Earth Model

As part of the Phase 2 activities, the Utah FORGE team developed a comprehensive earth model of the Milford Site and surrounding area using Leapfrog Geothermal. The interactive viewer and links below provide an overview of the earth model and access to the individual datasets (version February 15th, 2020). The earth model will be updated as existing data are analyzed and new data are acquired.

Click here to see model in full-page view.

Earth modeling data are in SI units and in UTM, Zone 12N, NAD83, NAVD88 referenced coordinates.

FORGE Site data are available on the Geothermal Data Repository. For questions contact us at utahforge@utah.edu.

 

Numerical Modeling

A numerical implementation of the conceptual and earth models was developed of the FORGE reservoir and surrounding area to estimate the spatial distribution of native state pressure, temperature, and stress conditions. The primary goal of this effort was threefold.

1. Incorporate detailed 3D parameter distributions and complex boundary conditions identified from characterizing the site.
2. Better understand the spatial distribution of stress at the site and how it may influence reservoir stimulation.
3. Establish a reference (or baseline) set of parameter and property distributions that can be used among the team (and modeling community at large) to ensure consistency and comparability of simulation results.

The data and results included here are from the reference models completed in November 2019, at the approximate end of Phase 2 of FORGE. A complete report on the Phase 2 Modeling and Simulation can be found here.

Model Location and Dimensions

	The Phase 3 numerical model domain sized to enclose a volume of the reservoir intersected by Wells 56-32, 58-32, 16A, 78B-32, and 78-32 and a significant subsurface volume below the FORGE site footprint. The model domain of 4.0 km x 4.0 km x 4.2 km is located approximately between depths of 4000 to 4200 meters below land surface. A nonuniform mesh spacing of average 40 m was used, with a total of 0.24 million tetrahedron elements. Finite element mesh of the model can be downloaded here. The image to the left shows the model geometries, the upper sedimentary layer (red), the lower granitoid layer mesh. This image also presents the global coordinate system where Z-axis is along vertical direction and X-axis is along the minimum horizontal direction.
	The Phase 2 numerical model domain sized to enclose a volume of the reservoir intersected by Well 58-32 and a significant subsurface volume below the FORGE site footprint. The model domain of 2.5 km x 2.5 km x 2.75 km is located approximately between depths of 400 to 3200 meters below land surface and aligned with the principle stress direction. A uniform mesh spacing of 50 m was used, with a total of 137,500 grid cells. Meshes were formulated in both UTM and a local coordinate systems, and can be downloaded here. The image to the left shows the boundaries of model domain (gray box), the Utah FORGE site outline projected into the subsurface (red), and the top of the granitoid surface colored by temperature.
	The lithology at the site was divided into two broadly defined units, consisting of crystalline granitic basement rock (granitoid) and the overlying sedimentary basin fill. Vertices of points defining the contact surface can be downloaded here. The contact surface interpolated onto the numerical model mesh can be downloaded here. The image to the left shows the land surface (gray), the Utah FORGE site outline projected into the subsurface (red), and the top of the granitoid surface (green).

	Boundary conditions for pressure, temperature, and stress were based on results obtained and compiled during Phase 2, which relied on both new data collection and information obtained from the literature. Due to the complex nature of the distributions of pressure, temperature, and stress, all boundary conditions in the native state model have varying degrees of spatial variability, which were implemented using Dirichlet conditions. Boundary conditions can be downloaded here.
	Initial pore pressure and temperature conditions used for the native state model were based on estimates in the earth model and assigned to the numerical model cells in a similar fashion used to assign the boundary conditions. Values for pore pressure were estimated based on the top boundary condition and interpolated downward as a function of depth. Temperate was directly interpolated from the earth model to the numerical model cells. Initial pore pressure and temperature conditions can be downloaded here. The image to the left shows the temperature applied to top of the model domain in degrees Celsius, in the local coordinate system implementation of the native state model. The location of Well 58-32 is shown for reference.
	Initial stress conditions used for the native state model are based on calculations of the displacement of the rock matrix, and as such, are difficult to assign a priori. A value of zero displacement is assigned as an initial condition, letting the model iterate a few extra times to come to a converged solution. Initial stress conditions can be downloaded here. The image to the left shows the total pressure applied to top of the model domain in megapascals, in the local coordinate system implementation of the native state model. The location of Well 58-32 is shown for reference.
	A representative population of natural fractures are included in the reference Discrete Fracture Network Model (DFN), which incorporates measured surface and well data to create planer fractures that communicate as a single hydrological and mechanical system. The reference DFN consists of a deterministic set of fractures intersecting Well 58-32 where fracture locations and orientations are known, plus a stochastic set of fractures away from well control. The image to the left shows the orientations of the fractures in the reference DFN in an upper hemisphere stereonet (plotting fracture poles). The table below provides a summary of the three fracture sets identified.

