Assessing induced seismicity due to fluid injection in the subsurface is required for all CO2 storage projects. There is great uncertainty at the preparation phase of such a project, when no fluid is injected yet. Hydrologically, it is possible to simulate the fluid flow and its impact in the geological layers through numerical simulations. Then, one can also simulate mechanical scenarios of faults reactivation (if any are identified) assuming a probable boundary condition. This assessment is necessary not only to estimate the worst scenario possible on a given fault but also to evaluate the possible temporal evolution of the microseismicity according to the injection planning. We propose a statistical modelling of the seismicity according to the hydraulic simulation of fluid flow. In the framework of the French national project MISS-CO2 (Modeling of Induced Seismicity for Storage of CO2), we target a reservoir in the Paris basin for the synthetic CO2 injection scenarios. The injection simulations have been previously carried out based upon a detailed 3D geological model assuming a CO2 injection during 30 years, and significantly different possible volume of injection leading to different pressure changes in the injection formation. Then, we adopt an Epidemic-Type Aftershock Sequence (ETAS) model considering non-stationarity of the background seismicity rate. We set the model parameters such that the natural seismicity (without injection) is consistent with the known catalog of the area and the triggering impact is adjusted from the regression analyses of the known induced seismicity catalogs. A single simulation contains the 30 years of injection and 20 years of post-injection phase. As the ETAS modelling naturally provide a randomness of the occurrence of seismicity, we ran more than a hundred simulations and analyse the results statistically. The maximum magnitude ranges from 0 to 2.5 under the same configuration and its timing also varies. Thus, such variation and uncertainty should be considered for the assessment of the risk and performance of the project. Further validation processes will be necessary with the on-going monitoring of the seismicity for establishing a data-driven, dynamic Traffic-Light-System to assess better the seismic hazard and finally optimize the injection operation on real-time.

This book provides a quantitative introduction to the physics, application, interpretation, and hazard aspects of fluid-induced seismicity, focussing on spatio-temporal dynamics. Including many real data examples, this is a valuable reference for researchers and graduate students of geophysics, geomechanics and petrophysics, and a practical guide for petroleum geoscientists and engineers.

It has long been recognized that pumping fluids into or out of the Earth has the potential to cause earthquakes. Some of the earliest field evidence dates to the 1960s, when earthquakes were turned on and off by water injection in Rangely, Colorado. More recently, induced seismicity has been reported worldwide in connection with many subsurface technologies, including wastewater disposal, natural gas storage, enhanced geothermal systems, and hydraulic fracturing. As a result, there has been a growing public concern around the world about the potential seismic hazard and environmental impact of subsurface energy technologies. Understanding the physical mechanisms that lead to induced seismicity is essential in efforts to mitigate the risk associated with subsurface operations. As a first step in this thesis, we develop a spring-poroslider model of frictional slip as an analogue for induced seismicity, and analyze conditions for the emergence of stick-slip frictional instability--the mechanism for earthquakes--by carrying out a linear stability analysis and nonlinear simulations. We found that the likelihood of triggering earthquakes depends largely on the rate of increase in pore pressure rather than its magnitude. Thus, the model explains the common observation that abrupt increases in injection rate increase the seismic risk. Second, we perform an energy analysis using the same spring-poroslider model to shed light into the partitioning of energy released into frictional and radiated energy-since the latter is associated with the overall size of the earthquake and its potential for damage to man-made structures. Two key elements of the analysis are: (1) incorporating seismic radiation within the model using a precisely-defined viscous damper, and (2) partitioning the energy supplied by fluid injection into dissipated and stored energy in fluid and skeleton. The analysis shows how the rate of increase in pore pressure controls the radiated energy, stress drop, and total slip of the earthquake. Third, we study the effect of heterogeneity on the dynamics of frictional faults. In particular, we develop an objective (frame-indifferent) formulation of frictional contact between heterogeneous surfaces at a small scale, and introduce the notion that friction is a function of the states of the two surfaces in contact, each representing roughness and microstructural details for the surface. We then conduct dynamic simulations of a spring-slider model and show that heterogeneous Coulomb friction alone is capable of reproducing the transitions in complex frictional behavior, from stable creep to regular earthquakes and slow slip. This thesis, as a whole, enhances our understanding of the mechanics of fluid-injection-induced earthquakes and suggests strategies that mitigate or minimize the seismic risk associated with a wide range of subsurface operations, from hydraulic fracturing and geothermal energy extraction to wastewater injection and geologic CO2 sequestration.

The study of earthquakes is a multidisciplinary field, an amalgam of geodynamics, mathematics, engineering and more. The overriding commonality between them all is the presence of natural randomness. Stochastic studies (probability, stochastic processes and statistics) can be of different types, for example, the black box approach (one state), the white box approach (multi-state), the simulation of different aspects, and so on. This book has the advantage of bringing together a group of international authors, known for their earthquake-specific approaches, to cover a wide array of these myriad aspects. A variety of topics are presented, including statistical nonparametric and parametric methods, a multi-state system approach, earthquake simulators, post-seismic activity models, time series Markov models with regression, scaling properties and multifractal approaches, selfcorrecting models, the linked stress release model, Markovian arrival models, Poisson-based detection techniques, change point detection techniques on seismicity models, and, finally, semi-Markov models for earthquake forecasting.

The perturbation of the earth by mankind causes earthquakes in a variety of situations. This phenomenon continues to be a major concern to engineers and scientists concerned with the mitigation of the consequences of this seismicity, as well as better understanding the processes at its origin. The present volume contains twelve papers from six countries, dealing with observations of triggered and induced seismicity in four continents. The reported cases include seismicity due to hard-rock mines, coal mines, underground research facilities for nuclear waste disposal, water injections, reservoirs, acquifers and oil fields. This volume provides case studies of previously unavailable observations of this phenomenon, investigations of the cause and source mechanism of seismic events, studies of source location distributions, determinations of seismic source parameters, cases of the use of such parameters in assessing rockburst hazard in mines, and measurements of velocity an attenuation properties of rock masses. The present collection of papers provides an excellent indication of the current state of the art and new developments in this area of research.

Under certain circumstances, the increased pore pressure resulting from fluid injection, whether for waste disposal, secondary recovery, geothermal energy, or solution mining, can trigger earthquakes. This report discusses known cases of injection-induced seismicity and how and why earthquakes may be triggered, as well as conditions under which the triggering is most likely to occur. Criteria are established to assist in regulating well operations so as to minimize the seismic hazard associated with deep well fluid injection.

Reprint from Pure and Applied Geophysics (PAGEOPH), Volume 150 (1997), No. 3/4