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  • Writer's pictureKristin Cooper

Innovative Methods to Control Hydraulic Properties of Enhanced Geothermal Systems

FOA Issue Date: 04/30/2021

Submission Deadline for Full Applications: 06/15/2021 5:00pm ET

Expected Date for EERE Selection Notifications: 08/31/2021

Expected Timeframe for Award Negotiations: September 2021 - January 2022

Technology Space and Strategic Goals

As identified in the GeoVision report, improving the tools, technologies, and methodologies used to explore, identify, access, create, and manage geothermal resources will reduce costs and risks associated with geothermal development. These reductions, possible via the development of EGS-enabling technologies, could increase geothermal power generation nearly 26-fold from today, representing 60 GWe of ‘always-on’, flexible electricity-generation capacity by 20503 . This would comprise 3.7% of total U.S. installed capacity and 8.5% of all U.S. electricity generation in 2050. The GeoVision analysis demonstrated that EGS resources have the potential to provide extreme growth in the electric sector and can also support significant growth within the non-electric sector for district heating and other direct-use applications.

The technology developments targeted in this FOA are intended ultimately to bring EGS technology closer to market. Strategic goals identified in the GeoVision Roadmap, outline a compilation of technical, economic, and institutional actions that the entire geothermal community including DOE, Industry, and Academia must address in order for geothermal technologies to play a larger role in the Nation’s energy supply.

Released in 2019, Frontier Observatory for Research in Geothermal Energy: A Roadmap4 (the FORGE Roadmap) outlines the key technical challenges necessary for EGS to become a reliable and reproducible energy supplier nationwide. The critical research areas from the FORGE roadmap relevant to this FOA are outlined in Figure 1. Technology-focused FOA Goals are also listed in Figure 1. These metrics track specific success indicators along the EGS technology development spectrum that are relevant to the higher level goals.

Topic Area 1 (of 1): Innovative Methods to Control Hydraulic Properties of Enhanced Geothermal Systems

Optimization of the subsurface heat exchange performance in fractured reservoirs is critical to building sustainable and economic EGS. A multitude of factors play a role in the thermal delivery of an EGS system. In low-permeability rock targeted for EGS development, fractures are the conduits for heat transfer and therefore the distribution, length, aperture, connectivity, flowing pressure, thermal conductivity, poroelastic properties of the rock mass, and fluid residence time all control the performance and sustainability of an EGS reservoir. These properties can evolve over time as well. The physical attributes of these fractures (either existing or created) in the subsurface are naturally heterogeneous and, therefore, some fractures will take more flow than others, potentially leading to heat extraction from only a small portion of the reservoir, which causes rapid thermal decline of the heat extraction fluid11 .

Thermal effects can exasperate the challenges associated with faster flowing fractures. In fractures that naturally allow greater flow, the temperature of the bounding matrix rock is depleted faster than the matrix rock surrounding lower permeability fractures leading to increased thermal contraction of the matrix rock and a graeter disparity in the distribution of flow through the reservoir. An opposing issue can occur when certain fractures support lower than optimal flow. Fractures with low hydraulic conductivity force fractures with higher conductivity to take more fluid at a given injection rate. This reinforces the tendency for accelerated thermal depletion and contraction in matrix rock bounding the higher flowing fractures, causing even larger flow rate disparities between high and low flowing fractures, which again limits the efficiency of heat extraction from the stimulated reservoir volume. The ability for EGS developers to predict changes in and adjust to the ever-evolving thermal balances within a reservoir are critical to long term EGS performance.

The focus of this FOA is the development and deployment of disruptive technologies and techniques that will affect the flow within the fracture network in the reservoir rather than controlling the flow into and out of the reservoir at the borehole. Successful projects will develop targeted and controllable in-reservoir fracture permeability modification systems / methods that yield long-term reservoir productivity improvements with the ability to reverse imparted effects.

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