Production capacity assessment of the Hoffell low-temperature geothermal system, SE-Iceland

Abstract

Recently a new geothermal well named HF-1 was drilled for the purpose of space heating in the low-temperature geothermal field at Hoffell, Nesjar, in southeast Iceland. To evaluate the production capacity of the geothermal system a conceptual model of the system was established and a volumetric assessment performed. Based on the analysis of the conceptual model and the volumetric assessment, together with the Monte Carlo method, the total energy in the system is estimated as 46.9 TJ (most likely), the recoverable energy as 9.4 TJ (most likely) and the thermal power for 50 and 100 years is estimated as 6.0 and 3.0 MWth. (most likely), respectively. Well test data from two step-rate tests were analysed with WellTester to estimate the parameters of the system. Permeability was, thus, estimated to be around 4.8-7.0 mDarcy. The well’s skin factor was around -0.1 after the well was deepened. Lumped parameter modelling was consequently used to simulate the behaviour of the reservoir to exploitation during a long-term test from April 9 to September 8, 2013. A two-tank closed model and a two-tank open model (using Lumpfit) could simulate the monitoring data very well.

The parameters resulting from the Lumpfit and WellTester evaluations are quite comparable, with the permeability thickness of the reservoir around Well HF-1 estimated to be in the range of 2-5 Darcy·m. Finally, future water level predictions were calculated to estimate the probable response of the reservoir to different production scenarios for the next 10 years, both with a conservative and an optimistic model. It seems that if the system is closed, Well HF-1 can only sustain around 10 l/s for the next 10 years and if it is open, Well HF-1 will sustain around 30 l/s for 10 years. It is unlikely that the system is completely closed so 15-20 l/s seems to be the most likely production range. If the system is closed, reinjection will be necessary to increase and maintain the production capacity.