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GMEG - Geology, Minerals, Energy, & Geophysics Science Center

Geothermal Resource Investigations

 

 

 

 

Enhanced Geothermal Systems

Issue: Enhanced Geothermal Systems (EGS) are the focus of substantial research and development investment in the United States and abroad. The provisional evaluation of EGS included in the new geothermal resource assessment indicates that the electric power production potential from EGS is substantially larger than that from conventional geothermal resources (~500,000 MW vs ~30,000 MW), yet significant questions remain regarding the constraints on EGS development, including the roles of temperature, lithology, stress, depth,fluid composition, and induced seismicity. New and expanded USGS research studies will be directed at understanding the geologic, geomechanical and hydrologic aspects of EGS exploration and development and providing a framework for future assessments of EGS resource potential.

Objectives: Several studies in a wide range of tectonic settings have demonstrated that fractures that are optimally oriented and critically stressed for frictional failure often dominate fluid flow in low-porosity crystalline rocks. Similar observations and analyses indicate that actively slipping (i.e., shearing) fractures help maintain geothermal reservoir permeability despite crack sealing and other geochemical fluid-rock interactions that should destroy that permeability. Thus, characterization of the geometrical and hydrologic properties of natural fractures in relation to the in-situ state of stress is critical to stimulation planning and evaluation for EGS projects. Understanding the factors controlling the viability of the stimulation and development of an EGS well requires a complete characterization of borehole geology, hydrology, and stress state. Elements of this evaluation include direct measurements of 3-D stress orientations and magnitudes, the distribution, orientation and hydrologic properties of natural fractures, the locations of primary structures such as bedding, foliation and formation contacts, and rock properties from geophysical logs and testing of core. The identification of intervals for hydraulic stimulation should be on the basis of borehole condition, large-scale reservoir structure and hydrology, and a number of local geologic criteria applicable to the immediate borehole environment. These local geologic criteria can include:

  1. the interval should be below any zones of significant clay alteration because clay-filled fractures are not prone to dilation/permeability enhancement during slip;
  2. the interval should exhibit high formation temperatures;
  3. the interval should intersect highly stressed, slightly permeable natural fractures well oriented for frictional shear failure;
  4. the mechanical properties of rocks in the interval should be such that they would be susceptible to “self-propping” dilatation and permeability enhancement during fluid injection and shearing.

Objectives under this task are threefold. First, to evaluate the issues mentioned above to develop improved quantitative criteria for classifying EGS resources. Second, to collect critical data to better characterize the EGS resource distribution in the western United States. Third, to address issues of potential induced seismicity related to EGS projects, understand the physical mechanisms responsible for the induced earthquakes, and characterize the possible impact on EGS resource development.

   

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