Water Saturation Modelling From petrophysics to volumes and simulation

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Water Saturation ModellingFrom petrophysics to
volumes and simulation

• FORCE seminar 30-31 March 2004
• Jan C. Rivenæs
• J.I. Kristiansen, Tor Barkve, Morten Fejerskov,
  Stephen Rodgers, Julio Marre, Geir Håskjold

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Topics

• Introduction
• Definitions
• Overall workflow
• Sw in Petrophysics
• Sw in Geology modelling
• Sw in Reservoir Simulation
• Upscaling issues
• Summary

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Philosofical background
Philosephor Thales from Miletus ( 500 B.C
) Everything is water.
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In the beginning, there was water
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An integrated work process
CORE ANALYSIS LAB
PETROPHYSICS
RESERVOIR
CAN WE FIND A common language? A common
understanding? A common model?
GEOLOGY
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The Sw modelling 2003 project
CORE ANALYSIS
JAN INGOLF KRISTIANSEN
GEIR HÅSKJOLD
STEPHEN RODGERS
PETROPHYSICS
RESERVOIR
Sw Boss
MORTEN FEJERSKOV
GEOLOGY
TOR BARKVE
JULIO MARRE
JAN CHR RIVENÆS
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Historic view on Sw modelling

• 1935 Realisation of existence of capillary water
• Garrison 1935
• Schilthuis 1938
• 1940 Methods for predicting water saturation
• Leverett 1941
• Archie 1942
• 70ties The first reservoir simulator
• Beta II 1975
• Eclipse - 1982
• 80ties Geological modelling using computers
• Mapping of zone averages
• Integrate Sw function over zones thickness - 1985
• 90ties 3D modelling and visualisation
• 2003 Sw modelling is still a "black box"

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Main deliverables from Sw modelling

• STOOIP/GIP
• Function of bulk rock volume, NTG, PHI, Sw and
  PVT
• HCPV distribution
• In which segments are the oil/gas accumulated?
• Input to reservoir simulation
• Initial hydrocarbon distribution prior to
  production
• Integrated part of SCAL model

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General workflow
Petrophysical modelling
Geological modelling
Reservoir simulation
Establish a Sw function honoring both log and
core data and define fluid levels
Defined segment contacts and populate the
Geo-model with water utilising an established Sw
function and mapped rock properties
Initialize the Sim-model honoring upscaled
saturations from the Geo-model and SCAL
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Topics

• Introduction
• Definitions
• Overall workflow
• Sw in Petrophysics
• Sw in Geology modelling
• Sw in Reservoir Simulation
• Upscaling issues
• Summary

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Water saturation - Sw
Fraction of water in the pore volume
Sw Sh 1 - Sw
Sw 1 Sh 0
Sw lt 1 Sh gt 0
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Vertical equilibrium
The initial fluid distribution is defined by
vertical equilibrium between capillary pressure
and gravity forces.
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FLUID DISTRIBUTION AND CONTACTS ROCK QUALITY
DEPENDENCE OF WATER SATURATION- HEIGHT PROFILE

• Saturation depends on rock quality (average pore
  throat radius) as well as height above the FWL.

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The basic levels
Free water level FWL Depth at which oil/water
capillary pressure is zero. Free oil level
FOL Depth at which gas/oil capillary pressure is
zero. Oil/water contact OWC Minimum depth
where the water saturation is equal to its
maximum value.
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Water saturation definitions
SWIR
Drainage
Imbibition
SWIR
Sw
SWCR
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Is OWC precise?
No, refer to fluid contacts based on pressures
instead Good sand OWC FWL Poor sand OWC ¹
FWL
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FOL and appearant FWL for gas
GAS
OIL
WATER
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Wettability and capillary pressure
WATER
The contact angle can be used to distinguish
between a water wet, mixed wet, and oil wet
system.
OIL
ROCK
Drainage Non-wetting phase saturation is
increasing. Imbibition Wetting phase saturation
is increasing.
The wettability is a function of rock and fluid
properties, but also on history.
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Sources for Sw data
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Sw and the other properties
Porosity Apparent, non-apparent, absolute,
effective, total, residual, intergranular,
intermatrix, primary, secondary,...
Permeability etc Appearant, effective, relative,
Klinkenberg, air, horisontal, vertical, syntetic,
well-test, mobility, wettability
?
Scale Core plugs, log scale, geomodel, simulation
model, upscaling, modelling approach,
Net-to-gross Cut-off, reservoir, non-reservoir
net sand, net pay, Vshale,
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The petrophysical porosities
Matrix (sand, silt)
Dry clay
Clay bound water
Free water
Free Water
Ft
Fc
Fe
Hydrocarbon
Log analysis Fe PHIE and Ft PHIT Fc
(low temperature humidity drying) Fe Fc (high
temperature oven drying) Ft
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Should we use NTG and cut-off?
If you model effective props (PHIE and Keff),
cut-off in a 3D model is not needed NTG should
not be modelled as a property However, for
up-scaling Sw, post-model cutoff may be needed in
some cases
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Topics

