Direct heat production by double wells is widely used to extract thermal energy from geothermal reservoirs. However, geothermal water is often type of brine with high dissolved salts. Injection and extraction of the geothermal fluid may cause strong chemical reactions, leading to scaling in the wellbore and reservoir. Such process was seldom considered in the heat production optimization. This study numerically investigated the mechanism of chemical damage to the reservoir in the Zhacang geothermal field, China, and optimized the heat production with constraint of weakening chemical damage. It was found that direct injection of the tail geothermal water can lead the porosity to reduce by 0.06%. The degree of precipitation can be controlled by artificial factors, including injection temperature and mixing ratio of the geothermal water with river water. The comparative computation indicated that at an injection temperature of 35 degrees C, and a mixing ratio of the river to the geothermal water of 6:4, the reservoir porosity was unaffected by chemical reactions in 100 years and geothermal energy production can be maintained above 0.11 MW. The methodology can be applied in other geothermal fields for sustainable geothermal energy production regarding the formation damage in geothermal reservoirs due to fluid-rock interaction. (C) 2019 Elsevier Ltd. All rights reserved.