Finite Difference vs. Finite Element Models
Also see:
From: Reservoir Simulation General Discussion discussion <SIM@communities.spe.org>
Discussion: Reservoir Simulation General Discussion
Author: Mr. Brian K Coats (Mr. Brian K Coats)
Date: 20060611
Subject: Re: Future Directions in Reservoir Simulation
While I realize
we are talking about future directions, any statements or conclusions that
current technology is inaccurate or insufficient or that some newer
technology is superior should be accompanied by (the simplest possible)
demonstrable and reproducible evidence, or reference to such.
It is through
such evidence that we are able to improve our empirical or partly empirical
macroscopic (blockaverage) representations of porescale structure,
phenomena, and properties. These models and properties include
permeability, relative permeability, capillary pressure, pvt,
compaction/fracturing/geomechanics, and generally represent things that we
are not able to accurately characterize or model or define distributions of
on the microscopic scale, at which the true physics apply, but are able to
sample, observe, and tune our models to on a macroscopic scale. When
these blockaverage representations are found to be inadequate, we respond
by defining multiple systems, such as in dual porosity models for naturally
fractured reservoirs.
For any given
case, any given simulator may or may not have functional dependencies or
options/couplings available for sufficient representation of blockaverage
porosities and permeabilities. I have no reason to believe that finite
element models offer any advantages in simulating geomechanical effects.
With regard to
Mr. McCracken’s statement about finite difference models not describing
radial wellbore flow properly in rectilinear coordinate systems, I
constructed the following test:
The second SPE
comparative solution project gives a single well radial coning problem in a
10 x 15 rz grid. Outer radius of the reservoir is 2050 ft. 3
cartesian grid systems were created with constant areal grid spacing,
computed to give the same total reservoir volume as the radial case.
Cartesian grids of 11x11x15, 51x 51x15, and 101x101x15 were used, the
areally square blocks having sides of lengths 330.32, 71.25, and 35.98 ft,
respectively. The well was placed at the center of each of the
Cartesian grids. Well indices are computed internally using the
Peaceman formula. All 4 runs in Sensor give virtually the same results
in terms of production up to 50 days of simulation. The original well
constraints were retained, and change from an initial oil rate of 1000 stb/d
to 100 stb/d at 10 days. Comparison of bottomhole pressures shows that the
Cartesian cases give 1 to 2% error in bhp at 10 days relative to the radial
case, but bhp is otherwise essentially the same in all runs out to 50 days.
The 51x51 and 101x101 cases give virtually identical results out to 900
days (end of run), with water and gas cumulatives about 10% higher than in
the radial case. This difference is attributed mainly to the different
representation of the outer boundary, at which a pressure response is
evident. The 11x11 case exhibits some numerical dispersion reflected
in cumulatives at 900 days, with water and gas about 10% lower than in the
finer Cartesian cases. Use of the 9 point differencing option did
reduce this difference somewhat, but had no effect on results up to 50 days
(The xy vs. radial nature of the flow has a more pronounced effect on the
11x11 case). The radial (spe2.dat) and 11x11 Cartesian cases are
reproducible using the downloadable Sensor version from our Coats
Engineering website.
Perhaps Mr.
McCracken would like to construct a finite element model for the rectangular
reservoir boundary case, and demonstrate the problem with representation of
well flow, say compared to our 51x51x15 case. I would be glad to
provide any output files, or even short term license files, for use in
making comparisons. As our cases are taking initial 10 day steps,
perhaps it would be desirable to take smaller steps to more accurately
capture the transient, and then examine the effect on production of any
differences in the transient. Our models show no such effect or
differences.
Also of
interest would be FDM vs FEM efficiency. In Sensor, these 4 cases take
0.06, 1.39, 91.3, and 554 cpu seconds (in the order described), on a 2.4 Ghz
Windows desktop pc, using the implicit formulation. For the Cartesian
cases, a 1/8 element of symmetry could have been used to obtain the same
results with significantly reduced run times. If it is desired to
eliminate linear solver times from those timings (since it is claimed that
improvements in linear solver technology will make FEM more viable), linear
solver time in the radial case is too small to be measured accurately, but
in the 3 Cartesian cases is 0.9, 80.5, and 497.4 cpu seconds.
