Sept 2009 The Geomodeling Network Newsletter

The Geomodeling Network Newsletter September 2009

W elcome to the latest version of the Geomodeling Network

newsletter. Last month was a quiet one in terms of receiving articles from
our members. I put this down to a lot of people being away on vacation,
where the most important thing on their minds was not sending in an
interesting article, but wondering where their next Pina Colada was
coming from. Because of this lackadaisical attitude  I decided to hold
back the release until now – but I hope you agree that it was worth
waiting for.

Talking of members, our network has grown remarkably quickly and we
have now shot past the 900 mark and are well on our way to 1000
members (unbelievable!). As befits any significant milestone, the 1000’th
member will be recognized in a suitable fashion (a knighthood, a Nobel
prize or lifetime membership to the Geomodeling Network – can’t decide
yet).

Now would be an excellent time to run the Workflow Manager. Set it for
100 realizations – that should be long enough to enable you to read this
month’s newsletter. Go and grab yourself a coffee (or a beer) depending
on how good or bad your day is going and put your feet up. For those
members who took the time to contribute the interesting articles
contained in this version – thank-you very much, it’s highly appreciated.

Finally, if there’s anyone out there just itching to contribute an article, I’d
be more than happy to receive them. Don’t worry if your article is radical
or off-the-wall. Both the newsletters and discussion forums are proving to
be great places for voicing opinions and stimulating conversations on all
aspects relating to geomodeling.

Cheers for now,

Mitch Sutherland

mitch.sutherland@blueback-reservoir.com

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Table of Contents

1. How to Model Separate Facies for Channel Thalwags
This is a response to an article raised in a previous Geomodeling Network
Newsletter and documents a workflow solution to this problem.
Sebastien Duclert –Geology & Geophysics Team Leader at Schlumberger SIS Page 3

2. Blueback Toolbox
The September 2009 release of this popular and FREE PETREL Plug-in is now
available – see below for recently added functionality
Paul Hovdenak – Software Manager at Blueback Reservoir AS Page 7

3. A New Multiple-Point Statistics Software
Ephesia Consult (formerly FSS Consultants) and the Stochastic Hydrogeology
Group of the University of Neuchâtel (Switzerland) have joined forces during the
last two years to develop new commercial software for efficient 3D multiple-
point simulation.
Dimitri D’Or – Environmental Resources Consultant at Ephesia Consult Page 14

4. Size is not important when you have the right Aspect Ratio
The importance of Throw-Length Ratios in Fault Seal Analysis
William Harvey & Titus Murray – FaultSeal Pty Ltd Page 18

5. Towards a set of Geological Rules for Characterizing Faults
Computing a Fault Network using Geostatistics and the Potential Field Method
DJ Fitzgerald, Intrepid Geophysics and Gabriel Courrioux, BRGM Page 23

6. Rigorous upscaling of net to gross: Is it possible? Does it have
value?
This is a good example of a question placed on the Geomodeling Network
discussion forum which has started to generate some response from our
members. Please feel free to comment further.
Tor Barkve –Technical Product Manager at Roxar Page 30



Member Articles, Reviews & Questions

1. How to model Separate Facies for Channel Thalwags?
Sebastien Duclert, Schlumberger SIS
I fell madly in love with
Geology.
In Object modeling, Petrel models channel/levee facies associations. Channel
Erin Brockovich sands are typically higher quality in the upper middle part of the channel.
Ideally, we would like to be able to model different petrophysical characteristics
than on the margins and at the bottom of the channel in this thalweg. The
workflow presented here allows the user to create a separate facies
representing the thalweg within previously modelled channels.

How to spot a geologist (1):

Someone who only includes The first part of the workflow is to generate a Bodies and Depth trend
properties from the Other output tab in Facies modeling. Tick on Body property
people in photos for scale
(only for the channel facies) and Depth trend (Leave the default options). Press
the Create buttons to generate the two blank properties that are stored in the
Properties folder.



You then need to re-run the modeling to populate those two properties.

The Bodies property is a simple body index, that will allow us to perform
operations on each channel independently inside the workflow manager.

The Depth trend shows the depth of each channel cell from the channel top,
relative to the total thickness of the channel. For each channel it has values
comprised between 0 at the top of the channel and 1 at the bottom.