Fracture Set Parameters

		EW Vertical	NS Inclined Dipping West	NE Steeply Dipping SE
Set Intensity	P32	0.78	1.41	0.31
	[%]	31	56	12
Mean Set Orientation	Strike [deg]	96	185	215
	Dip [deg]	80 S	48 W	64 SE

 

Native State Modeling

Reservoir Properties

Initial reservoir properties used in the native state model are taken directly from characterization data. In many cases, a range of possible values are available, and the mean or median is used, with the values being adjusted within the measured range during model calibration. In all cases, uniform reservoir properties are used within the alluvium. For the granitoid, heterogeneous property distributions are applied where appropriate and data are available. Tables below summarize the property values used. These are the “reference” values for the current state of FORGE.

Parameter	Units	Min	Max	Source/Comment	Link to data
Compressibility	1/kPA	2.52E-12	8.51E-08	Upscaled DFN
Kii	m2	1.75E-21	1.20E-16	Core and reservoir testing, upscaled DFN	1160
Kjj	m2	2.44E-21	1.28E-16	Core and reservoir testing, upscaled DFN	1160
Kkk	m2	2.93E-21	1.10E-16	Core and reservoir testing, upscaled DFN	1160
Porosity	—	1.00E-07	0.0118	Core and cuttings analysis, upscaled DFN	1052
Rock grain density	kg/m3	2750.00		Core and cuttings analysis, native state calibration	1052
Specific heat capacity	J/kg K	790.00		Literature
Grain thermal conductivity	W/m K	3.05		Core and cuttings analysis, native state calibration	58-32 thermal conductivity data
Young’s Modulus	Pa	5.50E+10	6.20E+10	Core analysis	1162
Drained Poisson’s Ratio	—	0.26	0.3	Core analysis	1162
Undrained Poisson’s Ratio	—	0.35	0.4	Assume B=0.8
Biot coef	—	0.5	0.7	Literature
Thermal expansion coef	—	2.00E-06		Literature
Fracture asperity (Lognormal)	m	µ=1e-4, s=1e-8		Literature
Cohesion (Lognormal)	MPa	µ=3, s=0.5ˑ10^6		Literature
Frictional angle (Lognormal)	MPa	µ=0.6, s=0.001		Literature
Mode 1 fracture toughness	—	2.48MP√M		Core analysis	1140

Modeled Basin Fill Parameters

Parameter	Units	Value	Source/Comment	Link to data
K	m2	1.70E-14	Aquifer test	1140
Porosity	—	0.12	Core cuttings analysis, native state calibration	1052
Rock grain density	kg/m3	2500.00	Core cuttings analysis, native state calibration	1052
Specific heat capacity	J/kg K	830.00	Literature
Grain thermal conductivity	W/m K	2.00	Core cuttings analysis, native state calibration	58-32_thermal conductivity data
Young’s Modulus	Pa	3.00E10	Literature
Drained Poisson’s Ratio	—	0.30	Literature
Biot coef	—	0.60	Literature
Thermal expansion coef	—	2.00E-06	Literature

 

Native State Model Results

The modeled pressure, temperature, and stress along the trajectory of Well 58-32 match the field measured data reasonably well. The pressure distribution is largely linear along the length of Well 58-32 within the model domain and shows little to no differentiation between the overlying sedimentary basin fill and the granitoid-hosted reservoir. The modeled temperature distribution also matches the field measured data and shows a break in slope at the sediment fill-granitoid contact. The image to the left shows subsampled results for the pore pressure, temperature, and stress obtained in the native state model (red X). Also shown are values from logging and testing Well 58-32 (lines). Note that the pore pressure and temperature were logged over the entire length of Well 58-32, while the stress is estimated from injection tests in the toe.

The vertical stress was calibrated by adjusting the sediment density and porosity, as well as the density of the granitoid within the range of measured values until the modeled vertical stress match the field measurements at the toe of Well 58-32. Native State modeling results can be downloaded here. The image to the left shows the estimated total vertical stress within the granitoid.