Exploring Hydraulic Fracture Stimulation Patterns in the FORGE Reservoir Using Multiple Stochastic DFN Realizations and Variable Stress Conditions

ALETA FINNILA (GOLDER)

FORGE Modeling and Simulation ForumMay 20, 2020

Acknowledgments

Co-Author of this WorkRobert Podgorney

Other Co-Authors from Previous FORGE DFN DevelopmentBryan Forbes, Hai Huang

FundingFunding for this work was provided by the U.S. DOE under grant DE-EE0007080

“Enhanced Geothermal System Concept Testing and Development at the Milford City, Utah FORGE Site”.

___AGENDA

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FORGE DFN development

DFN as a tool for FORGE modeling

Hydraulic fracture stimulation in FracMan

Variability due to stochastic realization

Variability due to regional stress uncertainty

Sharing the DFN with other modelers

___FORGE: develop geothermal reservoir in granite

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N E E D T O C R E AT E M O R E P E R M E A B I L I T Y

Location of the FORGE site (Moore et al., 2019)

Discrete Fracture Network (DFN)

DEFINITION needed of:

• Fracture Spatial Pattern including distributions of size, orientation, shape and intensity

• Fracture Hydraulic Properties including hydraulic aperture, permeability plus compressibility or transmissivity plus storativity

bulk rock permeability estimated at 4.7x10-17 m2 (48 µd) from well tests

FORGE DFN development

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For details on the FORGE DFN development:

Finnila, A., Forbes, B., and Podgorney, R.: Building and Utilizing a Discrete Fracture Network Model of the FORGE Utah Site, Proceedings, 44th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA (2019).

___DFN deterministic fracture set

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F M I D ATA F R O M P I L O T W E L L 5 8 - 3 2

Honor the data that we know: create fractures matching FMI log Along Well 58-32

What is known:• One point on the fracture surface• Orientation

What needs to be assumed:• Fracture size• Fracture shape

___DFN stochastic fracture set - orientation

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U T I L I Z E D ATA F R O M O U T C R O P S I N T H E M I N E R A L M O U N TA I N S

Comparison of fracture orientations from well and outcrop

Well 58-32 weighted FMI data shows three fracture sets: • E-W striking and subvertical• N-S striking and moderately W-dipping• NE-SW striking and steeply inclined dipping

NE

Outcrop patterns include three fracture sets: • E-W to WNW-ESE striking and subvertical• NE-SW to NNE-SSW striking and steeply dipping• NW to N to NE striking and gently W-dipping

___DFN stochastic fracture set - size

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U T I L I Z E D ATA F R O M O U T C R O P S I N T H E M I N E R A L M O U N TA I N S

Trace lengths from the various near-by outcrops fit truncated power law distributions with exponents ranging from 2.3 to 2.9. The average value of 2.6 is used in the DFN for generating fracture sizes.

___DFN stochastic fracture set - intensity

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F M I D ATA F R O M P I L O T W E L L 5 8 - 3 2

Use FracMan capability to match P10 on a well during fracture generation. Fractures are added to the model until the correct number are intersecting the well in the specified interval. Two zones of fracture intensity are evident, a weathered horizon in the top 300 m of the granitoid and a deeper zone having lower fracture intensity.

___DFN fracture hydraulic calibration

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D I F F E R E N T F O R E A C H S I Z E P O P U L AT I O N C O N S I D E R E D

Fracture Aperture• Estimated rock porosity1-2% from the neutron porosity log so assume bulk fracture

porosity ~ 0.5%• Assume aperture is proportional to sqrt(fracture size)• Solve for linear coefficient, a, by upscaling DFN

Fracture Compressibility• Estimate rock compressibility based on Young’s Modulus (61 GPa) and Poisson’s

ratio (0.27) ~ 2.26e-5 1/MPa• Solve for fracture compressibility using:

Fracture compressibility = (rock compressibility)/(fracture porosity) Fracture Permeability

• Estimated average permeability 48 µd from well tests• Assume fracture permeability is proportional to fracture aperture• Solve for linear coefficient, b, by upscaling DFN

aperture = a Re

bulk compressibility = 3 1−2𝜈𝜈𝐸𝐸

permeability = b aperture 1.5

DFN as a tool for FORGE modeling

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___DFN Subsets

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W H I C H D F N S H O U L D B E U S E D D E P E N D S O N M O D E L I N G Q U E S T I O N

