During August 2024, a geophysical measurement, was carried out in one of the boreholes in the Ringen Research Centre. The measurements were made in 190 m deep borehole LT2. Due to its shallow depth, this borehole only penetrates sedimentary layers, which are mainly sandstones and siltstones.
"The logs allow us to determine whether the casing (in this case a steel pipe inserted as a reinforcement in the borehole) is tightly adhering to the rock by means of cementation. If there were cavities, future temperature measurements with the fibre-optic cable would not be accurate. The leak check is carried out using ultrasonic signals. In places where the casing firmly adheres to the rock, the signal is weak. Where there are cavities between the casing and the rock, the casing 'resonates' and the signal is stronger, indicating their presence." explains geologist Lucie Janků from the Faculty of Science at Charles University.
Apart from the presence of cavities, measured by the ultrasonic probe, other parameters are important as well. What exactly is measured and why?
Verticality
A series of boreholes just a few meters apart will be used for heat storage. Maintaining the vertical direction is therefore very important, otherwise the boreholes would intersect and the system would not work optimally. An inclinometer is used to detect not only the degree of deviation, but even the azimuth, i.e. the direction of deviation. The borehole should ideally be completely vertical. This is achieved both by a suitably selected drilling technology and by careful, unhurried work. If a mistake is made and the borehole is not perpendicular, the verticality can be corrected with directional drilling heads. However, only a few companies have these special drilling heads and their work is expensive. Patience brings not only roses, but in this case a nice perpendicular borehole and less associated costs.
Natural radioactivity
This is not strong radioactivity, just the weak radiation emitted by, for example, the potassium isotope 40K, which is present in most rocks. Sandstones tend to have little of it, clays have a lot of it, so we can tell which rock is which by measuring the radioactivity, without having to pull samples to the surface.
Other radiometric methods, which use an active emitter mounted directly on the probe, also measure the density and porosity of rocks using back-emitted gamma photons or neutrons. Using this technology, we obtain not only the information about the rock type, but also about the properties of the rocks.
Optical and acoustic televiewer measurements
This type of probe cannot be used in our borehole because the signal would interfere with the steel casing. However, in other cases where the borehole is without reinforcement, this method is very effective. It shows the inside of the borehole realistically, so that even a layman is able to distinguish the boundaries of individual rock layers, the existence of cavities and cracks. Based on the data obtained, an impressive digital 3D model of the borehole can be modeled.
Here is an example of a 3D model and a 2D view (basically a 3D model "expanded" into a surface) obtained by the acoustic method in another borehole.
Measurements using acoustic and radiometric methods take a very long time, unlike verticality measurements. The probes are lowered into the borehole at a speed of 1-2 m per minute. In boreholes several kilometers deep, a single measurement can take several days. In the case of the 190 m deep LT2 borehole, it took a full day to measure five methods. Once the logging is complete, a downhole seismometer and a fibre-optic cable for temperature measurements will be installed in the borehole.
Temperature and groundwater flow
If the results of the logging show that the casing is well sealed, a 'thermal response test' will be carried out in the borehole. The borehole is heated and then the rock temperature is measured over a given period using a fibre-optic cable permanently cemented to the inner surface of the casing. The thermal diffusivity of rocks, i.e. their ability to heat up or cool down over time, is known from laboratory measurements. If the rate at which the temperature settles does not match the measured thermal diffusivity, this indicates groundwater flow around the borehole.
"This parameter is essential if we want to use the boreholes for heat storage, as is planned for the SYNERGYS project in Litoměřice. The water-saturated rock layers, where the water flows, will cool much faster and take the heat away. On a smaller scale, we can imagine the borehole as a heating coil that we want to use to heat water for tea. If we put it under a running tap (or layer of sediment where groundwater flows), the heated water will drain away immediately. So the goal is to place the boreholes where the groundwater does not flow (or place the heating coil in a kettle full of water). Otherwise, the summer heat would "flow away" and there would not be much left by winter," explains geologist Lucie Janků from the Faculty of Science of Charles University.
