Connectivity and Performance Evaluations for Fractured and Closed Loop Geothermal Systems

Abstract

The thermal energy stored in the Earth’s interior to depths of 10 km is vast compared to the annual global consumption of primary energy. Despite these large resource estimates, the installed worldwide geothermal production capacity is limited to 16 GW electric and 30 GW thermal. The reason for the limited capacity primarily lies in the fact that profitable high enthalpy geothermal systems also called Hydrothermal systems are geographically rare. Two technologies are being proposed to expand deep geothermal development and to make it globally accessible, namely-Enhanced Geothermal Systems (EGS) and Closed Loop Geothermal Systems (CLG). The primary difference between the two systems being the circulation of fluid takes place through hydraulically stimulated fractures in the rock for the former and the circulation of fluid takes place in a closed pipe for the latter. This thesis aims at dealing with the connectivity and thermal hydraulic performance of both these systems through analytical and numerical modeling techniques. The connectivity and thermal hydraulic performance in the context of EGS/CLG refers to the flow and thermal profile evaluation of a working fluid as it advances through the fractures in the subsurface between an injection and production well or through a closed pipe in the subsurface. A characterization technique for evaluating the hydraulic connectivity of a fractured bedrock is developed using periodic pumping tests. Traditional pumping tests performed to characterize the subsurface are often not suitable for fractured EGS systems since the diffusion length or radius of penetration quickly expands beyond the inter-well distance. Periodic pumping tests allow us to control the diffusion length by varying the period or frequency of the oscillation. Therefore, testing at multiple frequencies help probe different distances in the system. Analytical models and signal processing techniques are developed and employed to analyze the data generated from the periodic pumping tests conducted at a meso scale field site known to exhibit extreme flow channeling. The width of the channel formed was computed for various periods or frequency of oscillations and were found to decrease with period. Besides, the storage of fluid in the medium was compared with that of the storage in the monitoring wells and the former was found to be the dominating storage mechanism at the meso scale field site. Numerical Simulations are performed using a developed Finite Element Method (FEM) technique to evaluate CLG technologies. Sensitivity analysis is performed and their thermal performance is evaluated for various operating conditions. The developed models are also validated with Slender Body Theory (SBT) based approaches. In all scenarios, the results exhibit a large rapid drop in production temperature immediately after initiation of operations that levels off to a lower near steady state value. CLG is also compared with EGS using analytical techniques. In summary, it is found that CLG may be more appropriate when traditional hydrothermal systems and EGS are off the table, in situations with existing wells and for direct-use heat applications.