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Combined Geological Modelling and Flow SimulationJ. Florian Wellmann, Lynn Reid, Klaus Regenauer-Lieb and the Western Australian Geothermal Centre of Excellence
TIG-10 Workshop, Adelaide 11/2010
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Outline
• Addressing the problem that geological modelling and geothermal simulation are usually separated (and geological uncertainty not considered – even if significant)
• Workflow to integrate both steps into one framework
• Two example models:
1. Hypothesis testing for different geological scenarios2. Combination with geological uncertainty simulation
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Geological Modelling
• Construction of a structural representation of the subsurface
• Interpolation based on discretized geological observations (e.g. from drillholes, seismics, field work)
• Applied modelling tool: GeoModeller (Intrepid Geophysics, BRGM)
(Calcgano et al., 2008)
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Geothermal Flow Simulation
• Coupled simulation of fluid and heat transport equations in the subsurface
• Based on property distribution (e.g. permeability, porosity, thermal conductivity, heat capacity) in subsurface and boundary conditions (e.g. basal heat flux)
• Applied simulators: TOUGH2, SHEMAT
Permeability Porosity
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Boundary conditions
Geological Model
Mesh
Property assignment
Simulation
Discretized geological model
Geological Data
Manual steps
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Critical steps
• Mainly related to
– model construction, – mesh generation and – processing to flow simulation
Once constructed, the geological model is rarely changed or extended, even if significant source of uncertainty!
Steps before flow simulation
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Automation steps
Geological modeling
Discretization
Model simulation setup
Simulation and analysis
Implicit potential-field method (GeoModellerTM); enables direct model update
Automated rectilinear mesh discretization (python scripts)
Direct update of input files for simulation with SHEMAT and TOUGH2 (python scripts)
Simulation with available codes, post-processing and analysis (python scripts)
Change one data point
Evaluate effect on flow field
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Geological Hypothesis Testing
near-surface heat flux (z-dir)
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Combination with Uncertainty Simulation
• Consider uncertainties in structural geological models (one of main sources of uncertainty)
• Approach: random change of input data (discretized surface position, orientation data)
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Wells don’t penetrate basement!
Assume: structure more or less well defined (seismics) but exact position at depth unknown
Example model North Perth Basin
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Change bottom of formations randomly
Formation Name Standard deviation
Cadda 20m
Woodada-Kockatea 100m
Permian 200m
Standard deviation for data points defined at bottom of formation
Position of formation bottoms changed about random value
Create 20 different input data sets and 20 different models
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Results of simulation
• For 20 geological models, we obtain 20 simulated flow and heat flow fields (drawing from the uncertainty distributions)
Example of one temperature model
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Local mean and standard deviation of Temperature
mean
stdev
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Conclusion
• Uncertainties in structural model influence simulated geothermal flow field but they are usually not considered
• Developed integrated workflow
– enables hypothesis testing and consideration of geological uncertainty
– compliments and extends other approaches (e.g. stochastic simulation, as presented by Tony Meixner), e.g.:
• physics (multi-phase, thermo-hydro)• mesh (rectilinear)• consideration of uncertainties in geological data (not the
model)
• Specifically suited for early exploration stages and resource evaluation where uncertainties in the structural model are dominant.
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Outlook
• Complete implementation on supercomputer
• Optimal mesh construction for geological models (e.g. automatic rectilinear refinement, extruded triangular for TOUGH2)
• Coupling to advanced resource estimation methods (talk at AGEC)
• Combination with GIS methods
Thank you for your attention!
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Appendix
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Uncertainty in Geology models: different types
Incomplete knowledge
Are all relevant structures known?
How to analyse uncertainties in structural models?
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Uncertainty in Geology models: different types
Uncertainty of interpolation
How good is the interpolation between data points?
How to analyse uncertainties in structural models?
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Uncertainty in Geology models: different types
How exact is the data?
Uncertainty in raw data
How to analyse uncertainties in structural models?
• Applies specifically to interpreted data and assumptions• We consider this to be a significant part of model uncertainties(Wellmann et al, 2010)
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“Complex” and “simple” geological settings
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Mesh geometries
• Regular mesh
• Rectilinear mesh
• Extruded triangular mesh (only TOUGH2)
(work in progress: optimal mesh generation from geological models)
(Include example extruded triangular?)
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Processing simulated models to simulation
Coupled fluid and heat flow simulation in a 2-D subset of the model
Discretization in a regular grid
Two highly permeable formations
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Convective vs. conductive heat transfer
• Local Peclet number
Pe = l v /
In our case:- l: characteristic length- v: fluid velocity (model
result)- : thermal diffusivity
(10-6 m2/s)Conduction dominated: low Pe-Number
Convection dominated: high Pe-Number
As characteristic length scale, we use engineering lifetime of 30 years and get l approx 60 m
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Example of local Pe-Numbers for one model
Pex
Pez
left right
down up
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Local mean and standard deviation: Pe z-direction