Brian Coats
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From: Reservoir Simulation General Discussion discussion <SIM@communities.spe.org>
Discussion: Reservoir Simulation General Discussion
Author: Mr. Brian K Coats (Mr. Brian K Coats)
Date: 20061116 17:25:45
Subject: Part 1: Re: Effect of cell dimensions on Kv/Kh ratio
Kes, your last posting is indeed controversial!
Firstly, there is no link between the Effect of Cell Dimensions on Kv/Kh
Ratio issue to the issue of FD vs FEM modeling. The observation that
vertical perms often need reduction below measured values to match behavior
was I believe correctly explained as a sampling issue by Mr. Hicks in his
postings last August 24. It is in no way related to the numerical model
formulation as has been suspected and implied. The opposite problem can also
occur  you might capture a portion of a discontinuous shale layer in your
core that yields a lab measurement of zero Kv, when in fact, on the
gridblock scale, there is vertical communication.
But since the issue has been raised... There are many reasons that the
industry has not substantially moved in the direction of unstructured FEM
modeling, but I would argue that they do not include, as in Kes's loaded
statement, tradition or inertia. The main reason is that FD models are the
still the simplest and most efficient models that are sufficient for full
field simulation.
Major problems with unstructured grids include upscaling (alignment of
coarse and fine scale grid boundaries), conformance to boundaries (small
partial cells along boundaries), very complex control volume shapes in 3D
spacefilling grids, and substantially increased coefficient generation and
possibly linear solver cpu times (as opposed to structured grids with same
number of cells). Put more simply, gravity has affected nature to provide
reservoirs with features and properties that are most closely and easily
represented by rectilinear shapes (blocks) and smooth but possibly
discontinuous surfaces (faults and layer boundaries).
Mr. McCracken's comment about minimization of grid orientation effects
applies only to high mobility ratio displacements such as in steam flooding,
and even then, ninepoint difference approximations generally reduce those
effects in FD models to acceptable levels. And I disagree with his comment
that FEM models flow in the true flow direction, unless he means with a
dynamic grid.
The full tensor permeability issue is simply not a significant problem
with conventional methods. We can construct our grids along the bedding
planes to approximate as best we can the korthogonal condition for a
diagonal permeability tensor. We can measure or infer permeability along and
perpendicular to the bedding plane (have you ever seen measurements of the
off diagonal perm tensor components?). We can stairstep any faults or fluid
contacts with sufficient grid resolution such that the stairstepping has no
adverse effect on results. In an unstructured grid, control volume
interfaces that are not perpendicular to one of the 3 principal axes of any
permeability tensor create the REQUIREMENT for tensorial treatment of flow
at those interfaces. Conventional gridding techniques and FD models at least
have the ability to approximate and assume korthogonality.
<continued in Part 2>
From: Reservoir Simulation General Discussion discussion
<SIM@communities.spe.org>
Discussion: Reservoir Simulation General Discussion
Author: Mr. Brian K Coats (Mr. Brian K Coats)
Date: 20061116 17:27:43
Subject: Part 2: Re: Effect of cell dimensions on Kv/Kh ratio
And perhaps the biggest justifications here are that 1) we are going to
history match away any differences that might be caused by the differences
in flow representation, and 2) there is so much uncertainty in the local and
blockscale property distributions (as indicated by the Kv/Kh ratio
discussions  perhaps there's a link after all!) that anything more detailed
than the conventional gridding and FD flow treatment is unnecessary.
There have been several publications we are aware of demonstrating the
advantages of FEM over FD. In all such cases using reproducible examples, we
have found that our FD model was capable of producing results equal to or
better than the FEM results presented. In one such recent paper, an FEM
model was shown to give significantly less numerical dispersion for a 1D
miscible, zero cap pressure Buckley Leverett displacement. But the authors
either were unaware of or ignored the fact that the FD model exactly
reproduces the analytical (shock front) solution if it is run at the impes
stable step limit! Any desired level of increasing numerical dispersion (and
increased cpu time) can be introduced into the FD model results by running
at increasingly smaller timesteps below the stable value.
We would be glad to compare our FD model with FEM model results for those
or other cases that can be equivalently and easily represented. I suggest
that FEM model results and performance might be presented for some of the
SPE comparative solution problems, spe1, spe9, and spe10 (black oil) and/or
spe3 and spe5 (compositional) (our results are available on our website).
