Scaling of seismicity induced by nonlinear fluid-rock interaction after an injection stop

Fluid injections into unconventional reservoirs, performed for fluid-mobility enhancement, are accompanied by microseismic activity also after the injection. Previous studies revealed that the triggering of seismic events can be effectively described by nonlinear diffusion of pore fluid pressure perturbations where the hydraulic diffusivity becomes pressure dependent. The spatiotemporal distribution of postinjection-induced microseismicity has two important features: the triggering front, corresponding to early and distant events, and the back front, representing the time-dependent spatial envelope of the growing seismic quiescence zone. Here for the first time, we describe analytically the temporal behavior of these two fronts after the injection stop in the case of nonlinear pore fluid pressure diffusion. We propose a scaling law for the fronts and show that they are sensitive to the degree of nonlinearity and to the Euclidean dimension of the dominant growth of seismicity clouds. To validate the theoretical finding, we numerically model nonlinear pore fluid pressure diffusion and generate synthetic catalogs of seismicity. Additionally, we apply the new scaling relation to several case studies of injection-induced seismicity. The derived scaling laws describe well synthetic and real data

The surge of earthquakes in Central Oklahoma has features of reservoir-induced seismicity

The recent surge of seismicity in Oklahoma and Kansas is related to fluid disposal. Evidences suggest that critical parameters are the injection volume as well as injection depth but dominant physical processes and a corresponding model to describe the physics are still not clear. We analyse the spatio-temporal distribution of induced earthquakes in the basement and find visible signatures of pore pressure diffusion and poroelastic coupling, features which strongly resemble seismicity induced by the filling of artificial lakes, so-called reservoir-induced seismicity. We developed a first-principle model of underground reservoir-induced seismicity. The physics of the model are based upon the combined mechanisms of fluid mass added to the pore-space of the injection layer and acting as a normal stress on the basement surface, pore-fluid pressure diffusion in the basement as well as poroelastic coupling contributing to the pore-fluid pressure and stress. Furthermore, we demonstrate that underground reservoir-induced seismicity occurs preferably in normal faulting and strike-slip settings, the latter being prevalent in Oklahoma. Our model explains observed injection volume and depth dependence of the seismicity and should be considered as a basis for future hazard prediction and prevention as well as for planning possible disposal sites

Autonomous decision-making against induced seismicity in deep fluid injections

The rise in the frequency of anthropogenic earthquakes due to deep fluid injections is posing serious economic, societal, and legal challenges to geo-energy and waste-disposal projects. We propose an actuarial approach to mitigate this risk, first by defining an autonomous decision-making process based on an adaptive traffic light system (ATLS) to stop risky injections, and second by quantifying a "cost of public safety" based on the probability of an injection-well being abandoned. The ATLS underlying statistical model is first confirmed to be representative of injection-induced seismicity, with examples taken from past reservoir stimulation experiments (mostly from Enhanced Geothermal Systems, EGS). Then the decision strategy is formalized: Being integrable, the model yields a closed-form ATLS solution that maps a risk-based safety standard or norm to an earthquake magnitude not to exceed during stimulation. Finally, the EGS levelized cost of electricity (LCOE) is reformulated in terms of null expectation, with the cost of abandoned injection-well implemented. We find that the price increase to mitigate the increased seismic risk in populated areas can counterbalance the heat credit. However this "public safety cost" disappears if buildings are based on earthquake-resistant designs or if a more relaxed risk safety standard or norm is chosen.Comment: 8 pages, 4 figures, conference (International Symposium on Energy Geotechnics, 26-28 September 2018, Lausanne, Switzerland

