Geochemical modelling to evaluate mineral scaling risk in a geothermal loop.

Geothermal energy is a low-carbon energy solution obtained by harnessing the heat of the Earth’s interior, stored in rocks and groundwater. Despite its advantages as competitive renewable energy, geothermal development is full of challenges including tubing scaling risk due to important pressure (P) and temperature (T) changes in thermal fluids during the geothermal loop. Geochemical models are developed to determine the geochemical processes that control scaling processes, allowing creation of efficient geothermal plants and scaling risk evaluation for new and existing facilities. Four major scaling minerals such as silicates, carbonates, sulfides, and oxides, have been reported to dominate the geothermal environment. Among them, the silica scaling is one of the biggest problems occurring in many geothermal fields worldwide.

In this study, a PHREEQC geochemical model is carried out to evaluate the mineral scaling risk associated to a new potential geothermal loop using water from a carbonate reservoir. The effect of gradual pressure and temperature changes on the evolution of the water chemistry is assessed by defining (thermodynamic) equilibrium simulations during the three steps of the geothermal loop: (i) P decrease from 285 bar to 18 bar in the isothermal water pumping, (ii) T decrease from 107.5 °C to 40 °C in the isobaric cooling, and (iii) both P and T increase up to 285 bar and 107.5 °C, in the re-injection to reservoir. The amount of mineral precipitation at a given location in the tubing is provided by PhreeqC in kg/L of water. Hence, the rate of mineral precipitation at this location is calculated by multiplying this amount by the water flow rate (L/d).

During the pumping, no silicate and sulphate precipitation is revealed. However, calcite precipitation occurs, reaching the highest amount of precipitation at the lowest P. Fe-bearing phases also precipitate during the pumping due to the high Fe concentration released from tubing corrosion. Different precipitation reactions are revealed in the cooling where no carbonate, but barite precipitation takes place during the process, reaching the highest amount of precipitation at the lowest T. Even if barite precipitates at any T (from 107.5 °C to 40 °C) within the whole cooling, it does not show the highest rate of precipitation. Despite chalcedony precipitates in a closer T window (from 55 to 40 °C), it reaches a higher precipitation rate than barite (65,1 kg/d for chalcedony versus 11,4 kg/d for barite). The last step of the geothermal loop (re-injection to reservoir) does not show any possible mineral precipitation.

In addition to the ‘batch’ equilibrium simulations presented here, we plan to improve our model by performing reactive transport simulations in which scaling mineral precipitation kinetics as well as water flowing in the tubing will be considered.