3D Full Region3D Near Well 58-32

3D for Regions around Proposed Well Toes2D Trace planes around 58-32

Can filter generated fracture sets by:• Location• Size• Slip tendency• Dilation tendency• Orientation• Random %

Provide fractures in various formats(point data, surfaces)Provide upscaled fracture properties(grids or point data)

___What questions can modeling address?

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T O P V I E W F O R G E F O O T P R I N T W I T H S A M P L E S T I M U L AT I O N S

What is the extent of stimulation? What is the expected microseismic response?Will the stimulated zone exceed the property boundaries?Etc.

FORGE boundary in red

stimulated fractures

stimulated fractures

___Hydraulic fracturing at the toe of new well

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W H AT D F N S H O U L D B E U S E D ?

Intensity P32 [1/m] 1.9 (deep) and 3.6 (shallow) before min size correction

Orientation based on Terzaghi-weighted FMI log from 58-32 with a Fisher concentration parameter of 50

Size Distribution Power Law (Pareto with Xmin=1 and α=2.6)

Min Size (Equivalent Radius) [m]

10

Max Size (Equivalent Radius) [m]

150

Shape regular polygon with 6 sidesAperture [mm] 0.607*EquivalentRadius^0.5Permeability [m2] 1.66x10-15*Aperture^1.5Compressibility [1/kPa] 7.14x10-6

How much variation in modeling results is due to the particular DFN realization used?

Hydraulic fracture stimulation in FracMan

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___Hydraulic stimulation of new FORGE well

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N E E D T O C R E AT E M O R E P E R M E A B I L I T Y

Proposed new injection well

Existing pilot well

Model Region

Model hydraulic stimulation of open hole section of toe of planned new injection well(60 m section)

DFN is generated in a 1200 m sided cubic region centered at the well toe while the stimulation modeling region is restricted to an 800 m sided cubic region

___Hydraulic fracture simulation in FracMan

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S O M E B E H A V I O R I S E S T I M A T E D T O A L L O W F O R R A P I D E V A L U A T I O N

Pumping duration [hr] 2Pumping rate [kg/s] 50Delta pressure [MPa] 6.9Flowback [%] 5Minimum injection ratio to new hydraulic fracture [%] 10Water density [kg/m3] 1000Water viscosity [Pa s] 0.00089Rock lithology granitoidYoung's modulus [GPa] 55Poisson's ratio 0.27Frictional model Mohr-CoulombFriction angle [deg] 36Cohesion [MPa] 2New tensile fracture location best oriented with respect to the maximum horizontal stressBounding box 800 m region boxElement size [m] 10Fracture set full stochastic set in the 800 m box regionFracture pressure mode Secor and PollardPore pressure slope with distance 0.5Flow to fracture order (transmissivity*orientation)/ (connection_level)Maximum aperture for inflated fractures [mm] 5Pressure decrease with distance from inflated fracture linearReservoir pore pressure [MPa] 24.6Hydroshear element size [m] 10Hydroshear distance [m] 250

Strategy: Create a large number of DFN realizations, run a hydraulic fracture simulation on each, measure some parameter of interest, and compare the results

___Hydraulic fracture simulation in FracMan

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U S E D T O S C R E E N D F N R E A L I Z AT I O N S – Q U A L I TAT I V E R E S U LT S

What do we expect from these simulations?• Include realistic fracture

orientations, intensities, sizes and connectivity in 3D

• Good relative response to increased pore pressure based on fracture orientation and size –i.e. which fractures will inflate, which will be susceptible to shear failure and which will be stable with respect to regional stresses

• Identification of which DFN realizations show average vs end-member response to stimulation at the toe of the new well

What do we not expect from these simulations?• Absolute values for quantitative

measurements such as stimulation distance

• Stimulation results that reveal how localized stress variations affect fracture response

• Details of stimulation such as maximum aperture for inflated fractures or pressure dissipation along the fractures with distance

___Hydraulic fracture simulation in FracMan

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R E S P O N S E T O I N C R E A S E D P O R E P R E S S U R E

Dilation TendencySlip TendencyUpper hemisphere, fracture pole stereonets along with a Mohr circle showing fracture stability under the regional stress field in the FORGE reservoir at a depth of 2 km

