
Finally, we will examine how our observations compare with field examples on a range of scales in space and time. The results will determine whether we could add continuously monitored velocity change, and other seismic properties in response to stress perturbation to our armoury in quantifying the risk at the planning stage (by choosing less sensitive sites) and during operations (through continuous monitoring and control).


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