Seismic Methods for Observing Geothermal Field Evolution

In this overview we report results of analysing induced seismicity in geothermal reservoirs in various tectonic settings within the framework of the European Geothermal Engineering Integrating Mitigation of Induced Seismicity in Reservoirs (GEISER) project. In the scoping phase of a field, the subsurface fault mapping, in situ stress and the seismic network are of primary interest in order to help assess the geothermal resource. The hypocentres of the observed seismic events (seismic cloud) are dependent on the design of the installed network, the used velocity model and the applied location technique. During the stimulation phase, the attention is turned to reservoir hydraulics (e.g., fluid pressure, injection volume) and its relation to larger magnitude seismic events, their source characteristics and occurrence in space and time. A change in isotropic components of the full waveform moment tensor is observed for events close to the injection well (tensile character) as compared to events further away from the injection well (shear character). Tensile events coincide with high Gutenberg-Richter b-values and low Brune stress drop values. The stress regime in the reservoir controls the direction of the fracture growth at depth, as indicated by the extent of the seismic cloud observed. Stress magnitudes are important in multiple stimulation of wells, where little or no seismicity is seen until the previous maximum stress level is exceeded (Kaiser Effect). Prior to drilling, obtaining a 3D P-wave (Vp) and S-wave velocity (Vs) models down to reservoir depth is recommended. In the stimulation phase, we recommend to monitor and to locate seismicity with high precision (decametre) in real-time and to perform local 4D tomography for velocity ratio (Vp/Vs). During exploitation, one should use observed and model induced seismicity to forward estimate seismic hazard so that field operators are in a position to adjust well hydraulics (rate and volume of the fluid injected) when induced events start to occur far away from the boundary of the seismic cloud.

2006

Large velocity contrasts are regularly encountered in geothermal fields due to poorly consolidated and hydro-thermally altered rocks. The appropriate processing of seismic data is therefore crucial to delineate the geological structure. To assess the benefits of surface seismic surveys in such settings, we applied different migration procedures to image a synthetic reservoir model and seismic data from the Coso Geothermal

2019

We present a review summary of the theory of seismi c wave propagation in geothermal reservoir, including temperature and pressure effects based on the Arrhenius equation and poro-viscoelasticity. We quantify the effects of the melting rate point on t he seismic velocities, and consider surface and boreh ole acquisition geometries. We perform wavefield simulations for geothermal areas located at differe nt depths in dissimilar geological contexts.

1995

Surface seismic reflection surveys and VSP's have been extensively carried out in the Monteverdi area, within the Larderello region. Calibration of these seismic observations with data from numerous deep wells allowed the interpretation of the 2-D traverses so that a good geologic-structural reconstruction for the whole area was achieved. Many important reflectors were detected inside the Metamorphic basement by VSP's acquired in several wells in the area. These reflections were attributed to changes in petrophysical characteristics of the medium, particularly changes in fracture density, rather than to contrasts in lithology. Since at this time the main goal of the exploration in the Larderello region is the location of producing layers inside the basement, a major effort was applied to acquiring and modeling numerous VSP's in existing wells. In particular, a multioffset VSP was carried out in the COLLA 2 well with the specific goal of detecting fractured horizons withi...

2021

We apply a recently developed approach for inferring in situ fluid pressure changes from induced seismicity observations to datasets from geothermal reservoirs at St. Gallen (Switzerland), Paralana (Australia), and Cooper Basin (Australia), respectively. The approach, referred to as seismohydraulic pressure mapping (SHPM), is based on mapping the seismic moment of induced earthquakes. Relative fluid pressure changes are inferred from the stress deficit of fracture patches slipping repeatedly. The SHPM approach was developed for the specific scenario, where induced earthquakes occur on a single, larger-scale plane with slip being driven by the regional stress field. We demonstrate that this scenario applies to the three datasets under investigation, indicating that geothermal systems in crystalline rock could typically be fault-dominated. For all datasets, individual earthquake source geometry could not be determined from source spectra due to the attenuation of the high signal frequ...

