Addressing the root cause of calcite precipitation that leads to energy loss in geothermal systems

Abstract Geothermal sources are an essential energy resource in many cities around the world, in which a deep hot water reservoir has two energetic bases: thermal and hydraulic. The use of this energy can produce both electricity and heat. A geothermal well is a way in which the energy flows from the reservoir to the surface. A known operational problem can directly consume hydraulic energy and indirectly consume thermal energy. Calcium carbonate (CaCO3) precipitation and scaling can reduce and even block the flow area of a geothermal well, deferring, degrading, or even closing the production of water, steam, heat, and power. CO2 degassing, which is intrinsic to the deepwater system, is often the cause of this problem. This relationship is evident in the chemical balance: Ca2+(aq) + 2 HCO3–(aq) ⇆ H2O + CaCO3(s) + CO2(aq). With the head loss provided by the water flow, CO2(aq) changes to the gaseous state (CO2(aq) → CO2(g)), which disfavors the CaCO3 dissolution and implies its mineralization. The CO2(aq) closely links the two mentioned chemical reactions and makes them mutually dependent. Therefore, two causes favor the CaCO3 precipitation: changes in thermodynamic conditions (pressure and temperature) and CO2 exsolution. It takes place even if the hot water is not boiling or flashing. This paper aims to quantify these two causes by applying our techniques in some geothermal fields in different locations in the world. In all scenarios investigated, the release of CO2 contributes from 66.1% to 92.6% of the CaCO3 precipitation. These results can lead engineers to adequately remedy the problem and suitably design and operate a geothermal system as a whole.