Thermal response tests are the most popular in-situ technique to assess ground thermal conductivity and borehole thermal resistance. Conventional tests with a constant heat injection rate do not allow to infer the ground heat capacity. Oscillatory tests are proposed as a tool to evaluate subsurface thermal diffusivity and estimate the heat capacity. Numerical simulation results showed that the frequency of heat injection and the radius of the U-pipe have a significant influence on the temperature response of such tests. The grout’s thermal properties and the borehole heat storage effect are main issues to face in order to evaluate the ground heat capacity. Oscillation periods longer than 12 h can however help to have proper penetration depth and limit the borehole influence.