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.

In many hydrothermal systems, fracture permeability along faults provides pathways for groundwater to transport heat from depth. Faulting generates a range of deformation styles that cross-cut heterogeneous geology, resulting in complex patterns of permeability, porosity, and hydraulic conductivity. Vertical connectivity (a through going network of permeable areas that allows advection of heat from depth to the shallow subsurface) is rare and is confined to relatively small volumes that have highly variable spatial distribution. This local compartmentalization of connectivity represents a significant challenge to understanding hydrothermal circulation and for exploring, developing, and managing hydrothermal resources. Here, we present an evaluation of the geologic characteristics that control this compartmentalization in hydrothermal systems through 3-D analysis of the Brady geothermal field in western Nevada. A published 3-D geologic map of the Brady area is used as a basis to develop structural and geological variables that are hypothesized to control or effect permeability or connectivity. The 3-D distribution of these variables is compared to the distribution of productive and non-productive fluid flow intervals along production wells and non-productive wells via principal component analysis (PCA). This comparison elucidates which geologic and structural variables are most closely associated with productive fluid flow intervals. Results indicate that production intervals at Brady are located: (1) within or near to known and stress-loaded macro-scale faults, and (2) in areas of high fault and fracture density. This submission includes the published journal article detailing this work, the published 3-D geologic map of the Brady Geothermal Area used as a basis to develop structural and geological variables that are hypothesized to control or effect permeability or connectivity, 3-D well data, along which geologic data were sampled for PCA analyses, and associated metadata file. This work was done using existing R programs.

In many hydrothermal systems, fracture permeability along faults provides pathways for groundwater to transport heat from depth. Faulting generates a range of deformation styles that cross-cut heterogeneous geology, resulting in complex patterns of permeability, porosity, and hydraulic conductivity. Vertical connectivity (a through going network of permeable areas that allows advection of heat from depth to the shallow subsurface) is rare and is confined to relatively small volumes that have highly variable spatial distribution. This local compartmentalization of connectivity represents a significant challenge to understanding hydrothermal circulation and for exploring, developing, and managing hydrothermal resources. Here, we present an evaluation of the geologic characteristics that control this compartmentalization in hydrothermal systems through 3-D analysis of the Brady geothermal field in western Nevada. A published 3-D geologic map of the Brady area is used as a basis to develop structural and geological variables that are hypothesized to control or effect permeability or connectivity. The 3-D distribution of these variables is compared to the distribution of productive and non-productive fluid flow intervals along production wells and non-productive wells via principal component analysis (PCA). This comparison elucidates which geologic and structural variables are most closely associated with productive fluid flow intervals. Results indicate that production intervals at Brady are located: (1) within or near to known and stress-loaded macro-scale faults, and (2) in areas of high fault and fracture density. This submission includes the published journal article detailing this work, the published 3-D geologic map of the Brady Geothermal Area used as a basis to develop structural and geological variables that are hypothesized to control or effect permeability or connectivity, 3-D well data, along which geologic data were sampled for PCA analyses, and associated metadata file. This work was done using existing R programs.

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.

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)

The PoroTomo team has completed inverse modeling of the three data sets (seismology, geodesy, and hydrology) individually, as described previously. The estimated values of the material properties are registered on a three-dimensional grid with a spacing of 25 meters between nodes. The material properties are listed an Excel file. Figures show planar slices in three sets: horizontal slices in a planes normal to the vertical Z axis (Z normal), vertical slices in planes perpendicular to the dominant strike of the fault system (X normal), and vertical slices in planes parallel to the dominant strike of the fault system (Y normal). The results agree on the following points. The material is unconsolidated and/or fractured, especially in the shallow layers. The structural trends follow the fault system in strike and dip. The geodetic measurements favor the hypothesis of thermal contraction. Temporal changes in pressure, subsidence rate, and seismic amplitude are associated with changes in pumping rates during the four stages of the deployment in 2016. The modeled hydraulic conductivity is high in fault damage zones. All the observations are consistent with the conceptual model: highly permeable conduits along faults channel fluids from shallow aquifers to the deep geothermal reservoir tapped by the production wells.

The PoroTomo team has completed inverse modeling of the three data sets (seismology, geodesy, and hydrology) individually, as described previously. The estimated values of the material properties are registered on a three-dimensional grid with a spacing of 25 meters between nodes. The material properties are listed an Excel file. Figures show planar slices in three sets: horizontal slices in a planes normal to the vertical Z axis (Z normal), vertical slices in planes perpendicular to the dominant strike of the fault system (X normal), and vertical slices in planes parallel to the dominant strike of the fault system (Y normal). The results agree on the following points. The material is unconsolidated and/or fractured, especially in the shallow layers. The structural trends follow the fault system in strike and dip. The geodetic measurements favor the hypothesis of thermal contraction. Temporal changes in pressure, subsidence rate, and seismic amplitude are associated with changes in pumping rates during the four stages of the deployment in 2016. The modeled hydraulic conductivity is high in fault damage zones. All the observations are consistent with the conceptual model: highly permeable conduits along faults channel fluids from shallow aquifers to the deep geothermal reservoir tapped by the production wells.

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.