Title: Numerical modelling of subsidence in geothermal reservoirs: A case study of the Svartsengi Geothermal System, SW-Iceland

Abstract

The Svartsengi-Eldvörp geothermal system is a high-temperature system situated within the Reykjanes
Peninsula oblique rift in south-west Iceland, a part of the boundary between the North America and
Eurasia plates. Extensive regional ground subsidence ranging from 7 to 14 mm/year has been observed
from 1975 to 2015, with the maximum changes in elevation detected in the production field at
Svartsengi. Numerous studies done throughout the years have sought to identify and isolate the various
signals that contribute to the observed subsidence. Recently, the combined analysis of a variety of
geodetic studies have proposed a natural subsidence velocity of 6 mm/year along the central volcanic
rift within the Reykjanes Peninsula. This project however, seeks to contribute to the existing
deformation studies at Svartsengi by setting up a TOUGH2 numerical model of the Svartsengi
geothermal system, calibrated against the average annual mass extraction and reinjection rates from
1975 to 2015. One-dimensional subsidence modelling was performed with the model, using a
newly developed subsidence module in iTOUGH2. The model reveals that high permeabilities, pressure
drawdown and changes in the rates of production and reinjection have been major factors that have
influenced the contribution of geothermal production to the total observed vertical deformation at
Svartsengi. Modelled results indicate an average subsidence velocity of 3-4 mm/year as a result of mass
extraction due to geothermal activity. Coupled with the natural subsidence value previously determined,
this value is capable of accurately representing the total subsidence observed at Svartsengi. This
numerical model is therefore a valuable tool for predicting subsidence rates due to future production at
Svartsengi and Eldvörp.