In order to reach Europe's 2020 and 2050 targets on greenhouse gas emissions, geothermal resources have to contribute substantially to carbon-free energy needs. Deep geothermal developments, however, are often accompanied by induced seismicity due to stimulation. The induced seismicity can be a threat for the development of future large scale application of deep geothermal power plants. Therefore, understanding the physical processes at the origin of the seismicity induced by forced fluid circulation in geothermal fields is essential, and this paper reviews the current knowledge in connection with field cases. The driving force of a seismic event is a change of the stress state in the crust. To asses this quantitatively, one needs to know the initial stress state, the spatio-temporal stress changes, the failure criterion, and the rupture dynamics that describes how a seismic event is produced. Several existing geomechanical-numerical models are, in theory, capable of predicting the spatio-temporal changes of the stress state and few of their effects on induced seismicity. They consider coupling between geomechanical, fluid flow, and heat transport processes, with different levels of complexity. The characteristics of the recorded seismicity induced in geothermal fields play a major role to calibrate and to assess the models. The Soultz-sous-Forêts enhanced geothermal system in France, where thousands of seismic events were induced during stimulations, is an example representative for fields developed in deep crystalline rocks. In such formations, induced seismicity mainly occurs on a network of pre-existing faults and fractures oriented in accordance with the stress field, and shearing on these structures is apparently the dominating failure mechanism. Several physics-based models have been tested on this well-documented field to reproduce the observations. By contrast, the hydraulic fracturing operations carried out in Groß Schönebeck (Germany) geothermal reservoir, which is located in sedimentary formations at a depth similar to Soultz-sous-Forêts, induced very few and weak seismic events. This behavior is consistent with a less seismogenic tensile fracture opening as being the dominant failure mechanism. Interestingly, the few recorded and located seismic events at this site likely occurred on a pre-existing fault. To characterize a geothermal field with regards to the expected induced seismicity, the following key factors are proposed: natural seismicity at a field scale, stress field, structural fracture/fault characterization, rock type, history of pressure and injection/circulation rates, and past induced seismicity. To mitigate induced seismicity during major hydraulic stimulations, and to prevent large-magnitude event occurrence once injection stopped, early-warning systems and decision support systems are required. To feed these systems, we advocate the application of hybrid methods and the development of fast models. Hybrid methods would combine the best a priori knowledge of the expected behavior of the field underground, inherited from geomechanical-numerical modeling, with a statistical approach based on the real-time observation of the induced seismicity. Fast models would capture the essential physics while minimizing computing time. The quantitative understanding of induced seismicity, however, remains a challenging and complex matter. Only an integration of all current research and development efforts, in the fields of modeling, measuring, monitoring, and matching, will make a chance on success.

Geothermics, 2017

We present a numerical modeling aimed at investigating nature and role of the self-potential (SP) anomalies induced by water injection in boreholes at the Soultz-sous-Forêts (SsF) hot dry rock enhanced geothermal field. The overpressure due to the fluid stimulation is considered as source of the streaming potential effects in rocks, responsible on their turn of the SP anomalies observed at the ground surface. The numerical simulations have been realized by a combined application of the TOUGH2 and Comsol Multiphysics codes, which had already been successfully used to predict Coulomb stress changes in rocks induced by a fluid injection cycle. Two synthetic cases are investigated. At first, a simulated injection cycle in a single borehole has been modeled, consisting in the reconstruction of the overpressure and SP temporal and spatial evolutions induced by the hydraulic stimulation of the rock. The main result is that the front of the SP anomaly follows the overpressure front, with the time delay between the two fronts decreasing at increasing distance from the well. The second case takes into consideration a real injection experiment performed in 2003 at SsF, which has allowed to examine the induced seismicity. The simulated SP response to this real injection cycle shows that the SP temporal evolution is essentially a post-seismic effect. The conclusion from the simulations is that SP measurements can be used to localize the main features of the fluid flow into the reservoir.

Journal of Geophysical Research, 2010

Seismic wide-angle data were collected along a 40-km-long profile centered at the geothermal research well GrSk 3/90 in the Northeast German Basin. Tomographic inversion of travel time data provided a velocity and a vertical velocity gradient model, indicative of Cenozoic to Pre-Permian sediments. Wide-angle reflections are modelled and interpreted as top Zechstein and top Pre-Permian. Changes in velocity gradients are interpreted as the transition from mechanical to chemical compaction at 2-3 km depth, and localized salt structures are imaged, suggesting a previously unknown salt pillow in the southern part of the seismic profile. The Zechstein salt shows decreased velocities in the adjacent salt pillows compared to the salt lows, which is confirmed by sonic log data. This decrease in velocity could be explained by the mobilization of less dense salt, which moved and formed the salt pillows, whereas the denser salt remained in the lithological structure, major faults with reduced velocity, and fluid-bearing horizons. The combination of velocity tomography with pre-existing or new seismic reflection data can significantly improve the knowledge of the structure and composition. Deeper levels underneath the geothermal system can be resolved by seismic tomography, whereas seismic reflection data exhibit less certain information, e.g. on the existence of deep reaching faults. Faults are important for the geothermal system because they can channel rising deeper (hotter) fluids which may represent a major heat source . Furthermore, the results from tomography can be combined with thermal modeling using the relationship between seismic velocity and 3 temperature [e.g., . The idea of such a combined analysis would be to test if the velocity structure reflects variations in the temperature field as an important outcome for the geothermal exploration.

2019

That seismic wave speeds in geothermal systems might change with time is an expected consequence both of natural processes and of industrial activities such as fluid withdrawal and injection, and indeed detections of such changes have been reported, based on repeated microearthquake seismic tomography. Seismic tomography suffers, however, from limitations that cast doubt on some of these reports. Specifying a three-dimensional model of seismic wave speed in even a small volume requires at least thousands of parameter values, but because of the limited distribution of both earthquakes and seismometers, real data sets virtually never contain enough information. Seismic rays tend to occur in bundles that sample the Earth very unevenly, and standard inversion methods impose assumptions that result in structural features that mimic these bundles. Any changes in the ray distributions, such as those caused by seismicity variations, can therefore produce spurious apparent changes in the wav...