In enhanced geothermal systems (EGS) the reservoir permeability is often enhanced or created using hydraulic fracturing. In hydraulic fracturing, high fluid pressures are applied to confined zones in the subsurface usually using packers to fracture the host rock. This enhances rock permeability and therefore conductive heat transfer to the circulating geothermal fluid (e.g. water or supercritical carbon dioxide). The ultimate goal is to increase or improve the thermal energy production from the subsurface by either optimal designs of injection and production wells or by altering the fracture permeability to create different zones of circulation that can be exploited in geothermal heat extraction. Moreover, hydraulic fracturing can lead to the creation of undesirable short-circuits or fast flow-paths between the injection and extraction wells leading to a short thermal residence time, low heat recovery, and thus a short-life of the EGS. A potential remedy to these problems is to deploy a cementing (blocking, diverting) agent to minimize short-cuts and/or create new circulation cells for heat extraction. A potential diverting agent is the colloidal silica by-product that can be co-produced from geothermal fluids. Silica gels are abundant in various surface and subsurface applications, yet they have not been evaluated for EGS applications. In this study we are investigating the benefits of silica gel deployment on thermal response of an EGS, either by blocking short-circuiting undesirable pathways as a result of diverting the geofluid to other fractures; or creating, within fractures, new circulation cells for harvesting heat through newly active surface area contact. A significant advantage of colloidal silica is that it can be co-produced from geothermal fluids using an inexpensive membrane-based separation technology that was developed previously using DOE-GTP funding. This co-produced silica has properties that potentially make it useful as a fluid diversion agent for subsurface applications. Colloidal silica solutions exist as low-viscosity fluids during their "induction period" but then undergo a rapid increase in viscosity (gelation) to form a solid gel. The length of the induction period can be manipulated by varying the properties of the solution, such as silica concentration and colloid size. We believe it is possible to produce colloidal silica gels suitable for use as diverting agents for blocking undesirable fast-paths which result in short-circuiting the EGS once hydraulic fracturing has been deployed. In addition, the gels could be used in conventional geothermal fields to increase overall energy recovery by modifying flow.

Stratigraphic reservoirs with high permeability and temperature at economically accessible depths are attractive for power generation because of their large areal extent (> 100 km2) compared to the fault controlled hydrothermal reservoirs (< 10 km2) found throughout much of the western US. A preliminary screening of the geothermal power potential of sedimentary basins in the U.S. assuming present day drilling costs, a levelized cost of electricity over 30 years of $10/Wh, and realistic reservoir permeabilities, indicates that basins with heat flows of more than about 80 mW/m2, reservoir temperatures of more than 175 degrees C, and a reservoir depth of less than 4 km are required. This puts the focus for future geothermal power generation on high heat flow regions of California (e.g. the Imperial Valley and regions adjacent to The Geysers), the Rio Grande rift system of New Mexico and Colorado (especially the Denver Basin), the Great Basin of the western U.S., and high heat flow parts of Hawaii and the Alaska volcanic arc. This submission includes a Stage Gate Report on "Novel Geothermal Development of Deep Sedimentary Systems in the United States" in addition to the following resources compiled into a single PDF: Fluid-Mineral and Reactional Path Calculations (Simmons, S.F. 2012) Summary of Coupled Fluid Geochemistry with Depth Analyses in the Great Basin and Adjoining Regions (Kirby, S.M. 2012) Summary of Compiled Permeability with Depth Measurements for Basin Fill, Igneous, Carbonate, and Siliciclastic Rocks in the Great Basin and Adjoining Regions (Kirby, S.M. 2012) Review of Permeability Characteristics in Drilled, Sediment-Hosted, Geothermal Systems (Anderson, T.C. 2012) Structural Geology of the Eastern Basin and Range; Structural Cross Sections Across Western Utah and Northeastern Nevada (Schelling, D.D. 2012) Stratigraphic Reservoirs in the Great Basin-The Bridge to Development of Enhanced Geothermal Systems in the U.S. (Allis et al. 2012) Presentation: Stratigraphic Reservoirs in the Great Basin-the Bridge to Development of Enhanced Geothermal Systems in the U.S. (Allis et al. 2012) Presentation: Novel Geothermal Development of Deep Sedimentary Systems in the United States (Moore, J. and R. Allis, 2012) The Potential for Basin-Centered Geothermal Resources in the Great Basin (Allis et al. 2011) Presentation: The Potential for Basin-Centered Geothermal Resources in the Great Basin (Allis et al. 2011) Geothermal Resources in Southwestern Utah: Gravity and Magnetotelluric Investigations (Hardwick, C. 2012) Geophysical Delineation of the Crater Bench, Utah, Geothermal System (Hardwick C.L. and D.S. Chapman, 2011) Geothermal Resources in the Black Rock Desert, Utah: MT and Gravity Surveys (Hardwick, C.L and D.S. Chapman, 2012) Simulation of Heat Exchange Processes and Thermal Evolution of Deep Sedimentary Resevoirs (2012) Performance of Air-Cooled Binary Power Plants: An Analysis using Pacificorp's Blundell plant near Milford, Utah (Allis, R. and G. Larsen, 2012) Chapter 4: Reservoir Implications of CO2 in Produced Fluids and as Co-Injected Fluid (2012) Developing Geothermal Resources beneath Hot Basins (stratigraphic reservoirs) Economic Constraints - draft notes for report (Spencer, T. and R. Allis 2012) Using Hydrogeologic Data to Evaluate Geothermal Potential in the Eastern Great Basin, Western U.S. (Heilweil et al. 2012) Subsidence in Sedimentary Basins due to Groundwater Withdrawal for Geothermal Energy Development (Lowe, M. 2012) Induced Seismicity [associated with deep sedimentary basin EGS development] (McPherson, B. 2012)