The depth trend will allow us to isolate the cells located near the top of the
channel. By using the workflow below, we can create a new Depth Trend Sum
property.



The workflow sums vertically the depth trend values independently for each
Someone who scoffs when channel, then divides the sum by the maximum value of the sum for each
they see your compass channel. The resulting property is equivalent to a pseudo relative thickness of
doesnt have a clinometer as the channel.
standard

We can now create our separate thalweg facies by applying a combined if
statement on both depth trend properties in the property calculator.

1. Copy the initial facies property.
2. Create a thalweg facies in the property template
3. Use the Property filter to filter out the background and levee facies (Keep
the channel and thalweg facies ON) as well as the upscaled cells.

4. Run the following equation in the property calculator with the filter ON.



In plain text, this can be translated as:

To the cells of the copy of the original facies property where the relative depth
from the top is lower than 0.5 and where the relative thickness is greater than
0.5 (near the channel axis), assign the facies thalweg (code 15 in this case),
otherwise keep the existing facies (vary the thresholds according to the amount
of thalweg you want to create).

A different porosity or permeability distribution can then be used in the thalweg
facies.

An obvious limitation of this workflow is that, if the higher quality thalweg facies
has been identified in the wells, there is no guarantee that the axis of the
channel will run through it. Setting the thickness of the channels smaller than
the thickness of the channel intervals in the well will help. If the thickness of the
modelled channels is larger than the interval, the wells will usually tie to the
margins of the channels.

The channels have to be modelled with a simplified channel facies (ignoring the
She looks like a cocktail distinction between thalweg and non-thalweg) and several realizations can be
compared to find the one that will match the best the high quality sand in the
waitress on an oil rig “Best in
well.
Show”
In order to re-associate the detailed facies model with the result of the
Anonymous
workflow:



1. Upscale the detailed log (including the thalweg facies).
2. Copy the result property (Channels with thalwegs).
3. Set the Property filter to Upscaled cells only.
4. Run the equation below in the property calculator (with the filter ON):

Filtered

(Upscaled
=
cells only)

For this particular realization and well, the match between
the thalweg facies in the upscaled cells and modelled
thalweg is pretty good, except for the lowermost channel.
Another realization might do a better job. Changing the
cut-offs on the depth trend properties might also help.

2. Blueback Toolbox
September 2009 version has been released

The new version of the Blueback Toolbox is now available for download. Several
hundred Petrel users now have installed our Toolbox, and its great to see that
we can help expanding and optimising reservoir characterization workflows in
Petrel.



All the new features in this release have been developed based on feedback and
suggestions from Petrel users, so feel free to forward your ideas for the next
It was the foolish man that built release of the Toolbox!
his house upon the sand.
Frequency power spectre
Matthew 7:26 - A tool for seismic interpreters to better understand the frequency content of the
seismic signal

This much requested little tool allows an interpreter to easily compute the
Power-Spectrum within a defined area "Probe" for any Seismic Line.
The use of such an information helps the interpreter to better understand the
frequency content of the seismic signal at any given depth range.
From this data, if some knowledge of local velocities is at hand, geologists can
for example estimate the actual resolution of the seismic signal.
Power spectra plots can be also used to estimate the frequency content
characteristics of certain areas, for example in order to derive analytic wavelets.

The tool also allows the user to display and compare the Power Spectra of areas
across different lines.

Analyse the frequency spectre in your seismic

This feature was developed based on input from Gaston Bejarano, former
Geophysics Champion at Schlumberger SIS. Gaston has been working with
seismic software for 30 years, and has forthe last 6 years worked in the Petrel
team.



Gaston Bejarano, Geophysics Champion Schlumberger SIS

Unflatten interpretation
- Option to unflatten a seismic interpretation done on flattened seismic

This feature was added based on feedback from users using the Flatten Seismic
Cube feature in the Toolbox. When interpreting on a flattened seismic cube, the
interpretation now can be unflattened according to the seismic pick on the
original unflattended seismic.

The functionality was requested from one of the geophysicists and expert
volume interpreters at a major American oil company.

Import images as 2D seismic lines
- This feature allows for display of any kind of image onto a seismic 2D section

The functionality allows to position a digital profile (image) along a crooked line.
Because the image is represented as a seismic section, it can be used to visualize
in the 3D window together with satellite images and in the Interpretation
window to perform actual interpretation on it.

