Drilling isn’t bore-ing

Jaz Andress, Senior Geo-Environmental Consultant at Tetra Tech Europe, delves ‘beneath our feet’ to share how drilling techniques can unearth geothermal energy at the Stormont Estate. 

In May this year, the GeoEnergy NI project reached an exciting new chapter as we commenced the drilling phase of our shallow geothermal feasibility study on the Stormont Estate, starting with the ‘stratigraphic’ borehole. Stratigraphic means the study of the composition and positions of rock strata in order to determine their geological history. The initial results have already provided invaluable information about the subsurface, setting the tone for what’s to come as we explore what’s beneath our feet. 

To start the process, our team of expert drillers began by advancing the borehole, using robust 5.5-inch diameter casing. Casing is used to line the borehole walls, providing stability in looser ground that may otherwise collapse into the hole. Most boreholes are cased through the unconsolidated sediment nearest to the surface, such as loose layers of soils, sands, and gravels. All these deposits are grouped together under the umbrella term ‘overburden’, which overlies the structurally sound bedrock. In this case, the overburden was a quaternary glacial deposit, including sediments deposited during the last ice age, between 10,000 and 16,000 years ago.  

The stratigraphic borehole was cased to a depth of 62.50 meters and as we advanced the borehole through the overburden, we used a smaller 3.5-inch diameter casing for the remainder of the drilling. Once the borehole reached its final depth, the 3.5-inch casing was removed, while the 5.5-inch diameter casing remains in the ground to help maintain the upper portion of the borehole and prevent collapse. 

As drilling progressed, the drillers completed 3 meter long ’core runs’, extracting 3m of rock core at a time. The rock cores are often extracted from the barrel broken due to natural faults, fractures, and other geological features. Following extraction to the surface, cores were transferred to a geologist who logged the findings. The geologist records various parameters and features including engineering descriptions (total core recovered, degree of fracturing and competency etc.) and geological descriptions such as colour, weathering, grain size, mineralogy and rock type. This gives us a better understanding of the subsurface and the hydrogeological and geothermal potential of the ground in that area.  

After drilling of the stratigraphic borehole was completed, we conducted geophysical logging to gather more details about the geology at the Stormont Estate. We measured electrical resistivity, porosity, density, natural radioactivity, borehole diameter, and the overall condition of the borehole. We then compared these results with the rock samples that were collected during drilling. After completing the drilling and geophysical logging, we secured the borehole with a manhole cover, allowing us to return to the site for additional testing if needed.  

The Stormont site is underlain by a near surface aquifer containing groundwater in the region of 15 °C. This is the first of five boreholes we are drilling and testing within the grounds of the Estate as part of the GeoEnergy NI project. The remaining 4 boreholes are primarily hydrogeological (groundwater) boreholes which will be used to complete hydrogeological testing once drilling has been completed. It is hoped the project on the Stormont Estate will result in a future geothermal heat network that will replace the current fossil fuel heating systems at some of the buildings on the Estate. Ultimately this will help Northern Ireland meet the Executive’s carbon reduction targets as well as reducing and stabilising heating costs. 

You can find out more about the GeoEnergy NI project and Northern Ireland’s overall geothermal potential on the project website www.GeoEnergyNI.org or by following @GeoEnergyNI on social media channels.  

Our GeoEnergy NI Discovery Centre, located at the Mo Mowlam playpark is packed full of interactive experiences including VR headsets where you can immerse yourself down the borehole, learning about the heat beneath your feet.