Stratigraphic reservoirs with high permeability and temperature at economically accessible depths are attractive for power generation because of their large areal extent (> 100 km2) compared to the fault controlled hydrothermal reservoirs (< 10 km2) found throughout much of the western US. A preliminary screening of the geothermal power potential of sedimentary basins in the U.S. assuming present day drilling costs, a levelized cost of electricity over 30 years of $10/Wh, and realistic reservoir permeabilities, indicates that basins with heat flows of more than about 80 mW/m2, reservoir temperatures of more than 175 degrees C, and a reservoir depth of less than 4 km are required. This puts the focus for future geothermal power generation on high heat flow regions of California (e.g. the Imperial Valley and regions adjacent to The Geysers), the Rio Grande rift system of New Mexico and Colorado (especially the Denver Basin), the Great Basin of the western U.S., and high heat flow parts of Hawaii and the Alaska volcanic arc. This submission includes a Stage Gate Report on "Novel Geothermal Development of Deep Sedimentary Systems in the United States" in addition to the following resources compiled into a single PDF: Fluid-Mineral and Reactional Path Calculations (Simmons, S.F. 2012) Summary of Coupled Fluid Geochemistry with Depth Analyses in the Great Basin and Adjoining Regions (Kirby, S.M. 2012) Summary of Compiled Permeability with Depth Measurements for Basin Fill, Igneous, Carbonate, and Siliciclastic Rocks in the Great Basin and Adjoining Regions (Kirby, S.M. 2012) Review of Permeability Characteristics in Drilled, Sediment-Hosted, Geothermal Systems (Anderson, T.C. 2012) Structural Geology of the Eastern Basin and Range; Structural Cross Sections Across Western Utah and Northeastern Nevada (Schelling, D.D. 2012) Stratigraphic Reservoirs in the Great Basin-The Bridge to Development of Enhanced Geothermal Systems in the U.S. (Allis et al. 2012) Presentation: Stratigraphic Reservoirs in the Great Basin-the Bridge to Development of Enhanced Geothermal Systems in the U.S. (Allis et al. 2012) Presentation: Novel Geothermal Development of Deep Sedimentary Systems in the United States (Moore, J. and R. Allis, 2012) The Potential for Basin-Centered Geothermal Resources in the Great Basin (Allis et al. 2011) Presentation: The Potential for Basin-Centered Geothermal Resources in the Great Basin (Allis et al. 2011) Geothermal Resources in Southwestern Utah: Gravity and Magnetotelluric Investigations (Hardwick, C. 2012) Geophysical Delineation of the Crater Bench, Utah, Geothermal System (Hardwick C.L. and D.S. Chapman, 2011) Geothermal Resources in the Black Rock Desert, Utah: MT and Gravity Surveys (Hardwick, C.L and D.S. Chapman, 2012) Simulation of Heat Exchange Processes and Thermal Evolution of Deep Sedimentary Resevoirs (2012) Performance of Air-Cooled Binary Power Plants: An Analysis using Pacificorp's Blundell plant near Milford, Utah (Allis, R. and G. Larsen, 2012) Chapter 4: Reservoir Implications of CO2 in Produced Fluids and as Co-Injected Fluid (2012) Developing Geothermal Resources beneath Hot Basins (stratigraphic reservoirs) Economic Constraints - draft notes for report (Spencer, T. and R. Allis 2012) Using Hydrogeologic Data to Evaluate Geothermal Potential in the Eastern Great Basin, Western U.S. (Heilweil et al. 2012) Subsidence in Sedimentary Basins due to Groundwater Withdrawal for Geothermal Energy Development (Lowe, M. 2012) Induced Seismicity [associated with deep sedimentary basin EGS development] (McPherson, B. 2012)

