| Title | Assessment of Reservoir Processes Induced by Geothermal Utilization at Reykjanes, SW Iceland, Inferred from Interferometric Analysis of Satellite Images 2005-2019 |
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| Authors | Mylene RECEVEUR, Freysteinn SIGMUNDSSON, Michelle PARKS, Vincent DROUIN, Halldór GEIRSSON, Sigrún HREINSDÓTTIR, Ómar SIGURDSSON |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | High-temperature geothermal system, radar interferometry, time-series analysis, ground deformation modeling, steam zone |
| Abstract | We study local deformation induced by geothermal utilization of the high-temperature Reykjanes system, SW-Iceland, since 2006 when a new 100 MWe power plant began operation. Using interferometric analyses of synthetic aperture radar (InSAR) of images acquired by the Sentinel-1 satellites, we provide an update of the time series of surface displacement above the geothermal reservoir for the 2015-2019 time period. Results from previous studies showed that the highest rates were observed for the 2005-2008 period, where the main deformation area was aligned along the Reykjanes fissure swarm in the vicinity of the power plant. From 2009, the deformation zone narrowed to a 2-km wide circular subsidence bowl centered on the well field accompanied by a decrease in the maximum subsidence rate. The decrease in the rate of volume change together with the migration of the best-fitting source toward shallower depth between the 2005–2008 and 2009-2019 periods was attributed to a change in the deformation processes within the reservoir. While deformation in the 2005-2008 period was related to the compaction of deep rocks as a result of a 3 MPa pressure drop in the initial years of geothermal production, the continuous subsidence since 2009 was attributed to a combination of both poro- and thermo-elastic compaction within or near a steam zone, formed in the topmost part of the reservoir in response to this pressure drop. Time-series of range change in the satellite line-of-sight (LOS) for the 2015-2019 period confirm the continuous deflation in the area of maximum geothermal utilization, at an increased rate during the 2018-2019 interval. A total subsidence of about 330 mm together with horizontal contraction towards the center of deflation of up to about 170 mm is inferred for the 2006-2019 period. The average ascending and descending LOS velocities are used to invert for the characteristics of the deformation source for the 2015-2019 and 2018-2019 time periods, modeling the reservoir as a body of simple geometry within an elastic half space. Results for both periods indicate a best-fitting source at ~1.4 km depth contracting at a rate of about −0.9×10^5 m3yr−1. This is comparable to values found since 2006, despite the higher deformation rates in the central area of deformation. Pressure measurements in boreholes since the end of 2017 indicate renewed pressure decline at 1.6 km depth that correlates well with the increase rate of subsidence and contraction rates observed in the center of the deformation field since 2018. However, we suggest that more time and further work is necessary for the effects of this pressure drop to reach the surface and to be able to determinate more precisely the characteristics of the deformation source from 2018, through inverse modeling of geodetic data. |