A review of geothermal reservoir properties of Triassic, Permian and Carboniferous sandstones in Northern Ireland

Raine, R.; Reay, D.M.. 2019 A review of geothermal reservoir properties of Triassic, Permian and Carboniferous sandstones in Northern Ireland. Belfast, UK, Geological Survey of Northern Ireland, 59pp. (INTERNAL REPORT 19/EM/01) (Unpublished)

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This report presents the results of a review of the available porosity and permeability data for potential deep geothermal sandstone reservoirs in Northern Ireland. It is based on information from published and unpublished sources and data collected as part of a GSNI reservoir characterisation project involving a detailed petrography study borehole core and sample material by one of the authors (Dr R Raine). The geology of the north-eastern part of Northern Ireland is characterised by a number of deep sedimentary basins largely concealed below the extensive outcrop of Palaeogene basalts covering most of County Antrim and parts of counties Londonderry, Tyrone, Armagh and Down. The Larne, Lough Neagh, Rathlin and Foyle basins contain thick sequences of sedimentary rocks of Carboniferous to Cretaceous age, including Carboniferous, Permian and Triassic sandstones which are potential hydrocarbon and/or hydrogeothermal reservoirs. Sandstones of similar age and types are being used to produce heat in geothermal energy systems in the Netherlands, Denmark, Germany and Great Britain. There is the potential to develop similar geothermal energy systems in Northern Ireland. Geothermal energy systems can play a role in the decarbonisation of heat and are characterised by their high deliverability and availability approaching 24/7, 365 days a year. Deep geothermal energy systems involve relatively high initial capex associated with the drilling of deep boreholes so it is important to be able to predict accurately the nature and performance of the target geothermal aquifers. In particular, we are interested in the thickness, depth of these sandstones and the temperature of the formation fluids they contain, how much fluid is stored within them and how easily it can move through the rocks. This report focuses on the porosity and permeability of the reservoir rocks, i.e. their storage capacity and fluid flow characteristics. Surface outcrops of these sandstones are very limited in extent so that most of the information about them has been derived from rock samples collected from deep boreholes. This report draws together data included from previous published and unpublished data together with the results from recent laboratory analyses and petrographic studies carried out on core and cuttings samples by GSNI geologists or by external contractors on behalf of GSNI. Overall the results reinforce earlier observations that the Permo-Triassic sandstones are potentially productive geothermal aquifers, albeit with significant variations in porosity and permeability both within individual borehole sequences and between boreholes from the same and different sedimentary basins. The Sherwood Sandstone Group (SSG) is a prolific aquifer at shallow depths in the Lagan Valley and Newtownards Trough and, although reservoir quality generally decreases with depth, the available data indicate that some intervals within the upper part of the SSG could be productive geothermal aquifers at depths below 1500m. When the results from pumping tests in water production boreholes are compared to laboratory derived permeabilities this suggests that fracture flow is important in the SSG. There is very limited information from the Carboniferous sandstones – mostly from outcrops, shallow boreholes and moderate depths – but some of the samples exhibit surprisingly good reservoir properties, even below 1000 m depth. However, the petrographic analysis suggests that the porosity and permeability might be expected to reduce significantly at greater depths. These are the oldest reservoirs considered and, as a result, a better understanding of the distribution of sandstones with the greatest potential for the preservation of primary porosity or creation of secondary porosity would be helpful. The Permian outcrop in Northern Ireland is very limited in extent and most of the available information about Permian sandstones has been obtained from core samples in a relatively small number of boreholes, both shallow and deep. Permian sandstones exhibit a wide variation in sedimentary characteristics and consequently in reservoir quality, although some borehole samples yield good porosity and permeability values. vi Available temperature data from deep boreholes in Northern Ireland yield geothermal gradients of about 34oC/km, 32oC/km and 28oC/km for the Rathlin, Lough Neagh and Larne sedimentary basins, respectively, significantly higher than the average (26oC/km) for Upper Palaeozoic and Mesozoic sedimentary basins in Great Britain. The geothermal gradients for the Lough Neagh and Larne basins may be underestimates because the temperatures may not have reached thermal equilibrium with the formation fluids at the time of measurement. As a result formation fluids are likely to be close to 80oC at depths of 2000 metres, temperatures suitable for large scale direct heating uses. A temperature of 97.8oC was recorded from a depth of 2565 m in the Ballinlea No. 1 well in the Rathlin Basin. Gravity modelling has been used with seismic survey and borehole data to predict the depths of the potential geothermal aquifers in the deepest parts of the sedimentary basins. Although gravity modelling cannot give unique or definitive depth solutions, they do indicate that the Sherwood Sandstone should lie below 2000 metres depth beneath the northeast corner of Lough Neagh and below 1500 metres in the Rathlin Basin near Ballymoney and Ballycastle, with the Permian and Carboniferous sandstones at still greater depths. Magnetotelluric (MT) surveys are a relatively low-cost geophysical method for determining the subsurface distribution of rocks with low electrical resistivity and are routinely used in exploration for deep geothermal energy resources. In Northern Ireland MT surveys have been carried out in the Rathlin Basin and the northern part of the Lough Neagh Basin that show low resistivity rocks (potential geothermal aquifers) extending down below 2000 metres depth, thus increasing confidence in the results of the gravity modelling. The results from this review of reservoir quality, taken together with the information on temperatures, geothermal gradients and likely depths of potential geothermal sandstone aquifers, show that there are several areas in Northern Ireland where deep geothermal energy resources may be suitable for direct heating use. Heat demand mapping will indicate where in Northern Ireland geothermal energy could provide the energy source for heat networks.

Item Type: Publication - Report
Funders/Sponsors: Department for the Economy, Geological Survey of Northern Ireland, British Geological Survey
Additional Information. Not used in RCUK Gateway to Research.: This item has been internally reviewed, but not externally peer-reviewed.
NORA Subject Terms: Earth Sciences
Date made live: 20 Mar 2020 14:12 +0 (UTC)

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