Abstract Induced earthquakes pose a substantial challenge to many geo-energy applications, and in particular to Enhanced Geothermal Systems. We demonstrate that the key factor controlling the seismic hazard is the relative size distribution of earthquakes, the b -value, because it is closely coupled to the stress conditions in the underground. By comparing high resolution observations from an Enhanced Geothermal System project in Basel with a loosely coupled hydro-mechanical-stochastic model, we establish a highly systematic behaviour of the b -value and resulting hazard through the injection cycle. This time evolution is controlled not only by the specific site conditions and the proximity of nearby faults but also by the injection strategy followed. Our results open up new approaches to assess and mitigate seismic hazard and risk through careful site selection and adequate injection strategy, coupled to real-time monitoring and modelling during reservoir stimulation.
Fault activation and induced seismicity associated with both geologic carbon storage (GCS) and shale-gas fracturing were simulated using a coupled multiphase flow and geomechanical numerical model with a strain-softening Mohr-Coulomb model for fault seismic rupture. The analysis shows that for the case representative of GCS, felt seismic events (e.g., moment magnitude 4) could be induced whereas in the case representing shale-gas fracturing, only unfelt, smaller magnitude events were calculated. In the GCS case, a fault, crossing or bounding the reservoir, could be uniformly pressurized over a much larger fault surface area that could then rupture in one instance. In the case of shale-gas fracturing, the analysis shows that the expected low permeability of faults intersecting gas saturated shales could be a limiting factor for the possible rupture size and seismic magnitude. In any case, the initial stress field as well as the rock properties, whether more ductile or brittle, are important factors that will determine whether larger seismic events could be induced.
Sea wave reflection from coastal protection structures is one of the main issues in the coastal design process. Several empirical formulas have been proposed so far to predict reflection coefficient from rubble mound breakwaters and smooth slopes. The aim of this study is to investigate wave reflection from a rubble mound structure placed in front of a vertical concrete seawall. Several experimental tests were performed on a two-dimensional wave flume by reproducing on a rubble mound structure with a steep single primary layer armored with a novel artificial unit. A new approach for the prediction of the reflection coefficient based on dimensional analysis is also proposed, and a new empirical equation is derived. The performance of the proposed equation was compared with widespread existing formulas, and a good accuracy was found.
Abstract Changes in water level are commonly reported in regions struck by a seismic event. The sign and amplitude of such changes depend on the relative position of measuring points with respect to the hypocenter, and on the poroelastic properties of the rock. We apply a porous media flow model (TOUGH2) to describe groundwater flow and water‐level changes associated with the first M L 5.9 mainshock of the 2012 seismic sequence in Emilia (Italy). We represent the earthquake as an instantaneous pressure step, whose amplitude was inferred from the properties of the seismic source inverted from geodetic data. The results are consistent with the evolution recorded in both deep and shallow water wells in the area and suggest that our description of the seismic event is suitable to capture both timing and magnitude of water‐level changes. We draw some conclusions about the influence of material heterogeneity on the pore pressure evolution, and we show that to reproduce the observed maximum amplitude it is necessary to take into account compaction in the shallow layer.
Abstract The sealing characteristics of the geological formation located above a CO 2 storage reservoir, the so-called caprock, are essential to ensure efficient geological carbon storage. If CO 2 were to leak through the caprock, temporal changes in fluid geochemistry can reveal fundamental information on migration mechanisms and induced fluid–rock interactions. Here, we present the results from a unique in-situ injection experiment, where CO 2 -enriched fluid was continuously injected in a faulted caprock analogue. Our results show that the CO 2 migration follows complex pathways within the fault structure. The joint analysis of noble gases, ion concentrations and carbon isotopes allow us to quantify mixing between injected CO 2 -enriched fluid and resident formation water and to describe the temporal evolution of water–rock interaction processes. The results presented here are a crucial complement to the geophysical monitoring at the fracture scale highlighting a unique migration of CO 2 in fault zones.
Abstract The presence of fluid within a fault zone can cause overpressure and trigger earthquakes. In this work, we study the influence of fault‐zone architecture on pore pressure distribution and on the resulting fault reactivation caused by CO 2 injection. In particular, we investigate the effect of the variation and distribution of lithological and rock physical properties within a fault zone embedded in a multi‐layer sedimentary system. Through numerical analysis, we compare several models where the complexity of the fault‐zone architecture and different layers (such as caprock and injection reservoir) are incrementally included. Results show how the presence of hydraulic and mechanical heterogeneity along the fault influences the pressure diffusion, as well as the effective normal and shear stress evolution. Hydromechanical heterogeneities (i) strengthen the fault zone resulting in earthquakes of small magnitude, and (ii) impede fluid migration upward along the fault. We also study the effects of the caprock and aquifer thickness on the resulting induced seismicity and CO 2 leakage, both in heterogeneous and homogeneous fault zones. Results show that a thin caprock or aquifer allows smaller events, but a much higher percentage of leakage through the caprock and into the upper aquifer. The amount of leakage reduces drastically in the case of a multi‐caprock, multi‐aquifer system.