Workflow:

How to spot a geologist (3): 1. Position interpreted sections and geological interpretations from an
electronic document along an existing seismic line.
Someone whose shorts expose 2. Interpret directly on geological sections using seismic horizons and faults
way more leg than you ever 3. Co-visualize regional geological profiles with project data in 3D, perfect for
wanted to see exploration work

This tool is not converting images to real seismic amplitudes/wiggles. It is merely
a grayscale/pseudocolor image represented in seismic format.
In Petrel, a "pseudo color" to the sections is introduced by associating it to a
color template.



This functionality does unfortunately not work on Petrel 2008 version. This is
because of OCEAN extensions affecting color table handling introduced in Petrel
2009.

Schematic regional structural model displayed on a seismic line

The feature was requested by Edo Hoekstra at Schlumberger SIS. Edo has been
working with seismic interpretation tools for the last 12 years and is now a
Product Champion for Petrel geophysics.

Extend seismic 2D lines
- Extend the length of a 2D seismic lines
Geologists are 'scientists' with
This feature extends a 2D seismic section with empty traces, so that you can
an unnatural obsession with
make composite 2D line displays of seismic lines which do not intersect. Where
geology (rocks and alcohol). there are empty traces, you get a blank area in the section.
Often too intelligent to do This tool is very useful and you can quickly make composite sections without
monotonous sciences like having crossing 2D lines.
biology, chemistry, or physics,
geologists devote their time to
mud-worrying, volcano poking,
fault finding, bouldering, dust-
collecting, and high-risk
colouring.

Anonymous

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10


The functionality was requested by Petrel users at a major oil company

Create seismic cubes with data specified resolution
- Tool for creating a cube with lateral dimensions from another cube, 3D grid or
surface

This is an extension to the Make Cube feature in the Toolbox. You can now use
Petrel data objects to specify the lateral resolution and rotation of the cube you
create.

This enables a workflow where you can sample all types of seismic into a new
cube. Most capabilities in Petrel assume identical inline/crossline/depth to
seismic cubes, and multiple seismic vintages (particularly for 4D seismic) can be
a bit of a nightmare to deal with.

Workflow:
Make a simple 3D grid model with correct number of layers relative to the
seismic increment. And with a rotation that would best capture the data.
Sample seismic from various vintages into the simple model using standard
Petrel functionality
Use the Make Cube feature in the Toolbox to create a seismic cube with the
same resolution as the grid model
Convert the grid model into the new seismic cube

This new workflow would give a quick way to bring several seismic vintages in
line, within a subset volume.

Since you also can control the rotation of the cube, you can ensure that it is
canyon running at 45 degrees to the original seismic acquisition, you could
reduce seismic volume size greatly).

11


This feature was requested by Petrel users at a major oil company.

Server license option

A server license option is now available through a hardware key licensing
system. A dongle must be plugged into your server, with a license file defining
how many licenses (concurrent users) you need in your offices. This licensing
option has an associated cost to cover for the the third party licensing system
(SafeNet/Sentinel). Contact sales@blueback-reservoir.com for more details.

Submittal form for functionality requests

You can now easily submit a functionality request from within the Toolbox. On
the Info tab in the Toolbox dialog, you can start a Word document using our
submittal template, write up your suggestion, and send it to us. We encourage
you to send us your input! Many smaller upgrades are very quickly
implemented.

Bug fixes

The only larger bug fix to mention is on the Flatten cube feature. There was a
problem when the input cube was cropped and the interpretation was not
cropped. This bug is now fixed.

Blueback Toolbox - development requests

All new features are developed based on feedback from the users. With the
Petrel development kit (OCEAN) there are limitations to what can be developed,
but in many cases we see that small development efforts can make a big
difference to Petrel users :-)

Do you have functionality ideas yourself ?

Are there some specific missing features in Petrel you would like to add?
Get in touch with us to discuss how we can help you and your company to
optimize your workflows and Petrel usage.

For each new Blueback Toolbox version, a Blueback iPod is the award for the
best new feature included.

12


For the August'09 version the winner was Gaston Bejarano at Schlumberger SIS in Stavanger

Blueback Toolbox - download info

The Blueback Toolbox is available to anyone interested and can be downloaded
from our FTP site.