The fact that no FEM model results have ever been submitted in these
projects or presented subsequent to them is a clue to the results that we
believe these comparisons would show. Or we might start with some very
simple single phase tests for unidirectional (xdirection) flow in a 2d
areal cross section of a rectangular reservoir, with constant pressure
boundary conditions on either end, and diagonal permeability tensor. Our
model can get the correct q/dp ratio using any value of nx>2, and the
results are completely independent of choice of Ky. It's not clear to me
that this would be the case using an unstructured grid using, for example,
hexagonal prisms. The problem could then be made progressively more complex
to demonstrate diagonal flow issues, etc.
Regards,
Brian Coats
Coats Engineering
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From: Reservoir Simulation General Discussion discussion <SIM@communities.spe.org>
Discussion: Reservoir Simulation General Discussion
Author: Mr. Brian K Coats (Mr. Brian K Coats)
Date: 20061120 09:48:43
Subject: Re: FEM vs FD
Deepankar,
I did not dismiss FEM, but it's interesting that you think so. I'm sure
it can be valuable for detailed nearwell modeling, and it definitely has
many valuable applications in other areas, although I think the problems I
noted for reservoir simulation still apply. My opinions are not based on
conjecture. I developed a 2.5D unstructured simulator in the mid 90's that
used a full perm tensor. Sait Kocberber did the CVFE and Pebi gridding. He
was often able to construct korthogonal grids, but not always, in which
case he computed the 'transmissibility matrix' for each connection
representing our form of the multipoint flux approximation. However, your
implementation could be better than ours was.
The main thrust of my post was that FD models are the 'simplest and most
efficient models that are sufficient for FULL FIELD simulation'. I'll extend
that a little and say they are also the most suitable for constructing
simple prototype models as engineers often do to answer basic questions. You
specify dx,dy,dz,depth,perms directly to the simulator and run. There's
nothing simpler or faster in gridding and modeling these simple approximate
systems.
Perhaps you, or others, could point us to or provide us with the simplest
possible reproducible examples, and FEM results and performance, using
diagonal perm tensors wrt Cartesian coordinates, demonstrating any or all of
the following:
1. That hybrid gridding is necessary in full field modeling.
2. Your model's ability, using hexagonal prisms, to give the correct
answer to the unidirectional flow problem I described, independent of Ky.
I've described a test problem and given our data and output files on the web
page
http://www.coatsengineering.com/unidirectional.htm
. The correct steady state rate is 474.7 stb/d. Sensor can obtain this same
result using as little as 2 total grid cells (without demonstrating
independence of results on Ky, that requires 4 cells). An additional test 
in your model, does a tracer injected in the middle of the high pressure
boundary disperse in the y direction?
3. The need or benefit of using dynamic grids. We have not been able to
obtain such, despite requests.
4. Your solutions to any of the SPE comparative solution problems.
We could endlessly trade claims and arguments, but no progress or
conclusions can be made without simple reproducible examples and results.
Thanks,
Brian@CoatsEngineering.com
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From: Reservoir Simulation General Discussion discussion <SIM@communities.spe.org>
Discussion: Reservoir Simulation General Discussion
Author: Mr. Brian K Coats (Mr. Brian K Coats)
Date: 20061202 20:41:58
Subject: Unstructured Grid Orientation Effects, part 1
Unstructured grid orientation effects
We've all heard many times about the problems with Cartesian grid
orientation effects that unstructured gridding can eliminate. But somehow,
it does not seem to be recognized that some commonly used types of
unstructured grids introduce orientation effects that can be as bad or worse
than the ones that they eliminate for nearwell flow in Cartesian grids, and
that make them inappropriate for use in full field modeling. This is one of
the things that the unidirectional flow problem I posed is intended to
illustrate.
I wonder if anyone has run that case using a hexagonal grid? It is the
simplest possible test that the model either uses a korthogonal grid or
implements a robust multipoint flux approximation, or both. Without going
into detail, the need to conform to boundaries and permeability
discontinuities adds great complexity to the already complex requirements.
And 3D is very much more difficult than 2.5D. But I won't at all be
surprised if the correct flow rate can be obtained. Random Ky would be the
best test, followed by added internal conformation to a zerothrow 100%
conductive angled fault. Solutions using more advanced unstructured grids
would also be very interesting.
No results are needed to predict what the answer is to the additional
question I asked regarding dispersion of an injected tracer, which would
give an indication of the modeled fluid flow path in the regular or
irregular hexagonal (or any shape other than rectangular) grid. The
unstructured solution will exhibit massive numerical dispersion in the y
direction, since flow will necessarily occur from the point of injection on
the left towards the right across all interfaces not parallel to the x
direction, i.e. flow will occur across diagonal hexagonal interfaces (those
not oriented in the x or y directions), obviously not in the true flow
direction, even if the grid is korthogonal! This is purely a gridding
effect, caused by having staggered blocks in the direction of flow.
