Lumped and distributed parameter models of the Seltjarnarnes geothermal field, SW-Iceland

Abstract

Many mathematical models have been developed for geothermal modelling. Analytical methods are used in lumped parameter models and numerical methods are used to solve the distributed parameter models. For the Seltjarnarnes geothermal field both these methods are used for simulating the fluid flow, chemical mass and heat transport. Data observations from about 30 years, for water-level and chloride concentration are used in the modelling. The main purpose in using a lumped model is to determine two kinds of storages. One is controlled by elastic storage that depends on liquid and formation compressibility in the reservoir and the other by mobility of the free surface of the aquifer. The results are compared with the results of the distributed model. The distributed parameter model is used for simulation of flow, mass and heat transport in the field. Both leakage and free surface are considered in the model. Future predictions of the behaviour in the field have been calculated from the year 1994 to 2005. Two cases are considered, one with a constant average yearly rate of 35 l/s another with a 2% annual increase from 35 l/s pumping rate. The two cases are considered with and without injection. The injection rate is constant at 20 l/s with the temperature at 50°C and chloride concentration of 500 ppm. The final water level is -55 and -70 m respectively, for the two cases without reinjection, and -5 and -20 m with 20 l/s reinjection. Chloride concentration increases to 2000 ppm in the year 2005 without reinjection. With reinjection, there is little difference between the two cases of pumping rates, but the chloride concentration decreases to 1000 ppm in the year 2005. There is no change in temperature for the future predictions.