Use our FTP site
ftp.blueback-reservoir.com
User: TOOLBOX
Password: xxToolbox2009

There are 3 zip-files with different versions:
1 – TOOLBOX 1.3 2008-1035-040909.zip.
This is the installation file if you are running Petrel 2008 on XP.

2 – TOOLBOX 1.3 2009.1 32bit-1034-040909.zip.
This is the installation file if you are running Petrel 2009 on Vista or XP 32 bit.

3 – TOOLBOX 1.3 2009.1 64bit-1034-040909.zip.
This is the installation file for those running Petrel 2009 on Vista 64 bit.

Download the file you need, unzip it and run the installation on the PC you will
be running the Toolbox on.
Then start Petrel and open the Blueback License dialog from the HELP pulldown
menu.
To activate the Toolbox – you must send to support@blueback-reservoir.com
the COMPUTER CODE for your installation. This is found if you click the Manage
Licenses button. Send us the computer code for the TOOLBOX.

Contact support@blueback-reservoir.com if you have any problems installing
the Toolbox.

13


3. A New Multiple-Point Statistics Software
IMPALA – Improved Multiple Point Algorithm using a List Approach
Dimitri D’Or, Environmental resources Consultant, Ephesia Consult

Ephesia Consult (formerly FSS Consultants) and the Stochastic
Hydrogeology Group of the University of Neuchâtel (Switzerland) have
joined forces during the last two years to develop a new commercial
software for efficient 3D multiple-point simulation.

The challenge was twofold:

Develop a fast (CPU) and efficient (RAM) algorithm, with as little
hardware induced limitations as possible.
Ensure that its results are geologically realistic even when based on a
structurally complex training image.

The beta version of this software, named IMPALA, is now ready and we
believe that the challenges have been met:

It is a fully parallelized algorithm which dramatically improves the CPU
performances
The multi-point statistics are stored in lists rather than search-trees,
thus resulting in a substantial reduction in RAM requirements
It implements an innovative synchro-processing which provides
superior results compared to traditional post-processing
It proposes new methods for conditioning to existing hard data
It offers a wide choice of simulation path strategies
It can account for non-stationarity, using a-priori expert information
It embeds a novel algorithm to account for secondary attribute

Impala performances
The use of lists instead of trees results in a large reduction of RAM usage
(Figure 1). In addition, the parallelization of the code allows for drastic
reduction of the computation times. This is shown on the speed-up curves
(Figure 2) where the speed-up is the ratio between the computation time
with p processors and the computation time with 1 processor. Ideally, the
speed-up curve must be close to S_p=p, which is the case here.

14


Figure 1: Comparison of RAM usage between lists and trees.

Figure 2: Speed-up curves for 3 test situations.

15


Examples
Example 1: The Lena Delta

This example uses the Lena Delta as training image along with auxiliary
information about the distance to the sea (Figure 3). On the simulation
grid, two auxiliary variables are available: the distance to the sea and the
orientation (Figure 4). The results of the simulation are given at Figure 5.

Figure 3: training image and auxiliary information on the training image.

Figure 4: Auxiliary information available on the simulation domain.

16


Figure 5: One realization produced by Impala.

Example 2: 3D simulation

This example aims at simulating on various 3D grids using a 100 x 100 x
60 cells training image (Figure 6a). Results are shown at Figure 6b.

Figure 6a: Training image.

Figure 6b: Some realizations.

17


Other examples can be found at :
http://www.ephesia-
consult.com/Table/Impala/

If you are interested in becoming a
partner of Ephesia Consult to
commercialize this software please
contact:

Roland Froidevaux
E-mail: roland.froidevaux@ephesia-
consult.com
Phone: +41 (0)22 300 4629

4. Size is not important when you have the right Aspect Ratio
The importance of Throw-Length Ratios in Fault Seal Analysis
William Harvey & Titus Murray, FaultSeal Pty Ltd

When building a static reservoir model, the absence and inaccuracy of fault data
can result in the misinterpretation of structures, with the outcome often being
poor reserves and history matches.
The important thing to remember most of structural geology is “Not Rocket
Surgery”. It is all about simple geometry that any good geologist or interpreter
understands.
Last issue we investigated the importance of displacement (throw) profiles on
the correct interpretation and subsequent analysis of faults. This issue we
illustrate another straightforward structural geological tool Throw Length Ratios
and how they can be used to assess the fault validity.
In the article are examples from our software application FaultRisk . Where we
have imbedded structural validation into tools and work flows for used by both
our customers and us so we can more accurately calculate the probability of
faults sealing or leaking.

