| Title | Analysis and Modelling of Gravity Changes in the Reykjanes Geothermal System in Iceland, During 2004–2010 |
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| Authors | Egill Á. GUÐNASON, Andri ARNALDSSON, Guðni AXELSSON, Jean-Claude C. BERTHET, Sæunn HALLDÓRSDÓTTIR, Ingvar Þ. MAGNÚSSON |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Reykjanes, gravity, monitoring, mass renewal |
| Abstract | An analysis of gravity changes observed in the Reykjanes geothermal system during the last decade is presented, with the principal purpose of estimating the mass changes in the geothermal system during the period 2006–2010 and hence the renewal of the fluid reserves in the system. The gravity surveys this work was based on were conducted by Iceland GeoSurvey during the summers of 2004 (prior to start-up of the Reykjanes 100 MWe power plant), 2008 and 2010. The analysis involved three main steps. First, an estimation of the mass changes in the geothermal system through a Gauss-integral of the observed gravity changes during two periods, 2004–2008 and 2008–2010. Secondly, a simulation of the gravity-change anomaly for 2008–2010 by two simple mass change models. Thirdly, a calculation of gravity changes at the observation points of the gravity grid on basis of mass changes in the numerical TOUGH2-model of the geothermal system set up by Vatnaskil Engineers for HS Orka. The results of a Gauss-integration of the gravity-change anomaly for the period 2008–2010 indicates that renewal of the reservoir mass during that period is in the range of 30–50%. The renewal for the period 2006–2008 was probably somewhat less, but can’t be estimated because of the loss of a centrally located measuring point in 2006. A simulation of the 2008–2010 anomaly by a spherical mass-change volume indicates that the center of the volume is at a depth of 1400–1700 m. Modelling of the gravity changes on basis of the mass changes in the TOUGH2 numerical reservoir model of the Reykjanes geothermal system result in quite comparable results for the 2008–2010 period. The observed and calculated anomalies are comparable in shape, even though the calculated one doesn’t extend as much to the NE and SW as the observed one and the calculated maximum anomaly is a little smaller. A smaller and lower gravity-change anomaly for the 2008–2010 period indicates that the numerical model allows somewhat more recharge to the geothermal system than is the case in reality. These results show that the gravity data can now be used to support the calibration of the reservoir model. |