Stress Drop, Seismogenic Index and Fault Cohesion of Fluid-Induced Earthquakes

Sometimes, a rather high stress drop characterizes earthquakes induced by underground fluid injections or productions. In addition, long-term fluid operations in the underground can influence a seismogenic reaction of the rock per unit volume of the fluid involved. The seismogenic index is a quantitative characteristic of such a reaction. We derive a relationship between the seismogenic index and stress drop. This relationship shows that the seismogenic index increases with the average stress drop of induced seismicity. Further, we formulate a simple and rather general phenomenological model of stress drop of induced earthquakes. This model shows that both a decrease of fault cohesion during the earthquake rupture process and an enhanced level of effective stresses could lead to high stress drop. Using these two formulations, we propose the following mechanism of increasing induced seismicity rates observed, e.g., by long-term gas production at Groningen. Pore pressure depletion can lead to a systematic increase of the average stress drop (and thus, of magnitudes) due to gradually destabilizing cohesive faults and due to a general increase of effective stresses. Consequently, elevated average stress drop increases seismogenic index. This can lead to seismic risk increasing with the operation time of an underground reservoir.PHASE University consortium project of Freie Universität BerlinFreie Universität Berlin (1008

Interpretation fluid-induzierter Erdbeben im Geothermie-Reservoir Basel und im Kohlenwasserstoff-Reservoir Cotton Valley

