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© 2004 BRGM

Ray Seikel (Intrepid Geophysics), Kurt Stüwe (Graz University), Helen Gibson (Intrepid Geophysics), Betina

Bendall (Petratherm), Louise McAllister (Petratherm), Peter Reid (Petratherm), Anthony Budd (GeoScience

Australia)

Forward Prediction of Spatial Temperature Variation From 3D Geology Models

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1) Building 3D geology models2) Prediction of spatial temperature variation from

3D geology model. Develop a method for rapid computation directly from a 3D geology model

3) Case Study - Compare predictions with measured

Collaboration

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© 2004 BRGM

ConductionProduction Radiogenically Mechanically

ChemicallyAdvection by Fluids

by Erosion by Deformation by Magma

)/()(2p

cmech

Schem

Srad

SUdt

dT

Heat Transfer Processes

+ Schem + Smech

Summary of heat flow theory

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© 2004 BRGM

Heat production via radioactive sources is important

In contrast no highly active tectonism, metamorphism or volcanism is occuring in the upper crust today, which might otherwise contribute to mechanical or chemical heat production. So we do not take these into account

Assumptions for Australia

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It is sufficient to consider only the case of thermal steady state for the Australian crust

We must take into account the variation of conductivity with rock types

Assumptions for Australia (Cont)

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© 2004 BRGM

Conduction *

Production Radiogenically * Mechanically

Chemically

Advection by Fluids * by Erosion by Deformation by Magma

)/()(2p

cmech

Schem

Srad

SUdt

dT

Heat Transfer Processes

+ Schem + Smech X X

Simplified Equations

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© 2004 BRGM

Software Implementation

• Fourier’s first and second laws• Steady state• Variable thermal conductivity (k) & heat production rate (S)

The final equation allows us to solve in 3D using finite difference approximation

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© 2004 BRGM

Surface Topography

Surface: Mean surface temperature

Sides: Neumann-type

Base: Constant Heat Flow or

Constant Temperature

Support for•surface topography•fixed internal temperatures

Isotherms with Increasing depth

Boundary Conditions

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© 2004 BRGM

Solve in Voxet space<Discretise the model>

<Assign: Thermal ConductivitiesHeat Production Rates

<Assign:Boundary Conditions

<InputVoxet> <ForwardModel3DTemperature> <OutputVoxet(s)>

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© 2004 BRGM

Case 1 - Constant qbase and layered geology• Case 2 – Constant temperature at base and layered geology• Case 3 – Uniform thermal conductivity and heat production rate through out• Case 4 – Step heat production rate• Case 5 – Same as Case 2 expect one voxel is held at fixed temperature• Case 6 – Topo test• Case 7 – Uniform advection through out• Case 8 – Advection through a 3x3 vertical column

Unit testing: 8 cases

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© 2004 BRGM

•To validate FD approximations against analytical solutions and expected T distributions

•Different initial settings and boundary conditions

all passed

Unit testing: 8 cases

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© 2004 BRGM

Example: Unit Test 3 Results Uniform conductivityUniform radiogenic heat production

TemperatureHeat Production

De

pth

Conductivity

De

pth

De

pth

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© 2004 BRGM

Heat ProductionConductivity Temperature

De

pth

De

pth

De

pth

Uniform conductivityNo heat productionSetting drill hole temperature data as fixed(Unrealistic scenario but ok for testing!)

Example: Unit Test 5 Results

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© 2004 BRGM

Example: Test Localised advection

Assuming

Uniform conductivity and constant basal heat flow

Fluid flow upwards through 150x150m vertical column

Properties adjusted to give visible results

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© 2004 BRGM

1) Compare with measured values2) Consider potential field data, re-fine the model,

repeat

This case study assists in software testing

Brief overview of Paralana Case Study

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© 2004 BRGM

Petratherm Ltd’sParalana Project Tenements

~20 km east of Mt Painter Inlier

Northern Flinders RangesSouth Australia

ParalanaProject

Mt Painter Inlier

•Adelaide

•

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© 2004 BRGM

Generalised W-E section: Poontana Graben

126-129 mW/m2

Paralana-1B

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© 2004 BRGM

Paralana Case Study

Geology model constraints

• 9 interpreted seismic sections (Petratherm) simple, linear depth conversion (in GeoModeller)

• Paralana-1B well (Petratherm)

• ~50 shallow drill holes (SARIG dataset, PIRSA)

• SEEBASE economic basement depth (PIRSA / SRK)

• 1:700,000 Basement map Arrowie Basin (PIRSA / SRK)

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Tops and faults from seismic

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© 2004 BRGM

Paralana-1B

SeeBase: Top Curnamona

Paralana Fault(s) Shallow drill holes

Seismic

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Solid geology model

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Forward temperature modelling

• Conduction

• Heat Production (U, Th, K)

Advection x (but soon possible)

Possible small heat contribution from fluids fluxing via Paralana Fault and (?) deeper fracture networks/pathways

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Steps <Assign Thermal ConductivitiesHeat Production Rates

<SetBoundary Conditions

<Discretise the model

<Input Voxet<Forward Model 3D Temperature<Output Voxet(s)

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© 2004 BRGM

model inputs

run24

Thermal Conductivity

Watts m–1

K –1

Heat Production Rate

Watts / m3

Rec - Mesozoic 1.5 ~1 x10-6

Carboniferous 2.0 ~1 x10-6

Lake Frome Gp 5.3 ~1 x10-6

Lwr Arrowie 3.2 ~1 x10-6

Brachina Sh 2.0 ~1 x10-6

Lwr Adelaidean 2.4 ~1 x10-6

Moolawatana 3.2 ~22 x10-6

Mt Painter MesoP 3.2 ~22 x10-6

U-depleted base 3.2 ~2 x10-6

BOUNDARY

CONDITIONS

Top: 19°C Bottom: 0.035 Wm-2

Constant heat flow

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Model discretisation: run 24Input model

extents

Number of

cells

Discretisation

cell size

X 55 km 40 1350 m

Y30 km 40 750 m

Z10 km 40 250 m

Total voxels: 64,000 Run-time: 14 mins

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Set iterations controls (run24)

• Maximum residual in Degrees C: 0.0001(the maximum change allowed in temperature in any cell)

• Maximum Iterations: 15,000

When either condition is met, iterations cease, as thermal equilibrium is said to be reached

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Model outputs

• Voxet: x, y, z, lithologies (initial earth model)

• Voxet of results: temperature, vertical heat flow, vertical temperature gradient,

total horizontal temperature gradient

• jpegs (for every 2D section in the geology model)

• grid files (ditto)

• record of run (inversions.xml; COMPUTE_LOG.txt)

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‘Verify‘: W-E section•Temperature•Modelled geology•Wells (projected to section)Result ~103°C at bottom hole / Paralana–1B(compared measured 109 °C)

286 degC

19 degC

Paralana-1B

Section line: 55 km long

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140 mWm-2

63 mWm-2

Horizontal section at -500m•Vertical Heat FlowResult ~108 mWm-2

(compared measured 129 mWm-2 within Paralana–1B)

Paralana-1B

55 km x 30 km

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© 2004 BRGM

Concluding points•Initial geology model is reasonable

Together with estimated thermal properties:•Measured T data in Paralana-1B can be matched•Surface Heat Flow data can be (~) matched•Software implementation performs to specifications

•Geology model still needs refining (assisted by forward modelled gravity and magnetics in GeoModeller)

•Geology dominates the T distribution- Hence true 3D modelling is crucial !

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grateful acknowledgement !

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© 2004 BRGM

Disclaimer

Data have been manipulated to show software features and may not reflect

actual conditions at Paralana