The project I-GET is aimed at developing an innovative geothermal exploration approach based on advanced geophysical methods. The Project Acronym stands for Integrated Geophysical Exploration Technologies for deep fractured geothermal systems. The objective of the project is to improve the detection, prior to drilling, of fluid bearing zones in naturally and/or artificially fractured geothermal reservoirs. This new approach has been tested in four European geothermal systems with different geological and thermodynamic reservoir characteristics: two high enthalpy (metamorphic in Travale, Italy and volcanic rocks at Hengill, Iceland), and two middle enthalpy geothermal system (deep sedimentary rocks in Gross-Schoenebeck, Germany and Skierniewice, Poland).
For Travale, a recently acquired 3-D seismic dataset was reprocessed and the seismic amplitudes with azimuth as well as the amplitude versus offset (AVO) of signals reflected from the subsurface targets were analysed. For the analysis of amplitude with azimuth in the data gathers with sufficient coverage the sinusoids matched to the observed amplitudes all show one specific orientation. The available geological information and borehole data are currently studied to see if the observed preferred orientation corresponds to a preferential fracture orientation.
In the AVO analysis, the top of the geothermal reservoir corresponds (at least in this area) with high acoustic impedance contrasts. Consistently high values of the intercept attribute correspond with target reflections. In addition, the amplitude gradient of the reflections from the target shows an increase of the amplitudes with increasing offset. Well information is currently analysed and synthetic seismograms based on borehole logs are developed to see if these observations can be matched with the presence of fractures and fluids.
In addition to seismic data, magnetotelluric (MT) measurements at low and high frequencies were carried out at 22 and 35 sites respectively in the Travale area, in addition to the already acquired 60 sites. The observed resistivities were analysed with respect to the known lithologies and alteration affecting the reservoir rocks, with particular regard to conductive and phyllosilicate minerals, the physico-chemical characteristics of the fluids, and their distribution and evolution with time. 1D inversion of MT soundings and 2D inversion along parallel profiles was performed and a 3D resistivity structure compiled from the individual models. Moreover, 3D forward models have defined the minimum size of the resistivity anomaly characterizing the Travale subsurface. By means of 3D forward models and 2D inversions, resolution of MT to different permeability features was analyzed..
At Hengill, an extensive MT survey was carried out. A total of 70 MT soundings were made within the framework of the I-GET project. 117 pre-existing soundings and additional 30 soundings funded by Reykjavik Energy are available to the project. The I-GET project therefore includes a total of 217 MT soundings in the Hengill area. Almost all the MT soundings were made at locations previously visited by TEM soundings which is now used for static shift corrections of the MT data. Joint 1D inversion of all MT and TEM soundings in the Hengill area was performed and a 3D reisistivity structure was compiled from the individual 1D models.
In addition to electromagnetic measurements, a broad-band passive seismic survey was carried out. Seven broad-band seismic stations were installed in and around the most seismically active part of the area and recorded continuously for four months. The data base includes 662 earthquakes recorded by at least 4 of the stations. 424 of the recorded events were micro-earthquakes with clear P and S waves. Finally, 19 low frequency earthquakes were recognised with sharp onset and resonance having a large peak at about 1.5 Hz. The analysis of location and magnitude of all the earthquakes is still in process, because of the large number of earthquakes recorded. The analysis of the relation between seismicity rate and steam production is ongoing (data supplied by the geothermal power company). Efforts will also concentrate on the location of earthquakes and magnitude for which P and S waves have been picked (about 50 % of them) and on trying waveform inversion on lower frequencies of the long period earthquakes. This data set is unique and complete analysis may need longer time than initially expected.
At Gross Schoenebeck, seismic experiments included two parts: First a 40 km long profile to derive a regional 2-D seismic model of the potential reservoir layers and overlying sediments. The profile is centered at the geothermal drill sites GrSk 3/90 and GrSk4/05 and is oriented parallel to the estimated strike of the regional stress field. Second, a star-like arrangement consisting of 4 profiles each of 6 km length was deployed and a low-fold (low budget) 3-D seismic experiment was conducted to identify fractures around the geothermal well location.
The signals of the first arriving P-waves from the long profile are used for the inversion of travel times to determine P wave velocities and vertical gradients, and represent the input data for the attenuation tomography. Both the P wave velocity tomography and, in particular, the derived vertical gradient of the velocity reveal new aspects how sedimentary sections can be imaged.
Besides the analysis of the first arrival travel time and spectral wave form information, new techniques were developed to benefit from the secondary wave field, which was also measured. In particular, the detection (filtering) of reflections and diffractions from steep dipping features would be an important input information for the geological and geo-hydraulic modeling of the geothermal system.
MT measurements were carried out along a 40 km long profile coinciding with the seismic profile, with 58 MT stations; 20 new MT sites were recorded along a 20-km profile located 5 km to the east of the long profile. In addition, 159 new sites were recorded on a 3 km x 28 km grid around the Groß Schönebeck borehole, with a site spacing of 500m x 500m. Data analysis indicates a mainly two-dimensional conductivity structure. Ongoing interpretation of the results yields generally a good agreement with the known geology of the area. Several zones of high conductivity coincide with structural lows, probably due to sedimentation of more porous material or fluid accumulation. Two distinct conductors located at depths of 4-5 km coincide with the Lower Permian reservoir rocks
Data acquisition in Skierniewice was just finished. Data processing is ongoing.
Integration of different geophysical approaches is the key concept of the project. To this end, seismic and magnetotelluric data have been acquired in the test sites, and new acquisition and processing techniques are developed to solve problems related to the particular target such as high temperatures, anisotropy, phase condition, etc.. The static and dynamic three-dimensional model of geothermal reservoirs is reconstructed by means of all the data acquired. The input of the results of new geophysical prospecting into reservoir modelling is a crucial test of the quality of the new exploration method.
Parallel to in-situ data acquisition, petrophysical and geomechanical properties of the investigated rocks have been defined by laboratory measurements. With respect to the high enthalpy sites, elastic and electric rock properties are determined at the steam/liquid transition of the pore fillings. The validity of the laboratory and simulation results will be verified by the new field experiments.
The ultimate goal is the development of an efficient, low-cost exploration method, by taking advantage of the strengths of different disciplines and combining them to an integrated approach.