Impact of Microannulus on the efficiency of Closed-Loop Geothermal Systems

To ensure power generation feasibility from closed-loop geothermal wells, deeper wells are required to reach higher temperature zones. However, weak bonding between cement and casing or cement and formation may allow the development of a small gap (known as microannulus), which have a negative effect on the heat extraction rate and may compromise the entire investment. Previous projects have reported that the output temperatures were significantly lower than the expected values, and the cause is believed to be cement debonding. This study aims to develop a reliable simulation model to demonstrate the impact of microannulus in closed-loop geothermal systems. Our simulation model was constructed with a commercial multiphysics finite element software. The numerical results were initially verified with analytical results. The microannulus was modeled based on results from cement evaluation logs, with the gap varying between a few micrometers to the order of millimeters. In extreme cases, the presence of microannulus was found to decrease the geothermal well power in more than 35%. The results also highlight the importance of proper cementing design to ensure wellbore integrity and avoid heat loss.