Multi-scale Investigation of a Triassic Sherwood Sandstone Aquifer and Its Impact on Aquifer Thermal Energy Storage

The transition of energy supply from mainly fossil sources to low-carbon energy sources is essential for future environmental sustainability. The current worldwide energy demand for heating and cooling in buildings accounts for 40% of the primary energy consumption. The use of aquifer thermal energy storage (ATES) is predicted to play an increasing role as a regenerative energy source. The Triassic Sherwood Sandstone aquifer is the second most important aquifer in England and Wales, providing about 25% of groundwater resources. Hence, understanding its aquifer characteristics is important for assessing the suitability of its use for ATES.

This study utilizes samples of the Permo-Triassic Sherwood Sandstone from the Abbey Arms Wood borehole in Delamere, Cheshire, UK. This borehole was drilled to approximately 150m and penetrates the Helsby and Wilmslow Formation. The samples are selected from 5 major facies (coarse- and fine-grained fluvial channel fill sandstones, sandy sabkha, sand sheet and dune aeolian sandstones, and massive sandstone facies).

 XRD data show that samples are mainly composed of quartz, feldspar, and muscovite, with minor calcite and hematite, with some accessory phases. Quartz contents are between 58.4%- 88.6%, with an average of 76.9%. Feldspar contents are between 6.0%-23.9%, 16.0% in average. Muscovite contents are in the range of 3.3%-12.5%, the mean value is 5.6%. Thin section observation shows that selected Sherwood sandstone samples are characterized by high quartz and feldspar content with few lithic fragments, and hematite coatings are well-developed around mineral grains. The dominant sandstone types are arkose (37.5%) and lithic arkose (18.8%).

The porosity of the samples is between 19.2%-26.2%, with the majority being more than 20%. The permeability is between 3.6*10^-14m and 1.2*10^-11m, mean value is 2.2*10^-12m. At room temperature, the testing of 14 samples of the Sherwood sandstone group found that the average intrinsic thermal conductivity is 1.4 w/mK, thermal diffusivity is 1.2 mm²/s and heat capacity is 0.7 J/gK. The physical properties of each lithofacies show subtle systematic differences, with the fluvial sandstone facies and massive sandstone facies having a relatively higher permeability and thermal conductivity. However, significant heterogeneity is not present, suggesting that this aquifer is a good target for ATES.