Every injection and production operation are accompanied by heat transfer between the wellbore fluids and the formation. Often these fluids are only circulated inside the wellbore. However, the presence of microannulus, besides compromising wellbore integrity, could have a negative impact on the rate of heat transfer to and from the formation. Thermal conductivity could be critical in CO2 sequestration, thermal EOR and specially closed-loop geothermal wells. This study aims to evaluate the impact of microannulus on the heat exchange rate at the bottomhole by combining numerical results and field measurements. We propose to identify presence of microannulus by analyzing distributed temperature sensing (DTS) measurements acquired at different times from EOR and closed-loop geothermal wells. In a DTS system, temperatures are recorded continuously along an optical sensor cable placed in the wellbore. The analysis is combined with numerical simulations considering different operational conditions to estimate the severity of the microannulus. In extreme cases, the presence of microannulus was found to decrease the bottomhole temperature in 2.5%. The results also highlight the importance of proper cementing design to ensure wellbore integrity and avoid heat loss.