Impact of Carbonate Scaling on the Efficiency of Used Geothermal Water Reinjection from Low-Middle Temperature Geothermal Fluid in Xianyang Porous Geothermal Field, NW China

In low-middle temperature porous geothermal field, precipitation of carbonate from solution to form carbonate scale is confirmed as the main obstacle to efficient reinjection from the used hot fluids in Xianyang city, NW China. The carbonate deposits cause operational problems and may even clog pipeline and reinjection drillholes. For this reason, it's important to understand how impact of carbonate precipitation on the second-hand geothermal water before reinjection has been operated, which concern sustainable utilization of the geothermal water in Xianyang, the famous porous geothermal field in China. In this paper, firstly, different possible minerals precipitation from used geothermal fluid were discussed, and carbonate scaling was regarded as mineral for chemical clog by using combined with lab and chemical simulation method. Secondly, the key impacts of carbonate scaling on clog in reinjection holes such as temperature, degassing, pH, mixing, mineralization were demonstrated based on coupled simulation both in lab experiment and theoretical method. Lastly, the distribution of carbonate scaling and its degree in study area were calculated. The results show that, as the main mineral for precipitation, carbonate exist in most thermal-water wells of study area with the minor or medium scaling, and the carbonate scaling vary increasingly from bottom to top in a single reinjection drillhole, north to south in the study area. According to mineral-dissolution-precipitation equilibrium calculation carbonate saturated tend growth with the temperature and pH increases in study area. The influence of mineralization on carbonate clog is different, common-ion effect in the south of study area is given priority, but in the north salt effect is showed main effect. The analysis for impacts on carbonate clog reflect that carbonate deposit effect are very complicated, which interact each other.