• Introduction
• Definitions
• Overall workflow
• Sw in Petrophysics
• Sw in Geology modelling
• Sw in Reservoir Simulation
• Upscaling issues
• Summary

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Sw An integrated work process
CORE ANALYSIS LAB
COOPERATION EXTERNAL CONDITIONS WORKFLOW ORGANI
SATION
PETROPHYSICS
RESERVOIR
GEOLOGY
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A question of scale
Core scale
Geo model scale
Log scale
Flow model scale
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Topics

• Introduction
• Definitions
• Overall workflow
• Sw in Petrophysics
• Sw in Geology modelling
• Sw in Reservoir Simulation
• Upscaling issues
• Summary

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Workflow for petrophysical Sw-model
CORE ANALYSIS LAB
CORE DATA k, w, Pc
PETROPHYSICS
RAW LOGS
RESERVOIR
FLOW ZONES
FMP
CPIs Sw MODELS FLUID LEVELS
ZONATION FACIES
GEOLOGY
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Traditional Leverett model building
Using core data Pc(Sw)FcK?J-model?test
J (Sw)-model
Using log data SWEPHIEKHLOGH?J-model?test
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Sw-modelling preferences
Keep it simple
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Topics

• Introduction
• Definitions
• Overall workflow
• Sw in Petrophysics
• Sw in Geology modelling
• Sw in Reservoir Simulation
• Upscaling issues
• Summary

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Workflow for geological Sw-model
CORE ANALYSIS LAB
PETROPHYSICS
RESERVOIR
CORE DATA k, w
CPIs Sw MODELS FLUID LEVELS
ZONATION REGIONS GEO MODEL Sw VOLUMES
GEOLOGY
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3 steps for the geomodeller
Receive function(s)
Implement in model
Deliver products
Drainage part
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Handling input from petrophysicists
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Quality control is important
H
GEO MODEL
Ln(SwD)
Large (thick) cells may give problems
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Topics

• Introduction
• Definitions
• Overall workflow
• Sw in Petrophysics
• Sw in Geology modelling
• Sw in Reservoir Simulation
• Upscaling issues
• Summary

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Work flow for the simulation Sw-model
CORE ANALYSIS LAB
SCAL DATA PC AND REL.PERM
PETROPHYSICS
RESERVOIR
J FUNCTIONS FLUID LEVELS
GEO MODEL SW VOLUMES

GEOLOGY
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Upscaling - prone for confusion
Talk together The way R.T. choose to incorporate
Sw in Eclipse may influence the upscaling
I can perhaps use your upscaled Sw, but only
partially!
RT
Partially ...? Dont you like me?
The upscaling is a multidisciplinary task, but in
practice R.T. is in charge!
GEO
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The world as seen by a reservoir dog

• The geo model alone is not sufficient to define
  all necessary Sw data for the simulation model.
• The upscaled Geo Sw cannot be used directly in
  the simulation model.
• Sw data related to mobility must be taken from
  SCAL data.
• SCAL data are not always sufficiently available.
• SCAL and Geo data may seem inconsistent.

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Requirements for the simulation Sw model

• The initial volumes should be (approximately)
  stable.
• Capillary pressure/gravity equilibrium.
• Correct dynamic response.
• Imbibition or drainage data?
• Initial volumes should match geo model.
• Drainage Pc data.

STABILITY NOW
Use Pc!
LESS SW!
Give us SWCR
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Vertical equilibrium in the simulator
To achieve true vertical equilibrium Depth
values must be taken at grid cell centers.
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Accurate volumes versus stability
Pc 0
Inaccurate representation of fluid contact
FWL
Grid cells with saturation change larger than
0.05 after 100 days simulation without wells.
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Sw for R.T. is less straightforward

• If using upscaled Sw from geomodel
• Need to use rel.perm. end-point scaling
• No capillary pressure
• No water production in transition zone
• Problems around FWL (anyway)
• Or
• If using Sw from Capillary pressures
• Core plug J-curve does not apply to sim. scale
• Achieve volume match with Geo may be difficult
• Simulator will produce water in transition zone
• Which is commonly not observed in practice!
• Approach dependent on use of model!

