Balancing reservoir creation and seismic hazard in enhanced geothermal systems

Fracture shear-dilatancy is an essential process for enhancing the permeability of deep geothermal reservoirs, and is usually accompanied by the radiation of seismic waves. However, the hazard and risk perspective of induced seismicity research typically focuses only on the question of how to reduce the occurrence of induced earthquakes. Here we present a quantitative analysis of seismic hazard as a function of the two key factors defining an enhanced geothermal system: The permeability enhancement, and the size of the stimulated reservoir. Our model has two coupled components: (1) a pressure diffusion model and (2) a stochastic seismicity model. Permeability is increased in the source area of each induced earthquake depending on the amount of slip, which is determined by the magnitude. We show that the few largest earthquakes (i.e. 5-10 events with M ≥ 1.5) contribute more than half of the total reservoir stimulation. The results further indicate that planning and controlling of reservoir engineering operations may be compromised by the considerable variability of maximum observed magnitude, reservoir size, the Gutenberg-Richter b-value and Shapiro's seismogenic index (i.e. a measure of seismic reactivity of a reservoir) that arises from the intrinsic stochastic nature of induced seismicity. We also find that injection volume has a large impact on both reservoir size and seismic hazard. Injection rate and injection scheme have a negligible effect. The impact of site-specific parameters on seismicity and reservoir properties is greater than that of the injected volume. In particular, conditions that lead to high b-values—possibly a low differential stress level—have a high impact on seismic hazard, but also reduce the efficiency of the stimulation in terms of permeability enhancement. Under such conditions, target reservoir permeability can still be achieved without reaching an unacceptable level of seismic hazard, if either the initial reservoir permeability is high or if several fractures are stimulated. The proposed methodology is a first step towards including induced seismic hazard analysis into the design of reservoir stimulation in a quantitative and robust manne

Rockslide deformation monitoring with fiber optic strain sensors

With micro-strain resolution and the capability to sample at rates of 100 Hz and higher, fiber optic (FO) strain sensors offer exciting new possibilities for in-situ landslide monitoring. Here we describe a new FO monitoring system based on long-gauge fiber Bragg grating sensors installed at the Randa Rockslide Laboratory in southern Switzerland. The new FO monitoring system can detect sub-micrometer scale deformations in both triggered-dynamic and continuous measurements. Two types of sensors have been installed: (1) fully embedded borehole sensors and (2) surface extensometers. Dynamic measurements are triggered by sensor deformation and recorded at 100 Hz, while continuous data are logged every 5 min. Deformation time series for all sensors show displacements consistent with previous monitoring. Accelerated shortening following installation of the borehole sensors is likely related to long-term shrinkage of the grout. A number of transient signals have been observed, which in some cases were large enough to trigger rapid sampling. The combination of short- and long-term observation offers new insight into the deformation process. Accelerated surface crack opening in spring is shown to have a diurnal trend, which we attribute to the effect of snowmelt seeping into the crack void space and freezing at night to generate pressure on the crack walls. Controlled-source tests investigated the sensor response to dynamic inputs, which compared an independent measure of ground motion against the strain measured across a surface crack. Low frequency signals were comparable but the FO record suffered from aliasing, where undersampling of higher frequency signals generated spectral peaks not related to ground motion

Synthesis of palladium complexes derived from amido linked N-heterocyclic carbenes and their use in Suzuki cross coupling reactions

One of the authors (RSC) is grateful to DST for the financial support under the DST young scientist scheme YSS/2014/000797.Treatment of 1-(n-butyl)-3-N-(2-Ar)acetamido-1, 3-imidazolium chloride (Ar=furylmethyl,phenylmethyl) with excess K2CO3 and [PdCl2(L−L)] (L−L=2 PPh3, dppf) afforded orange compounds of composition [(1-(n-butyl)-3-N-(2-Ar)acetamido-1,3-imidazol-2-ylidene)]2Pd (Ar=furylmethyl; phenylmethyl). These complexes were characterized by NMR (1H and 13C{1H} NMR), IR and micro-analysis data. Subsequently, the catalytic efficiency of these complexes for cross coupling reactions between 4-haloarenens (halo=Br, I) and phenylboronic acid was studied under different solvents (acetonitrile, THF and DMF), temperatures with different catalyst loadings. The molecular structure of [(1-(n-butyl)-3-N-(2-furylmethyl)acetamido-1, 3-imidazol-2-ylidene)]2Pd was established by single crystal X-ray diffraction analysis.PostprintPeer reviewe

Balancing reservoir creation and seismic hazard in enhanced geothermal systems

ISSN:0956-540XISSN:1365-246

Multicomponent ensemble models to forecast induced seismicity

In recent years, human-induced seismicity has become a more and more relevant topic due to its economic and social implications. Several models and approaches have been developed to explain underlying physical processes or forecast induced seismicity. They range from simple statistical models to coupled numerical models incorporating complex physics. We advocate the need for forecast testing as currently the best method for ascertaining if models are capable to reasonably accounting for key physical governing processes—or not. Moreover, operational forecast models are of great interest to help on-site decision-making in projects entailing induced earthquakes. We previously introduced a standardized framework following the guidelines of the Collaboratory for the Study of Earthquake Predictability, the Induced Seismicity Test Bench, to test, validate, and rank induced seismicity models. In this study, we describe how to construct multicomponent ensemble models based on Bayesian weightings that deliver more accurate forecasts than individual models in the case of Basel 2006 and Soultz-sous-Forêts 2004 enhanced geothermal stimulation projects. For this, we examine five calibrated variants of two significantly different model groups: (1) Shapiro and Smoothed Seismicity based on the seismogenic index, simple modified Omori-law-type seismicity decay, and temporally weighted smoothed seismicity; (2) Hydraulics and Seismicity based on numerically modelled pore pressure evolution that triggers seismicity using the Mohr–Coulomb failure criterion. We also demonstrate how the individual and ensemble models would perform as part of an operational Adaptive Traffic Light System. Investigating seismicity forecasts based on a range of potential injection scenarios, we use forecast periods of different durations to compute the occurrence probabilities of seismic events M ≥ 3. We show that in the case of the Basel 2006 geothermal stimulation the models forecast hazardous levels of seismicity days before the occurrence of felt events.ISSN:0956-540XISSN:1365-246

The importance of earthquake interactions for injection-induced seismicity: Retrospective modeling of the Basel Enhanced Geothermal System

We explore the role of earthquake interactions during an injection-induced seismic sequence. We propose a model, which considers both a transient pressure and static stress redistribution due to event interactions as triggering mechanisms. By calibrating the model against observations at the Enhanced Geothermal System of Basel, Switzerland, we are able to reproduce the time behavior of the seismicity rate. We observe that considering earthquake interactions in the modeling leads to a larger number of expected seismic events (24% more) if compared to a pressure-induced seismicity only. The increase of the number of events is particularly evident after the end of the injection. We conclude that implementing a model for estimating the static stress changes due to mutual event interactions increases significantly the understanding of the process and the behavior of induced seismicity