18


Geologists, secure in their
vague estimates have forever Throw Length Ratios
conflicted with engineers and Structural Geologists use the ratio of maximum fault throw and length to
their need for a definitive, understand faulting processes. Throw-length ratios can also be used to assess
quantifiable answer since the the accuracy and validity of a fault interpretation.
building of the pyramids. We feel that it is a vital step in assessing the abilities of these faults to seal or
The ancient Egyptian leak hydrocarbons.
engineers had determined that
Factors affecting the interpretation include:
the Great Pyramid would
require 1. Factors affecting fault length:
a. Position and direction of measurement relative to seismic
6961105709.356732519874 acquisition;
886510 metric tons of stone b. Seismic resolution;
blocks to construct. The c. Inclusion or exclusion of damaged zones.
ancient Egyptian geologists 2. Factors affecting fault displacement:
a. Map censoring effect the: The maximum throw may be off the
yawned and disagreed. When map;
it turned out that only b. Time to Depth conversion
6961105709.356732519874 c. Well control to constrain growth and fault inversion
886509 metric tons were d. Velocity and geometry causing miss positioning of images
required, the geologists
sneered and said, "I told In exploration regional faults are frequently longer than they appear in models
you your calculations were or maps. This extra length results in frequent fault interaction and the
wrong." The geologists, subsequent segmentation of faults. Fault interaction affects the amount of
stress on faults, therefore altering the maximum displacement of a fault or fault
having been proven correct
segment (Shultz et al, 2005).
and superior, have been
envied by engineers since that In appraisal and development faults are often shorter than interpreted due to
fateful day. To this day, the miscorrelation of faults at seismic resolution. This leads to static models having
distinction between the two is inappropriate compartment sizes.
quite simple, an engineer is a Suitable throw length ratios
geologist with his brains Plotting the maximum displacement versus length of multiple normal faults
knocked out. results in a linear relationship (n=1) (Fig. 1). The relationship between the
Anonymous maximum displacement (dmax) and the maximum length of a fault can be
expressed as:

Dmax = cLn
Assuming that the linear scaling law is used (n=1) c is an expression of the
displacement / length ratio (dmax/ L) (Kim and Sanderson, 2005). The maximum
displacement versus length diagram of Schultz et aI (2008) suggests that throw

19


length ratios between 0.001 and 0.1 are acceptable. Throw length ratios are
therefore good indicators that a fault is more complex than first thought and
may require reanalyzing.

Figure 1 – Plot of maximum displacement versus length from a compilation of
faults (Source: Schultz et al, 2008)

Examples
As an example of this type of analysis we will look at the Gulfaks model. Fault
polygons have been made from the Petrel grid and imported into FaultRisk™.
The throw length ratio can be seen in the throw profile window (Fig. 2), the blue
example has a displacement length ratio of y=0.0138, which is well within our
acceptable range. This is further supported by reviewing the displacement
profile that has been created by FaultRisk™. The red example has a significantly
lower displacement length ratio of y=0.055 and is somewhat of an outlier when
plotted upon the diagram of Schultz et al (2005).

We have contingency plans for
anything that can possibly go
wrong ... except the volcano.

When Time Ran Out... – 1980

20


Figure 2 –Displacement profiles and displacement versus length plot
Further review of the displacement profile of the red fault indicates that fault a
series of faults may have been combined across branch lines or the maximum
displacement is off the map.

For a more accurate interpretation we can split the fault. (Fig 3).

21


“You want to compare Figure 3 – Displacement profiles for the new split faults
brainpans? I won the The fault was split into three sections. The new faults have throw/length ratios
Westinghouse prize when I (y=0.016, 0.0228 & 0.016) plot more appropriately (Figure 4).
was 12, big deal. Published Splitting faults can be extremely useful in the accurate analysis of fault seal
at 19, so what. I got a capacity of the compartment highlighting branch lines, genetically separate
double doctorate from faults and relay ramps acting all of which may act as breaches.
MIT at 22, Chemistry and Some care should be used when using Dmax/L ratios when combining different
Geology. I taught at data sets where discrepancies of fault type, tectonic history and analytical
Princeton for two and a techniques, result in scatter of results are plotted.
half years. Why do I do
this? Because the money's
good, the scenery changes
and they let me use
explosives.”

