Wellbore thermal effects during underground hydrogen storage

During the operation of underground hydrogen storage (UHS), the injection and production of hydrogen will alter the temperature of the wellbore and the target storage formation. These temperature changes can potentially affect wellbore materials, and alter reservoir processes such as geochemical and microbial reactions. This study applies standard wellbore heat transport models to calculate the temperature changes induced by the flow of hydrogen and some potential cushion gases: methane, carbon dioxide and nitrogen. These models incorporate the thermal conductivity of wellbore materials, and heat exchange with the surrounding formations, and are solved numerically for a range of flow rates, wellhead temperatures and depths. Two well designs are considered, one with casing only, and the other with injection through tubing, where the annulus is filled with either air or water. An explicit analytical solution is also derived for the simplified case where fluid properties are unchanged with depth, which provides insight into the dependence of the temperature distribution on the input parameters. At a fixed depth, the wellhead temperature and the injection flow rate are found to be the primary factors influencing the bottom hole temperature. At a typical target storage depth of 1500 meters, a wellhead temperature of 15 °C, and low injection rates (e.g. 2 tonnes/day, typical of pilot-scale UHS), the hydrogen at the bottom of the well was < 5 ° C cooler than the storage formation. At higher injection rates ( > 20 tonnes/day, more suitable for commercial UHS), the hydrogen was more than 20 °C cooler than the formation. A well design with an annulus leads to larger cooling effects, especially if the annulus is filled with air. If cushion gas injection is compared for the same bottom-hole pressure difference (rather than the same volumetric rate), then the cooling effects are less than for hydrogen. These wellbore thermal calculations then give the thermal boundary conditions required for non-isothermal reservoir simulations of UHS, which are crucial for assessing feasibility. • Wellbore heat transport models are applied to hydrogen and cushion gases injection. • Hydrogen and cushion gases remain cooler than reservoir at typical storage depths. • Temperature difference between injected gas and formation is sensitive to surface parameters. • Presence of annulus in well design creates an insulating effect. • Air in annulus has higher insulating effect than water.