Hydraulic fracturing is currently the primary method for stimulating low-permeability geothermal reservoirs and creating Enhanced (or Engineered) Geothermal Systems (EGS) with improved permeability and heat production efficiency. Complex natural fracture systems usually exist in the formations to be stimulated and it is therefore critical to understand the interactions between existing fractures and newly created fractures before optimal stimulation strategies can be developed. Our study aims to improve the understanding of EGS stimulation-response relationships by developing and applying computer-based models that can effectively reflect the key mechanisms governing interactions between complex existing fracture networks and newly created hydraulic fractures. In this paper, we first briefly describe the key modules of our methodology, namely a geomechanics solver, a discrete fracture flow solver, a rock joint response model, an adaptive remeshing module, and most importantly their effective coupling. After verifying the numerical model against classical closed-form solutions, we investigate responses of reservoirs with different preexisting natural fractures to a variety of stimulation strategies. The factors investigated include: the in situ stress states (orientation of the principal stresses and the degree of stress anisotropy), pumping pressure, and stimulation sequences of multiple wells.

Hydraulic fracturing is currently the primary method for stimulating low-permeability geothermal reservoirs and creating Enhanced (or Engineered) Geothermal Systems (EGS) with improved permeability and heat production efficiency. Complex natural fracture systems usually exist in the formations to be stimulated and it is therefore critical to understand the interactions between existing fractures and newly created fractures before optimal stimulation strategies can be developed. Our study aims to improve the understanding of EGS stimulation-response relationships by developing and applying computer-based models that can effectively reflect the key mechanisms governing interactions between complex existing fracture networks and newly created hydraulic fractures. In this paper, we first briefly describe the key modules of our methodology, namely a geomechanics solver, a discrete fracture flow solver, a rock joint response model, an adaptive remeshing module, and most importantly their effective coupling. After verifying the numerical model against classical closed-form solutions, we investigate responses of reservoirs with different preexisting natural fractures to a variety of stimulation strategies. The factors investigated include: the in situ stress states (orientation of the principal stresses and the degree of stress anisotropy), pumping pressure, and stimulation sequences of multiple wells.

Lab-scale stimulation work on non-porous fused silica (similar mechanical properties to igneous rock) was performed using pure water, pure CO2 and water/CO2 mixtures to compare back to back fracturing performance of these fluids with PNNL's StimuFrac.

Lab-scale stimulation work on non-porous fused silica (similar mechanical properties to igneous rock) was performed using pure water, pure CO2 and water/CO2 mixtures to compare back to back fracturing performance of these fluids with PNNL's StimuFrac.

Low permeability geothermal reservoirs can be stimulated by hydraulic fracturing to create Enhanced (or Engineered) Geothermal Systems (EGS) with higher permeability and improved heat transfer to increase heat production. In this paper, we document our effort to develop a numerical simulator with explicit geomechanics-discrete flow network coupling by utilizing and further advancing the simulation capabilities of the Livermore Distinct Element Code (LDEC). The important modules of the simulator include an explicit finite element solid solver, a finite volume method flow solver, a joint model using the combined FEM-DEM capability of LDEC, and an adaptive remeshing module. The numerical implementation is verified against the classical KGD model. The interaction between two fractures with simple geometry and the stimulation of a relatively complex existing fracture network under different in-situ stress conditions are studied with the simulator.

Low permeability geothermal reservoirs can be stimulated by hydraulic fracturing to create Enhanced (or Engineered) Geothermal Systems (EGS) with higher permeability and improved heat transfer to increase heat production. In this paper, we document our effort to develop a numerical simulator with explicit geomechanics-discrete flow network coupling by utilizing and further advancing the simulation capabilities of the Livermore Distinct Element Code (LDEC). The important modules of the simulator include an explicit finite element solid solver, a finite volume method flow solver, a joint model using the combined FEM-DEM capability of LDEC, and an adaptive remeshing module. The numerical implementation is verified against the classical KGD model. The interaction between two fractures with simple geometry and the stimulation of a relatively complex existing fracture network under different in-situ stress conditions are studied with the simulator.

A corrigendum was submitted to the journal of Geothermics on our article "Environmentally friendly, rheoreversible, hydraulic-fracturing fluids for enhanced geothermal systems" Shao et al Geothermics 58 (2015) 22-31. In the original article some permeability values were underestimated, in particular, for rock samples fractured by the stimuli-responsive fracking fluid (PAA-CO2). In addition, effective pressures were determined to be lower for three control experiments (deionized water-carbon dioxide, DIW-CO2). Therefore, we revised values of permeability and effective pressure as well as performed additional lab-scale stimulation experiments under identical conditions to further verify/update the deductions presented in the discussion section. This is the reason for the additional data introduced in the below Table 1 (grey color). The authors regret the following inadvertent errors and corresponding modifications. These modifications do not change the scientific conclusions of the article.