A cavern thermal energy storage (CTES) in Bålsta, Sweden is investigated for its potential integration with the district heating system operated by E-On in the neighborhood. In this project, as a bachelor's thesis project work, the heat and mass transfer of the hot water charged to and discharged form the CTES considered will be analysed, using COMSOL multiphysics. The thermal behavior of the CTES and its interaction with the surrounding rock will be here investigated for a number of years, using 2D and possibly 3D simulations.