Tube blocking

Cooling or lowering the pressure of the usually very salty thermal water when using geothermal energy can lead to precipitation and deposits of supersaturated salts (scales). Commercially available inhibitors can be used to reduce the formation of scales.

The minimum effective concentration of a scale inhibitor is usually determined using dynamic tube blocking tests (TBT) in laboratory tests (Kelland, 2014). Synthetic thermal water is used for this purpose. A mobile tube blocking system was set up as part of the EIKE project (Heberling, 2024). This made it possible to carry out tests with real thermal water directly at the power plant on the open bypass.

^{Fig. 1: Photo of the tube blocking plant}

In order to speed up the reaction, ions are added to the thermal water flow - both in the laboratory and in the field tests - which cause the precipitation. For example, when precipitating barite (= barium sulphate), the ions barium (in the form of a barium chloride solution) and sulphate (in the form of a sodium sulphate solution) are added separately.

The resulting supersaturated fluid flows through a capillary in which precipitating solids are deposited on the walls. The increasing formation and deposition of barium sulphate in the capillary tube causes an increase in pressure and a continuous reduction in the flow rate until it drops to zero. Effective inhibitors prevent an increase in pressure in the capillary or a reduction in the flow velocity within a predefined measuring time (here: 30 min).

Figures 2 to 4 show series of tests that were carried out with increasing inhibitor concentrations (0 mg/L, 6.25 mg/L and 9.00 mg/L) under otherwise identical operating conditions.

^{Fig. 2: Change in the flow rate in the experiments without inhibitor}

^{Fig. 3: Change in the flow rate in the experiments with 6.25 mg/L inhibitor}

^{Fig. 4: Change in the flow rate in the experiments with 9.00 mg/L inhibitor}

Within a figure, a scattering of measured values can be seen in successive tests carried out under the same test conditions (comparison of blue, red and green curves within a figure).

A comparison of Figures 2 to 4 shows that the time to reach zero flow velocity increases with increasing inhibitor concentration. In the third test series, the flow rate does not fall below 1 L/h within 30 min. The effective inhibitor concentration in the application case described is therefore 9.00 mg/L.

Literature:

KELLAND, M. A.: Production Chemicals for the Oil and Gas Industry. CRC Press, 2014

HEBERLING, F., KUHN, D., BAUR, S., OTTO, T., SEIBT, A. & BUSE, C.: EIKE - Development and testing of inhibitor combinations for the efficient use of hydrothermal reservoirs; sub-project: Selection and evaluation of an inhibitor combination. Final report on BMWK project 03EE4022A, 2024

GOLDBERG, V., BAUR, S., SEIBT, A., STERN, G. & KUHN, D.: A Novel Approach for Selecting and Evaluating BaSO4-scale Inhibitors under in-situ Conditions, Proceedings, DGMK/ÖGEW Spring Conference 2025