Estimating Effective Permittivity using Digital Rock Physics and Drill Cuttings – Preliminary Insights and Future Challenges

The EU-funded project GeoHEAT aims to develop new time- and cost-efficient methods for geothermal exploration. One part will be a georadar probe that can be used to characterize geological structures at elevated ambient temperatures. As the permittivity of the surrounding rock mass is of great importance for the optimal use of this probe, we are investigating the possibility of estimating this dielectric property using digital rock physics (DRP) and drill cuttings. With the latter being a by-product of exploration drilling, they represent a further potential approach at saving both time and costs, without the added expense of core sampling. Here we want to present the current state of our research and give an overview of preliminary insights and future challenges.

In order to determine the effective permittivity using DRP, a pore scale model is required. For this purpose, nano-computed tomography images are created and then segmented, whereby different phases are assigned to the gray value intensities of the scanned sample. The model is finalized by attributing physical properties to individual material particles in the location-dependent volume and is subsequently used to compute the effective permittivity with a solver utilizing the finite volume method. To optimize this process, standardized granite core samples from the Bedretto Lab in Switzerland are used, and the results are validated by comparing them to laboratory measurements of the same rock type for accuracy.

Since the extraction of these kinds of samples is expensive, requires interruption of drilling and can only be carried out for limited boreholes and depth sections, the next step is to investigate the use of cuttings, which are generally produced during drilling and transported to the surface with the drilling mud. We analyze to what extent the precision of the obtained results is affected by this more economic approach.

The analysis of core samples has yielded promising results, demonstrating a high level of accuracy in estimating effective permittivity values using DRP. In addition, ongoing investigations into the use of rock cuttings as a substitute have shown great potential for significantly reducing cost and time, while maintaining reliability of the determined rock property. These advances could improve the practical application of georadar probes, offer a more efficient approach to geothermal exploration and provide deeper insights into the geology of the subsurface.