Sustainable utilization of low-temperature geothermal systems with limited natural recharge

Abstract

Experience from long-term utilization of many geothermal systems has shown that there usually exists a level of maximum energy production, E0, below which it will be possible to maintain constant energy production from a geothermal system for a very long time (100-300 years). If the production rate is greater than E0 it cannot be maintained for this length of time. Geothermal energy production less than or equal to E0 is termed sustainable production. To maintain a production rate below E0, i.e. sustainable production, reinjection is an extremely useful method, in particular for a low-temperature geothermal system with limited natural recharge. This paper presents a successful example, namely sustainable utilization with reinjection in the Hofstadir system, W-Iceland. Based on the utilization analysis and the reinjection mode assessment, the paper studies the possible change in the production temperature and the water level after reinjection started by using simple models, i.e. lumped parameter and tracer test models. As a geothermal system with insufficient recharge, the production capacity of the Hofstadir field is limited and the heat stored in the 86-87°C reservoir can only be used to a limited extent. Since 22-04-2007, the water level of production well HO-1 has recovered continuously due to reinjection at a ratio of 65-73% of the production.

The reinjection has shown a good effect on supporting the reservoir pressure and improving heat mining. In order to evaluate production temperature change during the reinjection period, a tracer test was carried out from 29-08-2007 to 09-11-2010. The data from the tracer tests were simulated using a multiple flow-channel model. The results indicate that there are 3 direct paths between the injection and production wells. It is confirmed that there is quite good connectivity in the reservoir and the reextracted water includes almost 90% of the injection water. A cooling forecast was done by using program TRCOOL, indicating that the temperature of well HO-1 may decline by 18-26°C for different reinjection-ratios. The monitored water level data provides strong evidence to prove the reinjection effect. By lumped parameter modelling, water level predictions were calculated for different scenarios of reinjection and production, based on the present situation, for the next 50 years. The predicted results show that reinjection will allow production to continue at the present rate for the coming decades. All the study results show that reinjection is one of the most important measures for the management and sustainable utilization of a geothermal system with limited natural recharge.