The main objective of the thesis is a further development of the seismicity based reservoir characterization approach (SBRC). In general, the SBRC method is applied to earthquakes resulting from fluid injections into the subsurface. This method allows firstly, to estimate the fluid-transport properties of hydraulically stimulated reservoir, secondly, to examine the fluid-rock interaction, and thirdly, to reconstruct critical pressures of the activated fractures within the reservoir. To extend the applicability of SBRC the thesis focus on the following topics. The SBRC method so far assumes a constant injection source strength. This condition, however, is not always given, such as by the hydraulic stimulation of a geothermal reservoir in Basel (Switzerland). In the first part of the thesis, SBRC is extended in order to analyze seismicity resulting from fluid injections where the source strength is linearly increasing with time. For this purpose, an analytical solution of the diffusion equation is derived taking into account this special condition. The derived solution and the resulting expressions for the seismicity rate and the cumulative number of earthquakes are numerically verified using finite element modeling and synthetically generated seismicity. Afterwards, SBRC is applied to fluid-induced seismicity recorded in Basel providing consistent estimates of the permeability of the hydraulically stimulated reservoir and of the distribution of critical pressures. In the second part of the thesis, a model is introduced in order to interpret induced seismicity of single-planar hydraulic fractures. The model considers the growth of fracture and seismicity as a combined geometry- and diffusion-controlled process. It is confirmed by observations from fracturing-induced seismicity in the Cotton Valley gas reservoir (USA). The space-time diagrams (r-t diagrams) of induced earthquakes show signatures of fracture volume growth, loss of treatment fluid, and diffusion of injection-induced pore pressure perturbations. Evaluation of envelopes of the spatio-temporal distribution of induced seismicity allows to determine geometrical parameters and hydraulical properties of the fracture. Considering a volume balance principle of the injected fluid permits to quantify the fluid loss from the fracture into the reservoir and to estimate the reservoir permeability. The proposed interpretational approach is applied to earthquakes induced during three fracturing stages in Cotton Valley. Although the three stages differ with respect to the treatment design parameters, it is found that all stages resulted in similar fracture geometries. Ratios of fracture volume and total injected fluid volume are nearly identical. It means that the fracture growth process is likely decoupled from the type of treatment design. Estimates of fluid loss and reservoir permeability are consistent for the investigated fracturing stages. Fluid injections into the subsurface can sometimes induce earthquakes characterized by a significant magnitude. In particular, seismic events with larger magnitudes are reported from geothermal reservoirs. Understanding the scaling relations of magnitudes of fluid-induced seismicity is crucial for assessing the seismic risk by injection operations. In the last part of the thesis, a statistical model is introduced which describes the magnitude distribution of earthquakes induced during injections. It combines a Gutenberg-Richter statistics of magnitude probability with the cumulative number of induced earthquakes. Earthquake magnitudes presented in this thesis are in agreement with this model. Furthermore, the model allows to identify controlling parameters of the magnitude distribution. These include design parameters of a fluid injection, such as the injected fluid volume, and seismotectonic quantities like the probabilistic Gutenberg-Richter a- and b-value and the tectonic potential which is defined by statistical properties of pre-existing fractures.Die wesentliche Zielsetzung der Dissertation ist eine Weiterentwicklung des Ansatzes der Seismizitätsbasierten Reservoircharakterisierung (SBRC). Die SBRC Methode wird im Allgemeinen auf Erdbeben, die durch die Injektion von Fluiden in den Untergrund ausgelöst werden, angewendet. Dieses ermöglicht einerseits die Fluidtransporteigenschaften des hydraulisch stimulierten Reservoirgesteins abzuschätzen, zweitens, die Fluid-Gesteins-Wechselwirkungen zu untersuchen, und drittens, die kritischen Drücke aktivierter Bruch- und Störungssysteme zu rekonstruieren. Zur Erweiterung der Anwendbarkeit der SBRC Methode konzentriert sich die Dissertation auf die folgenden Themen. Die SBRC Methode basiert bisher auf der Annahme einer konstanten Quellstärke einer Injektion. Diese Bedingung ist jedoch nicht immer gegeben, wie beispielsweise bei der hydraulischen Stimulation des geothermischen Reservoirs in Basel (Schweiz). Im ersten Teil der Dissertation wird die SBRC Methode erweitert, um Seismizität resultierend aus Fluidinjektionen in denen die Quellstärke linear mit der Zeit ansteigt, zu analysieren. Zunächst wird eine analytische Lösung der Diffusionsgleichung für diese spezielle Randbedingung hergeleitet. Die Lösung und daraus ermittelte mathematische Formulierungen für die Rate und die Anzahl der Erdbeben werden mit Finite-Elementen-Modellierung und synthetisch erzeugter Seismizität numerisch verifiziert. Anschließend wird die SBRC Methode auf den Katalog fluid-induzierter Seismizität in Basel angewendet, welches konsistente Abschätzungen der Permeabilität des hydraulisch stimulierten Reservoirs und der Verteilung der kritischen Drücke ergibt. Im zweiten Teil der Dissertation wird ein Modell hergeleitet, das die Interpretation induzierter Seismizität bei einfach-planaren hydraulischen Brüchen ermöglicht. Das Modell betrachtet das Wachstum eines hydraulisch- generierten Bruches und der assoziierten Seismizität als einen kombinierten Geometrie- und Diffusions-kontrollierten Prozess. Dieses wird anhand von Beobachtungen aus dem Cotton Valley Gasreservoir (USA) bestätigt. Die Raum- Zeit-Diagramme (r-t Diagramme) der induzierten Erdbeben zeigen Signaturen der Bruchausbreitung, des Verlustes von Fluid vom Bruch in das umgebende Reservoirgestein und die Ausbreitung von injektions-induzierten Perturbationen des Porenfluiddrucks. Die Auswertung der Einhüllenden der räumlich-zeitlichen Verteilung der induzierten Seismizität erlaubt die Bestimmung von geometrischen und hydraulischen Parametern des erzeugten Bruches. Ausgehend von dem Prinzip einer Volumenbalance des injizierten Fluid können der Fluidverlust und die Permeabilität des Reservoirs quantifiziert werden. Der vorgestellte Interpretationsansatz wird auf Erdbeben, die während dreier Phasen hydraulischer Bruchbildung in Cotton Valley ausgelöst worden sind, angewendet. Obwohl die drei Phasen sich hinsichtlich der Konzeption der Injektion unterscheiden, ergaben sich sehr ähnliche Bruchgeometrien. Das Verhältnis aus neu geschaffenem Bruchvolumen zu injiziertem Gesamtvolumen ist nahezu identisch in allen Phasen. Die Abchätzungen von Fluidverlust und Reservoirpermeabilität sind konsistent für die untersuchten Phasen. Fluidinjektionen in den Untergrund können zum Teil Erdbeben hervorrufen, die durch eine signifikante Magnitude charakterisiert sind. Insbesondere in geothermischen Reservoiren sind seismische Ereignisse mit größerer Magnitude beobachtet worden. Das Verständnis der Skalierungsverhältnisse von Magnituden fluid-induzierter Seismizität ist entscheidend für eine Beurteilung der seismischen Gefährdung. Im letzten Teil der Dissertation wird ein statistisches Modell vorgestellt, welches die Magnitudenverteilung von Erdbeben, die während der Injektion auftreten, beschreibt. Das Modell kombiniert die Gutenberg-Richter Statistik der Magnitudenwahrscheinlichkeit mit der kumulativen Anzahl der induzierten Erdbeben. Erdbebenmagnituden in Basel und in Cotton Valley sind in Übereinstimmung mit diesem Modell. Darüber hinaus ermöglicht das Modell, kontrollierende Parameter der Magnitudenverteilung zu benennen. Dazu zählen einerseits Konzeptionsparameter einer Fluidinjektion, wie zum Beispiel das injizierte Fluidvolumen, als auch standort-spezifische seismotektonische Kenngrößen