<continued in part 2>
A pair of pictures here would beat a thousand words. A unique tracer
injected in each cell on the left boundary would show that the FD fluid flow
paths (streamlines) consist of a set of straight horizontal lines from left
to right, one starting at each injection cell. The unstructured flow paths
would make a very messy picture. If you took a map of the grid and started
from each cell on the left and scribbled up and down randomly and diagonally
towards the right, 500 times or so for each cell (assuming 10 hexagons in
the x direction), and added a single straight horizontal line from each
injection cell on the left to the right, you would be pretty close! For
nearwell radial flow, use of hexagonal grids will similarly result in
severe theta direction dispersion. I believe that this nearwell
unstructured dispersion is even greater than would be observed with 5point
Cartesian radial flow, because in the unstructured grid those staggered
blocks are directly connected (even though the unstructured grid tracks
fronts better, particularly for high mobility ratio displacements). Knowing
this behavior, it would be easy to construct any number of other cases that
clearly demonstrate that the incorrect answer is obtained when using
unstructured grids, where FD models can easily obtain the correct solution
through appropriate selection of the grid. For anisotropic nearwell flow,
structured radial grids can of course also be shown to have problems (since
the principal axes of the permeability tensor are most certainly not
generally in the r and theta directions!), unless perhaps a radial
multipoint flux approximation is implemented. This might make a very good
nearwell model, but it seems to me that all this indicates that either
cornerpoint or unstructured (rectilinear!) gridding based on approximate
streamlines (near the wells, the these might look similar to a radial grid
and would be korthogonal, and these can also be run in FD models) would
theoretically be, for deterministic systems, the best generalized
singlegrid full field gridding solutions, and would also make very good
detailed nearwell models. It also suggests that hybrid or dynamic grids
might be called for. But the real questions that we need to answer are (1)
what level of complexity in gridding is required, considering the
uncertainty in our descriptions, and (2) does detailed modeling of nearwell
effects have significant effect on full field recovery. These questions
might be addressed with respect to current capabilities through the other
examples I suggested regarding the need for refined, hybrid, or dynamic
grids in full field simulation. If anyone has a simple case demonstrating
any of these needs, please send it to me.
Occam and I disagree with the suggestion that complicated situations should
be considered before capabilities and needs are demonstrated through simple
tests.
Regards,
Brian
@CoatsEngineering.com
From: Reservoir Simulation General Discussion discussion <SIM@communities.spe.org>
Discussion: Reservoir Simulation General Discussion
Author: Mr. Brian K Coats (Mr. Brian K Coats)
Date: 20061206 13:56:46
Subject: Re: Unstructured Grid Orientation Effects, part 2
Can anyone demonstrate a more advanced meshbased FE solution to the
unidirectional flow problem? Does avoidance of transverse dispersion require
that the nodes be aligned in the direction of flow and/or transverse to it?
The pictures of 3D triangulated FE meshes that I've seen don't appear to do
that.
From: Reservoir Simulation General Discussion discussion <SIM@communities.spe.org>
Discussion: Reservoir Simulation General Discussion
Author: Mr. Brian K Coats (Mr. Brian K Coats)
Date: 20061213 12:17:01
Subject: Re: Unstructured Grid Orientation Effects, part 2
Since no one has responded, I'll again answer my own question with the
generalized prediction that avoidance of transverse numerical dispersion in
any numerical solution to the unidirectional flow problem, using a
blockcentered or a pointcentered grid, requires a 5point Cartesian
differencing scheme or its equivalent pointcentered RECTANGULAR node and
node connection pattern, since, even in the isotropic case, modeled flow
will occur through any existing diagonal connections due to nonzero flow
potential. If this is correct, global use of either unstructured
blockcentered grids (of similar block shapes, for example hexagons) or
triangulated FE meshes is inappropriate for full field modeling.
