AQUIFER THERMAL ENERGY STORAGE IN THE FRACTURED LONDON CHALK. A THERMAL INJECTION/WITHDRAWAL TEST AND ITS INTERPRETATION

摘要

To comply with current renewable energy targets substantial new building developments in London are using the water held within the Chalk aquifer as a source for heating or cooling. Owing to the fractured structure of the Chalk there is the possibility of rapid transport occurring between the abstraction and injection boreholes at a site. This has the potential to cause thermal interference between the boreholes and a consequent loss of performance and, ultimately, eventual failure of the system. The nature of the thermal transport that occurs at a site will primarily be dependent on the fracture frequency in the aquifer. To help determine the frequency of the fractures beneath a proposed site in central London a thermal injection and abstraction test was designed and undertaken. The test was the first of its kind in the United Kingdom and its interpretation will be used to assist with the design of the proposed heating and cooling system. The test consisted of heating and storing water on site, followed by injection into a packered section of a borehole. Once the hot water was injected the pump was reversed and the water abstracted. The temperature of the packered section of the borehole was monitored with thermistors throughout the test. The results were then interpreted with the United States Geological Survey (USGS) SUTRA 3D code. The results suggest that the fracture frequency beneath the site is sufficiently high to limit the possibility of rapid thermal transport occurring between the two boreholes. The results have been used in further numerical models that predict the long term performance of the proposed system in this flow regime.