Reliable and affordable backfilling of boreholes in closed thermal energy systems

Thermal energy is an important technique in the heat transition. Its use can therefore be expected to increase very rapidly in the decades ahead. The installation of closed thermal energy systems involves penetrating semi-permeable layers in the subsurface.

The sealing integrity of these layers must be restored after the drilling to prevent exposing groundwater quality to any risks. Because the installation involves deep wells (>100m) with relatively small diameters (<20cm), backfilling them is not easy. Innovation is needed to ensure that the right techniques are used to backfill boreholes in closed thermal energy systems effectively and efficiently.


This research project evaluated the techniques (methods and materials) used in the Netherlands to restore the sealing integrity of the penetrated layers after the installation of closed thermal energy systems. It explored possible ways of improving the backfilling of boreholes and making regulations more workable (work package 1), determining the quality of backfilling in situ (work package 2) and strengthening the quality assurance system (work package 3). Knowledge gaps were also identified.


Since groundwater is a key raw material for drinking water production, it is important, above all, to backfill the boreholes properly. The current techniques in widespread use are laborious, which leads to problems in practice. Since the demand for boring for closed thermal energy systems is expected to increase significantly, the drilling companies have to make decisions about major investments for the backfilling or sealing of boreholes. The efficiency of the techniques is therefore a major consideration as well. To prevent investments being made in the wrong equipment, it is important, in the short term, to make it clear which techniques meet the requirements.


The magnetometer is currently the most promising technology for determining whether backfill material is present at the correct depth. Measurements are relatively straightforward and quick to use, but they do require drilling firms to use magnetically detectable backfill material (with magnetite).

Four backfill methods were tested. The “full backfill with a fixed filling line” came out on top. The “full backfill with withdrawal of the filling line” and the “backfill in layers” approaches involved much higher risks of errors. “Reversed grouting” may be a workable alternative but further studies of reliability are required.

Measurements in shallow boreholes looking at both grouting and the Reuse of Bored Material (HOM) showed density differences over the height of the column. Only 1 of the 5 samples met the required standard of 10-9 m/s permeability. It is suspected that the materials were tested in laboratory conditions in a manner that is not representative of field conditions.

The study produced concrete recommendations, some of which are already being used in practice by the drilling companies involved, such as monitoring the backfill level during the backfilling. Furthermore, in addition to the geohydrological parameters, rheological properties and mixing instructions should be described in the product sheet for each backfill material (grout, HOM).

Knowledge gaps

The knowledge gaps identified by this study show that we need a clearer understanding of the relationship between in situ measurements in a ground loop (as in the case of a magnetometer) and the permeability of the sealing material. Once the work has been completed, it is possible on the basis of this information to determine with greater certainty the extent to which the sealing integrity of the penetrated clay layer has been restored. In addition, we need to know more about which grouts (and HOM formulations) are suitable for sealing boreholes effectively.

Improved quality assurance

The system of quality assurance can be strengthened by including the recommendations of this study in the BRL 2100 and BRL 11000 guidelines. The improved central registration of incidents and control measurements enhances the effectiveness of information-driven supervision and makes it possible to learn from mistakes and adapt regulations accordingly. In addition, steps should also be taken to identify drilling firms that operate outside the certification scheme. For example by introducing a more efficient whereabouts requirement with a GPS tracker. This will make it easier to identify illegally operating drill arrays in the field.

Finally, it is advisable to think about the organisation of quality assurance in the future. At present, supervision is the responsibility of different organisations (certifying bodies, environmental services, the Dutch Human Environment and Transport Inspectorate (ILT)) who lack the required time and expertise. A different system is perhaps needed in which certifying bodies are given a greater role and the competence to impose sanctions (as is the case in the asbestos remediation industry). Alternatively, there should be a stronger emphasis on clients (through fines imposed after the event) and financiers (through public reporting on the extent to which each drilling firm meets environmental performance requirements such as the reliable sealing of separating layers).

Schematic representation of a closed thermal energy system, highlighting the required restoration of the sealing integrity of the penetrated clay layer.