Traditional geothermal energy relies on naturally occurring reservoirs of hot water and steam, which are geographically limited to places with natural hydrothermal activity, like Yosemite, Iceland, or Japan. But what if we could create those reservoirs precisely where the energy demand is highest? That’s the idea behind Enhanced Geothermal Systems or EGS. In these enhanced systems, deep underground hot, dry rock formations are fractured, and fluids are circulated to extract the heat. This method opens up geothermal potential in a much broader geographical range, moving well beyond areas with natural hydrothermal activity. This chapter focuses on exactly how EGS works and the role geosynthetics play.
Emerging Technology
Enhanced Geothermal Systems (EGS) represent a significant leap forward in geothermal energy technology. Hot, dry rock formations have historically been unsuitable for geothermal power since the plants rely on steam for energy production. EGS unlocks access to vast, previously untapped geothermal potential by creating artificial reservoirs to replace the fluids found in hydrothermal formations. This expansion will help respond to the growing need for consistent renewable baseload energy.
What makes EGS truly ‘emerging’ is the ongoing development and refinement of the technologies involved. Drilling techniques, reservoir modeling, and fluid injection strategies are constantly evolving to improve the efficiency of EGS projects.
How EGS Works
The premise of Enhanced Geothermal Systems is simple: create a geothermal reservoir where none naturally exists. The design mimics processes found in traditional hydrothermal systems in hot, dry rock formations.
Traditional EGS begins with drilling deep wells into a suitable geological formation, often granite or other crystalline rock. These formations are usually hotter than typical hydrothermal systems, but the rock is too dense for fluid to circulate. To create openings, fluids are injected into a pair of wells, a process known as hydraulic fracturing. The high-pressure fluid cracks the rock and creates a network of interconnected fractures.
Once a fracture network is established, water (ideally the same fluid used in the stimulation step) is pumped into the injection well, where it circulates through the fractures and is heated by the hot rock. The heated water is extracted through an extraction well and used to generate electricity in a power plant, similar to hydrothermal systems.
Since natural reservoirs are not present in hot, dry rock formations, a closed-loop fluid cycle is used in EGS. Once heat has been extracted from circulating fluid, it’s continuously reinjected, minimizing water consumption. Closed-loop systems are a defining characteristic of EGS.
An exciting recent advancement in EGS technology is the patented Eavor-Loop system. It differs significantly from traditional EGS in that it uses advanced drilling techniques to avoid hydraulic fracturing altogether. In this system, two vertical wells are drilled and connected by multiple horizontal, multilateral wellbores. The network of horizontal sections creates a large, wholly contained subsurface heat exchanger so that the working fluid circulates exclusively within this well network, extracting heat without any exchange of fluids with the surrounding formation. This approach addresses concerns associated with hydraulic fracturing, such as induced seismicity and the possibility of damage to the reservoir.
Geosynthetics in Enhanced Geothermal Systems
While the subsurface operations of EGS draw the most attention, surface infrastructure is just as important for successful operations, and this is where geomembranes play a starring role.
Surface Reservoir Lining
In traditional EGS, surface reservoirs are frequently used for working fluid management. Since fluid loss and environmental contamination are significant concerns, the reservoir liners must withstand high temperatures and potentially corrosive fluids.
Solid Waste Management
Like other industrial operations, EGS generates solid waste, and proper management is essential to prevent contamination. Waste like drilling cuttings, spent fluids, additives, and scale accumulates during operations. Because these materials often contain heavy metals and residues, they must be contained to protect both soil and groundwater. Lined containment areas feature durable geomembrane liners to provide a secure barrier between the waste and adjacent soil. Integrated leachate collection systems are also installed to capture precipitation or other liquids percolating through the waste, directing them for proper treatment and disposal.
Secondary Containment and Fluid Management
It’s common to see long pipelines running above the ground at geothermal power plants, transporting fluids between wells and storage facilities. While these pipelines and other infrastructure are designed to be leakproof, issues like corrosion, mechanical damage, and land subsidence are always a risk, so geomembrane-lined trenches are often added to provide backup containment in case of a release.
Geomembranes also provide vital secondary containment around wellheads, storage tanks, and other fluid-handling equipment by lining containment structures and berms. These forms of ‘backup’ containment capture leaked fluids and prevent them from reaching environmentally sensitive areas, particularly near water sources.
Geomembranes are also used in fluid management facilities such as storage tanks for working fluids, treatment and conditioning ponds, and emergency overflow ponds.
Looking Ahead
At this point, it shouldn’t be surprising that geosynthetic liners provide even more essential services in geothermal projects. The next chapter explores their role in environmental remediation and soil stabilization.