Current FE/unstructured methods may seem more valuable to the user
because of their apparent ability to more quickly and easily represent
complex geometry, but they are less valuable if the grid is inappropriate
for flow modeling. I believe that for the general anisotropic case, an
optimal FE mesh would be essentially identical to an optimal cornerpoint
grid with 5point connectivity and arbitrarily missing cells (for which at
least some FD models pay no penalty), except for the
pointcentered/blockcentered difference, in regions where the flow is
locally unidirectional. Other optimally gridded regions would have
unstructured cell/mesh shapes, such as the use of a 5sided (or 3+n sided)
block or its pointcentered equivalent to properly represent divergence or
convergence of a pair of (or n) streamlines, or the use of a manysided cell
for well blocks (looking similar to a radial grid cell with many
thetaincrements). FE methods do seem to offer flexibility in their ability
to use different types of elements for features like fractures and
conductive faults, but these can be represented I think much more easily and
efficiently, and for all practical purposes just as accurately, in grids or
meshes using multiple porosity/permeability systems.
I should note that for singlewell radial anisotropic flow, the global
and more uniform numerical dispersion in FE and unstructured (and 9point
FD) methods over FD 5point Cartesian and cornerpoint methods can be and
has been shown to be beneficial in tracking diagonal movement of fronts (but
at the expense of creating dispersion for flow in x and y directions), and
has or at least can have little adverse effect on results for Darcy flow
when there is no composition variation in injected or insitu fluids. The
latter is best understood by considering the homogeneous isotropic
incompressible radial case, in which the exact radius to an ideal piston
displacement front is a function only of the volume of fluid injected,
regardless of the flow paths.
If I were promoting superior technology, I would certainly be offering a
large number of cases and results that demonstrate it. The fact that no one
has referenced or provided any demonstrable evidence of FE superiority
regarding the issues raised, presented results for any standard benchmarks
(which obviously would exist for any model), or has offered a solution to or
answered any questions related to one of the simplest possible flow problems
that should not take more than a few minutes to set up and run, is I think a
good indication that the superiority does not currently exist. Demonstrable
evidence of superiority is usually a prerequisite for significant industry
adoption of any new (or 40 year old) technology.
Regards,
Brian
@CoatsEngineering.com
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From: Reservoir Simulation TIG [specommunities@spe.org]
Title:
Re: Simulation model w/nonorthogonal cells due to low angle fault
Created at 10/7/2007 6:38 PM by Mr Brian K Coats
JP,
Mr. Balnaves gave excellent advice in warning to be
careful.
Errors will depend on case, method, and possibly model
(vendor) implementation choices. If possible you should evaluate
sensitivity of results to your gridding (and any numerical method) options.
If that requires too much time, or if determining which solution is most
accurate is a problem, then construct a prototype that exhibits the same
gridding problems, preferably one to which the correct solution is known or
can be easily determined. Almost a year ago I submitted to this forum (the
original post date is now (incorrectly) listed as 12/2/06, "Unstructured
Grid Orientation Effects, Part 2") a very simple problem that tests the
ability of the gridding/numerical method to give the correct flow rate for
unidirectional (xdirection) flow in a 2D areal grid with constant pressure
boundary conditions, independent of Ky. The correct rate will be
obtained if the model properly applies a method resulting in korthogonality.
Consider conformation to a nothrow sloping 100% conductive fault, rather
than your thrown fault. Let a vertical nothrow fault stairstepped in
the xy plane in the 2D areal (uni.dat) Cartesian grid represent your
sloping fault. If we use any arbitrarily stairstepped representation
of the fault, with no changes to Kx and Ky across the fault caused by the
presence of the fault, (there are no data changes required to the FD
Cartesian model and) obviously this FD model will give the (same 474.7 stb/d)
correct result. The difference between this solution and those using
each of your options including corner point or unstructured grids, or finite
element meshes, that exactly conform to the slope of the fault, gives each
of their errors due to nonorthogonality. Sensor data and output files
uni.dat, uni.out are attached and are at
http://www.coatsengineering.com/unidirectional.htm (the simple case
description in English is here, use it so you don’t have to interpret the
Sensor data file to construct an Eclipse deck).
Nobody responded to my yearago request for
demonstration of an unstructured grid or finite element mesh solution to
this problem, even though it shouldn't take more than a few minutes to set
up and run. So I can only assume (and repeat my 10month old
conclusion) that these accurate and efficient solutions, using anything but
a grid or mesh forming regular rectangular prisms, don’t exist. Actually,
it was back then that I first made the suggestion that we might look
specifically at the issues involved here by adding internal conformance to a
nothrow angled fault. I also suggested that we look at the flow path
of a tracer injected at a point on the left boundary.