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Summary

• The reservoir challenge is the combination of the
  SCAL data and the geo model data.
• The petrophysicist has already handled this
  problem, but on a different scale.
• A Sw implementation in the simulation model
  should preferably be based on a J function
  approach.
• Sw modelling for reservoir is time consuming.
  Plan for this!

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Topics

• Introduction
• Definitions
• Overall workflow
• Sw in Petrophysics
• Sw in Geology modelling
• Sw in Reservoir Simulation
• Upscaling issues
• Summary

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Upscaling

• Best practice
• Upscale porosity arithmetic, weighted on bulk
  cell volume
• Compute PoreVolume for each cell
• Upscale Sw weighted on pore-volume, arithmetic
• Consider use of "cut-off" in Geomodel before
  upscaling
• Challenge
• What input does the res.eng want?
• Sw
• Swir? (a rock parameter, independent of
  contacts)
• Swcr? (a rock parameter, independent of
  contacts)
• Be precise on delivery to R.T.

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Some upscaling of Sw issues

• The contact problem
• Cell below or above contact?
• The resolution problem
• Cut-off (filtering) or not?
• Sampling near faults
• Core-plug to ressim cell problem

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The contact problem
Cell properties refers to center point i.e. the
whole cell gets one value
FWL
Oil cell?
100 water?
Large (thick) cells may give problems
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Geomodel Integrating J over geocell
HdZ
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Sw model implementation, QC

• Usually the SCAL data for the Sw model
  development, and the RMS PHIE and Kh data are
  different
• -1- The synthetic Kh can differ substantially
  from the SCAL data
• -2- The synthetic Kh is blocked before being used
  for Sw computation (up scaling process)
• Grid structure influence the fluid contacts

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The resolution problem (tiger problem)
Having geocell vertical grid 0.5 to 1m is close
to Sw log resolution!
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The tiger problem
Same Sw(H) curve? What about relperm start point?
Should we try to back-compute PHINet SwNet,
KNet and NTG (for Sw calculations only?)
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Cut-off (filter) in Sw upscaling
Sw in geo model is often correlated with the
permeability k.
In a stochastical model, some grid cells may have
low k, but high porosity.
This may lead to artificially high mobile water
saturations in the simulation model. Cure Set
all cells with K lt x and w lty inactive. I.e. a
cut off before upscaling (On Oseberg Øst, x1mD,
y0.05 p.u.) This is espacilly important if some
kind of end-point scaling is used, but should
also be considered elsewhere
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Water sat upscaling Sampling crucial!
Up
Up
Down
Down
GeoModel
SimModel
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Quality control on upscaling effects
H
GEO MODEL
SIM MODEL
Ln(SwD)
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QC of Sw upscaling
Plot the upscaled Sw from the Geo model in the
same functional format as the petrophysical model.
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The quest for the correct Pc
How should I define the Pc curve in the simulator
to reproduce the upscaled Geo Sw distribution
SWGEO?
J curves are scale dependent. Eclipse takes
simple form of J curves only.
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The end point scaling solution
For each cell in Eclipse, define Pc h 0 SWIR
SWGEO
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Pc selection for the simulation model
SCAL DATACAPILLARY PRESSURE DATA CORE
SCALE DRAINAGE OR IMBIBITION? NO LITHOLOGICAL
GROUPING ON SIM.SCALE
PETROPHYSICAL MODEL J-CURVES GEO-MODEL
SCALE MAY HAVE TO BE SIMPLIFIED PRIMARY
DRAINAGE DATA NO LITHOLOGICAL GROUPING ON
SIM.SCALE
GEOLOGY
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Topics

• Introduction
• Definitions
• Overall workflow
• Sw in Petrophysics
• Sw in Geology modelling
• Sw in Reservoir Simulation
• Upscaling issues
• Summary

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Summary (remember this)

• Use effective porosities (PHIE)
• Avoid net-to-gross as a property
• Use FWL instead of OWC
• Use appearant FWL for gas above oil
• Use J-functions (and start simple)
• J-functions should be derived at the proper scale
• Since permeability is so scale-dependent
• Treat 3D blocks near FWL correctly
• Consider use of appearant properties
• RT have the most difficult task
• Sw modelling is truly interdisciplinary
• NH work Best practice document
• Yet, there are many unresolved issues

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Philosofical epilougue
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Thank you for participating

• Feedback from you are welcome

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THE END