Induced seismicity due to subsurface fluid injection: the role of fault properties.

Overview

Research Background: There is evidence that areas considered geologically stable have now experienced increased rates of seismicity (Ellsworth, 2013), particularly in the US. It is believed that this is due to fluid injection activities used in modern energy production (McGarr et al., 2015).
Amongst the main subsurface injection activities are carbon capture and storage (CCS), and enhanced geothermal systems (EGS) exploitation. Carbon capture and storage is a method of capturing CO2 emissions of electricity generation and industrial processes and injecting them into a geological reservoir. The injection of high pressure CO2 into a geological formation increases pore pressure and reduces effective stress within the formation. This can increase the chance of any pre-existing faults present to undergo reactivation and failure, and potentially generate seismic activity (Cappa and Rutqvist, 2012).
An enhanced geothermal system is comprised of: 1) a natural heat source, 2) a geothermal reservoir (granite, carbonates, etc.), 3) a wells loop system made by injection/production wells for cool water recharge and hot water discharge, 4) a fault system of open and connected fractures to close the injection and production wells loop. Coupling mechanisms between the source of fluids and the re-activation of the exploited fault system may potentially lead to induced seismicity.

Central Research Problem: During EGS activities, moderate to large events took place (e.g. 2006 ML 3.4 Basel; 2017 Mw 5.5 Pohang), which were larger than the forecasted maximum magnitude. Clearly, each of these subsurface injection practices represents a considerable hazard and potential risk in terms of induced seismicity. Fluid-induced seismicity can be investigated, for example, by monitoring active faults in a reservoir from in-situ borehole observations and measurements (e.g. strain, strain rate, pore pressure, fluid chemistry). However, these techniques are indirect in nature, and they have a high cost and a low resolution. As a consequence, our forecasting and mitigation strategies remain severely limited by our lack of specific knowledge about how rupture nucleation and propagation process are affected by injected fluids at depth (McGarr et al., 2015; Ellsworth, 2013).

Main Aim of the Project: The main aim of the proposed research is the systematic investigation of the role played by in situ heterogeneities of stress conditions and fault properties (e.g. lithology, roughness, rheology, structure) in controlling rupture propagation parameters (e.g. dynamic friction, slip weakening distance, dissipated fracture energy, slip and rupture velocity). In particular, the critical conditions leading to the transition between self-arrested ruptures, which are contained within the exploited reservoir volume, and large run-away ruptures, able to grow well beyond the exploited reservoir, will be investigated.
The expected results will allow gaining a deeper physical insight into the propagation and arrest process of natural and fluid injection induced earthquake rupture. They can be used by seismologists, geophysicists and geodesists to inform existing conceptual models used during risk and hazard assessment procedures.

Methodology

To achieve the main aims of the project, the Student will adopt a multidisciplinary research approach, integrating results from mechanical laboratory experiments and microstructural observations.
Friction Laboratory experiments: The Student will perform new sets of triaxial loading laboratory experiments on simulated faults using the relevant lithologies to CCS and EGS reservoir, overburden and underburden units (e.g. granite, carbonates). Experiments will be performed at reservoir P-T conditions, using saw-cut and/or direct shear experimental setup. We will use the experimental setup to induce spontaneous stick-slip events, analogue of natural earthquake events, using the triaxial loading apparatus hosted at the rock mechanics laboratory at Durham University. Coseismic strain and acoustic emissions will be continuously recorded at high frequency during the experiments, using strain gages and piezoceramic sensors. This allows to measure locally on the fault the stress and slip evolution during fast (ms) seismic slip events.
The expected outcome is to obtain quantitative, constitutive laws of fault friction evolution during spontaneous dynamic rupture propagation (i.e. stick-slip events). Such constitutive laws will link measured rupture parameters to fault properties. In particular, the Student will quantify the control exerted by local stress and fault property heterogeneities on the onset of fault reactivation, the rupture propagation parameters and how such heterogeneities affect rupture modes, e.g. arrested vs runaway rupture.
Microstructural Observations: High-resolution optical microscope (OM) and field emission scanning (FeSEM) electron microscope images will be taken from thin sections and chips of sliding surfaces of samples deformed during stick-slip experiments. Microstructural observations will be used to infer the deformation mechanisms which may control seismic vs. aseismic behaviours observed during the experiments.

Project Timeline

Year 1

Literature review on fault and earthquake mechanics. Training in laboratory experiment and microstructural analyses techniques. Attendance to TSG annual meeting.

Year 2

Perform rock mechanics experiments, microstructural analyses (Optical Microscopy and Scanning Electron Microscopy). Complete data processing and analyses. Attendance to TSG annual meeting.

Year 3

Data integration, thesis writing completion, papers writing for submission in International peer reviewed journals. Attendance to a conference of international relevance (e.g. EGU, AGU) to disseminate the main results obtained.

Year 3.5

Thesis revision and writing completion, submission of final thesis.

Training
& Skills

The Student will benefit from interactions with members of the supervisory team who have expertise in rock and fault mechanics, microstructures of fault rocks. He/She will learn how to use a range of high-level analytical and experimental methods, and how to integrate different data types and to understand their significance from both scientific and industrial perspectives. The Student will join the Rock Mechanics Laboratory Group and the Durham Structural Group, made of 7 staff and 11 PhD students. The groups meet weekly and host national and international visitors.
The Student will be expected to present posters and talks at conferences and are also likely to spend time away from the host universities.

References & further reading

Cappa, F. and Rutqvist, J. (2012). Seismic rupture and ground accelerations induced by CO2 injection in the shallow crust. Geophysical Journal International, Volume 190, p. 1784–1789.
– Ellsworth, W. L. (2013). Injection-induced earthquakes. Science, vol 341.
– McGarr, A. et al. (February 2015). Coping with earthquakes induced by fluid injection. Science, Vol. 347, Issue 6224.

Further Information

Dr Nicola De Paola
Rock Mechanics Laboratory, Department of Earth Sciences, University of Durham (UK).
e-mail: nicola.de-paola@durham.ac.uk

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