Stress Drop, Seismogenic Index and Fault Cohesion of Fluid-Induced Earthquakes

Sometimes, a rather high stress drop characterizes earthquakes induced by underground fluid injections or productions. In addition, long-term fluid operations in the underground can influence a seismogenic reaction of the rock per unit volume of the fluid involved. The seismogenic index is a quantitative characteristic of such a reaction. We derive a relationship between the seismogenic index and stress drop. This relationship shows that the seismogenic index increases with the average stress drop of induced seismicity. Further, we formulate a simple and rather general phenomenological model of stress drop of induced earthquakes. This model shows that both a decrease of fault cohesion during the earthquake rupture process and an enhanced level of effective stresses could lead to high stress drop. Using these two formulations, we propose the following mechanism of increasing induced seismicity rates observed, e.g., by long-term gas production at Groningen. Pore pressure depletion can lead to a systematic increase of the average stress drop (and thus, of magnitudes) due to gradually destabilizing cohesive faults and due to a general increase of effective stresses. Consequently, elevated average stress drop increases seismogenic index. This can lead to seismic risk increasing with the operation time of an underground reservoir

Roof water-farm: recommendations for action : hygienic aspects of water and nutrient recycling in building-integrated farming

Frischer Fisch und frisches Gemüse direkt vom Dach produziert mit aufbereitetem Wasser aus dem Gebäude, das ist die Vision von ROOF WATER-FARM. Das vom Bundesministerium für Bildung und Forschung geförderte Verbundprojekt untersuchte erstmalig, wie sich Leichtbaufarmtechnologien wie Aquaponik und Hydroponik mit dezentralen Technologien zur Betriebswassernutzung und Düngemittelproduktion im und am Gebäude kombinieren lassen. Die ROOF WATER-FARM Handlungsempfehlungen sind eine aktualisierte Fassung der bereits im ROOF WATER-FARM Handbuch 2018 veröffentlichten Praxisempfehlungen Hygiene von kombinierten Wasserrecycling- und Farmsystemen. Sie sind neben gesundheitsrelevanten Fragen die Voraussetzung für die Vermarktungsfähigkeit der Produkte und eine breite praktische Umsetzung dieser zukunftsweisenden Kreislauftechnologien.Fresh fish and fresh vegetables directly from the roof, produced with treated water from houses, that is the vision of ROOF WATER-FARM. The research project, funded by the German Federal Ministry of Education and Research from 2013-2016, investigated for the first time how lightweight farm technologies such as aquaponics and hydroponics can be combined with decentralized technologies for the use of process water and fertilizer production from domestic wastewater streams in buildings. The ROOF WATER-FARM recommendations for action on hygienic aspects complement the recommendations for the implementation of combined water recycling and farming systems already published in the ROOF WATER-FARM Handbook 2018. Controlling the hygienic safety of the systems is a prerequisite for the marketability of the products and thus paves the way for a broad practical implementation of such innovative projects.BMBF, 033W012, INIS - Verbundprojekt ROOF-WATER-FARM: Sektor- übergreifende Wasserressourcennutzung durch gebäudeintegrierte Farmwirtschaf