Abstract The growing importance of subsurface carbon storage for tackling carbon emissions requires an accurate characterization of potential reservoirs to understand their capabilities. In this context, the use of legacy data originally acquired in the last fifty years for scientific projects and petroleum exploration and production activities would represent a suitable cost-effective solution and help to maximize the value of this extended national asset. Core material represents the only direct observation of subsurface deposits and must be preserved from the current disposal process related to the decommissioning of hydrocarbon fields. In this contribution, a suite of samples from core material stored at national (i.e. British Geological Survey) and local (i.e. Department of Earth Sciences, Royal Holloway, University of London) core repositories, previously characterized at the micro scale using X-ray micro-computed tomographic (μCT) imaging are discussed. Using this technique, it has been possible to investigate how pore and grain geometries control crucial features of a suitable reservoir such as porosity and permeability. The aim of this contribution is to describe the methodology behind digital image analysis (DIA) following μCT imaging applied to core material. We show how DIA can be used to provide significant measures of reservoir suitability when making initial assessments of storage reservoirs, without the need for expensive and time-consuming analyses.

This paper focuses on characterization of the heat-transfer and water-flow processes in physical models of borehole heat exchanger arrays in unsaturated soil layers. The overall goal is to develop a data set that can be used to validate the coupled thermo-hydraulic flow models needed to simulate the efficiency of heat transfer in soil-borehole thermal energy storage systems. Two bench-scale physical models consisting of a triangular array of vertical heat exchangers within a layer of unsaturated silt were constructed in insulated cylindrical tanks to evaluate the impact of different boundary conditions on the heat-transfer and water-flow processes in the silt during heat injection into the array. In one model, the heat exchangers were placed at a radial location at 26% of the tank radius, while in the other model, the heat exchangers were placed on the inside of the tank wall. During circulation of heated fluid through the heat exchangers, the changes in soil temperature and volumetric water content along the centerline of the array at different depths were measured using dielectric sensors. The thermal conductivity and specific heat capacity of the silt were also monitored using a thermal probe at the center of the silt layer at midheight. Permanent drying was observed for the soil within the array with the smaller spacing, while an increase in water content was observed in the array with a spacing equal to the container diameter. An increase in thermal conductivity of the soil was observed within the array in the case of larger spacing, while the opposite was observed in the case of smaller spacing. The results indicate the possible formation of a convective cell within the larger array as water was driven inward from the heat exchangers. These results highlight the importance of coupled heat transfer and water flow in soil-borehole thermal energy storage systems in the vadose zone.