Armageddon – 1998

Figure 4 – Maximum displacement versus length plot for new split faults (Schultz
et al, 2008)
This style of analysis is quick and easy to do and will ensure your 3D models have
good-looking faults.
Most importantly to us when investigating the probability of fault leakage
and/or across fault flow this analysis is a vital step in the workflow.

22


If you want to try FaultRisk™, have any queries on this article or fault seal issues
please contact us at titus@faultseal.com and/or visit our website
www.faultseal.com.

5. Towards a Set of Geological Rules for Characterizing Faults
Computing a Fault Network using Geostatistics and the Potential Field
Method.
DJ Fitzgerald, Intrepid Geophysics and Gabriel Courrioux, BRGM

Faults network
Faults are bounded by the existence of other faults :
---> User has to define relations between faults

• Set of faults: f1, f2, f3 ….
f6
• Relations:
• f1 stops on f2
• f1 stops on f4
• f5 stops on f4
• f3 stops on f6
• f6 stops on f2

f5

f1
f3 f4
f6 f2

23


Deriving a consistent geometry from relations
Someone who can jump start a do we need to guarantee transitivity of relations ?

campfire in wet weather with the
judicious application of a beer
fart.

“Things get worse under By transitivity F4 stops on F2 (and F1)
pressure”

Murphy’s Law of Thermodynamics

24


”We tend to overestimate the effect of
a technology in the short run and
underestimate the effect in the long Mixing finite and infinite faults in fault networks
run” • The troubles start here -
Amara’s Law – What about stoping an infinite fault on a finite fault ?
– What about stoping a big finite fault on a small finite
fault
– Is it possible to anticipate these consistency problems
?

• assumption behind is that the relation « stops on » means that displacement
agregates from minor faults to major faults.

• Defining another relation type such as « is cut
by » could probably help

Infinite fault stoping on a finite fault.
M M belongs _to_fault_domain (i)
if center of (i) is visible from M
2

1

25


“Conclusion = the place where you got tired
Taking faults into account for
of thinking”
interfaces interpolation
Anonymous Discontinuous drift functions are added to the cokriging equations
They model the shape influence of the fault

1
1

0
0
Transversal profile radius Transversal profile
-1 1
1
0
0 Longitudinal profile
Longitudinal profile radius

Finite Faults Infinite Faults

Effect of faults on the interface modelling

26


Effect of fault radius on the interface modeling

Infinite fault Radius = 500 m

Radius = 400 m Radius = 300 m

Taking faultnetwork into account for
interfaces interpolation

The discontinuous drift functions is associated to each fault as for single faults
but takes into account the relations

0 (f1)
1 (f2)
0 (f1)

0 (f2) f2
0 (f2)
1 (f1)

f1

27


Limagne basement(30km x 35 km)
COPGEN project (COmpilation du Potentiel Géothermique National)
collaboration Ademe, BRGM
From Adrien Dagallier – Master thesis -

Note the effect of cumulative displacement
when secondary faults branch on a major fault

Pb with the drift function
Telescoping drift functions
There is a boundary 01
for f1 in this area
Depending if f1 center is
visible or not
0 (f1)

1 (f2)
0 (f1) -1 (f2) f2
0 (f2)

1 (f1)

f1

28



Geometry of faults may look ok, but may have an undesired impact on layers interpolation.…
Explanation on previous slide.

New: Display faults only in the formations they affect.

ONLAP

If pile in Top:
Fault are displayed within
the formation they affect and within the
upper next in the pile

If pile in Base convention
Faults are displayed within
the formation they affect and within the
lower next in the pile

29


Consistencies between Faults-Faults and Faults-Series relations

• A fault should affect at least all series affected by faults which stop on this one.
• In other words a fault must affect a subset of the series affected by the faults on
which it stops on.

Could work in some cases Why ?
F1 does not exist for f2_series

> What should we do :
• Automatically check and
F2 stops on F1 fix ?
• Forbid ?
Will always work
• Allow ?
• Warnings ?
• ….

6. Rigorous upscaling of net-gross: Is it possible? Does it have
value?
Tor Barkve – Technical Product Manager, Roxar

Modelling of net-gross in flow simulators is difficult for at least three related
reasons:

1) Net-gross is both a volume property (used to calculate pore volumes) and a
transport property (used to calculate transmissibilities and well indicies). Finding
a upscaling method that can preserve both these aspects from the fine scale to
the coarse scale is challenging.

