simulation of brine reflux and geothermal circulation in large … · 2011. 12. 1. · simulation...
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Simulation of Brine Reflux and Geothermal Circulation in Large
Carbonate Platforms: An Attempt to Predict Dolomite Geo-bodies
Mayur Pal & Conxita Taberner
Shell Intl. E&P, The Netherlands
COMSOL Conference, Stuttgart 2011
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Outline
• Dolomitization in large Carbonate platforms
• Dolomitization and density driven flows
• Modeling density driven flows
• Results
• Conclusions
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• Dolomitization – is a diagenetic process
– Replacement of Calcite (CaCO3) by Dolomite (CaMg(CO3)2)
• Why is dolomitization important ? – Dolomitization creates porosity
– Dolostones makes excellent reservoirs for oil/gas
Dolomitization in large Carbonate platforms
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• Dolomitization demand large scale supply of Mg2+
• Sea water is rich source of magnesium
• Processes supplying magnesium rich fluid
– Density driven flows, e.g.,
• Brine reflux
• Geothermal circulation
Dolomitization & density driven flows
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What causes density driven flows ?
• Density variation in natural fluid systems
• Density variation due to naturally occurring salts
• Subsurface temperature changes
• Migrating pollutants
• Migration of fluids due to compaction
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Some density driven flow systems
• Salt-Lakes
• Saline-disposal basins
• Dense contaminant and leachate plumes
• Geothermal reservoirs
• Shallow marine basins
• Saline brines in deep basins
Voss & Souza, Water Resource Research, Vol. 23, No. 10, 1987
Bjorlykke et.al., Marine and Petroleum Geology, Vol. 5, 1998
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Ref - I. SHARP ET AL.
Conceptual model & dolomite patterns
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Gareth Jones et.al., American Journal of Science, Vol.304, 2004.
Modeling density driven flow
• Coupled flow and transport through:
– Darcy’s law for flow in porous media
– Solute transport through advection and diffusion
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Physical parameters governing density driven flow
• Salinity differences in circulating fluids
• Density differences in circulating fluids
• Permeability/Hydraulic conductivity of the porous medium
• Presence or absence of local hydraulic gradient
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Dimensionless parameters governing density driven flow
• Rayleigh number Ra - ratio between buoyancy forces tending to cause flow and other forces resisting flow
• Peclet number Pe - ratio between rate of advection of a physical quantity to rate of diffusion of the same quantity
• Damkohler number Da represents rate of reaction relative to advection rate
• Ra , Pe & Da governs the flow and reaction in a porous medium
Elder, J. Fluid Mech. , Vol. 27, Part 3, 1967
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Shallow Marine Evaporites
Modeling density driven flow in Carbonates
• Density driven flow due to reflux circulation
• In absence of hydraulic gradient
• In presence of hydraulic gradient Model Setting in presence of hydraulic Gradient
Shallow Marine Evaporites
Model Setting in absence of hydraulic Gradient
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Modelling changes in porosity and permeability
J.R.Wood, Applied Geochemistry, Vol. 2, pp 629-638, 1987
Approach 1 Approach 2
Changes in Permeability
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Model without hydraulic gradient – static/dynamic properties
Porosity Permeability
Darcy Velocity Field Pressure Distribution
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Dolomitize rock fraction - 200 year
Model without hydraulic gradient – dolomitize rock fraction
Dolomitize rock fraction - 1 year Dolomitize rock fraction - 10 year
Dolomitize rock fraction - 100 year
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Estimate of poro/perm changes assuming 100% ion exchange & no dolomite
cementation
Porosity after 10 Years Permeability after 10 Years
Porosity after 100 Years Permeability after 100 Years
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Impact of density difference on reflux dolomitization
drho – 100kg/m3 drho – 50kg/m3
drho – 25kg/m3 drho – 5kg/m3
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Modeling density driven flow due to Geo-thermal heating
• Density driven flow due to geo-thermal gradient
• In absence of hydraulic gradient
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Simulation model geo-thermal circulation – dolomitize rock fraction
After 1 - Year After 5 - Year
After 20 - Year After 30 - Year
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Porosity after 10 Years
Porosity after 100 Years
Permeability after 10 Years
Permeability after 100 Years
Estimate of poro/perm changes assuming 100% ion exchange & no dolomite
cementation
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Impact of magnitude of geo-thermal heat flux
Q – 0.06 W/m2 Q – 0.6 W/m2
Q – 1.0 W/m2 Q – 2.0 W/m2
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Field example - geological analogues
Ref - I. SHARP ET AL.
Numerical Simulation
Ref - I. SHARP ET AL.
Numerical Simulation
Geological Analogue
Geological Analogue
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Conclusions • Density driven flow modeling should be used
prior to RTM: – To quantify extent of dolomitization
– To predict shapes of dolomite bodies
– To possibly predict the parameters controlling dolomitization process
– Identify end-members to use in RTM approach
• Density driven flow modelling has less uncertainty compared to RTM due to less control parameter dependency
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Question ?