The sandstones being investigated by the GeoEnergy NI project are also the focus of my postgraduate research at University College Dublin. I’m currently investigating what makes an underground reservoir suitable for geothermal resources. My main area of interest is the Triassic aged Sherwood Sandstone across Ireland, Northern Ireland and Great Britain.

There are many things to consider when investigating whether a rock makes a good reservoir. Some processes that occur over geological time can improve or reduce the reservoir rock’s ability to hold and allow the flow of hot water. Here, I’ll focus on two key properties; porosity and permeability, and use some of my study examples from Northern Ireland to help explain these concepts.

Porosity

Porosity is the amount of open space within a rock, think of a sponge with holes. The more holes there are, the more water can be held inside these open spaces. For geothermal, we want a reservoir rock with good porosity as this means it can hold greater amounts of the geothermal resource, in this case warm or hot water.

Looking down a microscope you can see whether a sample has high or low porosity,  in the preparation, the rock slice is set in a blue resin, which also fills the pore space. This helps to highlight all the open space within the rock.

The images below are two thin sections collected from a borehole, drilled in 2009 near Kilroot Power Station (Kilroot-GT01), near Carrickfergus. In the left image, a sample from a measured depth of 329.1 metres below ground, shows a lot of blue areas which indicates good porosity. The image on the right in comparison is from deeper in the borehole (741.25 metres below ground) and shows a lot less of the blue resin, indicating lower porosity.

Photo caption: Figure 1: Thin section images from the Kilroot GT-01 borehole samples, demonstrating the difference in porosity. Left: a thin section image from a sample at 329.1 m depth. Right: Thin section image from a sample at 741.25 m depth.

The Kilroot Borehole retrieved rock core from the whole thickness of the Sherwood Sandstone, some 488.8 metres. This rock unit is known to have good porosity on average and is a key target for both shallow and deep geothermal across a large part of Northern Ireland.

While many rocks remain buried, some are lifted up by tectonic Earth movements over geological time, bringing them closer to the surface. This has happened to the Sherwood Sandstone in Northern Ireland and it makes the reservoir quality harder to predict.

For deep geothermal we are on the hunt for the “sweet spot” – a reservoir buried deep enough to heat the fluid, in this case saline water, to the right temperature, but not too deep that the porosity has diminished so that there is no space for storing the fluid. This is part of the exploration process for geothermal resource evaluation.

Permeability

The other property of reservoir quality, permeability, is a measure of how easy it is for a fluid to move between the open spaces in a rock. If these holes are connected, then fluid can easily pass through the rock. However, sometimes there may be minerals like quartz or clay blocking the flow between the sand grains, or the holes are too small for a fluid to pass through.

The video below demonstrates what happened when I add some water onto two types of sandstone. In the first sample, the water droplet disappears quickly into the rock, compared to the second where it stays on the surface for longer.

 

This is because the second sample has minerals that have grown into the pore space, a process geoscientists call “cementation”. These minerals have now taken up the available pore space and reduced pathways for the fluid to flow through. Porosity and permeability are usually linked, as more porosity in the pore space usually leads to an increase in the permeability.

For a good geothermal reservoir, we need the reservoir to be as permeable as possible to allow movement of warm water, so heat then can be extracted from the boreholes. Analysis of the core from the Kilroot GT-01 borehole core, shows that the permeability and porosity decrease towards the base of the reservoir rock. This suggests that if we want to utilise this rock formation for geothermal resources, we should prioritise the upper parts rather than the lower section, because the porosity and permeability is higher.

The Kilroot Borehole Core

So, what happens to the hundreds of metres of rock core that was drilled at Kilroot? It’s all kept safely in the Geological Survey of Northern Ireland (GSNI) core store in Belfast. Here, we can investigate a multitude of things; what is the composition of the rock and what was the depositional setting that deposited these sandstones (spoiler alert – major ancient river systems)? Is it mainly sandstone? Are there pebbles or layers of mud which could affect the ability of the rock to hold or flow water? We can also sample the core to produce thin sections for microscope work and do chemical analysis to better understand the mineralogy and history of the rocks. This is the closest we can get our hands to the rock beneath the surface which helps us understand and quantify the geothermal potential.

Photo Caption: Figure 2: Investigating the Kilroot borehole core at the GSNI corestore in Belfast. Right: An example of a mud clast within the sandstone which can reduce permeability and hinder fluid flow.

The new results from the GeoEnergy NI project boreholes will provide a valuable dataset to this PhD and help to tie between rocks exposed at Scrabo Quarry or on the shores of Strangford Lough and existing boreholes to those that have been studied. The modelling of changes in reservoir quality as part of this PhD will enable us to make better predications of the deep geothermal potential where the GeoEnergy NI project has been carrying out geophysical surveys.

Rioko Moscardini (LinkedIn Profile)
Rioko Moscardini is a PhD researcher in iCRAG’s Earth Resources and Energy Transition research areas. iCRAG is the world leading Science Foundation Ireland Research Centre in Applied Geosciences hosted by University College Dublin.