2) The view of net-gross changes in the modelling chain. For a petrophysicist and
a geologist, net-gross is a pure volumetric property. For a reservoir engineer not.
We may be talking different languages and not communicate correctly.

3) The definition of horisontal and vertical permeability in the geo model may be
blurred - how does it relate to net-gross? Is the definition applied in sync with
the use in the flow model?

I would appreciate your views: Is it possible to treat net-gross rigorously in a
flow model? Does it have value to have net-gross separately defined in the flow
model or should we process it together with porosity/permeability before the

30


upscaling?

This discussion is branched from a previous discussion initiated by Ivan Nikulshin
on upscaling in Petrel and RMS.

Comment 1: Tim Wynn
Senior Reservoir Geologist at AGR-TRACS

Good question and it is a definite can of worms. Before getting into the
upscaling issue I think points 1) and 2) should be discussed. Although NTG is
often defined from a porosity cutoff, I have always understood it's origins to be
related to permeability. The two are linked via poroperm relationships which are
straightforward in many clastic systems. Porosity is usually used instead of
permeability because it is easily defined from logs. I think this poroperm link is
often forgotten by geologists and NTG can become defined and discussed purely
in terms of porosity.

Therefore, if the link between porosity and permeability is remembered, net to
gross is a definition of effective STOIIP/GIIP i.e. the component of the
hydrocarbon volume that will flow. This way of defining effective or flowable
hydrocarbons was probably valid in the days before full field 3D simulators.
However, the problem with a log derived porosity cutoff is that it is deciding a-
priori what will flow. Removing pure shale or other tombstone lithologies like
evaporites is valid but even quite tight porous rock with low saturations can
contribute energy to a system even if the hydrocarbons within it are unlikely to
be accessed.

Another problems is that NTG definitions are fluid dependent and I have seen
NTG logs with different porosity cutoffs in the oil and gas legs. This is because
the definition of what will flow has a different permeability threshold for oil vs.
gas due to viscosity contrasts. This maybe all well and good for a static log
derived definition of NTG but plays havoc in a simulator as soon as the GOC
starts to move.

A practical way to handle the problem is to be inclusive of all reservoir rock
types. Non reservoir rocks (e.g. shales and evaporites) can be defined as non-net
as they usually contribute no volume or flow capacity. Water saturations and
absolute and relative permeabilities then define what will flow hydrocarbons
during simulations. Obviously this has a computational overhead but does avoid
the problems mentioned.

Comment 2: Steve Flew
Technical Manager - Field Development & Production Services at Schlumberger

You raise a very valid point, and I think its often down to communication. You
detail cases 1) and 2) well - I'd just add to 2) that its really the transmissibility
31


(and not instantaneous or 'averaged' permeability) that is affected by NTG per
se.

You're 3rd point I suspect will go past all the non-upscalers - I think this is often
the crux of the challenge:

horizontally, the volume factor NTG is easy to understand - it is applied to
reduce the cell face/face area between 2 adjacent cells, thus reducing the
transmissibility between those 2 cells (but NOT affecting the perm used for any
well inflow calcs, usually...).

when you come to vertical flow its more of a challenge, there was a previous
thread some months ago on some rules of thumb for reducing vertical
transmissibility based on the NTG raised to some power. I can't recall the
gentleman who wrote an excellent article on that, but seem to recall he was
from Shell. The key here is you cannot mod the vertical perm in many cases, it
has to be the cell-cell transmissibility that is 'corrected' for poor vertical
connectivity, since the 'kludge' of kv/kh across big cells often means you can kill
well inflow performance, where in deviated or horizontal wells, kz comes back
in.

Hopefully, there'll be a few more comments to keep this important, but oft
ignored/overlooked thread alive

cheers
Steve

Requests for the newsletter No6

The next newsletter is planned for Oct/Nov 2009, so please send articles to me
at the following email address for inclusion (mitch.sutherland@blueback-
reservoir.com).

Finally, a bit of fun at the expense of engineers everywhere……

http://www.youtube.com/watch?v=LOle_cIXuyw

Fin

32

Sept 2009 The Geomodeling Network Newsletter

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