Fillings of geothermal wells

For filling the borehole of ground-water heat pumps, pure bentonite, a mixture of bentonite and cement, or even the original rock is still often used. This is a commonly used solution, but it has several disadvantages. These materials achieve a low thermal conductivity coefficient, which results in a higher thermal resistance of the borehole. Also significant is the very low resistance of these fillings to negative temperatures, which causes them to degrade over time, increasing their porosity and further decreasing thermal conductivity.

A much more effective solution is a mixture developed and manufactured specifically for injecting heat pump boreholes under the name ThermoCem PLUS. The filling of the borehole made from ThermoCem PLUS mortar achieves significantly higher thermal conductivity than standard bentonite filling. This reduces the thermal resistance of the borehole (Rb) and increases thermal gain. ThermoCem has proven resistance to freezing and the action of aggressive waters, and is produced from raw materials selected with an emphasis on high product quality and environmental protection.

Among the indicators that must be monitored when selecting the filling for a ground-water heat pump borehole are borehole thermal resistance, thermal conductivity, suspension properties, and its application during implementation.

Borehole Thermal Resistance
Heat transfer between the circulating fluid and the rock depends on the geometry of the borehole, the arrangement of the probes, and the properties of all materials used. In addition to having a sufficient temperature gradient, the thermal conductivity also plays a key role in heat transfer by convection. The higher the thermal conductivity, the lower the temperature gradient required.

The overall thermal resistance of the system depends on the following factors:

• Thermal conductivity of the borehole filling
• Material properties of the probe
• Arrangement of the probes, their distance from the borehole edge
• Dimensions of the borehole

In summary, the thermal resistance of all parts of the borehole construction is called the thermal resistance of the borehole (R_b). The filling of the borehole made from ThermoCem PLUS mortar achieves significantly higher thermal conductivity than standard bentonite filling. This reduces the thermal resistance of the borehole and increases thermal gain.

The reduction of the thermal resistance of the borehole Rb also facilitates its use for active cooling of the building during summer months, which is an economically attractive alternative to standard air-to-air air conditioning units. This also contributes to balancing the thermal budget of the rock environment, positively influencing the performance during the heating season and its sustainability over the long term.

Improving Thermal Conductivity
There are various ways to increase the thermal conductivity of a material. One method is to add quartz sand or generally aggregates. To achieve thermal conductivity around 2 W.m^-1.K^-1, a relatively large amount of aggregates needs to be added, which can lead to worsened processability and pumpability while also increasing the abrasiveness of the mixture.

In the case of the ThermoCem PLUS mixture, a different method was chosen. To achieve a thermal conductivity (λ) of 2 W.m^-1.K^-1, a special additive is used. The amount and processability of the suspension are tailored to the specific requirements for injecting heat pump boreholes. Assuming a thermal output of the borehole of 50 W/m, a temperature gradient of 6 K is required when using a cement-bentonite filling (λ = 0.8 W.m^-1.K^-1), while only 3.5 K is needed when using the ThermoCem mixture (λ = 2 W.m^-1.K^-1).

Laboratory results show that using the ThermoCem mixture significantly increases the efficiency of the heat pump due to a higher output temperature of the primary circuit fluid or the possibility of shortening the borehole length, resulting in substantial savings in operating or cost.

Suspension Properties
The clay minerals contained in the ThermoCem mixture quickly absorb excess water, thus thickening the suspension and supporting its thixotropic properties. This is advantageous when passing through horizontal cavities or disturbances in the substrate. The declared technical and quality parameters including thermal conductivity can only be achieved by adhering to the specified amount of mixing water. The most important factor when mixing the suspension is the water-to-solid ratio (w/s), i.e., the weight ratio of water to dry mixture. A well-prepared mixture is characterized by low water separation. The maximum value for a well-processable mixture should be less than 1% to 2%.

ThermoCem® PLUS is a dry mix for site preparation of injection suspension intended for filling boreholes of ground-water heat pumps. It meets the requirements of VDI 4640 guideline. If the prescribed composition of 810 kg of mixture and 650 l of water per 1 m³ of suspension is maintained, the finished filling has high thermal conductivity (λ ≥ 2 W/mK), low permeability (kf ≤ 10-10), and verified resistance to freezing.

Quality Control on Site
The most suitable method for checking the correct ratio of components in the suspension is to determine the bulk density. Regular monitoring can confirm the proper filling of the borehole. A professional study (DVGW, 2003) has shown that it is also advisable to compare the bulk density of the suspension in the mixer with the bulk density of the suspension coming from the borehole after the injection is completed. If both values match, it can be assumed that the residual water has been completely displaced from the borehole and thus it has been uniformly filled.
For an approximate determination of the viscosity or processability of the suspension, a Marsh test is used. The measured quantity is the time it takes for 0.5 liters of suspension to flow from a funnel with a volume of about 1.5 liters through an opening with a diameter of approximately 5 mm. Pure water takes about 26 seconds, and injection suspension with good processability about 40 to 100 seconds.

By mixing the dry ThermoCem mixture with water, a easily processable suspension is created, which is immediately ready for use on the construction site.

When selecting filling material, it is therefore wise to consider not only the direct costs resulting from the unit price of the suspension but also the thermal conductivity value, water tightness, and durability of the filling. The reward for using a special filling mixture can be savings in both the implementation and operational phases of the heat pump's life cycle.

Českomoravský cement, a.s.
www.thermocem.cz