Characteristics of geothermal water in the Xining Basin and risk of reinjection scaling

The Xining Basin in Qinghai Province is rich in geothermal resources of low to medium temperature, but the geothermal reservoir is dominated by weakly consolidated sandstone containing clay minerals, and the geothermal water has high salinity, which causes a significant risk of scaling during reinjection. In this paper, based on the analysis of the geothermal genesis and resource distribution characteristics in the Xining Basin, different methods, such as the mineral solubility method and saturation index method, are used to assess the scaling tendency and risk in typical geothermal water during reinjection. The results show that the “convex in concave” structure of the Xining Basin is beneficial for the enrichment and warming of thermal groundwater in the deep geothermal reservoir, and at the same time, a large number of dissolved minerals are brought to the central bulge in the Xining Basin. The geothermal reservoir in the Xining Bain is mainly buried at a depth of 700−1600 m with water temperature of 30−70 ℃. The hydrochemical type is mainly of SO4·Cl—Na type, and the salinity range from 1.85×103 to 4.80×104 mg/L. The main scaling product during reinjection is CaCO3. When the characteristics of reinjection water and geothermal water are similar, the risk of scaling mainly occurs in the reinjection wellbore, and the risk of formation scaling is relatively small. When the characteristics of reinjection water and geothermal water are quite different, the incompatibility will greatly increase the risk of formation scaling. Among them, when the water from Yaowangquan is mixed with that from DR2005Y by 1∶1, the maximum scaling amount can reach 177.57 mg/L, while the scaling amount from other geothermal water is smaller. Based on the above characteristics, three sets of comprehensive measures are proposed as the follows: physical anti-scaling + pipe anti-corrosion, system pressurization anti-scaling + pipe anti-corrosion and ground pretreatment + pipe anti-corrosion, supplemented by measures such as cathodic protection anticorrosion, optimized displacement, and pickling the wellbore. The results of this study can provide theoretical basis and technical support for the formulation of measures to ensure the reinjection capacity of geothermal water in the future.