Colors on the stereonets indicate slip tendency and dilation tendency with red indicating fracture orientations being close to failure and blue colors being very stable

The example shown in the Mohr circle is for a vertical fracture having an East-West strike

Vertical fractures aligned with the maximum horizontal stress direction of N25E would be the easiest to dilate

___What gets measured in each run

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T O P V I E W – I N F L AT E D F R A C T U R E S S H O W N

Put a box around the inflated fractures (AxBxVert) as well as a box around the full stimulation results which include both the inflated fractures and the portions of other fractures that fail in shear (CxDxVert)

E,F show the boundary of the hydraulic fracture simulation

G,H show the boundary of the DFN generation region

Note: measured distances will be displayed as normalized values based on the size of the hydraulic stimulation region box side length due to uncertainty in absolute values given the simplifying assumptions used in the FracMan simulation

Variability due to stochastic realization

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For details on this section:

Finnila, A., and R.K. Podgorney. 2020. Exploring hydraulic fracture stimulation patterns in the FORGE reservoir using multiple stochastic DFN realizations and variable stress conditions. In Proceedings, 45th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California.

___Extent of hydraulic stimulation

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1 0 0 R E A L I Z AT I O N S U N D E R T H E M O S T L I K E LY S T R E S S C O N D I T I O N S

___Extent of hydraulic stimulation

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1 0 0 R E A L I Z AT I O N S U N D E R T H E M O S T L I K E LY S T R E S S C O N D I T I O N S

Box plots can provide a compact view of large data sets

maximum value

minimum value

median value (orange line)

Each box plot represents one histogram

1st

quartile

3rd

quartileInterquartile range

___DFN realization variation

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I N I T I A L C O N C L U S I O N S

• Hydroshearing occurs over a much larger volume than simple fracture inflation

• East-West stimulation distances are smaller than North-South or vertical stimulation distances

• The variation between stochastic realizations is significant and can lead to differences where minimum values can be half of the maximum values seen.

• Vertical extents show a wider range of values compared to the other dimensions

Variability due to regional stress uncertainty

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For details on this section:

Finnila, A., and R.K. Podgorney. 2020. Exploring hydraulic fracture stimulation patterns in the FORGE reservoir using multiple stochastic DFN realizations and variable stress conditions. In Proceedings, 45th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California.

___How much does the stress state matter?

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S E N S I T I V I T Y T O U N C E R TA I N S T R E S S PA R A M E T E R S

Stress SensitivityRecommended Minimum Maximum

Pore Pressure [MPa/km] 9.7 N/A N/AVertical Stress [MPa/km] 25.6 N/A N/ASHmax [MPa/km] 21.3 17.0 25.6Shmin [MPa/m] 17.0 13.8 N/ADirection SHmax N25E N10E N40E

Orientation of SHmax(Simmons et al., 2018)Stress gradients determined from multiple injection test

types and interpretation methods (Xing et al., 2020)

The direction of Shmax and the magnitudes of SHmax and Shmin have some uncertainty (gold background in adjacent table)

Strategy: test all the permutations of these uncertain stress values (18 total: 3 orientation x 3 SHmax x 2 Shmin)

___Vertical extent of stimulation

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3 0 R E A L I Z AT I O N S X 1 8 S T R E S S S TAT E S = 5 4 0 S I M U L AT I O N S

Labels show:Orientation_SHmax_Shminwhere Rec = Recommended

Stress conditions are ordered by median value

Observations:

• Vertical stimulation extent is significantly affected by the value of Shmin

• Variation in vertical extent due to Shminis greater than that due to the DFN realization

grey box outlines the most likely stress condition

___Extent of stimulation in three directions

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Important Note: stress conditions are ordered by median value for each figure, so the order can change from figure to figure

Observations:

• Stimulation extents in all directions are significantly affected by the value of Shmin with interesting exceptions for the Any_Min_Rec combinations

• Variation in extents due to Shmin is greater than that due to the DFN realization

___Ratios of stimulation extents – colored by Shmin

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A N I S O T R O P YAs seen in the top figure, East-West stimulation extents are generally less than the North-South stimulation extents, with the ratio varying between 0.5 and slightly less than 1.0

