Investigating the impact of groundwater flow on multi-lateral, U-Type, advanced geothermal systems

Abstract

New innovative methods of extracting thermal energy from geothermal systems are required to ensure heat decarbonisation to meet net zero targets. This paper investigates advanced geothermal systems (also referred to as U-type deep borehole heat exchangers); more specifically, their application in settings with deep regional groundwater flow. Past work has focused on single-lateral systems, with few if any exploring the role regional groundwater flow has on multi-lateral systems. Numerical models were developed on OpenGeoSys software targeting the lateral sections which connect two vertical wellbores. Long-term simulations of 20 years were performed to understand how perpendicular, co-current or counter-current groundwater flow can impact the performance of a single or multi-lateral advanced geothermal systems.

At a depth of 2 km, with a geothermal gradient of 30°C /km, results indicate that a 1 km single lateral section could extract 110 kW more heat when groundwater flow of a Darcy Velocity of 1e-6 m/s is perpendicular to the wellbore in comparison to a purely conductive geological setting. Counter- and co-current flow only have a minor impact on the thermal performance. Similarly, for a multi-lateral system consisting of 9-laterals there was an increase in performance from 746 kW in a conductive setting to 1.2 MW when perpendicular groundwater flow is present. Overall, advanced geothermal systems with groundwater flow perpendicular to lateral sections can be beneficial to performance. It should also be noted that multi-lateral systems can extract more heat than single lateral systems; however, they are likely to be more costly.