Numerical modelling of water level changes in Tianjin low-temperature geothermal system, China

Abstract

The Tianjin geothermal field is a typical sedimentary basin low-temperature geothermal system which includes a 6-layer reservoir of two kinds. One is a closed clastic rock subsystem where geothermal water is present in the pores, the other a semi-open and semi-closed bedrock subsystem where karst geothermal water is present. This report focuses on the Wumishan group reservoir which is located in the deepest layer. The Wumishan group belongs to a bedrock geothermal system that mainly lies in the Cangxian uplift and is the main reservoir under exploration and utilization. The reservoir is used for space heating and swimming pools. The depth range of this reservoir is 988-3000 m and the temperature ranges from 75 to 105°C. There are 122 geothermal wells (35 geothermal reinjection wells) in the Tianjin area which belong to the Wumishan group reservoir. The total production rate was 1362.2 × 104 m3 in 2010, an increase of 72.8 × 104 m3 from 2009. The Wumishan group production rate is 46.6% of the total production in 2010. The reinjection rate was 550.4 × 104 m3 in 2010, which covers 40.4% of the total production.

In order to study the dynamic changes in the Wumishan group reservoir, the water level, temperature and production were monitored. The water level has dropped quickly in recent years as a result of increased production. However, the temperature shows almost no change. This report describes the development of numerical models of the Wumishan group reservoir. Two types of models were constructed, a lumped parameter model using the Lumpfit program and a simple distributed parameter model using TOUGH2. The models were then used to predict future water level changes for three production scenarios. The water level is expected to drop 26.6 m in the next 10 years with the same production as in 2010, 24.3 m once reinjection is increased by 10%, and 37.2 m after 10% production has been added. The results presented here show that increasing reinjection and reducing production are good methods for helping to prolong the life of geothermal systems.