Some have criticized and may criticize this example
because it can be represented and solved exactly by FD Cartesian methods.
It is useful because it is simple, it is representative of a typical
fieldscale flow situation, and it can be solved exactly by at least one
method, so that we can easily measure error resulting from use of other
methods. Maybe someone can present a simple and significant example
where instead, the unstructured or FE method yields the exact representation
and solution.
A side note to Andras Gilicz:
Since you are recommending both of these types of
models (FEM to David Ogbe for hydraulic fracture modeling and unstructured
to JP here), I assume that you have used them. Have you performed this
simple unidirectional flow test? If so, what were the results, and if
not, why not, i.e. what indications do you have that your recommendations
provide the best or even good solutions to the problems described?
Regards,
Brian Coats
Coats Engineering
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From: Reservoir Simulation TIG [specommunities@spe.org]
Title:
Re: Finite element modelling  a contrachallenge
Created at 7/22/2011 7:38 AM by Mr Brian K Coats
Kes,
You’ve hit on one of the main reasons
for our disagreements on the value of FD vs FE in reservoir simulation – we
seem to disagree on what reality is, let alone how to simulate it! I’m
referring to your statements “...been able to reproduce … the generic
stressrelated directionalities observed (mainly through tracers) in the
majority of realfield flooding schemes”, and “…a contrachallenge that
requires the model to face reality”.
Following is a quote of Dr. Keith H.
Coats from his July 5, 2005 post “Effect of pressure on fractured reservoir
effective permeability” (you also discussed your opinions counter to these
in that discussion):
Dr. Flavio da Silva (with Petrofina
circa 1990) had over 30 years of field operational and engineering
experience with many if not most of the world's naturally fractured
reservoirs. A quote of his remarks appears in my paper "Engineering and
Simulation", Fourth International Forum on Reservoir Simulation, Salzburg,
Austria, 1992. Following is an abbreviation of his remarks: "All fractured
reservoirs I have seen ... have exhibited no permeability loss due to
pressure decline...(application of isochronal tests) for North Sea fractured
reservoir wells indicated no effect of pressure on fracture permeability...
Virtually all fractured reservoir wells over large ranges of pressure
decline exhibit normal or conventional ratedecline behavior which occurs
only when effective permeability is constant. ... the 1030 year large
pressure depletion .. behavior of at least eight North ! Se! a fractured
reservoirs cannot be explained if one assumes any significant effect of
lower pressure causing lower fracture permeability."
I’m no geologist or geophysicist, and
maybe some will give their opinions, but mine is that if the effects you
speak of don’t exist near wells in naturally fractured reservoirs, it’s not
possible that they exist away from the wells where temperature and stress
changes are much smaller (in fractured or unfractured reservoirs). I don’t
see any solid evidence that supports your selfdescribed conjecture
regarding permeability changes, and I see much against it. One of the most
common mistakes in simulation is to assume that movement of results in the
direction of observations due to the addition of some dependency or option
necessarily indicates that the dependency exists or that the option is
valid (another example: Eclipse TZONE).
But, just for the sake of argument,
and even though I don’t believe it, let’s assume that permeability stress
and temperature dependence is needed to reproduce observed flow. The next
question is “how much complexity is needed to model it?” The simplest
solution in a finite difference model (if coupling to a geomechanical model
isn’t necessary) would be to use transmissibility modifiers (separate x,y,z
direction values) that are any (possibly hysteretic) functions of stress and
temperature we might envision or desire, similar to what is in some models.
I’ll bet that is much simpler than characterizing whatever empirical
relationships you’re using in your finite element model, and it would
probably work just as well if not better. You should try it before assuming
the need for additional complexity.
We also disagree in that I don’t think
that any progress can be made in resolving disagreements without model
comparisons (when I said this on 6/4/2007 in “Discussion of SPE papers”, you
agreed with me!). We need the simplest possible cases that are designed
specifically to demonstrate the claimed advantages, either by demonstrating
an improvement over an existing solution, or by giving a solution to a new
completely defined problem. These cases are most easily created by adding
whatever complexity is necessary to known cases. All papers claiming
improvements should include such evidence, and willingness to do so should
be a condition for acceptance by all publishers. We are having these
discussions now only because of these deficiencies. As I’ve said many times
now, the fact that those claiming improvements and! t! he need for
additional complexities are not willing to present proper evidence continues
to indicate that those improvements and needs don’t exist.
Regards,
Brian
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