In the bottom two figures, the data can be divided into two groups: those stress conditions with median ratios plotting below the 1.0 line where the lateral stimulation distance is less than the vertical stimulation distance, and those that plot above. The only three stress conditions that cause the lateral distance to exceed the vertical distance are N10E_Min_Rec, N25E_Min_Rec, and N40E_Min_Rec

Not a strong pattern evident with Shmin value

___Ratios of stimulation extents – colored by Shmaxorientation

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While it looks like there is strong pattern based on the Shmax value for the stimulated North-South vs vertical extents, the values are too similar to distinguish between different orientations

___Extent of inflation in three directions

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• Inflation extents in all directions are not as sensitive to the stress conditions as were the full stimulation extents (including hydroshearing)

• Variability due to DFN realization is higher with lower Shmin values except for the N40E_Min_Min and N25E_Min_Min combinations

Note: in this normal stress regime, inflated fractures tend to be sub-vertical while fractures experiencing shear failure tend to be more inclined

___Ratios of inflation extents – colored by Shmin

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A N I S O T R O P Y

While there appears to be a pattern in the ratios of extents due to the stress conditions for two of these three figures, it is not controlled by the magnitude of Shmin.

___Conclusions 1/2

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H Y D R A U L I C S T I M U L AT I O N PAT T E R N S AT F O R G E

• Stimulation in the East-West direction is consistently smaller than in the North-South or vertical directions

• North-South stimulation extent is generally very similar to the vertical extent (except for the stress state combination using the minimum SHmax gradient and the recommend Shmin gradient)

• Inflation of fractures is not as sensitive to the stress condition as slip

___Conclusions 2/2

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I D E N T I F I C AT I O N O F D F N R E A L I Z AT I O N S F O R F U R T H E R W O R K

Example Question: What will be the vertical extent of stimulation?

Most Likely:N25E_Rec_Rec_20

Minimum:N40E_Min_Rec_30

Maximum:N25E_Rec_Min_17

Sharing the DFN with other modelers

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___DFN Subsets

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W H I C H D F N S H O U L D B E U S E D D E P E N D S O N M O D E L I N G Q U E S T I O N

3D Full Region3D Near Well 58-32

3D for Regions around Proposed Well Toes2D Trace planes around 58-32

Can filter generated fracture sets by:• Location• Size• Slip tendency• Dilation tendency• Orientation• Random %

Provide fractures in various formats(point data, surfaces)Provide upscaled fracture properties(grids or point data)

___Sharing the DFN

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2 D D I S C R E T E M O D E L I N G E X A M P L E

Only fractures intersecting the plane having pole vectors within 20° of being orthogonal to the plane are included.

Vertical slice (yellow) used for the 2D simulations oriented N105E to align with the lateral section of the new proposed injection well. The end of the well is in the center of the 1400 m x 1400 m plane.

Traces from fractures filtered by orientation, size and distance from the open hole section of the new injection well. The smaller box outlines a region containing fractures within 50 m of the open hole section of the well while the larger box shows fractures within 200 m of the open hole section. The full 2D region is 1400 m x 1400 m.

46,000 fractures intersecting this 2D slice

Filtering by orientation removes half

Filtering by size and location leaves 800

___Using the DFN

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S O M E Q U E S T I O N S T H AT N E E D T O B E A D D R E S S E D

• What question(s) are you addressing with your modeling? Stimulation of the pilot well? Stimulation of the new well? Long term thermal evolution of an injector-producer pair? Other?

• Discrete or continuum modeling?• 2D or 3D? • Modeling region?• Do you want the fractures pre- or -post stimulation• Maximum number of fractures? (discrete)• Filter the fracture by size, orientation, location?• Transfer file format? • Do you have an upscaling grid available? (continuum)• What properties would you like upscaled? (continuum)

i.e. porosity, directional permeability, others

___Accessing FORGE reference DFN models

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F R A C T U R E S E T S W I L L B E AVA I L A B L E O N T H E G D R

If you have a need for a FORGE DFN to use in your modeling work, send email to:

Utahforge-Modeling@utah.edu

While we will not be providing individual groups with custom DFNs, we will be compiling a set of reference DFNs that will be available to everyone. These will be posted to the GDR and publicized on the numerical-modeling page on the FORGE web site.

https://utahforge.com/laboratory/numerical-modeling/

Thank you for your attention