Novel concept for energy efficient hard rock drilling towards cost-effective geothermal energy harvesting

In Novock project the concept of combining an alternating voltage (AV) and mechanical breaking for hybrid rotary drilling application is investigated by addressing experimentally and numerically the potential weakening of rock granite material. Rock samples have been immersed in dielectric fluid and subjected to electric field loading to investigate if/how an Alternative Voltage (AV) can be used to soften the Kuru granite rock. A large test matrix has been performed (in 2018-2021) to study the rock drillability for a broad range of alternating electric field (various voltages, frequencies and exposure time). Miniature drill tests were thus applied on the untreated, immersed and immersed + treated (AV loading) specimens. Results revealed (beyond the scatter between test repetitions) the existence of a bimodal distribution of the drillability results observed only after AV loading. This distribution shown, that the rock drillability, after AV loading, can increase by at least a factor 2.5 compared to untreated rock material and by at least a factor 2 compared to immersed specimen. These results confirm a preliminary internal study (performed at Sintef in 2017) where an increase of drillability by at least a factor 2 was achieved on the same Kuru granite rock. Also, experimental observations of potential micro-cracks network appearing at the microstructural scale of the rock have been done on rock samples before and after the AV treatment. The surface observations were done with use of Polarized Light Microscopy and Scanning Electron Microscopy. Electron backscatter diffraction and Raman spectroscopy technics were also applied to evaluated possible changes in residual stress and strain fields induced in the rock specimen after the alternating electric field treatment. Results show the presence of microcracks that can be observed at the surfaces of both, treated and untreated specimen. Due to the scatter of cracks distribution (and microstructures properties) between specimen, it is however not possible, at this stage, to conclude (and to quantify) on the level of localisation and cracks distribution increase induced by an Alternative Voltage (AV) loading. The test matrix has been completed by 3-point bend tests. These tests were not able to capture rock weakening effect. The stress distribution under bending seems too localized to reach potential zones of micro-damage randomly distributed in the material (where the damage is assumed to be dependent the distribution of grains orientation and phases). Drillability has been done on these bend specimens and shown a rock weakening by an increase of drillabilty (by at least factor 3) compared to the reference material and by about a factor 2 compared to immersed only specimen). Also, the possible correlation of bimodal distribution of drillabilty results with microstructural properties within the drill holes area have been investigated. Results shown a rather homogeneous distribution of mineral phases (fraction of quartz, plagioclase) in the different holes. This is indicating a possible dominant effect of grains crystallographic orientations with respect to the AV field and the localisation of damage in the microstructure. The existence of a critical thresholds (distance of electrodes, AV frequency and time of exposure) for the rock breaking efficiency has been evaluated for a given level of AV (2kV). In the range investigated from 0.4 mm to 4 mm, no critical distance was detected, In addition, the results obtained so far at 50 Hz indicated a threshold at least below 10 min to weaken the rock. The variation of this threshold remains nevertheless dependent of the inherent rock microstructural heterogeneities. This is however indicating a good potential for time reduction which pave the way for practical drilling applications. Also, a fruitful collaboration was established with Tampere University of Technology on a numerical method, based on embedded discontinuity finite elements for solving the coupled piezoelectro-mechanical problem. Based on these preliminary simulations, showing localisation and cracks initiation at grain boundaries, the principle to actuate the piezoelectric properties of Quartz present in hard crystalline rocks, such as Granite, seems to be a feasible method to weaken rock and thus facilitate the mechanical breakage. This model will be extended to account for cyclic (alternative) loading. The results have been disseminated through three proceedings. The need for further clarification and understanding of the fundamental mechanisms behind rock weakening has been also addressed by TUT and Sintef in a research project called Piezodrill granted in April 2021 by the "Targeted Academy Project 2020: International Co-Investigator Scheme for Finnish-Norwegian research cooperation in engineering".

The project revealed that electromechanical fatigue loading can be used for more efficient rock breaking. This will naturally create a major impact in the broad research field of rock drilling and excavation, notably in hard rock formations often met in Scandinavia. The confirmation of minidrill increase of efficiency by a factor 2 after AV pave the way for a transfer of knowledge for further development and design of the concept in engineering practice. The results indicate a dominant effect of stresses localisation at the grains boundaries triggered by the piezoelectric response of Quartz mineral. This is opening for new questions to address for instance the characterization and prediction of grain interfaces evolution during AV.This point has been further addressed by TUT and Sintef in a project called Piezodrill granted in April 2021 by the International Co-Investigator Scheme for Finnish-Norwegian research cooperation in engineering. The project of 3 years started in Oct. 2021.

The project will reveal how an electromechanical-fatigue loading can be used for more efficient rock breaking by taking advantage of the weak tensile rock properties, heterogeneities of the rock microstructure and the ability to apply high frequency alternating electric fields triggering an electromechanic effect through a piezoelectric properties of one of the rocks constituents. A preliminary (funded internally at SINTEF, during all 2016) study performed without further parametric investigation has already shown significant improvement of the drillability in hard rock. Such type of experimental investigation on rock material cannot be found in the literature, Therefore, the panel of scientists from SINTEF and NTNU behind this application is convinced that this unprecedented result in its field of research, exhibiting rock softening by AC electric fields , builds the basis for a totally new approach to hard rock material excavation efficiency. However, at this stage, more experimental data and analytical and/or numerical models are needed to provide valuable insights into the combined mechanisms responsible for the rock fracturing process under AV exposure , their scale of activation, and their link to macroscopic strength of the rock material.