Geochemical simulation of lake water injection into the geothermal reservoir in Tianjin, China

Abstract

With the development of geothermal resources in Tianjin, geothermal energy plays an important role in socio-economic development. Large scale production has already resulted in a rapid drawdown of the water level. Increasing the quantity of injection water may be a good method to supplement the natural recharge to geothermal systems. This report focuses on the geochemical feasibility of lake water injection into the bedrock geothermal reservoir in Tianjin Dongli Lake area. PHREEQC is used to simulate the geochemical effects of lake water injection. The results show that if the lake water is injected on its own into the reservoir in winter, it is more likely to cause significant precipitation than injection of geothermal water only. Precipitation dominates if the mixture contains 50% lake water. The amount of dolomite precipitated exceeds that of dissolved calcite. The dolomite precipitated is 0.55 g/kg, six times more than from spent geothermal water. Precipitation dominates if lake water only is used as has been the custom in summer. Then the dolomite precipitated is 0.97 g/kg, eleven times more than from spent geothermal water. In order to reduce scaling in the reservoir during lake water injection, the injection water is acidified with hydrochloric acid. Adding 0.529 ml of 30% industrial hydrochloric acid to 1 kg of a mixture containing 50% lake water can remove the precipitation effectively. When the pH of the mixture is 6.054, it is calculated that no deposition takes place. If a higher proportion of lake water is injected into the reservoir, more acid is needed.