CSD Seminar Series: Gunnar Gunnarsson - Geothermal Reservoirs

Gunnar Gunnarsson holds á MSc degree in experimental physics from the university of Copenhagen and a PhD in experimental physics from the University of Baael, Switzerland. He has worked for many years in geothermal. Sínce 2008 he has worked as a senior geoscientist at OR - Reykjavík Energy. His main responsibilities are reservoir modelling and production management.

He is visiting Bergen to participate in the SiGS midterm meeting 11.-12.5. 

Title of talk: The importance of thermal effects on permeability of fractured geothermal reservoirs.

Abstract: The Geothermal field in the Hengill Area are an important source of energy for space heating and electricity generation. The Hengill is a central volcanic system on a plate boundary. It consists of 40 km long SW-NE oriented fissure swarm intersecting the Mt. Hengill. The surface rocks in the area are mainly basaltic in form of hyaloclastites from glaciation periods and lavas from interglacial periods.  Three Holocene eruptions are known in the Hengill system 2000, 5800, and 10,000 years ago and hot springs and fossil geothermal activity can be found all over the area.

Two power plants are operated in the Hengill Area: Nesjavellir in the North and Hellisheiði in the South.  Both are co-generation power plants generating electrical power and thermal energy for space heating. The permeability of the reservoir is fracture dominated and the tectonic movements of the plate boundaries continuously open new fractures and maintain the permeability of older ones.  Wells drilled into the reservoir are characterized by relatively few narrow feed zones where the wells are believed to intersect faults and fractures.  Another manifestation of the fractured dominated nature of the reservoir is the strong anisotropy of the permeability.  The water tends to flow along the SW-NE aligned fractures in the area.  

One of the more interesting consequences of the fractured nature of the reservoir is how strongly the injectivity of the wells is dependent on the temperature of the reinjected water. This dependence can be explained by thermo-mechanical effects, and it is often seen during the operation of the injection well of the power plant.  Moreover, this effect has been used to maximize the performance of the injection wells. This effect could also help explain the cooling intrusive bodies and other superhot formations and how they interact with the water circulation in the roots of geothermal systems.